Noodlings

When referring to class object, you can refer to it directly by name. When creating an object, you can do:

[[Foo alloc] init]

There are cases though where you want it to be a bit more dynamic. Say you want a class method which creates an autoreleased instance. You might do something like:

+ (id)fooWithName:(NSString *)name
{
   return [[[Foo alloc] initWithName:name] autorelease];
}


The problem with this, of course, is that if you have a subclass, SubFoo, then +fooWithName: will return Foo objects, not SubFoo. To fix this, instead of hardcoding the class, refer to it dynamically:

+ (id)fooWithName:(NSString *)name
{
  return [[self alloc] initWithName:name] autorelease];
}


Since this is a class method, self refers to the class object. Now, no matter which subclass you are in, the proper type of instance will be created. And if you are calling class methods from other class methods in the same class:

+ (void)doBlah
{
  [self doBlech];
}


Hardcoding the class here would call that specific class’s version of the method, even in subclasses, which is usually undesirable.

If you are in an instance method, you can likewise do something like:

- (id)copyWithZone:(NSZone *)zone
{
    return [[[self class] alloc] initWithName:[self name]];
}


Now, this is all well and good. In general, it’s a good idea to to use [self class] instead of hardcoding the name. But let’s take this example:

bundle = [NSBundle bundleForClass:[self class]];

Seems ok. You want the bundle where this class comes from. Or do you? Suppose Foo is in a Framework and SubFoo is in an app using that framework. The above code will return a different bundle for SubFoo than it does for Foo. Depending on what you want to do with that bundle, that can be a bad thing. For instance, finding a nib. Unless it is expected that the subclasses provide their own nib, chances are, the nib you are looking for only exists in Foo’s bundle and the above will fail. The fix? Hardcode the class:

bundle = [NSBundle bundleForClass:[Foo class]]

Here’s an odd case that I ran into some time back. Suppose Foo implements a protocol that has an optional method -bingo. In SubFoo, you implement -bingo but you’re not sure if the superclass (Foo) implements -bingo itself as part of the protocol. If it does, we want to call super but if it doesn’t we can’t. The documentation is silent on the subject so we can’t rely on any particular behavior. How do we check it at run-time?

- (void)bingo
{
   ...
    if ([[[self class] superclass] instancesRespondToSelector:@selector(bingo)])
   {
       [super bingo];
   }
}


Besides the compiler warning that Foo may not implement -bingo, it looks like it works. If you are a SubFoo instance, it gets the Foo class and asks it if its instances can receive -bingo. Ah, but what happens when we have a subclass of SubFoo called ReallySubFoo. Let’s also say that Foo doesn’t implement -bingo. When the above code is called, from a ReallySubFoo instance, we get a SubFoo class which then gets asked if it responds to -bingo. It does because we implemented it in SubFoo. Problem is there is no super implementation of -bingo and we get a crash. Again, the fix is to hardcode the class:

- (void)bingo
{
  ...
   if ([Foo instancesRespondToSelector(bingo)])
   {
       [super bingo];
   }
}


In most cases, dynamically getting the class is the right thing to do but don’t do it indiscriminately. Take a moment to think about how it will play out in subclasses. You may be surprised.

In Hazel 3.1, I added the ability to upload files. Underneath the hood, I used ConnectionKit 2 to do the heavy lifting. It has an API similar to NSFileManager, making it very easy to add FTP, SFTP and WebDAV support without having to deal with the pesky details of those protocols. There were other factors on why I chose it, such as the fact that not only is it actively developed, but done so by people who use it in their own shipping commercial products. And it didn’t hurt that it’s maintained by my friends at Karelia.

If you have used this feature in Hazel, you would have encountered this interface:

It should look a bit familiar. It’s a homegrown, from scratch, implementation of NSOpenPanel made to work on top of ConnectionKit. I felt it important to provide a familiar interface instead of coming up with something new in this case. I did a bit of work to make sure it operated as closely to NSOpenPanel as was reasonable. It has multiple view types (icon, list, and column) and most of the controls you’d expect.

Of course, browsing a filesystem via various network protocols is a different beast so some things had to be done differently. The main thing is that the UI is asynchronous from the network stuff so no beachballing while waiting for responses from the server. The UI should be responsive at all times, even allowing you to cancel the panel at any point. In the upper right, there’s a reload button that changes to a progress indicator to indicate activity and you will see a loading message when trying to fetch the contents of directories that haven’t been loaded yet.

Visually, the biggest difference is the header up top noting which server is being browsed. Also, the sidebar is missing. There aren’t many standard locations that can be put there and I felt that favorite URLs should be managed elsewhere as it didn’t seem likely that users would want to hop from server to server by clicking different entries in the sidebar. The one common directory that I could think of was the home directory and so instead, I gave that its own button (note that not all servers/protocols have such a concept of a home directory and it also depends on what URL you use to connect to the server so YMMV).

Another omission is the search field. Since there’s no notion of Spotlight in any of these protocols, doing an exhaustive search of the remote filesystem would be resource intensive and not very friendly to the servers involved. While I may consider adding it back in to search the current directory, you can get much of the same effect by just typing out the name of the file. Just as in NSOpenPanel, it will end up selecting the entry starting with those characters. I also dropped the Arrange button and the Coverflow view, mainly because I didn’t think people actually used those things. Especially for Coverflow, it wouldn’t be particularly useful since we can’t get file previews without downloading the whole files themselves.

But besides all of that, I think you’ll find it to be a pretty decent facsimile of NSOpenPanel. You might be surprised by how much of NSOpenPanel’s behavior is replicated here, including some features many people don’t know that NSOpenPanel has.

And the resemblance is more than skin deep. CK2OpenPanel (the name of the NSOpenPanel implementation) has a nearly identical API to that of NSOpenPanel. It’s a mostly drop-in replacement for NSOpenPanel so programming to its API should be just as familiar. Why would you care? Well, you should care because I’ve contributed the code for CK2OpenPanel to be included in ConnectionKit. You can see all the code, warts and all. Play around with it. Use it in your own projects. Print it out and wallpaper your home with it. I felt it was the least I could do for all the work that’s been done on the backend. Also, I thought it would be nice if people adopted it so that there was a standard UI for this type of thing.

You can find everything on the ConnectionKit GitHub page. In particular, you want to check out the v2.x-beta branch. Note that there are separate targets/frameworks in the project for the back end (ConnectionKit) and front end (ConnectionKitUI), with the latter dependent on the former. Make sure to check out the README as it should answer a lot of your questions. And if you are interested in contributing to ConnectionKit, please do as there are a bunch of things that need work. S3 support would be nice, for instance.

Last but not least, a big thanks to Mike Abdullah at Karelia for working with me to provide the backend support for this. Rest assured that he will be receiving beers from me at the next conference we both attend.

Enjoy.

While working on the most recent patch to Hazel, I stumbled across some code that made me scratch my head. It seemed the code was unnecessary and was causing the bug I was trying to fix. I ended up removing that code while doing the fix thinking I must have been drunk/high/had a gun to my head when I wrote that code originally. Initial tests seemed to confirm it as things were working at first. A couple days later I discovered an older bug had resurfaced. Sure enough, I began to realize why I wrote that odd piece of code back when. I ended up backing out my recent changes and making the fix with the original code intact. I also made another change which was to comment the code explaining why the hell I wrote it that way in the first place.

[self now] != [self future]

One of the nice things about being a solo developer is that you don’t have to deal with the overhead involved when working on a team. You don’t have issues with miscommunication that you would when working with others. Or do you? Like it or not, you are still a team. Sure, you probably don’t need to communicate things with someone else about a current issue, but you, yourself, now, is not the same person as yourself a year from now. You’ll find many times that intentions are lost through the annals of time so documenting things as if there are other programmers on your team is a worthy exercise. Comment your code. Write descriptive commit messages. Edit your wiki. Your future self will thank you for it.

At the last NYC Cocoaheads meeting, I did a presentation entitled “Let’s Build a Text Editor: A Practical Introduction to the Cocoa Text System.” In it, I basically go through building a code editor from scratch, pointing out how to use the Cocoa text system along the way. I’m not going to reprise the whole thing here. The first part was an introduction to the basics of creating a document-based app and fitting the text system into that and the second part dealt with adding line numbering, which, if you’ve been paying attention at home, I covered here a while back. Instead, I’m going to focus on the third part, which is implementing syntax coloring.

One major aspect of syntax coloring is actually parsing the text. And guess what? I’m not going to cover that here either. There are books you should read on the topic. Like many people during their college years, I learned from the Dragon book though there are probably books more specific to parsing that you can find out there. Keep in mind, though, that you may not have to write as sophisticated a parser as you think. Sometimes, a lexical scanner is all that you need. Just parse the different elements you want to highlight (comments, strings, keywords, etc.) and ignore the syntactic structure. In some cases, this may be enough.

The focus of this article will be getting the Cocoa text system to do the coloring based on what your parser parses. Before we start, you should download the example project as this article refers to it throughout. I’ve created various snapshots at key points to show the progression. If you want, you can go through the various snapshots to see how the app was built up but for the purposes of this article, start at the snapshot called “Version 4”.

The Basic Approach

At the core of Cocoa text system is the NSTextStorage object. It stores the actual text as well as any formatting attributes. Being a subclass of NSMutableAttributedString, NSTextStorage allows you to apply different attributes to ranges of characters. For syntax highlighting, all we care about is the NSForegroundColorAttributeName attribute, which is the attribute that affects the font color.

If you look at the –parse: method in NoodleSyntaxHighlighter, you’ll see that it scans for C-style comments (/*...*/) and string literals ("..."). When it finds the range of a particular entity, it calls -addAttributes:range: on the NSTextStorage to annotate the characters with the appropriate color based on which entity we found. Note that this is how you modify any attributed string; it’s not specific to NSTextStorage. To make sure we keep up with the changes, we implement NSTextStorageDelegate protocol’s -textStorageDidProcessEditing: method. There’s an equivalent NSTextStorageDidProcessEditingNotification though I believe modifying the text storage is only allowed via the delegate method.

If you compile and run at this point, you’ll see that it pretty much works as advertised. We’re done, right? Not exactly. Try loading in a large-ish document. Around a couple thousand lines or so. Start typing really fast. It might depend on your machine, but you’ll find that the typing lags a bit. It seems that modifying the NSTextStorage is not quite as efficient as we’d like. Luckily, there’s a better way to do this.

Using Temporary Attributes

NSLayoutManager has a thing called temporary attributes. These are attributes just like attributes you use on an attributed string, except that they’re, well, temporary. They are more lightweight and designed specifically for cases like this, where the attributes are only used for display and aren’t something intrinsic to the document itself. It’s used for features like spellchecking and other places where you need to make temporary annotations to the text.

Imagine writing a rich text editor. You don’t want to make display-only changes to the NSTextStorage since those changes could end up being saved with the document. Sure, the code editor in this example doesn’t allow rich text but you can see where semantic division lies. The NSTextStorage is your “model” and you don’t want something like syntax highlighting, a “view” feature, to change the model.

In the example project, restore snapshot “Version 5”. This code isn’t much different. It’s adding attributes as before but instead of adding them to the NSTextStorage, they are added via the NSLayoutManager. Now, compile and run it and, if it wasn’t automatically restored from before, load up the large document you used earlier. You should find that it keeps up with your typing this time around.

Now you could stop here but I’d like to take it a step further. You have this nice parser and all. Surely, the ability to pick out different entities in the document is useful for more than just syntax coloring. Maybe you want to add more smarts to your editor. Maybe triple clicks will select the whole comment or string literal instead of just a paragraph/line. Maybe you want to know where code blocks begin and end for code folding. Or maybe you want special tool tips when hovering over different entities.

You can ask the layout manager for the temporary attributes at any character position. Just check for the color that’s set there and deduce which entity (comment or string) it is. Suppose, though, that the user can change the colors in the preferences. If entities are set to be the same color, this method won’t work.

Creating Your Own Custom Attributes

There’s nothing saying that you can’t create and add your own attributes to an attributed string. Sure, only ones defined by Cocoa will be used by the system for display but let’s decide for the moment to annotate our text semantically. Instead of setting the NSForegroundColorAttributeName, we set our own custom attribute to indicate the type of element we’ve found.

Restore snapshot “Version 6” in the example project. At the top of the NoodleSyntaxHighlighter implementation you’ll see that I’ve defined our own attribute (“noodleElementType”) and the two possible values (“comment” and “string”). The -parse: method is the same as before except instead of setting colors, we set our own custom attribute, marking regions of text as comments or strings.

That’s great but how do we get our colors? If you look at the NSLayoutManagerDelegate protocol, you’ll see a method called -layoutManager:shouldUseTemporaryAttributes:forDrawingToScreen:atCharacterIndex: effectiveRange:. What this method does is allow you to substitute different attributes for the ones that are there already. In our implementation, we check what element type we set during the parse phase, look up its color and return it as the NSForegroundColor. So, our NSTextStorage has semantic markup but when displayed, we can substitute in the proper display attributes to get it to color properly.

• • •

Now, there’s one thing I glossed over which is an obvious optimization. Notice how we re-parse the whole document on every change. When you get the text storage notification, you can query it for the range of characters that was changed via its -editedRange method. You can use this information to limit the range of the document you have to re-parse. What you can do is look at what element is there already (from your annotations from the last parse pass) and start parsing from the beginning of that element. You can even tighten it up further by limiting how far it has to parse. I leave implementing this as an exercise for the reader.

As you can see, doing the coloring part isn’t so bad though there are some slightly subtle details to keep in mind. The hard part is the parsing which I’ve conveniently (for myself) left for you to figure out on your own. Enjoy.

And in case you missed the link to the example project above: NoodleEdit.zip

[This was the topic of my presentation at the NYC Cocoaheads meeting last night. I thought it would be nice to also post on the topic here.]

[I’ve received email from Ken Ferry. See addendum at the bottom]

NSImage is a troublesome class. Over the years, it’s been misunderstood and abused. I think much of this is because of a lack of conceptual clarity in the docs and examples and the API itself can be confusing and misleading. Add to this having to mix with CGImage and CIImage and you can end up with a confused mess.

The way I like to think of NSImage is that it’s a semantic image. When you look at icons, they are made up of several versions at different resolutions. Technically, it’s 4 or 5 images but NSImage wraps those all up into one notion of an image. Semantically, it is an icon of, say, a house but underneath it’s made up of several actual images of a house. The main reason for these different versions is that some graphics do not scale well and it helps to have hand tuned versions, especially for the smaller sizes. You can also do things like omit certain features in the smaller sizes to simplify the graphics and make the icon more recognizable.

One key misunderstanding is the notion that NSImage has actual image data. While there are parts of the API that deal with a cached bitmap (more on this below), NSImage itself is not based on image data. It is best to think of NSImage as a mediator between the image data in the various representations and the drawing context.

Loading an image from a file and drawing it is usually straightforward. If there are multiple representations, NSImage will figure out the correct one to use when drawing it. Most of this is automatic and is an important key to understanding how to use NSImage. As you will see further on, bypassing this mechanism leads to many issues with size and resolution mismatches when processing NSImages.

I’ve created a little program to help illustrate the points in this article. Since it also includes a new reusable class, I’ve included it in my NoodleKit repo. I suggest checking it out. Just compile and run the ImageLab target (you may need to set the active executable in XCode in the project menu). The rest of this article will be referring to it so I recommend you run it while reading the rest.

When you launch the app, it will load the test image. It’s just a colored circle. Now resize the window. You’ll see that the color changes as you resize. The icon itself is made up of four different sized images and to make it clear which representation is being used, I made the circle different colors for each one. Whenever the color changes when you resize, NSImage is switching to a different representation based on the size. Here’s what the different reps look like in Preview:

Size is another confusing aspect of NSImage. One thing to remember is that size is not pixels. It represents a coordinate space. One way to think of it is that NSImage’s size is like NSView’s frame while NSImageRep’s size is like NSView’s bounds. For NSImage, its size is a suggested size to draw the image in the user coordinate space. If possible, it’s best to explicitly specify the destination rect.

Let’s take the case of drawing some custom graphics on top of an existing icon with several representations, like drawing a badge over your app icon. A common way to do this is to lock focus on the NSImage, do your drawing, unlock focus and then draw the resulting image. What -lockFocus actually does is allow you to draw into a cache. This has probably lead to a lot of the misunderstanding that NSImage holds image data. Unfortunately, there are issues with this approach. Mainly, because there is no context about where this image will be drawn so the resulting image is tied to a specific resolution. Also, you are editing a cache so none of this image data is reflected in the original image reps and actually, from poking around, it’s actually destructive in that it may end up removing any other representations.

In our case here we are modifying an icon with different sized representations, what we end up doing is locking the resulting image into the size that happened to be set on the original NSImage. In many cases, you may not know how and where this icon will be used but if any scaling is involved, you may end up having an icon with the wrong version being displayed. In the example program, select “Modified (lock focus)” in the pop-up. Here it picks the 64×64 version (as indicated by the green circle) which becomes a problem when you scale the image up as shown on the left in the image below.

The one on the right is a version which uses an  NSImageRep subclass (select “Modified (custom image rep)” in the pop-up). Notice that it picks the appropriate size as you resize the window. Why is this the case? It’s because NSImage doesn’t ask the image rep to draw until the NSImage itself is drawn. At that point, you actually have information about the destination, including how big it will actually appear on screen. NSImage is able to use this context to determine the right match between different sized images and the resolution of the output context. The same goes for any drawing code.

Subclassing NSImageRep is quite easy; you just need to override the -draw method. I’ve made it even easier by providing NoodleCustomImageRep which takes a block, allowing you to create images without creating a new subclass.

Say, though, you are drawing an image that should scale without pixellation, like drawing a square. Surely, you can just lock focus on an NSImage and draw a square and scale that as needed? Well, Mr Smarty Pants, take a look at the example program. Select “Drawn (lock focus)” in the pop-up.

Here, you’ll see odd fuzziness around the edges. What’s going on here? It’s not anti-aliasing but instead the graphics context where the image is being drawn has image interpolation turned on. As a result, the AppKit is trying to interpolate the image from it’s original size to a much bigger size.

How do you fix this? You could try turning off image interpolation in the destination graphics context but this isn’t always possible or desirable. The better solution is just like before: use a custom image rep to do the drawing (select “Drawn (custom image rep)” from the pop-up). Since the drawing occurs at drawing time, instead of image creation time, it knows about the context it is drawing into and therefore can provide your drawing code with a context at the correct resolution. The crisp square on the right speaks for itself.

Let’s take another example. Say we want to take an existing image and run a Core Image filter on it. Somehow you have to convert your NSImage into a CIImage. This usually entails a game of connecting up various methods that fit together until you got a CIImage. A common way to do this is:

ciImage = [CIImage imageWithData:[nsImage TIFFRepresentation]];

This dumps the whole image (all of its representations) into TIFF format which is in turn, re-parsed back into data. Now, CIImage is pixel based so it ends up picking one of the representations. It’s not documented which representation is picked since there’s no way to specify the context where it will be drawn so there’s a chance it won’t be the right one. Select “Core Image (TIFF Rep)” in the pop-up and you get something like the image on the left (or maybe you won’t; read on):

Now, it doesn’t look too bad here. It took the highest res representation and used that. That said, technically, it’s not correct. The version on the right (select “Core Image (custom rep)”) shows the correct image rep being used. Also, my experience has shown that the representation chosen by the -imageWithData: method can be different on different hardware and that it ignores the size set on the original NSImage so you may not be so lucky depending on what machine your code runs on.

Fortunately, Snow Leopard introduced a new method: -CGImageForProposedRect:context:hints:. As mentioned before, when you draw an NSImage into a context, it will automatically pick the right representation for that context. This method does basically the same thing but without the drawing part. Instead it returns a CGImage which you can then use to create your CIImage:

cgImage = [nsImage CGImageForProposedRect:&rect context:[NSGraphicsContext currentContext] hints:nil];
ciImage = [CIImage imageWithCGImage:cgImage];

Keep in mind, though, that it the image returned might not be the exact size you wanted. This becomes more of an issue when you are combining multiple images together in a CIImage pipeline and you need them all to be the same size. You can adjust for this by using CIImage’s -imageByApplyingTransform:.

In addition to the above method, Snow Leopard introduced -bestRepresentationForRect:context:hints: which does a similar thing but returns an image rep instead. Depending on your needs, you can use one or the other to tap NSImage’s image matching logic.

Finally, a note about performance. NSImage does keep a cache based on the drawing context. This helps for when you repeatedly draw the same image at the same size over and over. If you end up sharing an NSImage across different contexts, you’ll find that you are defeating the cache. For these cases, you should be copying the NSImages. Remember that NSImages are just mediators between image data and drawing context. NSImageReps are the actual image sources and, starting with 10.6, reps like NSBitmapImageRep do copy-on-write making it inexpensive to copy NSImages and their reps.

In the example app, there’s a field which shows the time taken to display the image. You’ll notice that when you resize the window, the cases which use a custom image rep are slower as it has to recache whereas the lockFocus cases don’t since the image is static. If this becomes an issue, you can turn off caching or use a fixed resolution image during a live resize. Another more subtle piece of business is performance when drawing from the cache. If you click the “Redisplay” button, it will cause the image to be displayed again. Since you aren’t changing the size, the cached version can be used. Notice how the versions using the custom image rep are usually a smidgen faster than the lockFocus versions. I suspect what is happening is when you lockFocus on the image, you lock the cache into a specific version and size. As a result, if you are drawing at any other size, it has to scale the cached image every time. With a custom image rep, it’s cached at the exact size so the cache can be used as is.

What are the lessons here?

1. When defining your image, you should use an NSImageRep subclass and override -draw. If you don’t want to create a whole subclass just to create an image, use my NoodleCustomImageRep (included in NoodleKit) which allows you to pass in a drawing block. Using an image rep gives your drawing code better contextual information than you would get just drawing in a -lockFocus.
2. If you follow point #1, then you can let NSImage make the decision of which representation to use. Use  one of the drawing methods, -CGImageForProposeRect:... or -bestRepresentationForRect:... and you’ll get the best sized representation for the job. Do not assume, though, that this representation will be the actual size you want. When drawing, it also helps to specify the rect to draw into.
3. Avoid using -lockFocus. It doesn’t produce the correct image in different contexts and can be destructive in terms of kicking out the other reps in the NSImage. While still ok in specific circumstances, you have to know what you are doing.
4. If using the same NSImage in different contexts, copy it. In 10.6 onwards, this is an inexpensive operation as bitmap data is copy-on-write. Copying NSImages is is also a good idea in case some decides to use -lockFocus on the image (see #3).

If you have access to it, I highly recommend watching the video for Ken Ferry’s session at WWDC 2009: Session 111: NSImage in Snow Leopard (you may need a developer account to view/download it). Much of this is derived from that presentation and it has even more interesting bits about NSImage than what I’ve presented here.

And in case you missed it above, you can find NoodleKit (which contains NoodleCustomImageRep as well as the example program) here.

Addendum (Apr. 18, 2011):

I received an email from Ken Ferry himself pointing out a couple things. Mainly, that as of 10.6, lock focusing doesn’t draw into the cache anymore. It creates a representation whose context is suited for the main display. It is still good for images which are meant for that context. Also as the NSImage context will maintain any size-to-pixel ratio that is on the main display, it can still be used in this situation should resolution independence come into play. It’s not much different than before in this respect but it’s not as volatile as a cache, which I believe is the key point here.

That said, all the caveats above about using -lockFocus still hold true. It’s not so great for when the image needs to be scaled or if you have representations you want to keep (it does remove all other reps when you lock focus). Also, because the representation is tied to the machine, it’s not very suitable for persisting.

In my last post, I introduced a little adapter class called NoodleGlue. It’s a simple class that wraps around a block allowing you to use it as the target and action for APIs that require it. It also optionally can take an extra block that is executed when the glue object is deallocated. This allows you to do any cleanup. In my NSTimer category from last time, I used this cleanup ability to allow the object to unregister from any notifications. Being a category, I couldn’t hook into the -invalidate or -dealloc methods of NSTimer so I created a glue object with a clean up block which I then set as an associated object on the timer. As a result, when the timer was deallocated, so would my glue object and my cleanup block would be triggered.

In more general terms, what does this mean? With my glue object plus the associative references feature, you can inject your code into any object’s deallocation sequence. It’s like being able to be notified if any object is freed. I’ve wrapped this up in an NSObject category with a new method, -addCleanupBlock:, which allows you to add a block to be invoked when the object is deallocated. It’s probably more of a proof of concept than something you’d want to use in production code but you can find it in the latest version of NoodleKit (it’s tacked onto the end of the NoodleGlue class).

Now, this sounds pretty cool on paper but this may be one of those cases of a solution looking for a problem. Not to be deterred, I’ve come up with some contrived cases that might actually be useful.

Add extra cleanup for extra stuff you attached to an object

It’s how I used it in my NSTimer category. You can unregister notifications and invalidate objects which need more than just a -release call.

Zero out references

One of the cool things about weak references in a GC environment is that they nil themselves out when the object is collected. You can simulate this behavior when using retain/release by setting up a cleanup block to nil out your reference to the object in question.

Tracing/debugging deallocations

Sure, you can do this in the debugger or Instruments but in some cases you want to trigger more involved code that you can’t do in gdb. Also, in a multithreaded program where the timing affects whether the bug shows up or not, this allows you to track deallocations without setting a breakpoint in the debugger which might otherwise change the timing.

Test if objects are actually deallocated (unit tests)

Credit goes to Guy English for this one. If you need to do a unit test where you want to test your memory management, you can “do the glue” to test that objects actually get deallocated. The problem here is if it’s not working (i.e. object doesn’t get deallocated), your block won’t get called so any assertions you put in there will just silently not happen. So, have your block set a variable and at the end of the test, do an STAssertTrue() on the variable.

Keep in mind that there are a couple of important caveats:

1. You cannot retain the object you are “observing” in the block itself as this will create a retain loop and the observed object (as well as the glue) will never be dealloc’ed. Note that just referencing an object retains it unless you assign the object to a variable declared with __block. This has already been done for you in my code as the cleanup block is sent a non-retaining reference to the object being freed so use that reference instead.

	[self addCleanupBlock:
		^(id object)
		{
			// Do not reference "self"
			[object doSomething];
		}];

2. It’s not defined when in the deallocation process the associated object is freed. As a result, you can’t rely on any state of the observed object as it may be the case that all its ivars have already been released. But, you can have the block “capture” variables and the block will retain and subsequently release them. For instance, if you want to log the “name” of an object as it’s deallocated, you can reference the name in the block and it will be retained. But be careful though, if you reference an ivar in the object directly, you end up retaining that object. So, if the observed object is creating the block and you access its ivar in that block, you end up implicitly retaining the observed object, which is in violation of point 1 above. In those cases, assign the ivar to a local variable and use that (in this case, do not use __block in the variable declaration as you want the block to retain the object.

	NSString	*localName = _name;	// _name is an ivar
	[self addCleanupBlock:
		^(id object)
		{
			// Do not reference _name here as that will implicitly retain self
			NSLog(@"Object %@ has been deallocated.", localName);
		}];

I suggest reading up on the memory management issues with blocks as described here. Joachim Bengtsson has a great article on blocks (that link goes to the section relevant to this article but I recommend reading the whole thing) and there is always Mike Ash’s reliable series of articles on the subject, which I have consulted many a time myself.

It’s been a long while. Part of it is that I’ve been busy working on Hazel 3. And since I’m not interested in writing a book, I’m finding it hard to be motivated to keep posting here. Nonetheless, every so often I have to let something squirt out.

Today, we are going to talk about NSTimer. One thing you may or may not have noticed is that in certain cases where time is suspended (putting your machine to sleep) or altered (changing the clock), NSTimer has a tendency to try and compensate. For instance, say you set a timer to fire in an hour. You close the lid on your MBP and come back 30 minutes later. You’ll see that the fire date for the timer has adjusted to take into account the time it was asleep. Here’s a quickie diagram for those not quite following:

Now, this is great if you wanted a timer to fire an hour later in the machine’s conception of time. But there are times when you want it to fire on the actual date you set on it. Typical example is setting a timer for a calendar appointment. That time is an absolute point in the user’s timeline and you don’t want that timer to shift to compensate for the machine’s timeline.
The basic fix here is when one of these time-altering events occurs, you reset the fire time on the timer to the original time that was set on it when it was created. Luckily, there are notifications for when the machine wakes from sleep as well as when the system clock is changed (this one being new in 10.6). Naturally, one would think to make an NSTimer subclass as the most straightforward way to do this. Unfortunately, it seems that it doesn’t work. Even if you get over the hurdle that NSTimer is actually abstract (like a class cluster), you have to add the timer to the run loop to schedule it. While NSRunLoop seems to accept the NSTimer subclass, it doesn’t ever fire. I don’t know if NSRunLoop is looking for the NSTimer class specifically or if it’s calling some private method that I need to override. If someone else has had any luck doing this, drop me a line.

The alternative is a category. The problem is that we need at least one ivar to store the original fire date. Normally, you can’t add ivars in a category but in this glorious post-10.6 age, we can, via associative references. It’s basically just a dictionary where you can store variables associated with an object. Sure, you could have implemented that yourself but the nice thing is that it handles memory management as well. When the object is dealloc’ed/collected, depending on the memory management you specify, it can also release/collect any associated objects.

So, NSTimer category it is. It supplies the method (among others) +scheduledTimerWithAbsoluteFireDate:target:selector:userInfo: which creates and returns an autoreleased and scheduled timer that will fire on the given date regardless of any time fluctuations/lapses. If all of a sudden the fire date has passed (like if the machine was asleep during the fire date or if the system clock was set ahead), it will fire immediately. Note that calling -setFireDate: won’t quite work in the sense that if the timer ever has to be reset from a time shift, it will be set to the date used when creating the timer, not the one that you set it to afterwards. This is an implementation limitation as I am using a category and can’t override methods (at least not without doing some method-swizzling which is not something I want to deal with). In cases where you need to change the fire date, I’d suggest just creating a new NSTimer.

I’ve also included a test harness where you can see it in action compared against a timer running in normal (non-absolute) mode. When run normally, both timers should fire at the same time but you can test things out by changing the system clock or putting your machine to sleep. You’ll see that the regular timer’s fire date will start to drift out while the “absolute” timer will stick with the time originally set.

I also went ahead and added methods to have NSTimer to use blocks instead of a target/selector combo. Note sure why this wasn’t in there already but I thought it’d be useful.

And that’s not all! Included is my NoodleGlue class. It’s a simple little class that just wraps a block (plus another block for cleanup, if you need it). It’s useful for cases where you want to set a target and selector for some object to use but don’t want to create a new class or method for it. Check out the source code for the NSTimer category to see how its used both with NSTimer (to implement the block API) as well as NSNotificationCenter. In the latter case, there is a blocks-based API but I had special memory management requirements that couldn’t be done with the existing API.

Needless to say, this extension only works on 10.6+. It’s in the latest version of NoodleKit and as a result I’ve made NoodleKit require 10.6 as well. Just build and run the TimerLab target. Fixes and suggestions welcome.

[Update 8:35pm EDT] I neglected to push the code to the github repository. Sorry about that. Everything should be there now.

I was just turned on to a post from Jesper of Waffle Software entitled How to Mimic the Artwork Column in Cocoa. There he approaches the problem of implementing cells in an NSTableView that can span multiple rows. This spurred on Jacob Xiao at Like Thought to tackle the problem in a different manner. I suggest reading both of the above to get a sense of the problem and the possible approaches.

With NSTableView still on my mind from my last diversion, this got me thinking about the problem and I decided to try my own hand at it. I preferred Jacob’s approach as I think it should be all contained within NSTableView if at all possible. I liked the notion of “slicing” up the cell instead of using another view. Playing with it, I came up with a generalized solution. Here are some of the interesting aspects:

• It can support any NSCell subclass. I do this by using a special wrapper cell. It draws the “full” version of the cell then draws out the slices for each row into the tableview.
• It determines the spans by finding contiguous rows that have the same object value for the column in question.
• The above two points are exploited to do some caching such that the “full” cell is only drawn once and the parts composited in place as needed.
• If the first column is set to span rows, it will only draw the grid for the last row in a span.
• You can designate multiple columns to do this. In the example project, if you click on the “Album” column, it will toggle between span and non-span mode. The “Artwork” column doesn’t do this because it looks cruddy in non-span mode. I haven’t tested it (see update below) but theoretically, the spans do not have to line up between different columns.

To use it, for any columns you want to have this row-spanning behavior, set the table column class to NoodleRowSpanningTableColumn. Also, it might help to get rid of any intercell spacing in your tableview (at least in the vertical direction). There are probably a few bugs. I do know that reordering columns may cause some grid drawing funniness but for the most part, it seems to work pretty well. Let me know if you find otherwise.

You can check the code for other implementation tidbits. I’ve included a self-contained project below but I’ll eventually polish it a bit and add it to my NoodleKit repository where it can join the rest of my goodies.

Many thanks for Jesper and Jacob for the inspiration and laying down the groundwork for this.

Update (Oct. 21, 2009): I observed odd behavior in that the program would get into a re-drawing frenzy eating up a bit of CPU. This was non-blocking in that I was still able to manipulate the UI (which is why I didn’t notice it at first). I finally figured out that it was the -drawGridInClipRect implementation. I was drawing the grid selectively by changing the grid mask and then calling super on subregions. Unfortunately, changing the grid mask queued up a redraw which started a loop (albeit one that didn’t tie up the event loop). Seeing as I needed to fix the grid-drawing logic anyways, I reworked the whole thing.

The new grid code should now be able to handle multiple spanning columns rearranged in whatever way you want. Here’s contrived example of having multiple spanning columns that don’t necessarily line up:

I also made some other fixes and improvements so it’s worth re-downloading the project even if you already did so before (I bumped this version to 0.37).

As for integration with the rest of NoodleKit, I still need to decide whether I want to merge this with my sticky row tableview to make some über-tableview. Problem is is that I can’t think of how the two features would interact so maybe it’s best to keep them separate. Thoughts on this are welcome.

Update (Oct. 23, 2009): I’ve just updated the project to version 0.68. There was a crasher when you would click on a non-span row and then move into a spanning cell. Also, the span cells were highlighting when you clicked on a non-span row. Both of these have been fixed. Thanks for Jesper for finding the crasher and Jacob for helping with the fix.

Update (Oct. 25, 2009): I’ve consolidated this functionality along with my previous sticky row header code into a single NoodleTableView class. You can find it in the NoodleKit repository. Future updates to this will be posted there from here on out.

Download NoodleRowSpanningTableViewTest.zip
[This project is no longer being updated here. You can get the latest in the NoodleKit repo]

I’ve finally gotten around to consolidating most of the bits of code I’ve posted on this blog over the years and put it in a repository. While the real NoodleKit is a much more extensive and cohesive toolkit that I use internally, this one will serve as a place where I put the odd scraps that I decide to open source. I guess I could’ve called it NoodleScraps but it doesn’t quite sound as nice.

If you’ve been following my blog for a while then there’s not much new here, codewise. There are a tweaks here and there plus the stuff should be 64-bit ready so it might be worth re-downloading just for that. It will be the place where I put future code so you should keep an eye on it.

The project is structured to build a framework but also has an examples directory (with corresponding targets) to demonstrate the use of the different classes. The current notion is to keep things compatible with 10.5. And, as usual, all code is released under the MIT license.

NoodleKit (hosted at github)

Enjoy.

Once again, I find myself with some cool code with nowhere to put it. And, yet once again, you get the reap the benefits.

UITableView on the iPhone has the neat feature where section headers stick at the top after the actual row for it has scrolled off. For instance, in Contacts, if you scroll down, the row for section “A” stays visible so that you know what section you’re in if the section is too large for the screen.

Unfortunately, someone asked how something like this could be done on desktop Cocoa. I say “unfortunately” because I was within earshot and it resulted in me embarking on a journey peppered with annoying run-ins with NSTableView’s idiosyncrasies. Nonetheless, I persevered and came up with something hopefully someone will find useful. As was the case before, I don’t have a use for it myself at the moment, but I feel it would be a shame to let the code go to waste.

I am presenting NoodleStickyRowTableView. I call the section headers “sticky rows” as one can specify any row to stick up top for whatever reason, though by default, it does it for group rows.

The bad stuff:

NSTableView makes it very difficult to cache a row visually. -drawRow:clipRect: should be able to do it but the problem is that it’s too smart for it’s own good. If the row is not visible, it will not draw anything. Add on top of that the impossibility of getting it to draw the special background the group rows use. I tried all sorts of techniques but none were general or reliable enough. My email on the cocoa-dev list was, not surprisingly, ignored. As a result, in the default implementation, I use my own look for the rows and then use a fade-in transition since the sticky row will look different from the actual one. You can see it in the video below where it has a ghost-like effect.

The good stuff:

The good news is that I managed to work it into an NSTableView category. You can think of it as an abstract category if you wish. In your subclass, at the very minimum, override -drawRect: and call -drawStickyRowHeader after calling super. From that one call you will get all the functionality.

Why do it this way? Well, ease of use for one. It makes it so that you can integrate it into your own NSTableView subclass without having to copy and paste large swaths of code or put it all sorts of odd hooks. Secondly, the changes get inherited by NSOutlineView so the functionality is available there and in its subclasses as well.

While I could use the new associated objects functionality in Snow Leopard, I opted for Leopard-compatiblity. This resulted in me coming up with a couple tricks to get around the lack of ivars, including the rarely used -viewWithTag: method. Check the “read me” file and code for details.

There are two different transitions. The default is the fade-in transition as shown here:

As mentioned before, since I couldn’t get the group rows to draw properly and consistently, I use a more generic look for the row. The transition makes it less jarring but it is a compromise of sorts. Not bad when you consider that you get all this by calling just one extra method in your -drawRect:.

But what if you want something more like the iPhone’s version? In your subclass, you can override -stickyRowHeaderTransition to return no transition and then do your own drawing of the rows so that they look the same between the regular and sticky versions. The NoodleIPhoneTableView included in the project shows how this is done with the result being something on par with UITableView:

I am releasing this under MIT license as usual. Use it however you want as long as you give me credit somewhere. Please send in any feedback, suggestions, bugs, etc but keep in mind that this is not something I use in my projects so I probably will not be actively maintaining it beyond a certain point. I’ve made a point of making the the API easy to use and to extend so have fun with it.

Download NoodleStickyRowTableViewTest.zip (version 0.18639)

Update (Sep. 29, 2009): I have included this class in my NoodleKit repository so you should check there for future updates.