Vincent Gable’s Blog

December 20, 2008

Automatically Freeing Every @property

Filed under: Cocoa,MacOSX,Objective-C,Programming,Sample Code | , ,
― Vincent Gable on December 20, 2008

Here’s a category of NSObject that can simplify a dealloc method. It adds the method, setEveryObjCObjectPropertyToNil, that sets every property of the receiver to nil. (Properties that are readonly or not an Objective-C object are ignored.) This frees the underlying object, if the property is declared copy or retain; and it does no harm if it was declared assign.

If every ivar (member variable) in your object has a property declared for it, then your dealloc method can often be replaced by this macro, or it’s two-line expansion:

#define PROPERTY_ONLY_OBJECT_DEALLOC \
- (void) dealloc { \
   [self setEveryObjCObjectPropertyToNil]; \
   [super dealloc]; \
}

Limitations

Pointers

Any pointers (eg char*) will not be set to NULL; this includes pointers to an Objective-C object (eg NSError** outError). Of course NSObject* obj will be set to nil since it is considered an Objective-C object, even though it is written as if it were a pointer.

It is easy to build on setEveryObjCObjectPropertyToNil and have something that sets pointers to NULL as well. But I felt it was too risky. Sending a message to nil is valid, but dereferencing a NULL pointer is a “bus error” crash. [nil release]; does no harm, but free(NULL); is bad news. A settable @property that takes a raw pointer is a hybrid Objective-C, and “old”-C creature — I’ve never seen such a thing, so I’m wary of assuming that feeding it a NULL would be valid. Plus, opening the door to pointers means dealing with handles (pointers-to-pointers) and their ilk.

ivars

Any ivars (member variables) with no settable @property declared on them will not be freed. You can inspect your own ivars like you can @propertys (example), but it would not be safe to automatically release them. Some of them might be weak-links, meaning the object they point to was not sent a retain message. Weak-links are not terribly rare, for example objects always have a weak link to their delegate.

The Code

You will still need to download the source to get helper functions like SetterNameFromPropertyName() for this to actually run, but this should give you an idea of how it works:


@implementation NSObject(CleanUpProperties)
- (void) setEveryObjCObjectPropertyToNil;
{
   unsigned int i, propertyCount = 0;
   objc_property_t *propertyList = class_copyPropertyList([self class], &propertyCount);
   if(propertyList){
      for(i = 0; i < propertyCount; i++){
         const char *propertyAttrs = property_getAttributes(propertyList[i]);
         if(PropertyIsObjCObject(propertyAttrs) && PropertyIsSettable(propertyAttrs)) {
            NSString *setterName = SetterNameFromAttributes(propertyAttrs);
            if(!setterName)
               setterName = SetterNameFromPropertyName(property_getName(propertyList[i]));
            [self performSelector:NSSelectorFromString(setterName) withObject:nil];
         }
      }
      free(propertyList);
   }
}
@end


Download The Code


And please let me know how it works for you. I’ve started using setEveryObjCObjectPropertyToNil even if I only have one @property, because it means I’ll never forget to free one.

Warning (Update 2009-05-29)

Uli Kusterer gives a good reason not to use this code,

Don’t Use Accessors in Constructors or Destructors

This may sound a bit odd, but there is a reason to this madness. Constructors (i.e. -init methods in ObjC) and destructors (i.e. -dealloc or -finalize) are special methods: They are called before your object has fully been initialized, or may be called after it has already been partially torn down.

If someone subclasses your class, your object is still an object of that subclass. So, by the time your -dealloc method is called, the subclass has already been asked to do its -dealloc, and most of the instance variables are gone. If you now call an accessor, and that accessor does anything more than change the instance variable (e.g. send out notifications to interested parties), it might pass a pointer to its half-destructed zombie self to those other objects, or make decisions based on half-valid object state. The same applies to the constructor, but of course in reverse.

Now, some people that accessors should not be doing anything but change instance variables, but that is utter nonsense. If that was all they’re supposed to do, we wouldn’t need them. Accessors are supposed to maintain encapsulation. They’re supposed to insulate you from the internal details of how a particular object does its work, so you can easily revise the object to work in a completely different way, without anyone on the outside noticing. If an accessor could only change an instance variable, you would have very limited means to change this internal implementation.

Moreover, I don’t think Apple would have introduced Key-Value-Coding and Key-Value-Observing if they didn’t agree at least partially that it’s fine to do a bunch of things in response to an accessor being used to mutate an object.

Update 2009-11-29

I’m amused at the prevalent “Apple knows best” attitude. Bindings, garbage collection, NSOperationQueue, and so many other things, Apple has gotten wrong and burned me in the process. I always trust my own evaluation over their recommendations.

Mike Ash, Using Accessors in Init and Dealloc

December 19, 2008

Undocumented Automator.framework Goodness: Actions Library

In Leopard, Apple introduced AMWorkflowView and AMWorkflowController, which let you easily add automator-editing capabilities to your application (modulo bugs!)

Obviously, for editing a workflow to be useful, you need to be able to add actions to it. And to do that, you need to be able to browse and search actions. But the current Automator.framework has no official support for doing this.

But there are undocumented private APIs, which Automator.app (link launches it) uses…

How to Find Them

class-dump is an amazingly useful command-line tool that lets you generate headers from a compiled Objective-C binary. Download it, and point it at /System/Library/Frameworks/Automator.framework/Automator, to see everything Automator.framework really lets you do.

Instant Library Panel

The most useful definition I found was,
@interface NSApplication (AMLibraryPanel)
- (void)orderFrontAutomatorLibraryPanel:(id)sender;
@end

Calling [NSApp orderFrontAutomatorLibraryPanel:nil]; will show a panel with the same action-library view Automator has.

This is what I’m using right now in IMLocation to let people find actions for workflows. I don’t like the idea of using private APIs. But I like the idea of implementing my own AMLibraryView replacement even less. It would be a lot of work to make it as good as the real thing. Any differences give users a fractured experience — one way to do something in Automator, another in my program. And I would get dismal ROI, because I expect Apple to expose this functionality in the (near) future.

(I also wouldn’t be so bullish on using private APIs if I didn’t have workarounds in place right now. The only way orderFrontAutomatorLibraryPanel: gets called is if a user presses a “Show Actions” button on the toolbar. That same toolbar has an “Open in Automator” button that opens the worklfow in Automator, where they can edit it without bugs. So even if orderFrontAutomatorLibraryPanel: stopped working tomorrow, users could still do everything they could before — albeit less elegantly).

Looking Just Like Automator

I don’t recommend doing this, but I’ve been able to embed an AMLibraryView in an NSSplitView next to an AMWorkflowView, to get a more Automator.app-like appearance.

Here’s how I did it, given the IBOutlets workflowViewAndLibrarySplitView which is an NSSplit view with an AMWorkflowView in one side, and the other side’s view connected to the outlet workflowLibraryView,

[workflowViewAndLibrarySplitView replaceSubview:workflowLibraryView with:[[AMLibraryPanel sharedLibraryPanel] _libraryView]];

Will put the library view inside your split view.

Of course this screws up orderFrontAutomatorLibraryPanel:. And I would not expect it to work in two windows. And it’s using an underscore-private method of an already private and undocumented API. That’s just to too risky for me; so I stick with the panel. It gets the job done with one line of code, and that’s good enough for me.

December 18, 2008

Automator Bugs

I’ve started using Automator in IMLocation to let people build a set of actions that are executed when they arrive somewhere (for example, muting their laptop’s speakers in a classroom).

Unfortunately, my impression of Automator.framework so far is: buggy.

Here are some issues I’ve run into so far.

-[AMWorkflowController stop:] does not stop the workflow! I do not yet have a workaround.

AMWorkflowController sends its delegate a -document message whenever an action is removed from the workflowView, by clicking the (x) button. If the delegate does not implement it, the action will not be removed!

The (x) button is enabled , but does nothing, in workflows that are not editable.

-[AMWorkflow initWithContentsOfURL:error:] throws an exception when the URL is nil.

This project demonstrates the first two issues.

December 16, 2008

isEmpty?

Checking if a Cocoa object is empty is a little harder then in other languages, say C++, (but easier in some ways). Because every object in Objective-C is actually a pointer to an object, there are two ways, obj, can be empty.

obj = {}

obj points to an object that is empty. Say an array with 0 items, or the string "", etc..

obj = nil

obj, the pointer obj, is NULL, nil, 0, or whatever you want to call it. You might argue that obj isn’t really an object, but it is empty, because there’s nothing in it.

Bug:

When I first started writing Objective-C, I made the mistake of writing code like: if([name isEqualToString:@""]){ ... }, to test for empty strings. And this code would work for a while until I used it in a situation where name was nil, and then, because sending any method called on nil “returns” NO, I would have mysterious errors. (Worse then a crash, because it’s harder to track down.)

Bug:

It’s tempting to avoid the previous bug, by explicitly testing for nil and {}. Say with code like:

if (email == nil || ![email isEqualTo:@""] )
   email = @"An email address is required";

But generally this is a bad idea. It means more code, which means more places for a bug. I know it’s only one trivial test, but I’m serious, when I say it’s asking for a bug — like the bug in the example above, which sets email to @"An email address is required", whenever it is not the empty string, rather then when it is empty. (Values have been changed tho protect the innocent but it’s a bug I’ve seen.)

Solutions:

Wil Shipley suggests using the global function:

static inline BOOL IsEmpty(id thing) {
    return thing == nil
        || ([thing respondsToSelector:@selector(length)]
        && [(NSData *)thing length] == 0)
        || ([thing respondsToSelector:@selector(count)]
        && [(NSArray *)thing count] == 0);
}

I’ve been using his IsEmpty() for about a year. I’ve had zero problems with it, while it’s made my code more readable and concise.

Another solution is to take advantage of what happens when you send a message to nil. (To over-simplify, you get back 0 or NO.) So you can just say “if ([obj count] == 0) then obj is empty.” This often means reversing your thinking, and testing “IsNotEmpty()” instead of “IsEmpty()”. I don’t think it’s as clear is IsEmpty() in general, but in cases where it is, there you have it.

December 12, 2008

Catching nil Member Variables

Filed under: MacOSX,Objective-C,Sample Code | , , ,
― Vincent Gable on December 12, 2008

When I wrote this method it seemed like it would be helpful,

//log a warning for any member variable that nil
- (void) warnOfNilIVars;
{
  unsigned int ivarCount = 0;
  Ivar * ivarList = class_copyIvarList([self class], &ivarCount);
  if(ivarList) {
    for(int i = 0; i < ivarCount; i++){
      const char *typeString = ivar_getTypeEncoding(ivarList[i]);
      if(typeString && typeString[0] == '@')
        if(!object_getIvar(self, ivarList[i]))
          NSLog(@"***WARNING: ivar %s of %@ is nil", ivar_getName(ivarList[i]), [self className]);
    }
    free(ivarList);
  }
}

But in practice I haven’t really used it (and when I have there were quite a few false-positives). Still, I think it’s pretty neat that you can do something like this in Objective-C.

If you find a use for it, please let me know!

One assert() You Need?

Accidently disconnected outlets in shipping Cocoa apps are legend.

Jonathan “Wolf” Rentzsch

From what I can tell assert() is slowly going the way of the goto in the programming world. Exceptions, unit-tests, and other modern software engineering practices, seem to have a better answer for testing something at runtime, and ensuring that you never enter a bad state.

But there is one case where I think you should put an assert() (and not an NSAssert()) in your code: in awakeFromNib assert() that every IBOutlet is connected.

It’s surprisingly easy to accidentally disconnect something in Interface Builder, or rename something in Xcode. I’ve done it before. More then once. assert()-ing IBOutlets has saved me a lot of debugging time.

Hard to Find In Code

An IBOutlet that is not connected is nil. Because Objective-C quietly ignores messages sent to nil, it’s very easy not to notice the problem for a while; then spend a long time debugging a side effect of the issue.

Hard to Find By Eye

A widget in a nib/xib file that is not connected will never change it’s state. But it typically it has a reasonable initial state. This makes it difficult to detect disconnected IBOutlets by eye, because things will look right until they are supposed have changed, but the eye is drawn to change, and de-emphasizes unchanging things.

Why assert()?

As Wolf says, the worst-case-scenario is that right before release you make a trivial change in Interface Builder (“We can’t ship with that window saying “Claculator”), an IBOutlet gets disconnected, and nobody notices in time.

But an assert() failing will be noticed with casual testing, and triggered if the nib is loaded at all, even if the series of interactions needed to cause the widget to change state are not performed.

An assert() is very light-weight and easy to do. It’s exactly one line of code, and you don’t have to add a unit testing framework to your project to do it. And that makes it perfect for test projects. I’m a believer in getting a feature working in a test project first, then copying it into your real product. (Honestly I don’t do it as much as I should, and I almost always kick myself for it.) It lets you test and learn without hacking-up your product. Plus, new things are often easier to try without the weight of a big code base.

Speaking of unit tests, I don’t have complete faith in them here. That’s because a disconnected IBOutlet in a shipping program is fundamentally an issue with the way the release-build is configured. So anything that’s not testing the the actual release build is not exhaustively testing for this problem. an OCUnit-test can catch a disconnected IBOutlet if it is loaded into the application. But in my experience, such tests are cumbersome. Running them involves launching your full application, and programatically manipulating it. This can take a while to run since all the UI will be displayed and animated. And you have to be careful about state since each test will be changing the state of your program.

Ultimately, assert() just seems to be optimal, in dependability and simplicity, for catching a disconnected IBOutlet.

But I suspect there’s a better way I don’t know.

How do you catch IBOutlets?

December 8, 2008

C Comment Trivia

Filed under: Bug Bite,C++,Objective-C,Programming | , ,
― Vincent Gable on December 8, 2008

//I'll bet \
you \
didn't \
know \
you \
could \
comment \
like \
this \
in \
C/C++/Objective-C

Well, you can. Finding out was a lot of fun, believe me.

At some point, I’m not sure when, I unknowingly put a ‘\’ at the end of a block of // comments. That commented out the method-call the block was referring to. As you might guess from the fact that it had a big comment talking about why it was there, this was an important method call. I spent more time then I want to admit debugging and staring at my code trying to see the problem. Finally I noticed “hey, the color of that code is like a comment, even though it’s not in a comment”.

The More You Know!

December 4, 2008

NSAssert Considered Harmful

Filed under: Bug Bite,Cocoa,MacOSX,Objective-C,Programming,Tips | , , , ,
― Vincent Gable on December 4, 2008

The NSAssert() function is unreliable, and therefore dangerous. Plain-old assert() is a better choice.

Note that #define NDEBUG disables assert(); and #define NS_BLOCK_ASSERTIONS disables NSAssert(). One kind of assert being disabled does not mean the other kind is. Nether kind of assertion is disabled by default when building in “Release” mode — you must be sure to disable them yourself. Be careful, a user will not appreciate the difference between a bug and a failed assertion — both are a crash.

assert() immediately stops your program with a SIGABORT signal. You can’t miss a failed assert(), its behavior is predictable, and it fails fast (link is a PDF).

But a failed NSAssert() behaves unpredictably. Sometimes it stops program execution, sometimes it does not and leaves the program in a strange inconsistent state. I have entirely missed NSAssert()s failing, because I did not look at the console. if(!expr) NSLog(); would have been a better choice in the those cases, because at least I would have known how it behaved.

According to Apple,

Assertion macros, such as NSAssert and NSCAssert … pass a string to an NSAssertionHandler object describing the failure. Each thread has its own NSAssertionHandler object. When invoked, an assertion handler prints an error message that includes the method and class (or function) containing the assertion and raises an NSInternalInconsistencyException.

Complicated! The complexity means it is possible to customize what happens when an NSAssert() fails. That sounds cool, but I’ve never heard of someone needing to actually do that.

If a framework catches NSInternalInconsistencyExceptions, then your program will keep right on running after a failed NSAssert(). I have had this happen to me several times. I apologize for not having taken the time to investigate what frameworks were catching what.

Apple could change what catches what with any software update.

Variability and complexity are the last things you want while debugging. There’s no reason to invite it them by using NSAssert() over assert(). Since NSAssert() is not guaranteed to stop your program, it can not be relied on to guard against data corruption, or anything worse then a predictable crash.

UPDATE 2009-06-01: You can annotate assert(), so it prints an explanation like NSAssert(), by &&ing in a string after the condition. For example assert(i < j) is a lot more useful with an explanation: assert(i < j && "unexpected cycle in graph") — on failure it prints

Assertion failed: (i < j && “unexpected cycle in graph”), function WillHalt(), file /path/to/code.m, line 30.

Cocoa Coding Style Minutia: All Forward Declarations on One Line is the One True Way

This is a very petty point, but I believe there’s a right answer, and since it’s come up before I’m going to take the time to justify the best choice.

Forward Declare Classes on One Line, Not Many Lines

Right:
@class Foo, Bar, Baz...;

This is the style that Apple’s headers generally follow, look at NSDocument.h for example.

Wrong:
@class Foo;
@class Bar;
@class Baz;
...

Programming languages are to be read by people first, and interpreted by computers second. I really do hope anyone in software engineering can take that as an axiom!

Therefore, always favor brevity above all else with statements that are only for the compiler. They are by far the least important part of the code base. Keeping them terse minimizes distractions from code that can and should be read.

Naturally, it’s very hard to find a statement that’s only for the compiler. I can only think of one such statement in Objective-C, forward declarations of an object with @class.

What Does @class Do?

@class Foo; tells the compiler that Foo is an Objective-C class, so that it knows how much space to reserve in memory for things of type Foo. It does not tell the compiler what methods and ivars a Foo has, to do that you need to #import "Foo.h" (or have an @interface Foo... in your .m file.)

@class Foo; #import "Foo.h"
Foo is a black box. You can only use it as an argument. You can also send messages to Foo.

What is @class Good For?

Since #import "Foo.h" does everything @class Foo; does and more, why would you ever use @class? The short answer is less time wasted compiling.

Lets say you have a Controller class, and it has an ivar that’s a Foo. To get it to compile, you put #import "Foo.h" inside Controller.h. So far so good. The problem comes when Foo.h is changed. Now any file that has #import "Foo.h" in it must be recompiled. So Controller.h has to be recompiled. So any file that has #import "Controller.h" in it must also be recompiled, and so on. Many objects that don’t use Foo objects, but do talk to the Controller (or to something that talks to something that talks to the Controller!) have to be rebuilt as well. It’s likely that the entire project would end up being rebuilt. With even a moderately sized project, that means a lot of needless compile time!

One solution is to put a forward-declaration, @class Foo;, in Controller.h, and #import "Foo.h" in Controller.m, and the few files that actually do something to Foo objects. @class Foo; gives the compiler just enough information to build an object with a Foo* ivar, because it knows how much space that ivar will need in memory. And since only objects that need to talk to Foo objects directly have any dependency on Foo.h, changes to Foo can be tested quickly. The first time I tried this solution in one of my projects, compile times dropped from minutes to seconds.

Readers Need Less @class

Forward declarations add value to the development process, as does anything that saves programmers time. But @class tells a human reader nothing. Which is why I say you should use them in your code, but minimize the space they take up, by putting them all on one line.

@class Foo; tells a reader that “Foo is black-box that is a class.” That adds no useful information.

If the reader does not care about what a Foo is (they are treating it as a black box), then knowing that Foo is a class is useless information. Foo could be a struct, or a bit-field, or a function pointer, or anything else and they still would not need to know to understand the code.

Conversely, if they need to know what a Foo is to understand the code, then they need to know more then “it is a class”. They need to see Foo.h or documentation — @class just isn’t enough.

Exceptions

I want to again emphasize that this isn’t a big deal. I strongly feel that putting all forward declarations on one line is the right answer, but doing it wrong won’t perceptibly matter in the end.

So I don’t think there’s any reason to address exceptions to the rule. Do what you gotta do.

Best Practices

Use @class to speed up compile-times as much as you can, but use the keyword @class as little as possible, since it is for the compiler, not the person reading the code.

November 24, 2008

How To Put a % in an NSString/NSLog/printf

Filed under: Cocoa,MacOSX,Objective-C,Programming | , , , , , ,
― Vincent Gable on November 24, 2008

%% is turned into a single % in a call to NSLog, or -[NSString stringWithFormat:], or the printf-family of functions.

Note that %%format will become %format, even if %format usually prints an argument. For example, the code

NSLog(@"%%a will print a float in a machine-readable format, so that *scanf can read it back in from a string with no loss of precision.", 1.0f);

prints:

%a will print a float in a machine-readable format, so that *scanf can read
it back in from a string with no loss of precision.

not:

%0x1p+0 will print a float in a machine-readable format, so that *scanf can read it back in from a string with no loss of precision.

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