Parmanoir

Snow Leopard introduced Blocks. They are pieces of code that can be saved and passed around, making ObjC even more dynamic. They capture the scope around them, that is the local variables of your functions, and let you use it. They take arguments you specify and let you call them anytime. Blocks are like function pointers, except they don't suck. Here's a few use cases for this new beast :

1 Custom sorting To sort an NSArray in a custom way, you need to write a new method and pass its selector to the sorting function. With blocks, just sort in-place :

id sortedArray = [myArray sortedArrayUsingComparator:
	^(id a, id b)
	{
		if (a.someValue > b.someValue && a.otherValue > b.otherValue) return NSOrderedAscending;
		... some more sorting code
	}];

2 Enumeration Looking for an object in an array can be done with a block enumerator. Set stop to YES to stop enumeration.

[myArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop)
	{
		NSLog(@"obj %@", obj);
		if (...terminatingCondition...)	*stop = YES;
	}];

This is the least interesting use of blocks. The for .. in is much more readable :

for (id obj in myArray)
{
	if (...terminatingCondition...) break;
}

The one advantage enumerateObjectsUsingBlock: has is the current index. enumerateObjectsWithOptions:usingBlock: lets you go backwards.

3 Just-this-scope functions You need to call a piece of code multiple times but don't feel like adding a new method to your class just to accomplish this one task. For instance, protocol_copyMethodDescriptionList(protocol, isRequired, isInstance, count) needs multiple calls to enumerate all protocol methods. Instead of creating a new method, use a block :

- (void)enumerateProtocolMethods:(Protocol*)p
{
	// Custom block, used only in this method
	void (^enumerate)(BOOL, BOOL) = ^(BOOL isRequired, BOOL isInstance) {
		unsigned int descriptionCount;
		struct objc_method_description* methodDescriptions = 
				protocol_copyMethodDescriptionList(p, isRequired, isInstance, &descriptionCount);
		for (int i=0; i<descriptionCount; i++)
		{
			struct objc_method_description d = methodDescriptions[i];
			NSLog(@"Protocol method %@ isRequired=%d isInstance=%d", 
				NSStringFromSelector(d.name), isRequired, isInstance);
		}
		if (methodDescriptions)	free(methodDescriptions);
	};
	// Call our block multiple times with different arguments 
	// to enumerate all class, instance, required and non-required methods
	enumerate(YES, YES);
	enumerate(YES, NO);
	enumerate(NO, YES);
	enumerate(NO, NO);
}

4 Common cleanup I trust we've all done this :

- (id)veryFragileDaisyChain
{
	... init 1
	if (condition1) {
		... init 2
		if (condition2) {
			... init 3
			if (condition3) {
				... init 4
				if (condition4) {
					// At last ! Our shiny new object !
				}
				... init 3 cleanup
			}
			... init 2 cleanup
		}
		... init 1 cleanup
	}
}

How about a block ?

- (id)veryFragileDaisyChain
{
	// Let's declare everything in advance
	__block id obj1 = nil, obj2 = nil, obj3 = nil;
	// Cleanup block
	void(^cleanup)() = ^{
		if (obj3)	... cleanup
		if (obj2)	... cleanup
		if (obj1)	... cleanup
	};

	// Onward.
	... init code 1
	if (!condition1)	return cleanup(), nil;

	... init code 2
	if (!condition2)	return cleanup(), nil;

	... init code 3
	if (!condition3)	return cleanup(), nil;

	... init code 4
	if (!condition4)	return cleanup(), nil;

	// Done.
	return shinyNewObject;
}

5 Functional style Nicolas seriot wrote about functional code in ObjC before Snow Leopard was out, hinting at using blocks to implement functional style code in ObjC. Strangely enough, Snow Leopard didn't implement these. NSArray does have a indexesOfObjectsPassingTest: method, but it returns a NSIndexSet instead of an array ?! With blocks, we can implement a filter method that will create a new array collecting objects matching our block condition.

@implementation NSArray(FunctionalStyle)
- (NSArray*)filter:(BOOL(^)(id elt))filterBlock
{
	// Create a new array
	id filteredArray = [NSMutableArray array];
	// Collect elements matching the block condition
	for (id elt in self)
		if (filterBlock(elt))	[filteredArray addObject:elt];
	return	filteredArray;
}
@end

This removes the need to loop over array element ourselves when we only want to extract elements. Just supply a condition and voilà ! A new array.

// Return a new array containing elements greater than 4
id filteredArray = [myArray filter:^(id elt)	{
			return [elt intValue] > 4;
		}];

6 List comprehensions are a shorthand way of creating arrays.

// This shorthand Javascript
var s = [2*i for (i in 100) if (i*i>3)]
// is equivalent to
var newArray = []
for (var i=0; i<100; i++)
	if (i*i>3) newArray.push(2*i)

We can implement the same with blocks, just a little more verbosely :

@implementation NSArray(ListComprehensions)
// Create a new array with a block applied to each index to create a new element
+ (NSArray*)arrayWithBlock:(id(^)(int index))block range:(NSRange)range
{
	id array = [NSMutableArray array];
	for (int i=range.location; i<range.location+range.length; i++)
		[array addObject:block(i)];
	return	array;
}
// The same with a condition
+ (NSArray*)arrayWithBlock:(id(^)(int index))block range:(NSRange)range if:(BOOL(^)(int index))blockTest
{
	id array = [NSMutableArray array];
	for (int i=range.location; i<range.location+range.length; i++)
		if (blockTest(i))		[array addObject:block(i)];
	return	array;
}
@end

Then call with a block and a range, and an optional condition :

// Just block and range
id newArray = [NSArray 
	arrayWithBlock:^(int i) {	return [NSNumber numberWithInt:i*2]; }
	range:NSMakeRange(5, 10)];

// Optional condition : keep only multiples of 3
id newArray = [NSArray 
	arrayWithBlock:^(int i) {	return [NSNumber numberWithInt:i*2]; }
	range:NSMakeRange(5, 10)
	if:^(int i) { return i%3 == 0; } ];

7 Async processing This is a simple piece of Grand Central Dispatch : enqueue an asynchronous job. The function will return immediately while the block performs in the background in another thread. Use performSelectorOnMainThread: to update your GUI afterwards.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
		
	// Some looong computation in here ...

	// [NSThread isMainThread] tells us that calling [self taskDone] is done with the dispatch thread
	// Use this to call our end of job notification on the main thread
	[self performSelectorOnMainThread:@selector(taskDone) withObject:nil waitUntilDone:NO];
	});

8 Async monitoring Another piece of GCD, lifted from the Concurrency Programming Guide.

You can use GCD to monitor asynchronously resources. In this example, a dispatch source is set to watch when a file is renamed. One block handles notification, another handles common cleanup.

// Create a dispatch source and start watching it
- (dispatch_source_t)watchFileRename:(NSString*)fileName
{
	int fd = open([fileName UTF8String], O_EVTONLY);
	if (fd == -1)	return NULL;
	[fileName retain];
	
	// Cleanup block
	void(^cleanup)() = ^{
		close(fd);
		[fileName release];
	};

	// Create source
	dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE,
		fd, DISPATCH_VNODE_RENAME, 
		dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0));
	if (!source)  return cleanup(), NULL;

	// Dispatch source handler
	dispatch_source_set_event_handler(source, ^{
		// *** Async event handler *** 
		// Called when the file was renamed.		
	});
	// Dispatch source destructor
	dispatch_source_set_cancel_handler(source, ^{ cleanup(); });
	// Start watching
	dispatch_resume(source);
	return	source;
}

// Release a dispatch source
- (void)releaseDispatchSource:(dispatch_source_t)source
{
	if (!source)	return;
	dispatch_source_cancel(source);
	dispatch_release(source);
}

// Start watching file rename. Call releaseDispatchSource when done.
dispatch_source_t source = [self watchFileRename:@"/FileToWatch"];

There are even more uses of blocks, as Grand Central Dispatch uses them extensively to perform concurrent tasks. I encourage you to read the Concurrency Programming Guide.