Passing blocks in Objective-C

问题:

When writing a method that accepts a block as an argument, do I need to do anything special such as copying the block to the heap before executing it? For example, if I had the following method:

- (void)testWithBlock:(void (^)(NSString *))block {
    NSString *testString = @"Test";
    block(testString);
}

Should I do anything with block before calling it, or when entering the method? Or is the above the correct way of using the passed-in block? Also, is the following way of calling the method correct, or should I do something with the block before passing it?

[object testWithBlock:^(NSString *test){
    NSLog(@"[%@]", test);
}];

Where do I need to copy the block? And how would this have been different if I wasn't using ARC?

回答1:

When you receive a block as a method parameter, that block could be the original that had been created on the stack or it could be copy (a block on the heap). As far as I'm aware, there's no way tell. So the general rule of thumb is if you're going to execute the block within the method that's receiving it, you don't need to copy it. If you intend to pass that block to another method (which may or may not execute it immediately) then you also don't need to copy it (the receiving method should copy it if it intends to keep it around). However, if you intend to store the block in any way for some place for later execution, you need to copy it. The primary example many people use is some sort of completion block held as an instance variable:

typedef void (^IDBlock) (id);
@implementation MyClass{
    IDBlock _completionBlock;
}

However, you also need to copy it if you're going to add it to any kind of collection class, like an NSArray or NSDictionary. Otherwise, you'll get errors (most likely EXC_BAD_ACCESS) or possibly data corruption when you try to execute the block later.

When you execute a block, it's important to test first if the block is nil. Objective-c will allow you to pass nil to a block method parameter. If that block is nil, you'll get EXC_BAD_ACCESS when you try to execute it. Luckily this is easy to do. In your example, you'd write:

- (void)testWithBlock:(void (^)(NSString *))block {
    NSString *testString = @"Test";
    if (block) block(testString);
}

There are performance considerations in copying blocks. Compared to creating a block on the stack, copying a block to the heap is not trivial. It's not a major deal in general, but if you're using a block iteratively or using a bunch of blocks iteratively and copying them on each execution, it'll create a performance hit. So if your method - (void)testWithBlock:(void (^)(NSString *))block; were in some kind of loop, copying that block might hurt your performance if you don't need to copy it.

Another place you need to copy a block is if you intend on calling that block in itself (block recursion). This isn't all that common, but you must copy the block if you intend to do this. See my question/answer on SO here: Recursive Blocks In Objective-C.

Finally, if you're going to store a block you need to be really careful about creating retain cycles. Blocks will retain any object passed into it, and if that object is an instance variable, it will retain the instance variable's class (self). I personally love blocks and use them all the time. But there's a reason Apple doesn't use/store blocks for their UIKit classes and instead stick with either a target/action or delegate pattern. If you (the class creating the block) are retaining the class that is receiving/copying/storing the block, and in that block you reference either yourself or ANY class instance variable, you've created a retain cycle (classA -> classB -> block -> classA). This is remarkably easy to do, and it's something I've done too many times. Moreover, "Leaks" in Instruments doesn't catch it. The method to get around this is easy: simply create a temporary __weak variable (for ARC) or __block variable (non-ARC) and the block won't retain that variable. So,for example, the following would be a retain cycle if the 'object' copies/stores the block:

[object testWithBlock:^(NSString *test){
    _iVar = test;
    NSLog(@"[%@]", test);
}];

However, to fix this (using ARC):

__weak IVarClass *iVar = _iVar;
[object testWithBlock:^(NSString *test){
    iVar = test;
    NSLog(@"[%@]", test);
}];

You can also do something like this:

__weak ClassOfSelf _self = self;
[object testWithBlock:^(NSString *test){
    _self->_iVar = test;
    NSLog(@"[%@]", test);
}];

Note that many people don't like the above because they consider it fragile, but it is a valid way of accessing variables. Update - The current compiler now warns if you try to directly access a variable using '->'. For this reason (as well as reasons of safety) it's best to create a property for the variables you want to access. So instead of _self->_iVar = test; you would use: _self.iVar = test;.

UPDATE (more info)

Generally, it's best to consider the method that receives the block as responsible for determining whether the block needs to be copied rather than the caller. This is because the receiving method can be the only one that knows just how long the block needs to be kept alive or if it needs to be copied. You (as the programmer) will obviously know this information when you write the call, but if you mentally consider the caller and receiver at separate objects, the caller gives the receiver the block and is done with it. Therefore, it shouldn't need to know what is done with the block after it's gone. On the flip side, it's quite possible that the caller may have already copied the block (maybe it stored the block and is now handing it off to another method) but the receiver (who also intends on storing the block) should still copy the block (even though the block as already been copied). The receiver can't know that the block has already been copied, and some blocks it receives may be copied while others may not be. Therefore the receiver should always copy a block it intends on keeping around? Make sense? This essentially is good Object Oriented Design Practices. Basically, whoever has the information is responsible for handling it.

Blocks are used extensively in Apple's GCD (Grand Central Dispatch) to easily enable multi-threading. In general, you don't need to copy a block when you dispatch it on GCD. Oddly, this is slightly counter-intuitive (if you think about it) because if you dispatch a block asynchronously, often the method the block was created in will return before the block has executed, which generally would mean the block would expire since it is a stack object. I don't think that GCD copies the block to the stack (I read that somewhere but have been unable to find it again), instead I think the life of the thread is extended by being put on another thread.

Mike Ash has extensive articles on blocks, GCD and ARC which you may find useful:

• Mike Ash: Practical Blocks
• Mike Ash: Objective-C Blocks vs C++0x Lambdas: Fight!
• Mike Ash: Q&A - More info on Blocks
• Mike Ash: Automatic Reference Counting He talks about blocks & ARC in this article.
• Mike Ash: GCD Is Not Blocks, Blocks Are Not GCD
• Apple Docs - Introducing Block & GCD

回答2:

This all looks good. Though, you might want to double-check the block parameter:

@property id myObject;
@property (copy) void (^myBlock)(NSString *);

....

- (void)testWithBlock: (void (^)(NSString *))block
{
    NSString *testString = @"Test";
    if (block)
    {
        block(test);
        myObject = Block_copy(block);
        myBlock = block;
    }
}

...

[object testWithBlock: ^(NSString *test)
{
    NSLog(@"[%@]", test);
}];

Should be fine. And I believe that they are even trying to phase out Block_copy(), but they haven't yet.

回答3:

As the blocks programming topics guide says under 'Copying Blocks':

Typically, you shouldn’t need to copy (or retain) a block. You only need to make a copy when you expect the block to be used after destruction of the scope within which it was declared.

In the case you are describing, you can basically think about the block as simply being a parameter to your method, just like as if it were an int or other primitive type. When the method gets called, space on the stack will be allocated for the method parameters and so the block will live on the stack during the entire execution of your method (just like all the other parameters). When the stack frame gets popped off the top of the stack at the return of the method, the stack memory allocated to the block will be deallocated. Thus, during the execution of your method, the block is guaranteed to be alive, so there's no memory management to deal with here (in both ARC and non-ARC cases). In other words, your code is fine. You can simply call the block inside the method.

As the quoted text suggests, the only time you need to explicitly copy a block is when you want it to be accessible from outside of the scope where it was created (in your case, beyond the lifetime of the stack frame of your method). As an example, suppose you want a method that fetches some data from the web, and runs a block of code when the fetch is complete. Your method signature might look like:

- (void)getDataFromURL:(NSURL *)url completionHandler:(void(^)(void))completionHandler;

Since the data fetch happens asynchronously, you would want to keep the block around (likely in a property of your class) and then run the block once the data has been fully fetched. In this case, your implementation might look like:

@interface MyClass

@property (nonatomic, copy) void(^dataCompletion)(NSData *);

@end



@implementation MyClass
@synthesize dataCompletion = _dataCompletion;

- (void)getDataFromURL:(NSURL *)url completionHandler:(void(^)(NSData *fetchedData))completionHandler {
    self.dataCompletion = completionHandler;
    [self fetchDataFromURL:url]; 
}

- (void)fetchDataFromURL:(NSURL *)url {
    // Data fetch starts here 
}

- (void)finishedFetchingData:(NSData *)fetchedData {
    // Called when the data is done being fetched
    self.dataCompletion(fetchedData)
    self.dataCompletion = nil; 
}

In this example, using a property with a copy semantic will perform a Block_copy() on the block and copy it to the heap. This happens in the line self.dataCompletion = completionHandler. Thus, the block gets moved from the stack frame of the -getDataFromURL:completionHandler: method to the heap which allows it to be called later in the finishedFetchingData: method. In the latter method, the line self.dataCompletion = nil nullifies the property and sends a Block_release() to the stored block, thus deallocating it.

Using a property in this way is nice since it will essentially handle all of the block memory management for you (just make sure it's a copy (or strong) property and not simply a retain) and will work in both non-ARC and ARC cases. If instead you wanted to use a raw instance variable to store your block and were working in a non-ARC environment, you would have to call Block_copy(), Block_retain(), and Block_release() yourself in all the proper places if you wanted to keep the block around any longer than the lifetime of the method in which it's passed as a parameter. The same code above written using an ivar instead of a property would look like this:

@interface MyClass {
    void(^dataCompletion)(NSData *);
}

@end



@implementation MyClass

- (void)getDataFromURL:(NSURL *)url completionHandler:(void(^)(NSData *fetchedData))completionHandler {
    dataCompletion = Block_copy(completionHandler);
    [self fetchDataFromURL:url]; 
}

- (void)fetchDataFromURL:(NSURL *)url {
    // Data fetch starts here 
}

- (void)finishedFetchingData:(NSData *)fetchedData {
    // Called when the data is done being fetched
    dataCompletion(fetchedData)
    Block_release(dataCompletion);
    dataCompletion = nil;
}

回答4:

You know there are two kinds of blocks:

1. blocks stored in the stack, the ones that you explicitly write as ^{...}, and that disappear as soon as the function they are created in returns, just as regular stack variables. When you call a stack block after the function it belonged to has returned, bad things happens.

2. blocks in the heap, the ones that you get when you copy another block, the ones that live as long as some other object keeps a reference to them, just like regular objects.

The only reason for you to copy a block is when you are given a block that is, or may be a stack block (explicit local block ^{...}, or method argument whose origin you don't know), AND that you want to extend its life span out of the limited one of the stack blocks, AND that the compiler doesn't already do that job for you.

Think: keeping a block in an instance variable.

Adding a block in a collection such as NSArray.

Those are common examples of cases where you should copy a block when you're not sure it is already a heap block.

Note that the compiler does it for you when a block is called in another block.