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Capturing 'self' strongly in this block is likely to lead to a retain cycle [duplicate]

2016-01-06 14:28 751 查看

转载至：http://stackoverflow.com/questions/17009966/capturing-self-strongly-in-this-block-is-likely-to-lead-to-a-retain-cycle

问题描述：

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This question already has an answer here:

capturing
self strongly in this block is likely to lead to a retain cycle 5 answers

I have reqest with block. But the compiler issues a warning

"Capturing 'self' strongly in this block is likely to lead to a retain cycle"

__weak typeof(self) weakSelf = self;
[generalInstaImage setImageWithURLRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:data[@"images"][@"low_resolution"][@"url"]]] placeholderImage:[UIImage imageNamed:@"Default"] success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
NSLog(@"success");
[generalInstaImage setImage: image];
[weakSelf saveImage:generalInstaImage.image withName:data[@"id"]];

} failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
NSLog(@"fail");
}];

I try example write like

weakSelf.generalInstaImage

,
but then the compiler generates an error and do not compile.

解决方法：

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When you receive this warning:

Capturing

self

strongly
in this block is likely to lead to a retain cycle

You should review your block for:

(a) any explicit references to

self

(b) any implicit references to

self

caused
by referencing any instance variables.

Let's imagine that we have some simple class property that was a block (this will experience the same "retain cycle" warnings as your question, but will keep my examples a little simpler):

@property (nonatomic, copy) void (^block)(void);

And let's assume we had some other class property we wanted to use inside our block:

@property (nonatomic, strong) NSString *someString;

If you reference

self

within
the block (in my example below, in process of accessing this property), you will obviously receive that warning about the retain cycle risk:

self.block = ^{
NSLog(@"%@", self.someString);
};

That is remedied via the pattern you suggested, namely:

__weak typeof(self) weakSelf = self;

self.block = ^{
NSLog(@"%@", weakSelf.someString);
};

Less obvious, you will also receive the "retain cycle" warning if you reference an instance variable of the class inside the block, for example:

self.block = ^{
NSLog(@"%@", _someString);
};

This is because the

_someString

instance
variable carries an implicit reference to

self

,
and is actually equivalent to:

self.block = ^{
NSLog(@"%@", self->_someString);
};

One might be inclined to just use

weakSelf

pattern
again, resulting in the

weakSelf->_someString

syntax,
but the compiler will warn you about this:

Dereferencing a

__weak

pointer
is not allowed due to possible null value caused by race condition, assign it to

strong

variable
first

You therefore resolve this by using the

weakSelf

pattern,
but also create a local

strong

variable
within the block and use that to dereference the instance variable:

__weak typeof(self) weakSelf = self;

self.block = ^{
__strong typeof(self) strongSelf = weakSelf;

if (strongSelf) {
NSLog(@"%@", strongSelf->_someString);

// or better, just use the property
//
// NSLog(@"%@", strongSelf.someString);
}
};

As an aside, this creation of a local

strong

reference,

strongSelf

,
inside the block has other advantages, too, namely that if the completion block is running asynchronously on a different thread, you don't have to worry about

self

being
deallocated while the block is executing, resulting in unintended consequences.

This

weakSelf

strongSelf

pattern
is very useful when dealing with block properties and you want to prevent retain cycles (aka strong reference cycles), but at the same time ensuring that

self

cannot
be deallocated in the middle of the execution of the completion block.

FYI, Apple discusses this pattern in the "non-trivial cycles" discussion further down in the Use
Lifetime Qualifiers to Avoid Strong Reference Cycles section of the Transitioning to ARC Release Notes.

You report that you received some "error" when you referenced

weakSelf.generalInstaImage

in
your example. This is the correct way to resolve this "retain cycle" warning, so if you received some warning, you should share that with us, as well as show us how you declared the property.

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