The common pattern to avoid capturing self within a Block is to create a weak self outside the Block and use this to create a "locally strong" version of self within the Block (inner self).
__weak ClassX *weakSelf = self;
[someOtherObject methodThatTakesCOmpletionBlock: ^{
ClassX innserSelf = weakSelf; //innserSelf creation?
[someObject send:innerSelf.prop;}];
What happens when the innserSelf creation line is executed? Is innerSelf a copy of self at the time the method methodThatTakesCompletionBlock: is sent to someOtherObject?
This question just focusses on what happens when the innserSelf line is executed. I've seen Strong reference to a weak references inside blocks which is related but doesn't address this point.
Consider:
__weak id weakSelf = self;
[other doSomething: ^{
__strong id strongSelf = weakSelf;
....
}];
When other copies the block, there is no strong reference.
When other executes the block, then the strong reference is created at the beginning of the block's execution. When the block is done, the execution scope is gone and, thus, the strongSelf reference is destroyed.
Whether other hangs onto the block or not is irrelevant; the strongSelf reference only exists during block execution.
Assigning a weak pointer to a strong one does not copy the object. Both pointers will point to the same object. The strong pointer retains thus adding +1 to the retain count. The weak pointer does not alter the retain count
Related
When I pass self as an argument to a function, does it make any difference if I weakify it first?
For example:
__weak __typeof(self) weakSelf = self;
[self.someObject doWorkWithDelegate: weakSelf];
In the body of the doWork function it assigns it to a strong property
#property (strong) Foo* fooDelegate;
(I know this is terrible, don't get sidetracked).
This will still be a strong reference and cause a memory cycle despite theh fact that I "weakified" the reference first, correct?
Bonus Question: How can I check this myself?
It is the variable weakSelf which is weak. That is, ARC does not emit a retain when a value is assigned to it, nor a release when it goes out of scope (or is assigned a different value).
Since the fooDelegate property is strong, assigning to it releases any old value it may have had and retains the newly-assigned value.
So, yes, that will be a strong reference. It's not clear from what you posted whether it will constitute a retain cycle. The name of the method suggests that the delegate will be cleared (thus released) after the work has been completed. In that case, it's not a problem. It would only be a problem if self.someObject maintained the strong reference until it itself was released and self maintained a strong reference to someObject until self was released.
An important aspect of the original question could be further clarified. "When I pass self as an argument to a function, does it make any difference if I weakify it first?"
Note that in the example code block:
__weak __typeof(self) weakSelf = self;
[self.someObject doWorkWithDelegate: weakSelf];
ARC will perform a retain on each of the objects passed, including the receiver (i.e. self.someObject and weakSelf), and then a release on each when complete, allowing weak objects to be safely used for the lifetime of the called method. So the answer would be no, it doesn't make a difference to make it weak first because it will be strong for the duration of the call.
If the weak variable was simply referenced within a code block callback from the method, then it will still be weak within that scope.
So this is a common pattern to use a weakSelf variable within a block callback to call another method, foo, which then can safely use self because it is no longer weak during that execution of foo. But if self goes away before the callback is executed, then [weakSelf foo] will simply never be called because weakSelf has gone nil.
__weak __typeof(self) weakSelf = self;
[self.someObject doWorkWithCallback:^{
[weakSelf foo];
}];
- (void)foo
{
// Safely use self here.
[self doMoreWork];
}
//Parent.m
#import "Parent.h"
#implementation Parent{
dispatch_block_t _block;
NSTimer *_timer;
}
- (instancetype)init
{
self = [super init];
if (self) {
[self commonInitialization];
}
return self;
}
-(void)commonInitialization{
__unsafe_unretained typeof(self) weakSelf=self;
//__weak typeof(self) weakSelf=self; the same conculsion
//apple doc:The object to which to send the message specified by aSelector when the timer fires. The timer maintains a strong reference to this object until it (the timer) is invalidated.
_timer=[NSTimer timerWithTimeInterval:0.5 target:weakSelf selector:#selector(sayHello) userInfo:nil repeats:YES];// make weakSelf retain count +1
_block=^{
__strong Parent *parent=weakSelf;//also make weakSelf retain count +1,but the actual is that this wont make self retain count +1
[parent sayHello];//
};
// my question is why weakSelf can make _block wont retain self,but _timer will retain self,it look like contradictory
}
-(void)sayHello{
NSLog(#"hello");
}
-(void)dealloc{
NSLog(#"Parent instance can dealloc");
}
#end
In _block I retain weakSelf again like NSTimer will retain target:weakSelf.
My question is why __unsafe_unretained or __weak can make _block unretained self but _timer not. It looks contradictory.
Retain cycles with blocks happen when a class instance has a strong reference to the class and the block, in turn, has a strong reference to that instance. By capturing a weak reference to the instance within the block (weakSelf), that captured reference can be set to nil. Making the reference within the block a strong reference only means that while the block is executing, the instance will not be dealloc'd (because the block still has a live reference).
The timer internally maintains a strong reference to the instance, just as the block does while it's executing. The main difference is that the timer is a long-lived object, and unless you invalidate it or nil all your references to it, the timer still exists, along with its strong reference to your instance.
I really do not know, whether I understood you correctly. Do you mean with "makes retained" "retains"?
However, the difference between the two pieces of code is the time of execution and how references are handled:
Keep in mind that not object references are retained, but the object they points to.
A. Timer
{
__weak typeof(self) weakSelf=self;
_timer=[NSTimer timerWithTimeInterval:0.5 target:weakSelf …];
}
With this code you create an additional local var called weakSelf, that does not retain the object it points to. Moreover, the extent ("lifetime") of weakSelf ends with the C-block (not the closure what you called __block), that means with the closing }. So we have:
{
__weak typeof(self) weakSelf=self;
// does not retain self; no problem
_timer=[NSTimer timerWithTimeInterval:0.5 target:weakSelf …];
// weakSelf dies, since it is weak nothing happens.
}
In such a case it is completly meaningless to weakify self:
{
id anotherSelf=self;
// does retain self: +1;
_timer=[NSTimer timerWithTimeInterval:0.5 target:weakSelf …];
// anotherSelf dies: -1
}
This is always balanced, because ARC care about it. No problem.
So why is there a retain cycle? It is "inside the timer." According to the documentation:
The object to which to send the message specified by aSelector when the timer fires. The timer maintains a strong reference to this object until it (the timer) is invalidated.
Therefore, lets go back to your example:
{
__weak typeof(self) weakSelf=self;
// does not retain self;
_timer=[NSTimer timerWithTimeInterval:0.5 target:weakSelf …];
// timer retains the object, weakSelf points to: +1.
// self retains the timer: +1
// result: retain cycle
// weakSelf dies, since it is weak nothing happens.
}
What -timerWithInterval… does, does not depend of the strength of weakSelf. The method does not even see the strength of the reference. It retains an object that the argument points to. Period.
B. Block
Having a block it is different:
{
__weak typeof(self) weakSelf=self;
// does not retain self; no problem
_block=^{
…
};
// weakSelf dies, since it is weak nothing happens.
}
As you can see, there is no problem. Why can be there a retain cycle? This is quite easy: Having a reference inside the block, the referred object is retained when the block is created (similar to timers) and the reference is strong(different to timers):
{
__weak typeof(self) weakSelf=self;
// does not retain self; no problem
_block=^{
… self … // the object self points to is retained, because self is strong. +1
… weakSelf … // the object weakSelf points to is not retained.
…
};
// weakSelf dies, since it is weak nothing happens.
}
This reference lives as long as the block lives. You have to read it correctly: It is like the reference itself has a longer lifetime. Therefore it depends of the strength of the reference.
So there is a big difference, whether you use weakSelf or self (or any other weak or strong reference.)
What is done inside the block …:
_block=^{
__strong id strongSelf=weakSelf;
};
… is meaningless, because this is done, when the block is executed and the local strongSelf will immediately lose its extent. (Again with the closing }.
ARC and block are all compiler work, compiler will add retain release and it will translate block to struct.
NSTimer will hold a strong reference to target, so it will send retain method (it may call objc_retain(id value) in ARC) to weakSelf, self hold timer as a instance variable, retain cycle happens.
Let's take about the block, by default block will retain any NSObject it captures when the block is move from stack to heap.
the _block(in your case) is a instance variable, when it is assigned a block, the block will be send a copy method, so it will move from stack to heap.
But if object it captured is with __weak or __unsafe_unretained attribute, it won't send retain method to it. The rules are defined by compiler, so it works.
If you want to learn the detail, check the source code runtime.c, you may also need to assemble your code because runtime.c doesn't have ARC code.
If you were confused about this line of code __strong Parent *parent=weakSelf , check this answer.
Here is the block in question. Im wondering if declaring strongSelf outside of the block will introduce a retain cycle in anyway. The reason for setting up the strongSelf outside the block is that I'm worried that weakSelf will be deallocated before the completion block.
__weak CollectionView *weakSelf = self;
CollectionView *strongSelf = weakSelf;
[self
performBatchUpdates:
^{
strongSelf->_cellSet = [strongSelf performDeletions:[strongSelf deletionsTowards:destinationSet from:strongSelf->_cellSet] from:strongSelf->_cellSet];
[(CollectionViewLayout *)strongSelf.collectionViewLayout setCellSet:strongSelf->_cellSet];
}
completion:
^(BOOL finished)
{
[strongSelf insertTowards:destinationSet];
}];
What you do is just pointless. weakSelf is a weak reference to self. strongSelf is a strong reference to self. It is captured by the block, so the block now also has a strong reference to self (that's the difference when you use weakSelf in the block: When the block captures weakSelf, the block has a weak reference to self). Instead of using strongSelf in the block, it would be much easier to just use self - it will be exactly the same thing. You might confuse the compiler enough to not give you any warnings when you use strongSelf where you would have got a warning for using self (if that warning was appropriate).
Whether you cause a retain cycle: Depends on the code. If a copy of the two blocks is stored in the object self and not set back to nil when the completion blocks are called, then you have a retain cycle. If the two blocks are not stored anywhere then the strong reference to self goes away eventually.
The method that actually works well is to assign weakSelf to a strong reference strongSelf inside each block. And then the first thing that you do is to test once whether strongSelf is nil. In that case you know that self has been deallocated, and you need to think about what actions you would want to do in that case.
In Objective-C, what does it mean to assign a weak to a strong within a block? What's happening behind the scene?
e.g.
__weak __typeof(self) wself = self;
void (^cmd)() = ^(){
__strong __typeof(self) sself = wself;
if (!sself) return;
...
};
The intent here is two-fold:
First, is the use of:
__weak __typeof(self) wself = self;
This ensures that the cmd block does not maintain a strong reference to self. This ensures that, if cmd was an instance variable of the class, that you don't end up with a strong reference cycle. If you don't use this wself pattern, the class that has cmd as an instance variable would never be released and you'd leak the object that has cmd as an instance variable.
For more information, see the Avoid Strong Reference Cycles when Capturing self section of the Programming with Objective-C: Working With Blocks document.
Second, the use of the following within the block:
__strong __typeof(self) sself = wself;
if (!sself) return;
This ensures that, if the block starts execution, if wself was already deallocated, the block would quit. But if wself has not yet been deallocated, by assigning sself, you're making sure that the object will be retained during the execution of the block.
Also, if you reference any ivars in the block, be aware that you want to dereference them (because otherwise there is an implicit reference to self in the block, potentially leading to that strong reference cycle). But you cannot dereference ivars using a weak pointer (e.g. wself->someIvar is not permitted), but you can with this local strong pointer (e.g. sself->someIvar is ok). Generally you shouldn't be dereferencing ivars anyway, but rather use properties, but nonetheless it's another reason to use a local strong reference to self.
Often, you'll see this construct in conjunction with a class property (or an ivar):
#property (nonatomic, copy) void (^commandBlock)(void);
And, as a matter of convention, you'll generally see more descriptive variable names, weakSelf and strongSelf, thus:
__weak __typeof(self) weakSelf = self;
self.commandBlock = ^(){
__strong __typeof(self) strongSelf = weakSelf;
if (!strongSelf) return;
...
};
This weakSelf/strongSelf pattern is very common when you have your own block properties to your class and you want to (a) prevent strong reference cycles (aka retain cycles); but (b) want to ensure that the object in question cannot be deallocated in the middle of the execution of the block.
If you don't assign weak reference to a strong the object referenced by the weak reference can be deallocated in the middle of the block execution - something you might not expect. if you assign to strong the object is retained for as long as the strong reference is in scope (unless the object has already been deallocated prior to the assignment).
When you assign a weak reference to a strong compiler inserts Objective-C runtime function call into the code that increments the object reference counter as needed to retain it. When the strong variable goes out of scope (or earlier, after the last use of the strong variable) another call inserted by compiler decrements the reference counter.
I'm currently thinking about how to prevent strong reference cycles when using blocks that retain self. The usual way seems to be to just use a weak reference to self:
#property (strong, nonatomic) NSOperationQueue *queue;
- (void)methodA {
__weak id *weakSelf = self;
NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
[weakSelf methodB];
}];
[self.queue addOperation:operation];
}
But what if methodB looks like this:
- (void)methodB {
[self someOtherMethod];
}
Would this still cause a strong reference cycle? Or would methodB receive the weak reference to self from methodA as its reference to self? (That is, is methodB's reference to self just a strong reference to the weak reference from methodA?)
Or would methodB receive the weak reference to self from methodA as
its reference to self? (That is, is methodB's reference to self just a
strong reference to the weak reference from methodA?)
A method does not "receive" a strong or weak reference. It receives a reference. "Strong" or "weak" only apply to a variable (usually, an instance variable or variable captured by a block).
weakSelf is a weak reference in the block. Because weakSelf is a zeroing weak reference (__weak), either it points to a valid object, or its value is nil. If the former, it points to a valid object, and methodB is called on it. If the latter, sending a message to nil does nothing, so nothing happens.
You ask if the self in methodB is a strong reference. A strong reference means that it is retained, so if self were a strong reference in methodB, that means it retains self at the beginning of the method and releases it at the end. But why does it matter whether the method retains an argument such as self? Retain cycles refer to objects strongly referencing each other. A function or method will run and then stop; any retains they do on local variables must be temporary by the memory management rules, and have no effect on retain cycles. (The technical answer is no, self is not retained in ARC, and arguments including self are generally never retained in MRC. But as I said this really isn't relevant.)
I finally found a part in the documentation that kind of answers this question. So it really looks like methodB receives the block's weak reference to self.