Referring to weak self inside a nested block - ios

Suppose I already create a weak self using
__weak typeof(self) weakSelf = self;
[self doABlockOperation:^{
...
}];
Inside that block, if I nest another block:
[weakSelf doAnotherBlockOperation:^{
[weakSelf doSomething];
}
will it create a retain cycle? Do I need to create another weak reference to the weakSelf?
__weak typeof(self) weakerSelf = weakSelf;
[weakSelf doAnotherBlockOperation:^{
[weakerSelf doSomething];
}

Your code will work fine: the weak reference will not cause a retain cycle as you explicitly instruct ARC not to increase the retainCount of your weak object. For best practice, however, you should create a strong reference of your object using the weak one. This won't create a retain cycle either as the strong pointer within the block will only exist until the block completes (it's only scope is the block itself).
__weak typeof(self) weakSelf = self;
[self doABlockOperation:^{
__strong typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf) {
...
}
}];

It depends.
You only create a retain cycle if you actually store the block (because self points to the block, and block points to self). If you don't intend to store either of the blocks, using the strong reference to self is good enough --- block will be released first after it got executed, and then it will release it's pointer to self.
In your particular example, unless you're performing more operations which are not shown, you don't need to create any weakerWeakerEvenWeakerSelfs.

Related

Should I use weakSelf in nested blocks?

I'm trying to correctly avoid retain cycles with blocks in Objective C, and am not sure about having nested blocks.
If I write a simple block like this:
[self doSomethingWithBlock:^{
[self doSomethingElse];
}];
The compiler catches and warns me that this could cause retain cycles. I change it as follows to avoid the cycle:
__weak __typeof(self)weakSelf = self;
[self doSomethingWithBlock:^{
__strong __typeof(weakSelf)strongSelf = weakSelf;
[strongSelf doSomethingElse];
}];
When I write something like this:
[self doSomethingWithBlock:^(MyObject* object){
[object doSomethingElseWithBlock:^{
[self doYetAnotherThing];
}];
}];
The compiler is happy, but I'm not convinced that it's safe. Even though there is an intermediary object in between, it still looks conceptually the same as above, but now it's a cycle with 3 retains.
Should it be like this instead?
[self doSomethingWithBlock:^(MyObject* object){
__weak __typeof(self)weakSelf = self;
[object doSomethingElseWithBlock:^{
__strong __typeof(weakSelf)strongSelf = weakSelf;
[strongSelf doYetAnotherThing];
}];
}];
Or like this?
__weak __typeof(self)weakSelf = self;
[self doSomethingWithBlock:^(MyObject* object){
[object doSomethingElseWithBlock:^{
__strong __typeof(weakSelf)strongSelf = weakSelf;
[strongSelf doYetAnotherThing];
}];
}];
In this situation, you are not worried about cyclic references. What you are worried about is a situation where the object self isn't actually needed anymore, but using self inside a nested block would keep it unnecessarily alive. For example, if you have a view controller that should go away when the view is removed by the screen, but you download an image that you would like to display in the controllers view. If the image arrives long after the view is already gone, you don't want the view controller alive anymore.
Best is
__weak typeof (self) weakSelf = self;
before calling the outermost method. Then within every block that ought to use self, you add
typeof (self) strongSelf = weakSelf;
and use strongSelf within that block. Depending on the situation, you might want to check that strongSelf isn't nil at that point, but sending messages to strongSelf when it is nil has no effect, so if all you do is sending messages and getting or setting properties, then a check for nil is not necessary.
What happens if you don't do this? The difference will be that self may be kept alive unnecessarily into the innermost block, if you use self everywhere (or just in the innermost block).
You should not capture something weakly just because you get a warning from the compiler; the compiler warning is just guessing; it doesn't know how the methods you call make the references. You should only do this if you understand the architecture of the references and determine that there is a cycle and determine that capturing a weak reference instead still preserves the intended behavior. You haven't shown us the code of -doSomethingWithBlock:. It would only create a retain cycle if inside that method it assigns the block to a property or instance variable of self. Does it do that or not? If not, then there is no retain cycle, and there is no point to the outer block capturing self weakly.
Assuming that the outer block capturing self weakly is right, the examples where the outer block captures self strongly are out of the question. The remaining questions would be whether the inner block should capture self (or whatever version of self, e.g. strongSelf, is appropriate) strongly. In other words, whether you would do something like this:
__weak __typeof(self) weakSelf = self;
[self doSomethingWithBlock:^(MyObject* object){
__strong __typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf) {
[object doSomethingElseWithBlock:^{
[strongSelf doYetAnotherThing];
}];
}
}];
or something like this:
__weak __typeof(self) weakSelf = self;
[self doSomethingWithBlock:^(MyObject* object){
__strong __typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf) {
[object doSomethingElseWithBlock:^{
__strong __typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf) {
[strongSelf doYetAnotherThing];
}
}];
}
}];
Again, the main issue to determine is whether there is a retain cycle if the inner block captures self strongly. There would only be a retain cycle if [object doSomethingElseWithBlock:... somehow assigns the block to a property or instance variable of self. But how could it? The method is called on object, not self. The method does not get self in any way. Unless there is something complicated going on, the method is not going to assign to a property or instance variable of self, so it is unlikely to create a retain cycle. This means that the inner block capturing self weakly is not necessary to prevent a retain cycle.
But whether the inner block captures self weakly or strongly could affect the behavior. Namely, if the inner block captures self weakly, self could be deallocated by the time the block is run, in which case [strongSelf doYetAnotherThing]; will not be executed, whereas if the inner block captured self strongly, it would keep self alive and [strongSelf doYetAnotherThing]; would be executed. So it depends on what -doYetAnotherThing does. If it performs some UI operation on self which is a UI view or something, then whether you perform it on a view that is no longer displayed doesn't make a difference. But if it for example sends something to the network or something, then whether or not it is executed can make a big difference.
Xcode 8 beta 4 underlines the self keyword, and warns of a possible retain cycle for having used it inside the block.
Per Apple Developer Connection's Programming in Objective-C (Working with Blocks):
Avoid Strong Reference Cycles when Capturing self If you need to
capture self in a block, such as when defining a callback block, it’s
important to consider the memory management implications.
Blocks maintain strong references to any captured objects, including
self, which means that it’s easy to end up with a strong reference
cycle if, for example, an object maintains a copy property for a block
that captures self:
#interface XYZBlockKeeper : NSObject
#property (copy) void (^block)(void);
#end
#implementation XYZBlockKeeper
- (void)configureBlock {
self.block = ^{
[self doSomething]; // capturing a strong reference to self
// creates a strong reference cycle
};
}
...
#end
The compiler will warn you for a simple example like this, but a more
complex example might involve multiple strong references between
objects to create the cycle, making it more difficult to diagnose.
To avoid this problem, it’s best practice to capture a weak reference
to self, like this:
- (void)configureBlock {
XYZBlockKeeper * __weak weakSelf = self;
self.block = ^{
[weakSelf doSomething]; // capture the weak reference
// to avoid the reference cycle
}
}
By capturing the weak pointer to self, the block won’t maintain a
strong relationship back to the XYZBlockKeeper object. If that object
is deallocated before the block is called, the weakSelf pointer will
simply be set to nil.
This site reportedly provides a means for making the self keyword weak whenever it's used inside a block; it also provides instructions for returning a weak self or class object formerly strong, strong again:
https://coderwall.com/p/vaj4tg/making-all-self-references-in-blocks-weak-by-default
Look at the answer for this question.
I'd do this
__weak typeof (self) weakSelf = self;
[self doSomethingWithBlock:^(MyObject* object){
[object doSomethingElseWithBlock:^{
[weakSelf doYetAnotherThing];
}];
}];

Can __weak self turn nil in the middle of the block?

When using a __weak self reference in my block that runs on a background thread, do I only need to check for nil in the beginning, or can the __weak self become nil even during execution after the first nil test has passed? I want to access some ivars from self in the block and I need the latest values at the time the block is executing.
If no one is holding a reference to self then yes. You can mitigate this by taking a strong reference in the block
__weak __typeof(self) weakSelf = self;
^{
__strong __typeof(weakSelf) strongSelf = weakSelf;
if (!strongSelf) {
return;
}
};
You can avoid having your reference set to nil by strongly referencing it from inside the block.
Get a strong reference to your weak pointer inside the block.
__weak MyObject *weakSelf = self; // a weak reference
myBlock = ^{
MyObject *innerSelf = weakSelf; // a block-local strong reference
NSLog(#"MyObject: %#", innerSelf);
};
Avoid using the variables directly, because it will cause retain cycles.
If you use an instance variable directly within a block, the block will capture self so you'll have to reference the instance variables using their accessors.
__weak MyObject *weakSelf = self;
myBlock = ^{
MyObject *innerSelf = weakSelf; // a block-local strong reference
NSLog(#"MyObject: %#", innerSelf);
NSLog(#"MyObject ID: %d", innerSelf.objectID);
};
If you use the instance variables directly like this:
NSLog(#"MyObject ID: %d", _objectID);
The compiler interprets _objectID as self->_objectID, where self is captured by your block.
It is in the nature of a weak reference that it can become nil at any time, when the last strong reference is removed in another thread. This is fatal if you access member variables because you will crash, and it is fatal if you call a method on that weak variable, since self is unsafe unretained.
There is a trivial way to fix this which everyone uses: Store the weak variable into a string one before doing anything, without any checks. Then you check whether the strong variable is nil; that check is needed once.
Turn all warnings on in Xcode, so you will get a warning if you do anything dangerous.

ARC, self and blocks

I thought I understood the usage of self in a block that is copied is a no no.
But in an attempt to clean my code i enabled a bunch of warnings in Xcode, one called "Sending messages to weak pointers"
so now in all my blocks, every time I use my created weakself reference __weak typeof(self) weakself = self;
I get this warning: Weak receiver may be unpredictably set to nil
a trivial example:
__weak typeof(self) weakself = self;
[aClass doSomethingInABlock:^{
[weakself doSomething]; //warning.
}];
I have seen answers which define a version of self within the block like so:
__weak typeof(self) weakself = self;
[aClass doSomethingInABlock:^{
typeof(self) selfref = weakself;
[selfref doSomething]; //no warning.
}];
So I'm wondering what actually happens here:
Am I just tricking the compiler?
what does a strong reference to a weak reference do?
anything else I'm missing.
Thanks.
I thought I understood the usage of self in a block is a no no.
This is not strictly correct. Blocks retain the objects in them, so don't use self in a block if your block is retained by self.
For example, you can use self just fine in a UIView animation block. This is because your view controller (or whatever code is calling the animation) doesn't have a pointer to the UIView animation block.)
Am I just tricking the compiler?
No.
What does a strong reference to a weak reference do?
If the weak reference is not nil, the retain count of the receiver is increased. This will stop the object from being deallocated while you're using it.
Remember, ARC will deallocate objects when there are no longer strong references to them. By creating a strong reference inside the block, you're preventing possible deallocation until you're done with it.
anything else I'm missing.
I recommend reading the Practical Memory Management section of the Advanced Memory Management Programming Guide. Especially, read the subsection "Use Weak References to Avoid Retain Cycles".
Remember that in ARC, an object will not be deallocated as long as there's a strong reference to it.
When an object has a weak reference, the object might be deallocated (if there's no other strong reference to the same object), so a weak reference doesn't ensure the objects life.
By doing this:
typeof(self) selfref = weakself;
you're ensuring that you have a strong reference to that object before you use it (selfref is the strong reference pointing to the same object weakself is referring to). If you don't do this, the object could be deallocated while you use it.
Strong reference to a weak reference retains an object. It could be important in following case
__weak typeof(self) weakself = self;
[aClass doSomethingInABlock:^{
[weakself allocateSomething]; // (1)
// ..... code (2)
[weakself freeSomething]; // (3)
}];
If Weak receiver will be unpredictably set to nil in line (2) resources could be successfully allocated in (1) but not freed in (3). To avoid such problems strong reference could be used.
[aClass doSomethingInABlock:^{
typeof(self) selfref = weakself;
[selfref allocateSomething]; // (1)
// ..... code (2)
[selfref freeSomething]; // (3)
}];
Now if selfref is not nil in (1) it will also be valid in (2) and (3).

strong reference in block

I've known about the best practice of block is like that
__weak SomeObjectClass *weakSelf = self;
SomeBlockType someBlock = ^{
SomeObjectClass *strongSelf = weakSelf;
if (strongSelf == nil) {
// The original self doesn't exist anymore.
// Ignore, notify or otherwise handle this case.
}
[self someMethod];
};
I understand using weakSelf is used to prevent retain cycle and using strongSelf in case of weakSelf might nil.
But I just wonder using strongSelf can cause retain cycle again because block capture and retain strongSelf and strongSelf is also pointer of self.
can someone give me a explain ,thanks.
Ask yourself: How long does strongSelf exists?
strongSelf only exists while the block is executing. So basically, it holds a strong reference while someMethod is executing, and not any longer.
I assume you meant
[strongSelf someMethod];
and not
[self someMethod];
because the former holds a strong reference to strongSelf (which is equal to self) while the block is executing while latter will hold a strong reference to self while the block exists.
From documentation:
You can use lifetime qualifiers to avoid strong reference cycles. For
example, typically if you have a graph of objects arranged in a
parent-child hierarchy and parents need to refer to their children and
vice versa, then you make the parent-to-child relationship strong and
the child-to-parent relationship weak. Other situations may be more
subtle, particularly when they involve block objects.*
In manual reference counting mode, __block id x; has the effect of not
retaining x. In ARC mode, __block id x; defaults to retaining x (just
like all other values). To get the manual reference counting mode
behavior under ARC, you could use __unsafe_unretained __block id x;.
As the name __unsafe_unretained implies, however, having a
non-retained variable is dangerous (because it can dangle) and is
therefore discouraged. Two better options are to either use __weak (if
you don’t need to support iOS 4 or OS X v10.6), or set the __block
value to nil to break the retain cycle.
And som more documentation:
If you need to capture self in a block, such as when defining a
callback block, it’s important to consider the memory management
implications.
Blocks maintain strong references to any captured objects, including
self, which means that it’s easy to end up with a strong reference
cycle
As to your example - no retain cycle there, as CouchDeveloper said, because block will keep reference to self only while being executed. But there will be, no matter your __weak declaration, if we change code to this:
__weak SomeObjectClass *weakSelf = self;
self.someBlock = ^{
SomeObjectClass *strongSelf = weakSelf;
if (strongSelf == nil) {
// The original self doesn't exist anymore.
// Ignore, notify or otherwise handle this case.
}
[self someMethod];
};
Now we have retain cycle - we have reference to block and inside block we have reference to self. To avoid this, you should use __weak approach:
__weak SomeObjectClass *weakSelf = self;
self.someBlock = ^{
SomeObjectClass *strongSelf = weakSelf;
if (strongSelf == nil) {
// The original self doesn't exist anymore.
// Ignore, notify or otherwise handle this case.
} else {
[strongSelf someMethod];
}
};
Some more articles:
https://coderwall.com/p/vaj4tg
http://teohm.com/blog/2012/09/03/breaking-arc-retain-cycle-in-objective-c-blocks/
First, you must understand life cycle of strongSelf, it only exists in someBlock, after someBlock finished the object referenced by strongSelf will be released.
When the someBlock translated to MRR will like this:
^{
SomeObjectClass *strongSelf = [weakSelf retain];
if (strongSelf == nil) {
// The original self doesn't exist anymore.
// Ignore, notify or otherwise handle this case.
}
[self someMethod];
[strongSelf release];
};

Messaging a __weak object?

What happens if I send a message to a weak object? Does sending the message possess the object and hold it in memory until return?
I'm thinking of this pattern:
__weak MyObject *weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
[weakSelf doSomeAction];
});
Assuming weakSelf is non-nil when the message is sent, might it be deallocated while doSomeAction is working or is it guaranteed to remain valid until doSomeAction returns?
From the Clang ARC documentation:
Reading occurs when performing a lvalue-to-rvalue conversion on an object lvalue.
For __weak objects, the current pointee is retained and then released at the end of the current full-expression. This must execute atomically with respect to assignments and to the final release of the pointee.
Messaging a weak reference performs an lvalue-to-rvalue conversion on the variable, which means the value of the weak reference will be retained and then released at the end of the current full-expression (basically, the statement). It's basically equivalent to assigning to a strong variable whose scope only lasts for the current statement, and then messaging that strong variable.
The takeaway here is if you want to message a weak variable once, and never touch it again, and you don't care about the side-effects of evaluating the arguments to the method in the case where the weak reference ends up nil, then go ahead and message the weak reference directly. But if you need to refer to the weak reference twice (in separate statements), or the side-effects of evaluating the arguments do matter, then you should assign to a strong variable and test for non-nil before proceeding.
You asked:
Assuming weakSelf is non-nil when the message is sent, might it be deallocated while doSomeAction is working or is it guaranteed to remain valid until doSomeAction returns?
This ARC behavior has changed over time. But nowadays, weak references can be released as soon as the last strong reference is removed.
Thus, consider the following:
- (void)dealloc {
NSLog(#"%s", __FUNCTION__);
}
- (void)startBackgroundOperation {
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[weakSelf doSomeAction];
[NSThread sleepForTimeInterval:5];
[weakSelf doSomeAction2];
});
}
- (void)doSomeAction {
NSLog(#"%s", __FUNCTION__);
}
- (void)doSomeAction2 {
NSLog(#"%s", __FUNCTION__);
}
If you have some code invoke startBackgroundOperation and let the object be deallocated in the intervening time between doSomeAction and doSomeAction2, you will see the former will be called and the latter will not. I.e. if there were no more strong references, the object could be deallocated in the middle of the block.
So, if you want weak reference, but want an “all or none” sort of behavior whereby it to be retained for the duration of the closure, we perform what is jokingly referred to as the “weakSelf-strongSelf dance”:
- (void)startBackgroundOperation {
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
typeof(self) strongSelf = weakSelf; // establish just-in-time strong reference (if `weakSelf` is not yet `nil`)
[strongSelf doSomeAction];
[NSThread sleepForTimeInterval:5];
[strongSelf doSomeAction2];
});
}
This will ensure that the block has a weak reference, but if it is not deallocated by the time it hits the assignment of strongSelf, then it will establish and maintain a strong reference for the duration of the block.
For what it is worth, this weakSelf-strongSelf pattern is essential when dereferencing ivars with -> (avoiding race conditions with weakSelf).
E.g.
- (void)badDeferenceExample {
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
if (!weakSelf) { return; }
NSInteger value = weakSelf->_someIVar; // this is race and can crash!!!
...
});
}
- (void)properDeferenceExample {
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
typeof(self) strongSelf = weakSelf; // establish just-in-time strong reference (if `weakSelf` is not yet `nil`)
if (!strongSelf) { return; }
NSInteger value = strongSelf->_someIVar; // this is safe
...
});
}

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