strong reference in block - ios

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];
};

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];
}];
}];

Two ways of block for preventing retain-cycle

I usually use block like this if there might be a retain-cycle:
- (void)someFunction {
__weak __typeof(self) weakSelf = self;
[self setHandler:^{
[weakSelf doSomething];
}];
}
But recently I saw another way like:
- (void)someFunctionWithParam:(id)param {
__weak __typeof(param) weakParam = param;
[self setHandler:^{
__typeof(weakParam) strongParam = weakParam;
[strongParam doSomething];
}];
}
What's the difference between them?
Edit1: Does it mean the param won't be release when self running the handler?
In the second example, there is no benefit to creating the strongSelf variable in that specific case, but I can show you an example where there is a benefit.
In the first example, the statement [weakSelf doSomething] loads the reference in weakSelf, retains it, sends the doSomething message, and then (after doSomething returns) releases the reference. The second example does essentially exactly the same steps “by hand”.
Here's a slightly different example:
- (void)someFunction {
__weak __typeof(self) weakSelf = self;
[self setHandler:^{
[weakSelf doSomething];
[weakSelf doAnotherThing];
}];
}
In my code, suppose there's only one strong reference to the self object at the time the block is called. The [weakSelf doSomething] statement creates a second, temporary strong reference to it. While doSomething is running, another thread releases the other strong reference. When doSomething returns, the statement releases its temporary strong reference. Now self has no more strong references, so it is deallocated and weakSelf is set to nil.
Then the [weakSelf doAnotherThing] statement runs. It wants to load and retain the contents of weakSelf, but because weakSelf is now nil, the statement just uses nil. It sends the doAnotherThing message to nil, which is allowed and doesn't crash. It just does nothing. It doesn't call the method.
This might not be behavior you want. Maybe you always want doAnotherThing to run on self if doSomething ran. That's when you need the pattern in your second example:
- (void)someFunctionWithParam:(id)param {
__weak __typeof(self) weakSelf = self;
[self setHandler:^{
__typeof(weakSelf) strongSelf = weakSelf;
[strongSelf doSomething];
[strongSelf doAnotherThing];
}];
}
Here, when the block is called, it immediately stores a strong reference to self in strongSelf (or it stores nil if weakSelf has already been set to nil). The strongSelf reference can't be released until after the last use of the strongSelf variable, so it's impossible for self to be deallocated after doSomething but before doAnotherThing.

Do I need to check for nil on my strongSelf = weakSelf assignment inside a block?

For example, I'm using SVInfiniteScrolling (https://github.com/alexanderedge/SVInfiniteScrolling).
I have some code that looks like this...
- (void)initializeInfiniteScrollingForTableView {
__weak MyViewController *weakSelf = self;
[self.tableView addInfiniteScrollingWithActionHandler:^{
MyViewController *strongSelf = weakSelf;
if (!strongSelf.endReached) {
[strongSelf fetchData];
}
else {
[strongSelf.tableView.infiniteScrollingView stopAnimating];
}
}];
}
What I'm wondering is... do I need to check strongSelf for nil before using like this...
...
[self.tableView addInfiniteScrollingWithActionHandler:^{
MyViewController *strongSelf = weakSelf;
if (strongSelf) { // <== ** Is this needed? **
if (!strongSelf.endReached) {
Here is why I ask. From point #3 on this link (http://www.apeth.com/iOSBook/ch12.html#EXstrongWeakDance) it says "The nil test is because, in a multithreaded situation, our weak reference to self may have vanished out from under us before the previous step; it would then be nil, because it’s an ARC weak reference, and in that case there would be no point continuing."
Is this check needed? I thought the first time you used the reference to weakSelf within the block, it is retained for the duration of the expression?
For the code you posted, checking for nil is unnecessary.
This line of code:
if (!strongSelf.endReached) {
Will eval to false if strongSelf is nil. Further reading: Sending a message to nil?
And then this line of code will execute:
[strongSelf.tableView.infiniteScrollingView stopAnimating];
That line will just do nothing at all (not even an error) if strongSelf is nil.
However, some developers consider it a best practice to check for nil anyway, incase somebody later adds the code and it does care about nil. So you should consider doing:
MyViewController *strongSelf = weakSelf;
if (!strongSelf) {
return;
}
... the rest of your code ...
I think in your line:
if (!strongSelf.endReached) {
this will evaluate to YES if self is nil, which is not probably not you intend.
It happens that your code does the correct thing (do nothing) because both clauses only use self to call methods and then nothing will happen. The end result is the same as if you had the nil-check but the execution path is wrong. This might have consequences in the future if someone adds code that doesn't use self.
Your source is referring to the fact that weakSelf may have been deallocated and nil'd before execution of the block could even begin; and therefore, before strongSelf acquired a strong reference and retained it. The strongSelf is to ensure that the object referenced by weakSelf is not nil'd during the block's execution.
There is no notion in ARC of "using" a variable making it necessary to retain it when dealing with __weak variables -- __weak is explicitly opting out of this.
With that said, the check isn't strictly speaking necessary in this case, since messages to nil are always no-ops. If you intended to insert strongSelf into an array, or if it being deallocated between messages would put your program into an invalid state, it would be another matter.
To conclude, whenever you need to guarentee that a __weak variable will not be nil'd for a period of time, use __strong, and check before using it that it is not nil.

Weak Self in Blocks

Do I need to check if weak self is nil in blocks?
I create weakSelf pointer like:
__weak typeof(self) weakSelf = self;
and in the beginning of the blocks I do
if(!weakSelf){return;}
is this unnecessary? or does it depend on whether I coded the rest correctly so that when the self dies, others die too?
That check is unnecessary, and is giving you a false sense of security.
Here's the problem:
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
if (!weakSelf) { return; }
// THE LINE OF INTEREST
[weakSelf doSomething];
});
At THE LINE OF INTEREST, some other thread might clear the last strong reference to self, at which point weakSelf is set to nil. So the doSomething message gets sent to nil, which is “safe” (it does nothing), but might not be what you expected!
It's worse if you want to take a different action when weakSelf is nil, e.g.
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
if (weakSelf) {
[weakSelf doSomething];
} else {
[someOtherObject doSomethingElse];
}
});
In this case, between the time the block verifies that weakSelf is not nil and the time it sends the doSomething message, weakSelf might become nil, and neither doSomething nor doSomethingElse will actually run.
The correct solution is this:
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
typeof(self) strongSelf = weakSelf;
if (strongSelf) {
[strongSelf doSomething];
} else {
[someOtherObject doSomethingElse];
}
});
In this case, copying weakSelf to strongSelf (which is strong by default) is atomic. If weakSelf was nil, strongSelf will be nil. If weakSelf was not nil, strongSelf will not be nil, and will be a strong reference to the object, preventing it from being deallocated before the doSomething message.
It seems quite unnecessary since calling a message on nil is a no-op. (Nothing happens)
^{
[weakSelf doSomething]; //Does nothing if weakSelf is nil
}
The only reason I can think you might want to do this is if other messages (not on self) shouldn't be called
^{
// Here I don't want to add weakSelf as an observer if it's nil
if (!weakSelf) return;
[OtherClass addObserverForSomething:weakSelf];
}
Weak references do not retain the referred object. If none else is retaining it, the object is released and the weak references refers to nil.
Therefore it is possible that your code is executed with a weakSelf that refers nil. But this is no reason to check for it at all. Especially in Objective-C you use a defined behavior, if you send a message nil. I. e. it is perfect code, if you set a property using a nil reference. It simply goes to nowhere.
Of course sometime you do not want to interact with nil. In such a case you have to check for it.
BTW: You need weakSelf only in some, rarely circumstances. It is an urban legend that in closures in general references to self has to be weak to prevent retain cycles. It has not been true, it is not true and it will never be true.

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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