By using weakSelf in the block ,you are avoiding retain cycle.But sometime you should hold weakSelf until block retain ,therefor you need use strongSelf just like
__weak __typeof__(self) weakSelf = self;
dispatch_group_async(_operationsGroup, _operationsQueue, ^
{
__typeof__(self) strongSelf = weakSelf;
[strongSelf doSomething];
[strongSelf doSomethingElse];
} );
I want know when weakSelf will be nil ,then we should add__strong typeof(self) strongSelf = weakSelf
Example: If your aunty asked you, "Please purchase the umbrella from the market before I leave for my flight". You went to the market and it was very hard to find an umbrella. Finally you found a nice umbrella after few hours and you reach home but you find out that your aunt has left and you feel bad. But that was the right thing for your aunt because the flight is more important than the umbrella.
But in your coding problem what you're trying to do is
You are visiting market and taking your aunt's passport with you so that she won't leave until you come back.
I guess that's rude, but if you still want to do that, use self instead of strongSelf
dispatch_group_async(_operationsGroup, _operationsQueue, ^
{
[self doSomething];
[self doSomethingElse];
} );
It all depends on your requirements. There's no one, right answer.
Capturing self weakly allows the instance to be deallocated, even if the block is still retained somewhere. If self is released before the block is executed, it will be nil within the block.
If the block should not do anything when self has already been deallocated, there is no reason to capture self strongly. Simply test for nil and exit early. Or do whatever work is needed in the block that doesn't act on self.
If self should not disappear until the block has executed, capture self strongly, but avoid retain cycles by ensuring that self does not have a strong reference to the block.
If it's OK for self to disappear before the block begins executing, but must stick around until the block finishes, the block should capture a strong reference to the weakly-captured self when it begins.
If question is: "when weakSelf will be nil".
The answer is: When no one own the instance of self
For example, you have a UIViewController instance named vc, vc have a dependency for query API named apiHandler, and api have a callback block named successCallback.
like this:
#interface ApiHandler: NSObject
#property (nonatomic, copy) void (^successCallback)();
#end
#interface vc : UIViewController
#property (nonatomic, strong) ApiHandler *apiHandler;
#end
#implementation vc
- (void)doQuery {
self.apiHandler.successCallback = ^{
[self doSomething];
};
}
- (void)doSomething {
}
If vc be pop or dismiss, the vc instance WILL NOT BE dealloc. Because of retain cycle.
vc own apiHandler, apiHandler own successCallback and successCallback own vc.
So, using weak vc in block can avoid retain cycle.
like this:
- (void)doQuery {
__weak __typeof__(self) weakSelf = self;
self.apiHandler.successCallback = ^{
[weakSelf doSomething];
};
}
And now, if your vc be pop or dismiss, the vc instance will be dealloc, and weakSelf will be nil.
When apiHandler do query success in background thread and invoke successCallback, the message doSomething will send to nil object. (Zombie)
This is why you need to use strongSelf in block like you said.
Related
With the recent XCode update some code blocks are displaying as warnings where "Block implicitly retains 'self'"
It is my understanding that the when you create blocks it is best practice to create a weak self to keep from creating a strong reference that will not be garbage collected.
In the below example I set the myArray to self->myArray as recommended by XCode. Does this create the strong reference? Why can't I use 'weakSelf->myArray`? Attempting to do so results in this error:
Dereferencing a __weak pointer is not allowed due to possible null
value caused by race condition, assign it to strong variable first
I thought the whole point was to create weak refrences? Isn't weakSelf just a pointer to self?
Is the self-> even necessary in the below instance?
#interface SomeViewController (){
NSMutableArray * myArray;
}
#end
- (void) doSomethingInBackground {
// Do NSURLSessionTask on the background and onCompletion call mySuccessBlock.
}
- (SomeBlock) mySuccessBlock {
__block __typeof__(SomeViewController) __weak * weakSelf = self;
return ^(NSDictionary* result){
//this line is my related to my question
self->myArray = [weakSelf sortResultsAlphabetically: result];
dispatch_async(dispatch_get_main_queue(), ^{
[weakSelf.tableView reloadData]
});
};
}
Would recasting to be the correct way?
SomeViewController * strongSelf = weakSelf;
strongSelf->myArray = [weakSelf sortResultsAlphabetically: result];
The error message is right. You have to do the "weak-strong dance". You are only doing half of the dance. Pass self into the block as weak, but then immediately assign it, inside the block, to a strong reference (as in your edited "Would recasting to be the correct way?").
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];
}];
}];
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.
I am new to blocks and while reading over the internet I found that I must use weak variables to blocks, because blocks retains the variables. I am little confuse while using self with blocks. lets have an example:
#interface ViewController : UIViewController
#property (copy, nonatomic) void (^cyclicSelf1)();
-(IBAction)refferingSelf:(id)sender;
-(void)doSomethingLarge;
#end
Here I have a ViewController and it has declared block property with copy attribute. I don't want to make a retain cycle so I know when using self in the block I need to create weak object of self eg:
__weak typeof(self) weakSelf = self;
What I want to make sure is my block executes on background thread and may be user hit back before it get finish. My block are performing some valuable task and I don't want that to loose. So I need self till the end of block. I did following in my implementation file:
-(IBAction)refferingSelf:(id)sender
{
__weak typeof(self) weakSelf = self; // Weak reference of block
self.cyclicSelf1 = ^{
//Strong reference to weak self to keep it till the end of block
typeof(weakSelf) strongSelf = weakSelf;
if(strongSelf){
[strongSelf longRunningTask];//This takes about 8-10 seconds, Mean while I pop the view controller
}
[strongSelf executeSomeThingElse]; //strongSelf get nil here
};
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), self.cyclicSelf1);
}
According to me, using typeof(weakSelf) strongSelf = weakSelf; should create a strong reference of my self and when user hit back, self will still have one strong reference inside block until the scope get over.
Please help me to understand why this get crash? Why my strongSelf is not holding the object.
Your reference is not strong. Just add __strong directive like this:
__strong typeof(weakSelf) strongSelf = weakSelf;
I've found an answer. I was really curious myself, because your code seemed legit to me. The idea at least. So I've set up a similar project and experimented a little. The problem is this line:
typeof(weakSelf) strongSelf = weakSelf;
Change it to either
__strong typeof(weakSelf) strongSelf = weakSelf;
as suggested by #LDNZh or
typeof(self) strongSelf = weakSelf;
And your code will work.
UPDATE: Since this question shows up a lot I've made some changes to my example project. I'm making it available on github for everyone for future reference.
`I admit that I am not an expert on ARC and retain cycles though through some research and some great articles (like this), I believe I get the basics.
However, I am currently stumped. I have a property defined as follows.
#property (nonatomic,retain) Foo *foo;
Inside my init, I do the following.
if(self = [super init]) {
_foo = [[Foo alloc] initWithDelegate:self];
// async the rest
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,
(unsigned long)NULL), ^(void) {
__strong typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf.foo != nil) {
[strongSelf.foo runTests];
}
});
}
return self;
}
and here is my dealloc
- (void)dealloc {
_foo = nil;
}
If the dispatch_aync block is commented out, I see my Foo dealloc get called immediately after foo is set to nil. With the block commented in, foo's delloc is not called.
I've got a retain cycle correct? Any idea how?
No, you do not necessarily have a retain cycle (now known as a "strong reference cycle" in ARC). You have code that, if foo existed by the time strongSelf was defined, foo will be retained until the dispatched code finishes.
The only potential strong reference cycle here is the delegate you passed to foo. If that delegate is defined as strong property of the Foo class, then you have a strong reference cycle. If it's defined as a weak property, then you have no strong reference cycle.