I want to use an ivar in a method that is being called from a block. Is it fine to call the ivar directly from it without causing a retain cycle?
I have seen other questions related to calling properties- saying that we 'can' use self. Just wanted to make sure about ivars, can we call ivar directly or do we need to use self->_myInstanceVariable explicitly?
__weak MyClassName *weakSelf = self;
[MyBlockCreator myBlock:^{
MyClassName *strongSelf = weakSelf;
[strongSelf doSomething];
}];
- (void) doSomething {
_myInstanceVariable = someObject
// is it fine to use the ivar directly above?
// or do we want use self explicitly?
// like self->_myInstanceVariable
}
In this case you don't need to make the reference weak/strong at all, the only situation in which you can cause a retain cycle is if self references a block and a strong version of self is captured within that block. Since self doesn't retain the block in question, there would be no retain cycle even using a strong reference.
[Foo doWithBlock:^{
bar.baz = #"Hello World"; // Totally fine without weak/strong since bar doesn't retain the block here.
}];
In situations where self holds a strong reference to a block and the block refers to self, you need to make the reference weak outside the block then make it strong again inside.
__weak id weakSelf = self;
[self doWithBlock:^{
id strongSelf = weakSelf;
strongSelf.baz = #"Ok";
}];
Related
If the code inside a block calls a method, will a retain cycle exist if that method references self? In other words, does all code downstream of a block need to use the weakSelf/strongSelf pattern?
For example:
__weak __typeof__(self) weakSelf = self;
Myblock block = ^{
[weakSelf doSomething];
};
. . .
- (void)doSomething
{
self.myProperty = 5; // Is this ok or does it need to use a weakSelf?
}
Objective-C is not scoped like you suggest, namely, you don't have access to weakSelf from within -doSomething. Furthermore, as you are calling -doSomething on weakSelf, "self" within that call is actually referring to the same object that weakSelf is.
In short, no, you shouldn't, you can't and you shouldn't.
Retain cycle will be triggered only if you retain self inside the block. Otherwise it will just throw a warning only.
This is fine you can use this. Because block retains every vars used inside, so retain cycle would be like
Self would retain block
If block retains self then
Self would again retain block
block would retain self, so cycle goes on
The thing you are doing in method is just message passing. Everytime block is called a message would be sent to self to doSomething. And you can retain self in doSomething method it wont trigger retain cycle because this method dont have cycle loop to self. Hope you understand :)
- (void)doSomething
{
self.myProperty = 5; // Is this ok or does it need to use a weakSelf?
}
you can do this to get rid of retain cycle problem.
[self class] __weak *weakSelf = self;
self.completionBlock = ^{
[self class] __strong *strongSelf = weakSelf
[weakSelf doSomething];
};
What's the correct way to reference 'self' (and ivars) within blocks without the block creating a strong reference (and thus incrementing the ref count)?
For instance, I've found the following increments the ref count for 'self':
^(id sender) {
[self.navigationController popViewControllerAnimated:YES];
}
In order to circumvent the above, I've been doing the following:
__weak WhateverController *weakSelf = self;
^(id sender) {
[weakSelf.navigationController popViewControllerAnimated:YES];
};
And yes, I realize this is pseudocode.
Also, an indirect reference to self also creates a retain on self. For example, if _ivar were an instance variable, accessing it is an implicit reference to self so the following would also retain self.
^(id sender) {
[_ivar popViewControllerAnimated:YES];
}
Furthermore, to expand on your weak example, it is OK to send a message to a weak reference. If it's nil, nothing will happen. If not, then the compiler will generate code that ensures the reference remains valid through the invocation of the method.
So, this is fine:
__weak Foo *weakSelf = self;
^(id sender) {
[weakSelf.foo doSomething];
}
because foo will either be nil or if not, it is guaranteed to remain non-nil throughout the execution of doSomething.
However, the following would be inadvisable because self could go to nil in-between calls, which is probably not what you want:
__weak Foo *weakSelf = self;
^(id sender) {
[weakSelf.foo doSomething];
[weakSelf.foo doSomethingElse];
}
In that case, you probably want to create your own strong reference inside the block, so you have a consistent value throughout the execution of the block.
On the other hand, if you access iVars directly through a weak reference, you must do the weak-strong dance because this code:
__weak Foo *weakSelf = self;
^(id sender) {
weakSelf->_foo = bar;
}
will blow up if weakSelf is nil.
Thus, in the last two situations above, you want to do something like:
__weak Foo *weakSelf = self;
^(id sender) {
Foo *strongSelf = weakSelf;
if (!strongSelf) return;
// Now, do anything with strongSelf, as it is guaranteed to be around
}
Of course, the iVar situation is only a problem if you actually access iVars directly...
Apple's notation is sself but other than that - you're fine.
In a non arc project use the following code to prevent 'self' from being retained by the block:
__block id sself = self
__weak MyClass *selfReference = self;
dispatch_async(dispatch_get_main_queue(), ^{
[selfReference performSomeAction];
});
When do you need to pass a weak reference to a block?
Does this rule apply to dispatch_async as well as custom blocks?
Does a block copy the iVars used in it or does it retain them?
Who owns the variables initialized inside a block? Who should release them?
1, 2) Blocks retain the object-pointers in it (any blocks, dispatch_async blocks are nothing special). This usually isn't a problem, but can lead to retain-cycles, because the block can be associated with an owner object and that owner object (often self) might be retained by the block. In that case you should use a weak variable and then reassign it to a strong capture:
__weak MyClass *weakSelf = self;
self.block = ^{
MyClass *strongSelf = weakSelf;
...
[strongSelf ...];
[strongSelf.property ...];
[strongSelf->iVar ...];
}
Note: If you access an iVar directly, the compiler will transform that into self->iVar and thus retains self!
3) Blocks only retain the pointers, they don't copy them.
4) Variables created inside a block belong to that block and will be released when that block goes out of scope.
I know this question has been asked before, but non of the solutions solve my problem, so I'm asking this again. I am trying to call a method on self as the result of a callback through a block. I'm getting the following error:
Capturing 'self' strongly in this block is likely to lead a retain cycle
- (void)viewDidLoad {
[super viewDidLoad];
self.webClient.completionHandler = ^{
[self populateData];
};
}
I tried doing something like the code below, and I'm still getting the same warning.
What's the solution?
__weak id myself = self;
[myself populateData];
Your code should look like this:
- (void)viewDidLoad {
[super viewDidLoad];
__weak id weakSelf = self;
self.webClient.completionHandler = ^{
[weakSelf populateData];
};
}
UIAdam gave the correct answer, but it's worth understanding why it is correct.
First, why did you get the warning?
self has a strong reference to webClient. webClient has a strong reference to completionHandler. completionHandler has a strong reference to self. So if all other references in your program go away, there is still a strong reference to each item in this cycle, so they can never be deallocated.
The attempt of writing
__weak id myself = self;
[myself populateData];
doesn't work of course. The block still references self because it is assigning it to myself. So no difference here.
UIAdam's solution of writing
__weak id weakSelf = self;
self.webClient.completionHandler = ^{
[weakSelf populateData];
};
means that weakSelf is a weak reference, and the block only contains a weak reference to self. So if all other strong references to self is gone, there's only a weak reference left. A weak reference doesn't keep self alive, so self gets deallocated.
Now what if that happens, but something else had a strong reference to webClient and your block is called? weakSelf is a weak reference, and weak references are set to nil when the object is deallocated. So you have to be prepared that weakSelf is nil when your block gets called. It is actually better to write
id strongSelf = weakSelf;
[strongSelf populatedData];
inside the block: strongSelf might be set to nil, or it will be set to self. But since it is a strong reference, it will stay non-nil until the block has finished. If it was not nil to start with.
I want a block that is available throughout a class, so it can be re-used many times by different methods in the instance.
I want that block to be able to reference self.
I want to not have the block create any nasty retain cycles by retaining self.
So far I am stumped. I managed to create block in the .m outside of any method definitions, and that got me partway - I could reuse the block everywhere, but I couldn't access self. I tried putting the block into an ivar but I'm doing something wrong there and now I'm getting random EXC_BAD_ACCESS. Can someone explain it simply, line by line?
Try the following:
typedef void (^MyBlock)();
#implementation MyClass
{
MyBlock block;
}
- (id) init
{
self = [super init];
if (!self)
return nil;
__block MyClass* _self = self;
block = [^ {
[_self sendSomeMsg];
} copy];
}
Note the __block storage type. Quoting this: "At function level are __block variables. These are mutable within the block (and the enclosing scope) and are preserved if any referencing block is copied to the heap."
This idiom may help you to remove the exc_bad_access (ARC code).
// get a weak reference to self
__weak id weakSelf = self;
block = ^()
{
// now the block is executing so we get a strong reference to self
// (this prevents self from disappearing until the block is done executing)
id strongSelf = weakSelf;
if (strongSelf != nil)
{
// do whatever work you intended for this block
}
};
I figured it out.
In MyClass.h:
typedef void (^DefaultFailureBlock)();
#property (copy) DefaultFailureBlock defaultFailureBlock;
in the init method:
__block MyClass *selfReq = self;
self.defaultFailureBlock = ^{
//use selfReq instead of self in here.
};
Interestingly, if you accidentally refer to self inside the block, you will have a retain cycle, and Analyze will not complain. I put an NSLog in dealloc to prove that it is actually being dealloced, and it is.
Oh and don't forget to [defaultFailureBlock release]; in dealloc too...