I know blocks in Obj-C allow you to reference local variables, which is nice. But can we safely pass local variables to another method from inside of a block?
I know referencing self can be dangerous from inside blocks, but does this extend to any other variables.
Example:
-(void)methodTakesObject(ObjectA *)object {
ObjectB *b = object.b;
__weak MyObject *weakSelf = self;
[b doInBackground:^(NSArray *results) {
[weakSelf doSomethingElseWith:results andWith:object andEvenWith:b];
}
}
There are three "types" of local variables here. the results from the block, the ObjectB created inside the method, and an ObjectA which was passed to the method to start.
Should I be concerned about using any of these variables, and indeed is there any difference/concerns between how these variables are treated from within the block
In fact, it makes no difference no matter what variables are involved.
The concern with a retain cycle is if an object holds a strong reference to a block that it isn't expected to release until it is deallocated while the block holds a strong reference to that same object. That's not happening here.
A method called -doInBackground: seems like it will run the block (starting immediately-ish) and then release the block when it finishes. So, there's no prospect of the block being kept around as long as some other object exists, thus there's no prospect of it keeping that other object around forever.
You don't even need to use the weak-self convention to avoid a retain cycle here. You would only use it in the unlikely case that you really want to allow self to sometimes be deallocated before the block calls its method. More likely, you should just use a normal, strong reference to self because it's normal and/or desirable for self to live long enough for the block to execute.
If you are worried about retain cycle, #ken gave you the perfect answer. As for the concerns about variables:
object is retained from the method that called
methodTakesObject: so you don't have to worry about it. (if it isn't nil, tho).
b is also retained by you, the reference count is likely 2 or
more, so you also don't have to worry about it.
In other words, you are safe.
Related
I think I understand strong and weak keywords well, but I don't understand how it's used in the code below. This code is from SDWebImage by Olivier Poitrey available on github. I understand strong and weak keywords as is described here: Explanation of strong and weak storage in iOS5
The code below uses __weak and __strong keywords in a way that is curious to me. It is not a child-parent relationship or delegate pattern as I am used to seeing weak used. However, I'm sure that this is a pattern that is used often, as I've seen it before in other code. It sets a __weak reference before a block that runs on another thread. Then, within the block, it sets the weak reference to a strong reference.
I am certain that this good and elegant code, so I'm trying to understand it. If "self" ceases to exist before the block runs, the weak self reference will zero out. When the block runs, the strong reference will be set to zero as well. Therefore, it will know to kill the rest of the operation since self doesn't exist anymore. Did I get this right?
Now, what would happen if we didn't use __weak and __strong keywords? What if we just checked inside the block whether self == nil. Would "self" never be nil since the block copies the entire tree?
Can someone help demystify this awesome piece of code? Can someone verify or repudiate my hypotheses?
- (void)setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletedBlock)completedBlock;
{
[self cancelCurrentImageLoad];
self.image = placeholder;
if (url)
{
__weak UIImageView *wself = self;
id<SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished)
{
__strong UIImageView *sself = wself;
if (!sself) return;
if (image)
{
sself.image = image;
[sself setNeedsLayout];
}
if (completedBlock && finished)
{
completedBlock(image, error, cacheType);
}
}];
objc_setAssociatedObject(self, &operationKey, operation, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
}
The downloadWithUrl: method might take a long time. In that time, the user might decide to navigate away, eliminating the need for the SDWebImage object. To facilitate early cleanup of the object, the outer self reference is weak. This way, downloadWithUrl won't prevent the SDWebImage from being deallocated.
Of course, if you actually want to work with self, you need a strong reference. So, the on-completion block of downloadWithUrl grabs a strong reference to self. If the object goes away in this time, sself will be nil. Otherwise, it will be a valid strong reference, indicating that the SDWebImage object is still around, and the object will finish its work at this time.
I am certain that this good and elegant code, so I'm trying to understand it. If "self" ceases to exist before the block runs, the weak self reference will zero out. When the block runs, the strong reference will be set to zero as well. Therefore, it will know to kill the rest of the operation since self doesn't exist anymore. Did I get this right?
Nope, you're over thinking this quite a bit. The __weak storage qualifier is just that: a qualifier. Objects that are __weak are explicitly unretained, but they aren't just automatically set to nil if assigned from a strong variable. In fact, that would defeat the purpose of a weak variable!
Now, what would happen if we didn't use __weak and __strong keywords? What if we just checked inside the block whether self == nil. Would "self" never be nil since the block copies the entire tree?
The checking is actually unnecessary because the runtime resolves messages to nil to nil (nevertheless, it may be important to the implementation later on, who knows). You are spot on with this one: without that little "weak to strong" dance there, then self would be retained by the block, with the potential to create a pretty nasty retain cycle. Which is where I can start tying this all together:
Because we don't want the block to retain our variable, but we also want it to be strong inside the scope of the block so nothing weird happens, self is assigned to a weak pointer. When the block happens upon our weak pointer, it isn't allowed to retain it, so self's reference count stays the same, then once inside the block, we go right back to a strong self variable so the weak one is released, and we don't have to worry about it any more. Practically, this means we have a solid guarantee that self is either a value or nil throughout the entire execution of the block. Pretty neat, huh?
I come from a heavy JavaScript-oriented background and I'm transitioning as best I can into Objective-C. Naturally, I always find myself jumping at the opportunity to utilize closure functions in my source code, such as:
#property (nonatomic, retain) void (^zoomCallback)(NSInteger width, NSInteger height, NSString *url);
However, each time I jot this into Xcode, it warns me:
Retain'ed block property does not copy the block - use copy attribute instead
It was my understanding that you no longer have to retain things manually anymore in Objective-C due to ARC, so I'm admittedly rather thrown by this warning. I'm assuming by block it's referring to my closure function, so as far as I can interpret it's telling me that assigning this variable:
myObject.zoomCallback = someMethod;
Would also cause someMethod to be retained, and thus the owner of someMethod to continue to exist? Did I get that right?
What are the negative repercussions of this? If I "copy" the block, wont it allow for the owner of someMethod to be destroyed, and thus within the closure method itself any time I refer to "self" it will no longer exist? Don't I almost always want to retain the block unless my closure method is doing something very trivial or something that doesn't reference member variables or methods?
Blocks need to be copied before they are stored (and in some cases just before you use them, but that's usually a strange bit of code) because blocks are created on the stack and need to be moved to the heap. If the block isn't copied and the context in which it was created is destroyed then it won't always work properly. This is the case when the block captures some external variables (instead of only using the passed parameters).
I have seen a lot of answers around, but I couldn't find any solution for my problem.
I basically have a Class where I only use class methods. I never allocate this class. So, I am passing a block to it and storing it on a static on the .h of the class, like this:
static ErrorBlock _errorBlock;
I am storing this way:
_errorBlock = [errorBlock copy];
I receive errorBlock as a parameter of a method. After some calculations, I invoke the block like this:
_errorBlock(error);
It's worth saying that I am invoking this from a Class's Category. The application basically returns:
EXC_BAD_ACCESSS(code=2, adress = 0xc)
When I check the _errorBlock value, it's nil. So my question is, how can I keep a live reference to the block?
If _errorBlock is "nil" it might be safe to say that it was assigned with nil and there is no reference to keep it alive. If you create a reference as in NSObject *reference = someObject and someObject is released your reference pointer will still point to location where someObject used to be and should not be nil. Check if you have some problems with assigning the block or if it even gets called. Also you might want to check if _errorBlock exists before calling it.
I was able to work it out, my removing completely the use of global blocks. At the moment I use the blocks my passing them as parameters.
When writing a method that accepts a block as an argument, do I need to do anything special such as copying the block to the heap before executing it? For example, if I had the following method:
- (void)testWithBlock:(void (^)(NSString *))block {
NSString *testString = #"Test";
block(testString);
}
Should I do anything with block before calling it, or when entering the method? Or is the above the correct way of using the passed-in block? Also, is the following way of calling the method correct, or should I do something with the block before passing it?
[object testWithBlock:^(NSString *test){
NSLog(#"[%#]", test);
}];
Where do I need to copy the block? And how would this have been different if I wasn't using ARC?
When you receive a block as a method parameter, that block could be the original that had been created on the stack or it could be copy (a block on the heap). As far as I'm aware, there's no way tell. So the general rule of thumb is if you're going to execute the block within the method that's receiving it, you don't need to copy it. If you intend to pass that block to another method (which may or may not execute it immediately) then you also don't need to copy it (the receiving method should copy it if it intends to keep it around). However, if you intend to store the block in any way for some place for later execution, you need to copy it. The primary example many people use is some sort of completion block held as an instance variable:
typedef void (^IDBlock) (id);
#implementation MyClass{
IDBlock _completionBlock;
}
However, you also need to copy it if you're going to add it to any kind of collection class, like an NSArray or NSDictionary. Otherwise, you'll get errors (most likely EXC_BAD_ACCESS) or possibly data corruption when you try to execute the block later.
When you execute a block, it's important to test first if the block is nil. Objective-c will allow you to pass nil to a block method parameter. If that block is nil, you'll get EXC_BAD_ACCESS when you try to execute it. Luckily this is easy to do. In your example, you'd write:
- (void)testWithBlock:(void (^)(NSString *))block {
NSString *testString = #"Test";
if (block) block(testString);
}
There are performance considerations in copying blocks. Compared to creating a block on the stack, copying a block to the heap is not trivial. It's not a major deal in general, but if you're using a block iteratively or using a bunch of blocks iteratively and copying them on each execution, it'll create a performance hit. So if your method - (void)testWithBlock:(void (^)(NSString *))block; were in some kind of loop, copying that block might hurt your performance if you don't need to copy it.
Another place you need to copy a block is if you intend on calling that block in itself (block recursion). This isn't all that common, but you must copy the block if you intend to do this. See my question/answer on SO here: Recursive Blocks In Objective-C.
Finally, if you're going to store a block you need to be really careful about creating retain cycles. Blocks will retain any object passed into it, and if that object is an instance variable, it will retain the instance variable's class (self). I personally love blocks and use them all the time. But there's a reason Apple doesn't use/store blocks for their UIKit classes and instead stick with either a target/action or delegate pattern. If you (the class creating the block) are retaining the class that is receiving/copying/storing the block, and in that block you reference either yourself or ANY class instance variable, you've created a retain cycle (classA -> classB -> block -> classA). This is remarkably easy to do, and it's something I've done too many times. Moreover, "Leaks" in Instruments doesn't catch it. The method to get around this is easy: simply create a temporary __weak variable (for ARC) or __block variable (non-ARC) and the block won't retain that variable. So,for example, the following would be a retain cycle if the 'object' copies/stores the block:
[object testWithBlock:^(NSString *test){
_iVar = test;
NSLog(#"[%#]", test);
}];
However, to fix this (using ARC):
__weak IVarClass *iVar = _iVar;
[object testWithBlock:^(NSString *test){
iVar = test;
NSLog(#"[%#]", test);
}];
You can also do something like this:
__weak ClassOfSelf _self = self;
[object testWithBlock:^(NSString *test){
_self->_iVar = test;
NSLog(#"[%#]", test);
}];
Note that many people don't like the above because they consider it fragile, but it is a valid way of accessing variables.
Update - The current compiler now warns if you try to directly access a variable using '->'. For this reason (as well as reasons of safety) it's best to create a property for the variables you want to access. So instead of _self->_iVar = test; you would use: _self.iVar = test;.
UPDATE (more info)
Generally, it's best to consider the method that receives the block as responsible for determining whether the block needs to be copied rather than the caller. This is because the receiving method can be the only one that knows just how long the block needs to be kept alive or if it needs to be copied. You (as the programmer) will obviously know this information when you write the call, but if you mentally consider the caller and receiver at separate objects, the caller gives the receiver the block and is done with it. Therefore, it shouldn't need to know what is done with the block after it's gone. On the flip side, it's quite possible that the caller may have already copied the block (maybe it stored the block and is now handing it off to another method) but the receiver (who also intends on storing the block) should still copy the block (even though the block as already been copied). The receiver can't know that the block has already been copied, and some blocks it receives may be copied while others may not be. Therefore the receiver should always copy a block it intends on keeping around? Make sense? This essentially is good Object Oriented Design Practices. Basically, whoever has the information is responsible for handling it.
Blocks are used extensively in Apple's GCD (Grand Central Dispatch) to easily enable multi-threading. In general, you don't need to copy a block when you dispatch it on GCD. Oddly, this is slightly counter-intuitive (if you think about it) because if you dispatch a block asynchronously, often the method the block was created in will return before the block has executed, which generally would mean the block would expire since it is a stack object. I don't think that GCD copies the block to the stack (I read that somewhere but have been unable to find it again), instead I think the life of the thread is extended by being put on another thread.
Mike Ash has extensive articles on blocks, GCD and ARC which you may find useful:
Mike Ash: Practical Blocks
Mike Ash: Objective-C Blocks vs C++0x Lambdas: Fight!
Mike Ash: Q&A - More info on Blocks
Mike Ash: Automatic Reference Counting He talks about blocks & ARC in this article.
Mike Ash: GCD Is Not Blocks, Blocks Are Not GCD
Apple Docs - Introducing Block & GCD
This all looks good. Though, you might want to double-check the block parameter:
#property id myObject;
#property (copy) void (^myBlock)(NSString *);
....
- (void)testWithBlock: (void (^)(NSString *))block
{
NSString *testString = #"Test";
if (block)
{
block(test);
myObject = Block_copy(block);
myBlock = block;
}
}
...
[object testWithBlock: ^(NSString *test)
{
NSLog(#"[%#]", test);
}];
Should be fine. And I believe that they are even trying to phase out Block_copy(), but they haven't yet.
As the blocks programming topics guide says under 'Copying Blocks':
Typically, you shouldn’t need to copy (or retain) a block. You only need to make a copy when you expect the block to be used after destruction of the scope within which it was declared.
In the case you are describing, you can basically think about the block as simply being a parameter to your method, just like as if it were an int or other primitive type. When the method gets called, space on the stack will be allocated for the method parameters and so the block will live on the stack during the entire execution of your method (just like all the other parameters). When the stack frame gets popped off the top of the stack at the return of the method, the stack memory allocated to the block will be deallocated. Thus, during the execution of your method, the block is guaranteed to be alive, so there's no memory management to deal with here (in both ARC and non-ARC cases). In other words, your code is fine. You can simply call the block inside the method.
As the quoted text suggests, the only time you need to explicitly copy a block is when you want it to be accessible from outside of the scope where it was created (in your case, beyond the lifetime of the stack frame of your method). As an example, suppose you want a method that fetches some data from the web, and runs a block of code when the fetch is complete. Your method signature might look like:
- (void)getDataFromURL:(NSURL *)url completionHandler:(void(^)(void))completionHandler;
Since the data fetch happens asynchronously, you would want to keep the block around (likely in a property of your class) and then run the block once the data has been fully fetched. In this case, your implementation might look like:
#interface MyClass
#property (nonatomic, copy) void(^dataCompletion)(NSData *);
#end
#implementation MyClass
#synthesize dataCompletion = _dataCompletion;
- (void)getDataFromURL:(NSURL *)url completionHandler:(void(^)(NSData *fetchedData))completionHandler {
self.dataCompletion = completionHandler;
[self fetchDataFromURL:url];
}
- (void)fetchDataFromURL:(NSURL *)url {
// Data fetch starts here
}
- (void)finishedFetchingData:(NSData *)fetchedData {
// Called when the data is done being fetched
self.dataCompletion(fetchedData)
self.dataCompletion = nil;
}
In this example, using a property with a copy semantic will perform a Block_copy() on the block and copy it to the heap. This happens in the line self.dataCompletion = completionHandler. Thus, the block gets moved from the stack frame of the -getDataFromURL:completionHandler: method to the heap which allows it to be called later in the finishedFetchingData: method. In the latter method, the line self.dataCompletion = nil nullifies the property and sends a Block_release() to the stored block, thus deallocating it.
Using a property in this way is nice since it will essentially handle all of the block memory management for you (just make sure it's a copy (or strong) property and not simply a retain) and will work in both non-ARC and ARC cases. If instead you wanted to use a raw instance variable to store your block and were working in a non-ARC environment, you would have to call Block_copy(), Block_retain(), and Block_release() yourself in all the proper places if you wanted to keep the block around any longer than the lifetime of the method in which it's passed as a parameter. The same code above written using an ivar instead of a property would look like this:
#interface MyClass {
void(^dataCompletion)(NSData *);
}
#end
#implementation MyClass
- (void)getDataFromURL:(NSURL *)url completionHandler:(void(^)(NSData *fetchedData))completionHandler {
dataCompletion = Block_copy(completionHandler);
[self fetchDataFromURL:url];
}
- (void)fetchDataFromURL:(NSURL *)url {
// Data fetch starts here
}
- (void)finishedFetchingData:(NSData *)fetchedData {
// Called when the data is done being fetched
dataCompletion(fetchedData)
Block_release(dataCompletion);
dataCompletion = nil;
}
You know there are two kinds of blocks:
blocks stored in the stack, the ones that you explicitly write as ^{...}, and that disappear as soon as the function they are created in returns, just as regular stack variables. When you call a stack block after the function it belonged to has returned, bad things happens.
blocks in the heap, the ones that you get when you copy another block, the ones that live as long as some other object keeps a reference to them, just like regular objects.
The only reason for you to copy a block is when you are given a block that is, or may be a stack block (explicit local block ^{...}, or method argument whose origin you don't know), AND that you want to extend its life span out of the limited one of the stack blocks, AND that the compiler doesn't already do that job for you.
Think: keeping a block in an instance variable.
Adding a block in a collection such as NSArray.
Those are common examples of cases where you should copy a block when you're not sure it is already a heap block.
Note that the compiler does it for you when a block is called in another block.
I'm working on a Objective-C project and we have ARC enabled. I'm currently dealing with two separate classes, let's call them A and B.
A has a strong reference to B and B has a block property with the copy attribute.
#interface A {
B *b;
...
}
#end
#interface B {
...
}
#property (copy) void (^myBlock)();
#end
Now, if inside a method owned by A I try to assign to myBlock:
b.myBlock = ^{ [self doSomething] };
The compiler (correctly) complains:
Capturing 'self' strongly in this block is likely to lead to a retain cycle.
I understand this behaviour and how to work around it, based on the answers to this question. (Basically, a cycle is formed because once the block is copied it will contain a strong reference to A, which has a strong reference to B, which in turn has a strong reference to the block itself.)
Now, if instead of directly assigning the block to the property I pass it to a method of B:
[b takeMyBlock:^{ [self doSomething] }];
And inside that method I do the assignment, the compiler will no longer complain.
My question is: Will that create a memory leak? If so, how to avoid it? Is it possible to annotate the method argument somehow so that the compiler knows the argument is going to be copied?
Thanks in advance.
I just encountered the same issue myself. While the issue didn't manifest itself as a leak per se, I actually found the problem while analysing heap growth using the Allocations tool in Instruments -- so it obviously does cause a memory issue.
I think the compiler would have to be pretty smart to pick up on the problem and issue a warning, so my guess is that one of two approaches is needed:
Documentation: As you suggested in your question, clearly document
that this method will copy the block passed to it. This might
involve naming the block parameter something like blockToCopy
(Xcode helpfully displays the block parameter name when using
autocompletion). Also, comment the method declaration. If you're
using a documentation tool like appledoc this is particularly nice,
as your documentation will also appear in Xcode's quick help dialog
when option clicking your code.
Never capture self strongly in a block: This is perhaps best
practise, as I don't think it's ever a good idea to keep a strong
reference to self inside a block. If you have to reference self,
use a weak reference. This will certainly avoid the situation you
describe. (N.B. referencing an instance variable will also keep a strong
reference to self).
In ARC, what it does is if a local variable or a WEAK variable loses it reference. The memory will be dealloced immediately since its reference count is zero. However, if you are reallocating a STRONG variable, it will definitely cause a memory leak for strong variable retains even if reference count decays to zero. In this case, you shall set this variable to NIL before reallocation or set the variable to weak if strong property is unnecessary.