Manage objective c object memory in Block

Look at the document from Apple:

When a block is copied, it creates strong references to object variables used within the block. If you use a block
within the implementation of a method:
●If you access an instance variable by reference, a strong reference is made to self;
●If you access an instance variable by value, a strong reference is made to the variable.
The following examples illustrate the two different situations:

dispatch_async(queue, ^{
// instanceVariable is used by reference, a strong reference is made to self
doSomethingWithObject(instanceVariable);
});
id localVariable = instanceVariable;
dispatch_async(queue, ^{
/*
localVariable is used by value, a strong reference is made to localVariable
(and not to self).
Blocks and Variables
Object and Block Variables
*/
doSomethingWithObject(localVariable);
});

To override this behavior for a particular object variable, you can mark it with the __block storage type
modifier.

使用方法总结如下：（只对应NON-ARC的情况）

• 如果block没有被copy，则block中的变量在stack空间中被创建。一旦此scope执行完毕，此stack空间被释放。
• 如果block被copy，则block中的所有变量被拷贝到heap中。
  此时，可以看作所有的变量都被重新拷贝了一份。
  1. 对于 int, float等基本类型的变量，不需要担心release的问题，但是赋值操作将不会影响到stack空间中的变量（即block scope外的原变量）。
  2. 对于objective c对象，如果直接调用self中变量，则self会被retain。由于在self中copy了这个block，会造成循环引用。
  解决方法是：声明__block blockSelf = self, 在block中调用blockSelf的变量。
  (注意：TestClass *blockSelf = self, blockSelf和self都指向同一个内存地址，当self先于blockSelf被释放时，blockSelf就变成了野指针！！
  真的是这样吗？实际测试过，如果加上了__block，self不会先释放，会等到block执行完之后才释放。)
• 如果是

id localVariable = instanceVariable;

把localVariable传进block，则localVariable会被retain。 在block在最后需要release。 在执行完此block后，localVariable会被自动release(前提是此block要被释放)。
例子：

@  property (nonatomic, copy) RequestCompleteBlock copiedCompleteBlock;
@  property (nonatomic, copy) RequestFailureBlock failure;
TestClass *testObject = [TestClass node];
    self.copiedCompleteBlock = ^(NSData *data) {
        [self performSelector:@selector(execute:data:) withObject:testObject afterDelay:3.0f];
    };
    
    self.failure = ^(NSError *error){
        testObject.name = @"wang";
    };
    
    // set request type, HTML
    FWURLRequest *defaultRequest = [FWURLRequest defaultRequest];
    defaultRequest.requestType   = FWE_REQUESTTYPE_HTML;
    [defaultRequest requestWithURL:url
                     paramsDicData:params
                     completeBlock:self.copiedCompleteBlock
                         failBlock:self.failure];

因为在copiedCompleteBlock和failure中testObject都被retain了，但是这两个block只有一个会被调用到，所以无论如何testObject都不会被释放。

3. 如果声明了__block的objective c对象，可以理解成，此对象的指针地址被拷贝到了heap（其指针实际指向的地址并没有被拷贝），它指向实际地址空间的retain count并没有增加，所以读写都有效。也不需要进行额外的release。
注意，如上例子，如果

__block TestClass *testObject = [TestClass node];

因为testObject不会被retain，当程序运行到当前scope外时，此testObject会被release，如果在此之后copiedCompleteBlock才被调用，则程序会崩溃。

建议：
1. 对于非self的object，不要使用__block，因为在block释放时会自动释放这些object。
2. 对于self的object，

You have a few options.

1) When not using ARC, the __block qualifier on an object pointer causes it to be non-retained. Thus:

__block Whatever *weakSelf = self;
^{ [weakSelf ...]; }

2) When using ARC on 10.7/iOS5, the __weak qualifier gets the same job done, but even better. __weak qualified variables are zeroing, so they're safer to use.

3) If you're using ARC but have to support 10.6/iOS4, the __unsafe_unretained qualifier will give you a non-zeroing weak pointer.

4) My MAZeroingWeakRef library provides functionality similar to __weak, but works without ARC and on older OSes. For example:

MAZeroingWeakRef *weakSelfRef = [MAZeroingWeakRef refWithTarget: self];
^{ [[weakSelfRef target] ...]; }

Since this is a zeroing weak reference, it's safer to use than __block or __unsafe_unretained. You can get it here with a liberal license:

https://github.com/mikeash/MAZeroingWeakRef

Ref:
http://lcwangchao.github.com/IOS/2012/07/16/block_retain_cycle/