This document summarizes key characteristics of blocks in Objective-C, including how blocks are copied to the heap, how to use __block to modify values from within a block, how to avoid strong reference cycles when using blocks that reference self, and how to recursively call blocks.

1. Block特性
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2. block在栈创建

3. block copy
• 我们用到的callback/GCD都是copy
• 第一次copy，block放入堆里
• 用到的值变量 制一份
• 引用到的所有对象reatainCount+1

6. typedef int(^Blocky)(void);
Blocky b[3];
for (int i=0; i<3; i++)
b[i] = ^{ return i;};
for (int i=0; i<3; i++)
printf("b %dn", b[i]());

7. __block

8. 把变量放入堆，指向同一个地址
typedef void(^Blocky)(void);
Blocky b;
int i = 0;
b = ^{
NSLog(@"i %d", i);
};
i ++;
b(); //0
typedef void(^Blocky)(void);
Blocky b;
__block int i = 0;
b = ^{
NSLog(@"i %d", i);
};
i ++;
b(); //1

9. 可修改
id x;
__block id y;
__block BOOL isRuned = NO;
void (^block)(void) =^{
x = [NSString string]; // error
y = [NSString string]; // works
isRuned = YES; //works
};

10. 递归调用block
typedef int(^Blocky)(int i);
__block Blocky b = ^(int i){
if (i == 0 || i == 1) return i;
return b(i-1) + b(i-2);
};
NSLog(@"%d", b(10));

11. 循环引用

12. 问题
[self setCompletionBlock:^{
NSLog(@"%@", self.error);
}];
self retainCount + 1
self keep completeBlock
- (void)setCompletionBlock:(ASIBasicBlock)aCompletionBlock
{
! [completionBlock release];
! completionBlock = [aCompletionBlock copy];
}
self retain completionBlock, complationBlock retain self

13. 解决(without ARC)
__block id that = self;
[self setCompletionBlock:^{
NSLog(@"%@", that.error);
}];
or
NSString *err = self.error;
[self setCompletionBlock:^{
NSLog(@"%@", err);
}];

14. 解决(with ARC)
iOS5:
__weak id that = self;
[self setCompletionBlock:^{
NSLog(@"%@", that.error);
}];
iOS4/5:
__unsafe_unretained id that = self;
[self setCompletionBlock:^{
NSLog(@"%@", that.error);
}];

15. 不易发现的循环引用
ViewController* settingsViewController =
[[[ViewController alloc] init] autorelease];
settingsViewController.onUpdate = ^{
[self doUpdate];
}
self.settingsViewController = settingsViewController;

17. END