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研究下__block在MRC/ARC下区别，直接上代码。
@property (nonatomic,copy) TestBlock block;//定义的block
一._NSConcreteStackBlock
- (void)stackBlock{
NSLog(@"stackBlock start....");
Person *person = [[Person alloc]init];person.personName = @"张三";
NSLog(@"person RetainCount is %ld", CFGetRetainCount((__bridge CFTypeRef)person));
__block Person *blockPerson = person;
NSLog(@"blockPerson RetainCount is %ld", CFGetRetainCount((__bridge CFTypeRef)blockPerson));
void (^block)(int a) = ^(int a){
NSLog(@"block blockPerson 111 is %ld", CFGetRetainCount((__bridge CFTypeRef)blockPerson));
NSLog(@"block person 111 is %ld", CFGetRetainCount((__bridge CFTypeRef)person));
NSLog(@"block self is %ld", CFGetRetainCount((__bridge CFTypeRef)self));
};
NSLog(@"block is %@",block);
block(2);
NSLog(@"block blockPerson 2222 is %ld", CFGetRetainCount((__bridge CFTypeRef)blockPerson));
NSLog(@"block person 222 is %ld", CFGetRetainCount((__bridge CFTypeRef)person));
#if !__has_feature(objc_arc)
[person release];
#endif
NSLog(@"stackBlock end....\r\n\r\n\r\n\r\n");
}
日志
MRC
2016-03-17 16:32:25.253 ARCAndMRC[12313:228133] stackBlock start....
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] person RetainCount is 1
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] blockPerson RetainCount is 1
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] block is <__NSStackBlock__: 0xbff9c100>
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] block blockPerson 111 is 1
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] block person 111 is 1
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] block blockPerson 2222 is 1
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] block person 222 is 1
2016-03-17 16:32:25.254 ARCAndMRC[12313:228133] Person release ..:张三
2016-03-17 16:32:25.255 ARCAndMRC[12313:228133] stackBlock end....
ARC下
2016-03-17 17:14:22.662 ARCAndMRC[13677:260528] stackBlock start....
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] person RetainCount is 1
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] blockPerson RetainCount is 2
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] block is <__NSMallocBlock__: 0x7c92cf40>
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] block blockPerson 111 is 4
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] block person 111 is 4
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] block self is 3
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] block blockPerson 2222 is 4
2016-03-17 17:14:22.663 ARCAndMRC[13677:260528] block person 222 is 4
2016-03-17 17:14:22.664 ARCAndMRC[13677:260528] stackBlock end....
2016-03-17 17:14:22.664 ARCAndMRC[13677:260528] Person release ..:张三
根据日志可以看出,MRC下__block修饰的变量，并不改变引用计数,同时block内部并不对引入的外部对象，更改引用计数。
ARC下block会被修改为__NSMallocBlock__ ,同时引用计数增加了。
2._NSConcreteMallocBlock
- (void)mallocStack{
NSLog(@"mallocStack start....");
Person *person = [[Person alloc]init];person.personName = @"张三";
NSLog(@"person RetainCount is %ld", CFGetRetainCount((__bridge CFTypeRef)person));
__block Person *blockPerson = person;
NSLog(@"blockPerson RetainCount is %ld", CFGetRetainCount((__bridge CFTypeRef)blockPerson));
self.block = ^(int a){
NSLog(@"block blockPerson 111 is %ld", CFGetRetainCount((__bridge CFTypeRef)blockPerson));
NSLog(@"block person 111 is %ld", CFGetRetainCount((__bridge CFTypeRef)person));
NSLog(@"block self is %ld", CFGetRetainCount((__bridge CFTypeRef)self));
};
NSLog(@"self.block is %@", self.block);
self.block(2);
NSLog(@"block blockPerson 2222 is %ld", CFGetRetainCount((__bridge CFTypeRef)blockPerson));
NSLog(@"block person 222 is %ld", CFGetRetainCount((__bridge CFTypeRef)person));
#if !__has_feature(objc_arc)
[person release];
#endif
NSLog(@"mallocStack end....\r\n\r\n\r\n\r\n");
}
日志
MRC
2016-03-17 17:27:33.739 ARCAndMRC[14065:269749] mallocStack start....
2016-03-17 17:27:33.739 ARCAndMRC[14065:269749] person RetainCount is 1
2016-03-17 17:27:33.739 ARCAndMRC[14065:269749] blockPerson RetainCount is 1
2016-03-17 17:27:33.739 ARCAndMRC[14065:269749] self.block retaincount is 1 <__NSMallocBlock__: 0x7ae09d30>
2016-03-17 17:27:33.739 ARCAndMRC[14065:269749] block blockPerson 111 is 2
2016-03-17 17:27:33.739 ARCAndMRC[14065:269749] block person 111 is 2
2016-03-17 17:27:33.740 ARCAndMRC[14065:269749] block self is 2
2016-03-17 17:27:33.740 ARCAndMRC[14065:269749] block blockPerson 2222 is 2
2016-03-17 17:27:33.740 ARCAndMRC[14065:269749] block person 222 is 2
2016-03-17 17:27:33.740 ARCAndMRC[14065:269749] Person release ..:张三
2016-03-17 17:27:33.740 ARCAndMRC[14065:269749] mallocStack end....
ARC
2016-03-17 17:28:45.643 ARCAndMRC[14118:271047] mallocStack start....
2016-03-17 17:28:45.643 ARCAndMRC[14118:271047] person RetainCount is 1
2016-03-17 17:28:45.643 ARCAndMRC[14118:271047] blockPerson RetainCount is 2
2016-03-17 17:28:45.643 ARCAndMRC[14118:271047] self.block retaincount is 1 <__NSMallocBlock__: 0x7b23ed00>
2016-03-17 17:28:45.643 ARCAndMRC[14118:271047] block blockPerson 111 is 4
2016-03-17 17:28:45.644 ARCAndMRC[14118:271047] block person 111 is 4
2016-03-17 17:28:45.644 ARCAndMRC[14118:271047] block self is 3
2016-03-17 17:28:45.644 ARCAndMRC[14118:271047] block blockPerson 2222 is 4
2016-03-17 17:28:45.644 ARCAndMRC[14118:271047] block person 222 is 4
2016-03-17 17:28:45.644 ARCAndMRC[14118:271047] mallocStack end....
根据日志可以看出,MRC下__block修饰的变量，并不改变引用计数,但是block内部对引入的外部对象，会更改引用计数。所以要及时对block进行release.
ARC下，block修饰的引用计数会增加，同时block内部持有的对象引用计数会增加，所以
person没有被释放，原因是block内部持有了self,导致self没有被释放，这就会导致循环引用,所以需要使用weak.
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原文地址：http://www.cnblogs.com/menchao/p/5288429.html
