__bridge __bridgeretained __bridgetransfer的区别
2013 年 4 月 7 日
iOS开发中，经常会接触到两种对象，Objective-C对象和Core Foundation对象，他们之间有所不同，可以互相转换。最大的不同之处在于，在ARC模式下，前者不用开发者手动管理内存，后者需要开发者手动管理内存，即调用CFRelease方法释放对象，否则会造成内存泄漏。转换的话主要会用到以下3个方法：
- __bridge,
- __bridge_retained
- __bridge_transfer
__bridge可以用于OC对象和CF对象互转，例如
NSObject *obj = [[NSObject alloc] init]; //retain count 1 CFTypeRef cfObj1 = (__bridge CFTypeRef)obj; //retain count 1 NSObject *obj1 = (__bridge id)cfObj1; //retain count 2
在这种转换方式下，如果是OC对象转换成CF对象，引用计数不变，如果是CF对象转换成OC对象，引用计数会+1。
__bridge_retained用于OC对象转换为CF对象，例如
NSObject *obj = [[NSObject alloc] init]; //retain count 1 CFTypeRef cfObj1 = (__bridge_retained CFTypeRef)obj; //retain count 2 //等价写法 NSObject *obj = [[NSObject alloc] init]; //retain count 1 CFTypeRef cfObj1 = (CFTypeRef)CFBridgingRetain(obj); //retain count 2
这种情况下，obj的引用计数会+1，obj的释放不会影响到cfObj1的使用
__bridge_transfer用于CF对象转换为OC对象，例如
NSObject *obj = [[NSObject alloc] init]; //retain count 1 CFTypeRef cfObj1 = (__bridge_retained CFTypeRef)obj; //retain count 2 NSObject *obj1 = (__bridge_transfer id)cfObj1; //retain count 2 //等价写法 NSObject *obj = [[NSObject alloc] init]; //retain count 1 CFTypeRef cfObj1 = (__bridge_retained CFTypeRef)obj; //retain count 2 NSObject *obj1 = (NSObject *)CFBridgingRelease(cfObj1); //retain count 2
最后来做个练习，看下以下代码输出什么
NSObject *obj = [[NSObject alloc] init]; //retain count 1 CFTypeRef cfObj = (__bridge_retained CFTypeRef)obj; //retain count 2 CFTypeRef cfObj1 = (__bridge CFTypeRef)obj; //retain count 2 CFTypeRef cfObj2 = (__bridge_retained CFTypeRef)obj; //retain count 3 NSObject *obj1 = (__bridge_transfer id)cfObj1; //retain count 3 NSObject *obj2 = (__bridge id)cfObj2; //retain count 4 NSLog(@"obj retainCount %ld", (long)CFGetRetainCount(cfObj)); NSString *str = [NSString stringWithFormat:@"testStr"]; CFStringRef cfStr = (__bridge CFStringRef)str; NSLog(@"str retainCount %ld", (long)CFGetRetainCount(cfStr)); //retain count 9223372036854775807 = 0x7FFFFFFFFFFFFFFF
答案是
obj retainCount 4 str retainCount 9223372036854775807
str会被当做是字符串常量，retainCount是一个最大值，保证不会被系统回收。
个人认为苹果的这套API设计得不是很好，比如__bridge，并不是说引用计数不增加，而是看是转换的关系是什么，需要理解记忆一下才行。
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