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Objective-C 学习笔记<二>

使用ARC能帮我们减轻不少内存管理方面的负担,尤其是对用惯了Java的程序员来说。

最近,在做获取本地通讯录时,遇到CFObject和NSObject转换的问题,由于ARC不能管理Core Foundation Object的生命周期,所以在Core Foundation和ARC之间,我们需要使用到__bridge,__bridge_retained和__bridge_transfer三个转换关键字。

 

在苹果官方文档,我们找到了:

If you cast between Objective-C and Core Foundation-style objects, you need to tell the compiler about the ownership semantics of the object using either a cast (defined in objc/runtime.h) or a Core Foundation-style macro (defined in NSObject.h):

  • __bridge transfers a pointer between Objective-C and Core Foundation with no transfer of ownership.

  • __bridge_retained or CFBridgingRetain casts an Objective-C pointer to a Core Foundation pointer and also transfers ownership to you.

    You are responsible for calling CFRelease or a related function to relinquish ownership of the object.

  • __bridge_transfer or CFBridgingRelease moves a non-Objective-C pointer to Objective-C and also transfers ownership to ARC.

    ARC is responsible for relinquishing ownership of the object.

__bridge只做类型转换,但是不修改对象(内存)管理权;

__bridge_retained(也可以使用CFBridgingRetain)将Objective-C的对象转换为Core Foundation的对象,同时将对象(内存)的管理权交给我们,后续需要使用CFRelease或者相关方法来释放对象;

__bridge_transfer(也可以使用CFBridgingRelease)将Core Foundation的对象转换为Objective-C的对象,同时将对象(内存)的管理权交给ARC。

 

例如:

- (void)logFirstNameOfPerson:(ABRecordRef)person {
 
    NSString *name = (NSString *)ABRecordCopyValue(person, kABPersonFirstNameProperty);
    NSLog(@"Person‘s first name: %@", name);
    [name release];
}

 

我们可以做这样的修改:

- (void)logFirstNameOfPerson:(ABRecordRef)person {
 
    NSString *name = (NSString *)CFBridgingRelease(ABRecordCopyValue(person, kABPersonFirstNameProperty));
    NSLog(@"Person‘s first name: %@", name);
}

 

Objective-C 学习笔记<二>