Objective-C 學習筆記<二>

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使用ARC能幫我們減輕不少記憶體管理方面的負擔，尤其是對用慣了Java的程式員來說。

最近，在做擷取本地通訊錄時，遇到CFObject和NSObject轉換的問題，由於ARC不能管理Core Foundation Object的生命週期，所以在Core Foundation和ARC之間，我們需要使用到__bridge,__bridge_retained和__bridge_transfer三個轉換關鍵字。

 

在蘋果官方文檔，我們找到了：

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只做類型轉換，但是不修改對象（記憶體）管理權；

__bridge_retained（也可以使用CFBridgingRetain）將Objective-C的對象轉換為Core Foundation的對象，同時將對象（記憶體）的管理權交給我們，後續需要使用CFRelease或者相關方法來釋放對象；

__bridge_transfer（也可以使用CFBridgingRelease）將Core Foundation的對象轉換為Objective-C的對象，同時將對象（記憶體）的管理權交給ARC。

 

例如：

- (void)logFirstNameOfPerson:(ABRecordRef)person {

NSString *name = (NSString *)ABRecordCopyValue(person, kABPersonFirstNameProperty);

NSLog(@"Person‘s first name: %@", name);

[name release];

}

 

我們可以做這樣的修改：

- (void)logFirstNameOfPerson:(ABRecordRef)person {

NSString *name = (NSString *)CFBridgingRelease(ABRecordCopyValue(person, kABPersonFirstNameProperty));

NSLog(@"Person‘s first name: %@", name);

}

 

Objective-C 學習筆記<二>