使用 ARC，我不能再将 CGColorRef 转换为 id。我了解到我需要进行桥接类型转换。根据 clang docs :

A bridged cast is a C-style cast annotated with one of three keywords:

(__bridge T) op casts the operand to the destination type T. If T is a retainable object pointer type, then op must have a non-retainable pointer type. If T is a non-retainable pointer type, then op must have a retainable object pointer type. Otherwise the cast is ill-formed. There is no transfer of ownership, and ARC inserts no retain operations.

(__bridge_retained T) op casts the operand, which must have retainable object pointer type, to the destination type, which must be a non-retainable pointer type. ARC retains the value, subject to the usual optimizations on local values, and the recipient is responsible for balancing that +1.

(__bridge_transfer T) op casts the operand, which must have non-retainable pointer type, to the destination type, which must be a retainable object pointer type. ARC will release the value at the end of the enclosing full-expression, subject to the usual optimizations on local values.

These casts are required in order to transfer objects in and out of ARC control; see the rationale in the section on conversion of retainable object pointers.

Using a __bridge_retained or __bridge_transfer cast purely to convince ARC to emit an unbalanced retain or release, respectively, is poor form.

我会在什么样的情况下使用它们？

例如，CAGradientLayer 有一个 colors 属性，该属性接受 CGColorRef 数组。我的猜测是我应该在这里使用 __brige，但我应该(或不应该)的确切原因尚不清楚。

最佳答案

我同意描述令人困惑。由于我刚刚掌握了它们，所以我将尝试总结一下:

• (__bridge_transfer <NSType>) op或者CFBridgingRelease(op)用于使用 CFTypeRef 的保留计数同时将其转移到ARC。这也可以表示为 id someObj = (__bridge <NSType>) op; CFRelease(op);

• (__bridge_retained <CFType>) op或者CFBridgingRetain(op)用于递NSObject转移到CF-land，同时给它+1的保留计数。您应该处理 CFTypeRef您创建这种方式与处理 CFStringCreateCopy() 的结果相同.这也可以表示为 CFRetain((__bridge CFType)op); CFTypeRef someTypeRef = (__bridge CFType)op;

• __bridge只是在指针域和 Objective-C 对象域之间转换。如果您不想使用上述转换，请使用此转换。

也许这会有所帮助。我自己，我更喜欢 CFBridging…宏在普通的 Actor 阵容中相当多。

https://stackoverflow.com/questions/7036350/