ARC 和桥接铸件

问题描述

使用 ARC，我不能再将 CGColorRef 转换为 id.我了解到我需要进行桥接演员表.根据 clang 文档:

With ARC, I can no longer cast CGColorRef to id. I learned that I need to do a bridged cast. According clang docs:

桥接演员表是使用以下三个关键字之一注释的 C 风格演员表:

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

(__bridge T) op 将操作数转换为目标类型 T.如果 T是一个可保留的对象指针类型，那么 op 必须有一个不可保留的指针类型.如果 T 是不可保留的指针类型，那么 op 必须有一个可保留的对象指针类型.否则演员表格式不正确.没有所有权转移，ARC插入没有保留操作.

(__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 转换操作数，它必须有可保留对象指针类型，指向目标类型，必须是不可保留的指针类型.ARC 保留价值，但须遵守通常对局部值进行优化，接收者负责用于平衡 +1.

(__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 转换操作数，它必须有不可保留的指针类型，指向目标类型，它必须是可保留对象指针类型.ARC 会在最后释放值封闭的完整表达式，服从通常的优化当地价值观.

(__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.

需要这些转换才能将对象移入和移出电弧控制；参见关于转换的部分的基本原理可保留的对象指针.

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.

使用 __bridge_retained 或 __bridge_transfer 转换纯粹是为了说服ARC分别发出不平衡的保留或释放很差表格.

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?

In what kind of situations would I use each?

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.

For example, CAGradientLayer has a colors property which accepts an array of CGColorRefs. My guess is that I should use __brige here, but exactly why I should (or should not) is unclear.

推荐答案

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:

I agree that the description is confusing. Since I just grasped them, I'll try to summarize:

• (__bridge_transfer <NSType>) op 或 CFBridgingRelease(op) 用于消耗 CFTypeRef同时将其转移到ARC.这也可以用 id someObj = (__bridge <NSType>) op; 来表示.CFRelease(op);