为什么Objective-C 委托通常被赋予属性assign 而不是retain? [英] Why are Objective-C delegates usually given the property assign instead of retain?

问题描述

我正在浏览由 Scott Stevenson 维护的精彩博客，我试图理解一个基本的 Objective-C 概念，即为代表分配分配"属性与保留".请注意，两者在垃圾收集环境中是相同的.我最关心的是非基于 GC 的环境(例如:iPhone).

I'm surfing through the wonderful blog maintained by Scott Stevenson, and I'm trying to understand a fundamental Objective-C concept of assigning delegates the 'assign' property vs 'retain'. Note, the both are the same in a garbage collected environment. I'm mostly concerned with a non-GC based environment (eg: iPhone).

直接来自 Scott 的博客:

Directly from Scott's blog:

"assign 关键字将生成一个 setter，将值直接分配给实例变量，而不是复制或保留它.这最适合 NSInteger 和 CGFloat 等原始类型，或您不直接拥有的对象，例如代表."

"The assign keyword will generate a setter which assigns the value to the instance variable directly, rather than copying or retaining it. This is best for primitive types like NSInteger and CGFloat, or objects you don't directly own, such as delegates."

您不直接拥有委​​托对象是什么意思?我通常会保留我的代表，因为如果我不想让他们掉入深渊，保留会为我解决这个问题.我通常也将 UITableViewController 从其各自的数据源和委托中抽象出来.我也保留了那个特定的对象.我想确保它永远不会消失，所以我的 UITableView 总是有它的委托.

What does it mean that you don't directly own the delegate object? I typically retain my delegates, because if I don't want them to go away into the abyss, retain will take care of that for me. I usually abstract UITableViewController away from its respective dataSource and delegate also. I also retain that particular object. I want to make sure it never goes away so my UITableView always has its delegate around.

有人可以进一步解释我错在哪里/为什么，所以我可以理解在 Objective-C 2.0 编程中使用分配属性而不是保留的这种常见范式吗?

Can someone further explain where/why I'm wrong, so I can understand this common paradigm in Objective-C 2.0 programming of using the assign property on delegates instead of retain?

谢谢！

推荐答案

你避免保留委托的原因是你需要避免一个保留循环:

The reason that you avoid retaining delegates is that you need to avoid a retain cycle:

A 创建 BA 将自己设置为 B 的代表…A 由其所有者释放

A creates B A sets itself as B's delegate … A is released by its owner

如果 B 保留了 A，A 不会被释放，因为 B 拥有 A，因此 A 的 dealloc 永远不会被调用，导致 A 和 B 泄漏.

If B had retained A, A wouldn't be released, as B owns A, thus A's dealloc would never get called, causing both A and B to leak.

您不必担心 A 会消失，因为它拥有 B，因此在 dealloc 中将其删除.

You shouldn't worry about A going away because it owns B and thus gets rid of it in dealloc.

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