在使用 ARC 和针对 iOS4.0时，如何替换弱引用？

我已经开始用 Xcode 4.2开发我的第一个 iOS 应用程序，并且用一个“实用程序”模板(FlipsideViewController 附带的模板)瞄准 iOS 5.0。

我了解到由于 ARC 是一个编译时特性，它也应该与 iOS 4兼容，所以我试图将我的应用程序定位于4.3，并尝试编译它。当我这样做时，我得到这个错误:

Error: AutomativeReferenceCounting 問題: 當前部署目標不支援 Automated _ 孱弱引用

它指的是这句话:

@synthesize delegate = _delegate;

该变量声明为:

@property (weak, nonatomic) IBOutlet id <FlipsideViewControllerDelegate> delegate;

我知道 iOS4不支持“弱引用”，但是我真的不明白为什么一开始就要使用弱引用，我也不知道如何重写来避免使用它，同时还要利用 ARC (毕竟，它应该在 iOS4和5中工作，对吧

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最佳答案

To target the older OS, you can use unsafe_unretained instead of weak in your property declaration, and it should mostly work the same way. weak references nil themselves when their target goes away, but unsafe_unretained leaves open the possibility that the object you're linking to could turn into a dangling pointer when it is deallocated. The latter is the same behavior as if you had used assign as a property declaration in manual memory management.

You do this to avoid retain cycles, which I mention in my answer here. You don't want to have a strong pointer to something that might have a strong pointer back to the original object. Then nothing would get released properly.

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If only using weak references for additional safety, manually call the new runtime functions if they're available and fallback to simple assignment on __unsafe_unretained variables if not.

ZWRCompatibility.h will simplify this somewhat.

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Thanks to Mike Ash's compatibility library PLWeakCompatibilty, you can now simply use __weak on iOS 4.x, as well.

It's incredibly easy to configure and requires no additional consideration or effort over 5.x.


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