objective-c - ARC 是否与 Core Graphics 对象一起使用？

Question

我最近开始了一个使用自动引用计数 (ARC) 的新项目。
当我分配 CALayer 的内容时:

UIView* view = ...
UIImage* image = ...
view.layer.contents = image.CGImage

我遇到了一个错误

Implicit conversion of a non-Objective-C pointer type 'CGImageRef' to 'id' is disallowed with ARC

只需将 CGImageRef 转换为 id 即可隐藏错误，但我想知道 ARC 是否仍能正常运行？

Answer 1

您真的应该看看 WWDC 2011 中的 ARC 视频。它们可以在开发者网站上找到并通过 iTunes 打开。特别是:

• Session 323 – Introducing Automatic Reference Counting

• Session 322 – Objective-C Advancements in Depth

此外，ARC 引用注释:

https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html

引用说明和视频都讨论了 Core Graphics(等)以及它们如何与 ARC 一起工作。

具体来说，请查看名为“管理免费桥接”

的部分

In many Cocoa applications, you need to use Core Foundation-style objects, whether from the Core Foundation framework itself (such asCFArrayRef or CFMutableDictionaryRef) or from frameworks that adoptCore Foundation conventions such as Core Graphics (you might use typeslike CGColorSpaceRef and CGGradientRef).

The compiler does not automatically manage the lifetimes of CoreFoundation objects; you must call CFRetain and CFRelease (or thecorresponding type-specific variants) as dictated by the CoreFoundation memory management rules (see Memory Management ProgrammingGuide for Core Foundation).

If you cast between Objective-C and Core Foundation-style objects, youneed to tell the compiler about the ownership semantics of the objectusing either a cast (defined in objc/runtime.h) or a CoreFoundation-style macro (defined in NSObject.h): [...]

Jörg Jacobsen 对桥接选项也有很好的总结概述:Managing Toll-free Bridging in an ARC’ed Environment .

__bridge_retained (n.b.: only use it when casting from object pointer to C type pointer): I (the programmer) need to reference this objectfor some time in the dark world of C type pointers which is opaque toyou, ARC. So please, please do not release this object while I stillneed it. I (the programmer) promise to release it myself (in the darkworld) when I’m done with it

__bridge_transfer (n.b.: only use it when casting from C type pointer to object pointer): I (the programmer) hand over to you, ARC, anobject that I own and that I am no longer interested in in the darkworld of C type pointers that is opaque to you. Whenever you, ARC, aredone with that object please release it yourself, because you know theright time and thus save me some work not having to do it myself.

__bridge: ARC, you keep balancing out your retains and releases as I keep balancing out mine in the dark world of C type pointers whichis…. Whenever I need to hold on to an object in the dark world I willretain it myself and release it when appropriate. I don’t need anyextra contract with you, ARC.