ios - 包含尖括号 <> 的奇数属性声明语法

Question

我刚刚从 2015 WWDC 示例代码 (https://developer.apple.com/sample-code/wwdc/2015/) 下载了 FourInARow 并注意到文件 AAPLViewController.m 中有一个奇怪的属性声明

@property NSArray<NSMutableArray<CAShapeLayer *> *> *chipLayers;

这是什么意思？

Answer 1

它是 Objective-C 的新增功能，称为 Lightweight Generics .它是在 iOS9/OS X 10.11 中引入的，目的是增强 Swift 和 Objective-C 之间的互操作性。正如文档所说:

Objective-C declarations of NSArray, NSSet and NSDictionary types using lightweight generic parameterization are imported by Swift with information about the type of their contents preserved.

For example, consider the following Objective-C property declarations:

@property NSArray<NSDate *>* dates; 
@property NSSet<NSString *>* words; 
@property NSDictionary<KeyType: NSURL *, NSData *>* cachedData;

Here’s how Swift imports them:

var dates: [NSDate]
var words: Set<String> 
var cachedData: [NSURL: NSData]