xcode 如何在 iOS 上启用新的 Objective-C 对象字面量？
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How to enable the new Objective-C object literals on iOS?
提问by darrinm
When I create a new project with Xcode 4.4 and add these lines:
当我使用 Xcode 4.4 创建一个新项目并添加这些行时：
NSDictionary *test = @{ @"key" : @"test value" };
NSString *value = test[@"key"];
NSLog(@"value is: %@", value);
it compiles with no warnings and executes as expected.
它编译时没有警告并按预期执行。
Adding the same lines to an existing project produces the compiler error:
将相同的行添加到现有项目会产生编译器错误：
NSString *value = test[@"key"]; <-- Expected method to read dictionary element not found on object of type 'NSDictionary *'
I compared both projects' target build settings but nothing leapt out at me.
我比较了两个项目的目标构建设置，但没有任何反应。
Update:The new project that successfully compiled was for OSX. I tried another new one for iOS with the above lines and it fails to compile, same as my pre-existing (iOS) project.
更新：成功编译的新项目适用于 OSX。我使用上述几行尝试了另一个适用于 iOS 的新项目，但它无法编译，与我预先存在的 (iOS) 项目相同。
回答by Steven Fisher
This has nothing to do with old vs. new project, but rather is a factor of the SDK you use. The problem you're running into is that while this is a compiler feature, it requires SDK support. The iOS 5 SDK does not provide that support, though the iOS 6 SDK does.
这与旧项目与新项目无关，而是您使用的 SDK 的一个因素。您遇到的问题是，虽然这是一个编译器功能，但它需要 SDK 支持。尽管 iOS 6 SDK 提供，但 iOS 5 SDK 不提供该支持。
For that reason, now you should just use the iOS 6 SDK. Read on if you want to use object subscripting with the iOS 5 SDK.
因此，现在您应该只使用 iOS 6 SDK。如果您想在 iOS 5 SDK 中使用对象下标，请继续阅读。
All you need to do is add a header file so that the compiler will try the call. There's no need to add an implementation; it's handled automatically by arclite. (If you are not using ARC, you will have to force the linker to include arclite. But you still don't have to actually switch to it.)
您需要做的就是添加一个头文件，以便编译器尝试调用。无需添加实现；它由 自动处理arclite。（如果您不使用 ARC，则必须强制链接器包含arclite。但您仍然不必实际切换到它。）
Create a new interface file, NSObject+subscripts.h.
创建一个新的接口文件NSObject+subscripts.h。
#if __IPHONE_OS_VERSION_MAX_ALLOWED < 60000
@interface NSDictionary(subscripts)
- (id)objectForKeyedSubscript:(id)key;
@end
@interface NSMutableDictionary(subscripts)
- (void)setObject:(id)obj forKeyedSubscript:(id <NSCopying>)key;
@end
@interface NSArray(subscripts)
- (id)objectAtIndexedSubscript:(NSUInteger)idx;
@end
@interface NSMutableArray(subscripts)
- (void)setObject:(id)obj atIndexedSubscript:(NSUInteger)idx;
@end
#endif
I've put this chunk on github.
我已经把这个块放在github 上。
Note: I used to suggest adding the required methods to NSObjectbefore explaining how to add them only to the relevant objects. In retrospect, I believe this was an error on my part; it led to errors being caught at runtime rather than compile time, unlike the approach now presented here. That approach is still on my blog, but I now believe it to be more of a cool hack than a useful approach.
注意：NSObject在解释如何将它们仅添加到相关对象之前，我曾经建议添加所需的方法。回想起来，我认为这是我的错误；它导致错误在运行时被捕获，而不是在编译时被捕获，这与此处介绍的方法不同。这种方法仍然在我的博客上，但我现在相信它更像是一种很酷的技巧，而不是一种有用的方法。
Source:
来源：
