问题
I have this line in Objective-C.
NSMutableArray *mutableArray;
[mutableArray addObject:@{ @"Something" : aObject, @"Otherthing" : anotherObject }];
What does the @{ ... } part do exactly? It is an object, but it seems to create some kind of key, value pair on the fly.
回答1:
It is creating NSDictionary object as you said. Syntax is simple
NSDictionary* dictionary = @{key: object, key: object};
In your example, keys are objects of NSString class. It is important to remember that dictionary copies keys and retains values.
回答2:
These are called Literals. Apple LLVM Compiler 4.0 and above can use this.
In your question, the expression creates a dictionary
NSDictionary *settings = @{ AVEncoderAudioQualityKey : @(AVAudioQualityMax) };
Similarly arrays which were created using NSArray arrayWithArray and other similar methods, can now be done easily
NSArray *array = @[ @"Hello", @"World"];
and you will not even need the nil sentinel.
More details here: http://clang.llvm.org/docs/ObjectiveCLiterals.html
回答3:
The @{ ... } syntax is a shorthand way of creating a NSDictionary introduced as part of Modern Objective-C. The syntax @{@"key1": object1, @"key2": object2} is just a shorthand for more verbose methods like [NSDictionary dictionaryWithObjectsAndKeys:] among a few others.
来源：https://stackoverflow.com/questions/22510996/objective-c-at-sign-and-curly-braces-what-does-it-mean