@() 在 Objective-C 中是什么意思? [英] What does @() mean in Objective-C?

问题描述

例如，

CABasicAnimation *rotate = [CABasicAnimation animationWithKeyPath:@"transform.rotation"];
[rotate setToValue:@(M_PI)];
[rotate setDuration:0.1f];            
[[aView layer] addAnimation:rotate forKey:@"myRotationAnimation"];

其中M_PI定义为math.h中的宏，

#define M_PI  3.14159265358979323846264338327950288   /* pi */

推荐答案

它是一个指向 NSNumber 对象的指针.它被称为装箱文字，因为心理图景是将表达式的原始值放入一个盒子"，即一个对象中.

It's a pointer to an NSNumber object. It's called a boxed literal, because the mental picture is of putting a primitive value of expression inside into a "box", that is, an object.

如有疑问，请参阅官方文档.请注意，指针可以指向真实的"NSNumber 对象，也可以(理论上，不知道这在实践中是否可行)是标记指针(请参阅，例如，我的问题).

See official documentation if in doubt. Note that pointer can be to a "real" NSNumber object or it can (theoretically, don't know whether this will work in practice) be a tagged pointer (see, e.g., my question).

请注意，您还可以执行诸如 @"string" 和 @5 之类的操作，它们将在编译时创建常量.但是你需要括号来使用不是文字的东西，例如@(2 + 3).括号形式可以用于任何表达式，即使是编译器在编译时无法计算的表达式(尽管如果可以，它只会将表达式结果放入代码中).

Note that you can also do things like @"string" and @5, which will create constants in compile time. But you need parentheses to use something which is not a literal, e.g. @(2 + 3). Parentheses form can be used for any expression, even those that compiler cannot compute at compile-time (although if it can, it will just put an expression result into code).

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