如何声明一个@property，但是阻止它的ivar被创建？ [英] How can I declare a @property but prevent its ivar from being created?

问题描述

我有类 A ，它暴露了获取和设置类型 Foo 的对象的方法。在属性说法中，我通常在界面中声明：

  @property（nonatomic，strong）Foo * foo; 
 

这在现代ObjC中生成访问器和一个ivar， _foo 用于存储。

如果我想在访问器中进行自定义工作，我可以自己实现一个或两个。但是，如果我不仅想做自定义工作，我实际上不想要ivar吗？换句话说，我正在使用Foo对象做其他事情，例如将它来回传递给我组成的另一个内部对象。我实际上不需要在 A 的实例中保存 foo 的存储。

看起来我有两个选择：

1. 声明属性，实现两个访问器忽略编译器为 _foo 创建存储器的事实，并且不要使用它。


2. 显式声明我的访问器： - （Foo *）foo 和 - （void）setFoo：



第一个看起来很像在运行时，第二个似乎不起眼（在那里我可能现在有一个混合的属性和属性的访问器）。

有一种方法

解决方案

如果你重写了setter和getter，并且没有使用setter和getter中的变量，不会创建一个变量。

例如，如果你有一个类，你想要一个 firstName 和 lastName 属性，但也许还有一个 setter 和 getter 对于 fullName 属性，如果您的 fullName c $ c> firstName 和 lastName 并将这些属性设置为（并且不会将完整字符串存储到 fullName 变量），并且 fullName getter简单地返回连接的 firstName + lastName 并且从不使用 fullName 变量， p>

这是根据苹果的官方文档。 向下滚动到您可以实现自定义访问器方法

I have class A, which exposes a way of getting and setting an object of type Foo. In property parlance, I generally declare this in the interface:

@property (nonatomic, strong) Foo * foo;

This (in modern ObjC) generates both the accessors and an ivar, _foo for storage.

If I want to do custom work in the accessors, I can implement one or both of them myself. But what if I not only want to do custom work, I actually don't want the ivar? In other words, I'm doing something else with the Foo object, like handing it back and forth to another internal object that I'm composed with. I don't actually need to keep storage for foo in the instance of A at all.

It seems like I have two choices:

1. Declare the property, implement both accessors, and simply ignore the fact that the compiler creates storage for _foo, and never use it.
2. Declare my accessors explicitly: - (Foo *)foo and - (void)setFoo:(Foo *)foo in the interface, like I used to in pre-modern ObjC.

The first seems inelegant at runtime, and the second seems inelegant in the declaration (where I'd probably now have a mix of properties and property-like accessors).

Is there a way to declare a property and have it serve as purely a declaration?

解决方案

If you override both the setter and getter and don't use the variable within the setter and getter, a variable will not be created.

For example, if you have a class for which you want a firstName and lastName property, but perhaps also a setter and getter for a fullName property, if your fullName setter simply parses a string into firstName and lastName and sets these properties to that (and never stores the full string into a fullName variable), and your fullName getter simply returns the concatenated firstName + lastName and never uses a fullName variable, one will never be created.

This is according to Apple's official documentation. Scroll down to "You Can Implement Custom Accessor Methods"

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