objective-c 在Objective-C中等效的静态构造函数？

Static constructor equivalent in Objective-C?

提问by Franklin Munoz

I'm new to Objective C and I haven't been able to find out if there is the equivalent of a static constructor in the language, that is a static method in a class that will automatically be called before the first instance of such class is instantiated. Or do I need to call the Initialization code myself?

我是 Objective C 的新手，我无法找出语言中是否有静态构造函数的等效项，即类中的静态方法，将在此类类的第一个实例之前自动调用被实例化。还是我需要自己调用初始化代码？

Thanks

谢谢

回答by Quinn Taylor

The +initializemethod is called automaticallythe first time a class is used, before any class methods are used or instances are created. You should never call +initializeyourself.

第一次使用类时会自动+initialize调用该方法，然后再使用任何类方法或创建实例。你永远不应该给自己打电话。+initialize

I also wanted to pass along a tidbit I learned that can bite you down the road: +initializeis inherited by subclasses, and is also called for each subclasses that doesn't implement an +initializeof their own. This can be especially problematic if you naively implement singleton initialization in +initialize. The solution is to check the type of the class variable like so:

我还想传递一个我学到的花絮，它可以让你一头雾水：+initialize由子类继承，并且还为每个没有实现+initialize自己的. 如果您天真地在+initialize. 解决方案是检查类变量的类型，如下所示：

+ (void) initialize {
  if (self == [MyParentClass class]) {
    // Once-only initializion
  }
  // Initialization for this class and any subclasses
}

All classes that descend from NSObject have both +classand -classmethods that return the Classobject. Since there is only one Class object for each class, we do want to test equality with the ==operator. You can use this to filter what should happen only once ever, versus once for each distinct class in a hierarchy (which may not yet exist) below a given class.

所有继承自 NSObject 的类都有返回对象的+class和-class方法Class。由于每个类只有一个 Class 对象，我们确实想用==运算符测试相等性。您可以使用它来过滤什么应该只发生一次，而不是为给定类下面的层次结构（可能尚不存在）中的每个不同类过滤一次。

On a tangential topic, it's worth learning about the following related methods, if you haven't already:

在切入主题上，如果您还没有学习以下相关方法，则值得学习：

• - isMemberOfClass:(Class)aClass(true only for aClassitself)
• - isKindOfClass:(Class)aClass(true for aClassand children)
• + isSubclassOfClass:(Class)aClass(same as above, but a class method)

• - isMemberOfClass:(Class)aClass（仅适用于aClass自身）
• - isKindOfClass:(Class)aClass（适用于aClass儿童）
• + isSubclassOfClass:(Class)aClass（同上，不过是一个类方法）

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Edit:Check out this post by @bbumthat explains more about +initialize: http://www.friday.com/bbum/2009/09/06/iniailize-can-be-executed-multiple-times-load-not-so-much/

编辑：查看@bbum 的这篇文章，其中解释了更多关于+initialize：http: //www.friday.com/bbum/2009/09/06/iniailize-can-be-executed-multiple-times-load-not-so-很多/

Also, Mike Ash wrote a nice detailed Friday Q&A about the +initializeand +loadmethods: https://www.mikeash.com/pyblog/friday-qa-2009-05-22-objective-c-class-loading-and-initialization.html

另外，Mike Ash 写了一篇关于+initialize和+load方法的详细的周五问答：https: //www.mikeash.com/pyblog/friday-qa-2009-05-22-objective-c-class-loading-and-initialization.html

回答by Nathan Kinsinger

There is the +initializeclass method that will be called before a class is used.

在使用类之前将调用+initialize类方法。

回答by Richard J. Ross III

A bit of an addendum to this topic:

本主题的一些附录：

There is another way to create a 'static constructor' in obj-c, using an __attributedirective:

还有另一种方法可以使用__attribute指令在 obj-c 中创建“静态构造函数” ：

// prototype
void myStaticInitMethod(void);

__attribute__((constructor))
void myStaticInitMethod()
{
    // code here will be called as soon as the binary is loaded into memory
    // before any other code has a chance to call +initialize.
    // useful for a situation where you have a struct that must be 
    // initialized before any calls are made to the class, 
    // as they would be used as parameters to the constructors.
    // e.g.
    myStructDef.myVariable1 = "some C string";
    myStructDef.myFlag1 = TRUE; 

    // so when the user calls the code [MyClass createClassFromStruct:myStructDef], 
    // myStructDef is not junk values.
}