Objective-C等效的Java枚举或“静态最终”对象 [英] Objective-C equivalent of Java enums or "static final" objects

问题描述

我正在尝试找到与Java枚举类型或公共静态最终对象等效的Objective-C，例如：

I'm trying to find an Objective-C equivalent to either Java enum types, or "public static final" objects, like:

public enum MyEnum {
    private String str;
    private int val;
    FOO( "foo string", 42 ),
    BAR( "bar string", 1337 );
    MyEnum( String str, int val ) {
        this.str = str;
        this.val = val;
    }
}

或

public static final MyObject FOO = new MyObject( "foo", 42 );

我需要创建常量（当然）的常量，并且可以在任何导入相关联的地方访问。 h文件，或全局。我试过以下但没有成功：

I need to create constants that are constants (of course), and accessible anywhere that imports the associated .h file, or globally. I've tried the following with no success:

Foo.h：

static MyEnumClass* FOO;

Foo.m：

+ (void)initialize {
    FOO = [[MyEnumClass alloc] initWithStr:@"foo string" andInt:42];
}

当我这样做并尝试使用 FOO时常量，它在 str 和 val 变量中没有值。我已经通过使用 NSLog 验证了 initialize 实际上正在被调用。

When I did this and tried to use the FOO constant, it had no values in the str and val variables. I have verified through the use of NSLog calls that initialize is in fact being called.

另外，即使我在测试代码块中引用 FOO 变量，Xcode也会突出显示上面显示的.h文件中的行。评论'FOO'定义但未使用。

Also, even though I am referencing the FOO variable in the test block of code, Xcode highlighted the line in the .h file shown above with the comment 'FOO' defined but not used.

我完全不知所措！感谢您的帮助！

I'm totally baffled! Thanks for any help!

推荐答案

使用 extern 而不是 static ：

Foo.h：

extern MyEnumClass* FOO;

Foo.m：

MyEnumClass* FOO = nil; // This is the actual instance of FOO that will be shared by anyone who includes "Foo.h".  That's what the extern keyword accomplishes.

+ (void)initialize {
    if (!FOO) {
        FOO = [[MyEnumClass alloc] initWithStr:@"foo string" andInt:42];
    }
}

static 表示变量在单个编译单元中是私有的（例如，单个.m文件）。因此，在头文件中使用 static 将为包含Foo.h的每个.m文件创建私有FOO实例，这不是您想要的。

static means that the variable is private within a single compilation unit (e.g., a single .m file). So using static in a header file will create private FOO instances for each .m file that includes Foo.h, which is not what you would ever want.

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