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  • o_c runtime

    When you send a message like [NSObject alloc] you are actually sending a message to the class object and that class object needs to be an instance of the MetaClass which itself is an instance of the root meta class

    The basic implementation of a class in Objective-C looks like 

    interface MyClass : NSObject {

    //vars

    NSInteger counter;

    }

    //methods

    -voiddoFoo;

    end

    but the runtime has more than that to keep track of 

    if !__OBJC2__

        Class super_class                                        OBJC2_UNAVAILABLE;

        const char *name                                         OBJC2_UNAVAILABLE;

        long version                                             OBJC2_UNAVAILABLE;

        long info                                                OBJC2_UNAVAILABLE;

        long instance_size                                       OBJC2_UNAVAILABLE;

        struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;

        struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;

        struct objc_cache *cache                                 OBJC2_UNAVAILABLE;

        struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;

    endif

    Objective-C Associated Objects
    One thing recently introduced in Mac OS X 10.6 Snow Leopard was called Associated References. Objective-C has no support for dynamically adding on variables to objects unlike some other languages that have native support for this. So up until now you would have had to go to great lengths to build the infrastructure to pretend that you are adding a variable onto a class. Now in Mac OS X 10.6 the Objective-C Runtime has native support for this. If we wanted to add a variable to every class that already exists like say NSView we could do so like this... 

    import < Cocoa/Cocoa.h> //Cocoa

    include < objc/runtime.h> //objc runtime api’s

    interface NSView CustomAdditions

    propertyretain NSImage *customImage;

    end

    implementation NSView CustomAdditions

    static char img_key; //has a unique address identifier

    -NSImage *customImage

    {

        return objc_getAssociatedObjectself&img_key;

    }

    -voidsetCustomImage:NSImage *image

    {

        objc_setAssociatedObjectself&img_keyimage

                                 OBJC_ASSOCIATION_RETAIN;

    }

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  • 原文地址:https://www.cnblogs.com/zengyou/p/2766204.html
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