initialized

  • +initialized方法会在类第一次接收到消息时调用
  • 调用顺序
    • 先调用父类的+initialize，再调用子类的+initialize
    • 先初始化父类，再初始化子类，每个类只会初始化1次
  • +initialize和+load的很大区别是，+initialize是通过objc_msgSend进行调用的，所以有以下特点
    • 如果子类没有实现+initialize，会调用父类的+initialize（所以父类的+initialize可能会被调用多次）
    • 如果分类实现了+initialize，就覆盖类本身的+initialize调用

objc4源码解读

  • objc-msg-arm64.s，汇编级别的源码
    • objc_msgSend，消息发送汇编源码
  • objc-runtime-new.mm
    • class_getInstanceMethod
    • lookUpImpOrNil
    • lookUpImpOrForward
    • _class_initialize
    • callInitialize
    • objc_msgSend(cls, SEL_initialize)

源码分析，objc-runtime-new.mm文件

class_getInstanceMethod

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/***********************************************************************
* class_getInstanceMethod.  Return the instance method for the
* specified class and selector.
**********************************************************************/
Method class_getInstanceMethod(Class cls, SEL sel)
{
    if (!cls  ||  !sel) return nil;

    // This deliberately avoids +initialize because it historically did so.

    // This implementation is a bit weird because it's the only place that 
    // wants a Method instead of an IMP.

#warning fixme build and search caches
        
    // Search method lists, try method resolver, etc.
    // 查找方法
    lookUpImpOrNil(cls, sel, nil, 
                   NO/*initialize*/, NO/*cache*/, YES/*resolver*/);

#warning fixme build and search caches

    return _class_getMethod(cls, sel);
}

lookUpImpOrNil

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/***********************************************************************
* lookUpImpOrNil.
* Like lookUpImpOrForward, but returns nil instead of _objc_msgForward_impcache
**********************************************************************/
IMP lookUpImpOrNil(Class cls, SEL sel, id inst, 
                   bool initialize, bool cache, bool resolver)
{
    IMP imp = lookUpImpOrForward(cls, sel, inst, initialize, cache, resolver);
    if (imp == _objc_msgForward_impcache) return nil;
    else return imp;
}

lookUpImpOrForward

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IMP lookUpImpOrForward(Class cls, SEL sel, id inst, 
                       bool initialize, bool cache, bool resolver)
{
    IMP imp = nil;
    bool triedResolver = NO;

    runtimeLock.assertUnlocked();

    // 省去代码...   

    // 判断是否初始化过类，没有就进行初始化
    if (initialize  &&  !cls->isInitialized()) {
        runtimeLock.unlockRead();
        
        // 初始化操作
        _class_initialize (_class_getNonMetaClass(cls, inst));
        
        runtimeLock.read();
        // If sel == initialize, _class_initialize will send +initialize and 
        // then the messenger will send +initialize again after this 
        // procedure finishes. Of course, if this is not being called 
        // from the messenger then it won't happen. 2778172
    }
    
    
}

_class_initialize

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/***********************************************************************
* class_initialize.  Send the '+initialize' message on demand to any
* uninitialized class. Force initialization of superclasses first.
**********************************************************************/
void _class_initialize(Class cls)
{    
    // 省去代码...   

        {
            // 调用自己的初始化方法
            callInitialize(cls);

            if (PrintInitializing) {
                _objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
                             pthread_self(), cls->nameForLogging());
            }
        }
    
    // 省去代码...   
}

callInitialize

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void callInitialize(Class cls)
{
    // 消息发送机制
    ((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
    asm("");
}

高频面试

  • Category中有load方法吗？load方法是什么时候调用的？load 方法能继承吗？

    • 有load方法
    • load方法在runtime加载类、分类的时候调用
    • load方法可以继承，但是一般情况下不会主动去调用load方法，都是让系统自动调用

  • load、initialize方法的区别什么？它们在category中的调用的顺序？以及出现继承时他们之间的调用过程？

    • 分别在两个笔记里面顶部体现

  • Category能否添加成员变量？如果可以，如何给Category添加成员变量？

    • 不能直接给Category添加成员变量，但是可以间接实现Category有成员变量的效果

参考