RunTime知识梳理

2016-02-16 | 阅读：次

写在开头

iOS开发这么久，接触和使用底层runtime也不止一次了，也总是没有去好好的系统研究一番~
总是很好奇，去试着学习，发现，创造~

前言

Objective-C是一门动态的语言，而Runtime铸就了它动态语言的特性。

深入

在苹果的官方文档Objective-C Runtime Programming Guide是这样组织的：

那么从messaging说起
恩，这点很重要，Objective-C根据Smalltalk发展而来，而Alan Kay(smalltalk核心开发成员)多次强调消息传递（message-passing）是Smalltalk重要的部分。

In Objective-C，The compiler converts a message expression into a call on a messaging function, objc_msgSend.

//
[receiver message] 
//
objc_msgSend(receiver, selector)
//
objc_msgSend(receiver, selector, arg1, arg2, ...)

消息传递的关键在于两个重要的因素：

objc_object 中的isa point
This pointer, called isa, gives the object access to its class and, through the class, to all the classes it inherits from.
objc_class中的 class dispatch table
This table has entries that associate method selectors with the class-specific addresses of the methods they identify.

相关描述。。。

那么在底层整个过程是怎样的呢? 底层的结构是如何定义的呢？

/// A pointer to an instance of a class.
typedef struct objc_object *id;

/// Represents an instance of a class.
struct objc_object {
    Class isa  OBJC_ISA_AVAILABILITY;
};

/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

/// 
struct objc_class {
    Class isa  OBJC_ISA_AVAILABILITY;

#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

} OBJC2_UNAVAILABLE;

/// An opaque type that represents a method selector.
typedef struct objc_selector *SEL;

/// A pointer to the function of a method implementation. 
#if !OBJC_OLD_DISPATCH_PROTOTYPES
typedef void (*IMP)(void /* id, SEL, ... */ ); 
#else
typedef id (*IMP)(id, SEL, ...); 
#endif

objc_msgSend(reciver,foo)调用时，查找SEL的IML的过程如下：

通过 reciver 的 isa 指针找到它的 class
在 class 的 method list 找 foo
如果 class 中没到foo，继续往它的 superclass 中找
一旦找到foo这个函数，就去执行它的实现IMP

那么问题来了，如果foo未找到呢？系统又是如何处理的？

苹果处理未找到方法的过程如下：

从图上看出主要有三个步骤：

Dynamic Method Resolution
我们可以通过resolveInstanceMethod: and resolveClassMethod:给一个实例方法或者类方法提供动态实现。
given the following function:

void dynamicMethodIMP(id self, SEL _cmd) {
// implementation ....
}

we can dynamically add it to a class as a method (called resolveThisMethodDynamically) using resolveInstanceMethod: like this:

@implementation MyClass
+ (BOOL)resolveInstanceMethod:(SEL)aSEL
{
    if (aSEL == @selector(resolveThisMethodDynamically)) {
          class_addMethod([self class], aSEL, (IMP) dynamicMethodIMP, "v@:");
          return YES;
    }
    return [super resolveInstanceMethod:aSEL];
}
@end
	

forwardingTargetForSelector
如果目标对象实现了forwardingTargetForSelector: ，Runtime 就会调用这个方法，把这个消息转发给其他对象。

- (id)forwardingTargetForSelector:(SEL)aSelector
{
    if(aSelector == @selector(mysteriousMethod:)){
        return alternateObject;
    }
    return [super forwardingTargetForSelector:aSelector];
}
	

forwardInvocation
If you send a message to an object that does not handle that message, before announcing an error the runtime sends the object a forwardInvocation: message with an NSInvocation object as its sole argument—the NSInvocation object encapsulates the original message and the arguments that were passed with it.

You can implement a forwardInvocation: method to give a default response to the message, or to avoid the error in some other way. As its name implies, forwardInvocation: is commonly used to forward the message to another object.

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    if ([someOtherObject respondsToSelector:
            [anInvocation selector]])
        [anInvocation invokeWithTarget:someOtherObject];
    else
        [super forwardInvocation:anInvocation];
}
	

– 那么可以总结如下：

如果没有找到，Runtime 会发送 +resolveInstanceMethod: 或者 +resolveClassMethod: 尝试去 resolve 这个消息
如果 resolve 方法返回 NO，Runtime 就发送 -forwardingTargetForSelector: 允许你把这个消息转发给另一个对象
如果没有新的目标对象返回， Runtime 就会发送 -methodSignatureForSelector: 和 -forwardInvocation: 消息。你可以发送 -invokeWithTarget: 消息来手动转发消息或者发送 -doesNotRecognizeSelector: 抛出异常

–

写在最后

补充

Objective-C RunTime是开源的。源代码：http://www.opensource.apple.com/source/objc4/

参考文章

https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40008048-CH1-SW1
http://tech.glowing.com/cn/objective-c-runtime/
http://zhangbuhuai.com/2015/04/27/unstanding-the-Objective-C-Runtime-part3/
http://cocoasamurai.blogspot.sg/2010/01/understanding-objective-c-runtime.html

NSHYJ

NSHYJ

RunTime知识梳理

写在开头

前言

深入

写在最后

补充

参考文章