iOS开发知识

从RunTime源码回看消息转发机制【三次拯救】

2019-07-31  本文已影响0人  太阳骑士索拉尔

关于我的仓库

前言

准备工作

一个说法

关于消息转发

三道防线

关系图

E81306B3-FD5F-4B68-B7DF-7FA084A20E62

动态方法解析【resolveInstanceMethod】

作用

runtime源码

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    return NO;
}

实际使用

直接返回YES

///Person.m
#import "Person.h"
@implementation Person
+ (BOOL)resolveInstanceMethod:(SEL)sel {
    return YES;
}
@end

///main.m
#import <Foundation/Foundation.h>
#import "Student.h"
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        NSLog(@"Hello, World!");
        Student *student = [[Student alloc] init];
        [student eat:@"123"];
    }
    return 0;
}

//-[Person eat:]: unrecognized selector sent to instance 0x100700160

正常调用

///Person.m
#import "Person.h"
#import <objc/runtime.h>
@implementation Person
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
    if (sel == @selector(eat:)) {
        class_addMethod(self, sel, (IMP)Ceat, "v@:I");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}
void Ceat(id self, SEL cmd, NSInteger num) {
    NSLog(@"void Ceat(id self, SEL cmd, int num):");
    NSLog(@"%@, %s, %ld", [self class], sel_getName(cmd), num);
}

@end

///main.m
#import <Foundation/Foundation.h>
#import "Student.h"
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        NSLog(@"Hello, World!");
        Student *student = [[Student alloc] init];
        [student eat:@"123"];
    }
    return 0;
}

// messageTransmit[1221:273136] void Ceat(id self, SEL cmd, int num):
//messageTransmit[1221:273136] Person, eat:, 123

加大难度

整理波思路

备用接受者【(id)forwardingTargetForSelector:(SEL)aSelector。】

作用

runtime源码

- (id)forwardingTargetForSelector:(SEL)sel {
    return nil;
}

正常使用

- (id)forwardingTargetForSelector:(SEL)aSelector {
    NSString *selStr = NSStringFromSelector(aSelector);
    
    if ([selStr isEqualToString:@"eat:"]) {
        return [[Student alloc] init];
    }
    
    return [super forwardingTargetForSelector:aSelector];
}

//Student:eat:(NSString *)str:123

意义

完整转发【(void)forwardInvocation:(NSInvocation )anInvocation】

runtime源码

// Replaced by CF (returns an NSMethodSignature)
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    _objc_fatal("-[NSObject methodSignatureForSelector:] "
                "not available without CoreFoundation");
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    [self doesNotRecognizeSelector:(invocation ? [invocation selector] : 0)];
}

// Replaced by CF (throws an NSException)
- (void)doesNotRecognizeSelector:(SEL)sel {
    _objc_fatal("-[%s %s]: unrecognized selector sent to instance %p", 
                object_getClassName(self), sel_getName(sel), self);
}

概念解释

NSMethodSignature

Apple文档

A record of the type information for the return value and parameters of a method.

Use an NSMethodSignature object to forward messages that the receiving object does not respond to—most notably in the case of distributed objects. You typically create an NSMethodSignatureobject using the NSObject methodSignatureForSelector:instance method (in macOS 10.5 and later you can also use signatureWithObjCTypes:). It is then used to create an NSInvocation object, which is passed as the argument to a forwardInvocation: message to send the invocation on to whatever other object can handle the message. In the default case, NSObject invokes doesNotRecognizeSelector:, which raises an exception. For distributed objects, the NSInvocation object is encoded using the information in the NSMethodSignature object and sent to the real object represented by the receiver of the message.

方法的返回值以及参数的类型信息的记录

使用一个NSMethodSignature对象来转发那些接受者无法响应的消息，尤其是在distributed对象的情况下。通常使用methodSignatureForSelector:方法来创建一个NSMethodSignature对象。该对象之后会被用作创建一个NSInvocation 对象。这个NSInvocation 对象回作为传给forwardInvocation: 方法的参数。这样子那些无法响应的消息会被发给NSInvocation 对象，转发给任意能处理这个消息的对象。在默认情况下，对象是会直接调用doesNotRecognizeSelector:方法抛出异常。对于distributed对象，NSInvocation 对象根据在NSMethodSignature对象中的信息被编码发给真正的执行对象代替消息的接受者【原对象】

解释

NSInvocation

Apple文档

An Objective-C message rendered as an object.

呈现一个对象的Objective-C信息

NSInvocation objects are used to store and forward messages between objects and between applications, primarily by NSTimerobjects and the distributed objects system. An NSInvocation object contains all the elements of an Objective-C message: a target, a selector, arguments, and the return value. Each of these elements can be set directly, and the return value is set automatically when the NSInvocation object is dispatched.

NSInvocation 对象时用于在对象以及APP之间转发信息而存在的，主要通过NSTimer对象和distributed objects system。一个NSInvocation 对象包含OC消息中的所有元素：a target, a selector, 参数, 和返回值。 每一部分都能被直接设置，并且在NSInvocation 对象被发送时自动设置

An NSInvocation object can be repeatedly dispatched to different targets; its arguments can be modified between dispatch for varying results; even its selector can be changed to another with the same method signature (argument and return types). This flexibility makes NSInvocation useful for repeating messages with many arguments and variations; rather than retyping a slightly different expression for each message, you modify the NSInvocation object as needed each time before dispatching it to a new target.

一个NSInvocation 对象可以被重复发送到不同的target，它的参数可以在为了改变结果而进行的发送之间被修改。甚至它的selector可以变成另一个有着相同的方法签名【参数以及返回值】。这种灵活性使得NSInvocation 对于重复发送有着许多参数和变化的消息大有益处。而不再需要为每一个消息的不同，而去修改的它的表达，你只需在每次发送给新的target前修改这个 NSInvocation 对象【有需要的话】

解释

正常使用

// Person.m

//我们必须重写该方法 消息转发机制使用从这个方法中获取的信息来创建NSInvocation对象。因此我们必须重写这个方法，为给定的selector提供一个合适的方法签名。
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    NSString *sel = NSStringFromSelector(aSelector);
   // 判断要转发的SEL
   if ([sel isEqualToString:@"sleep"]) {
       // 为转发的方法手动生成签名
       return [NSMethodSignature signatureWithObjCTypes:"v@:"];
       //那么NSMethodSignature又是什么？来看看
   }
   
   return [super methodSignatureForSelector:aSelector]; 
}

//NSObject的forwardInvocation:方法实现只是简单调用了doesNotRecognizeSelector:方法，它不会转发任何消息。这样，如果不在以上所述的三个步骤中处理未知消息，则会引发一个异常。
//转发消息
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
   //拿到消息
   SEL selector = [anInvocation selector];
   // 新建需要转发消息的对象 转发消息
   Child *child = [[Child alloc] init];
   if ([child respondsToSelector:selector]) {
       // 转发 唤醒这个方法
       [anInvocation invokeWithTarget:child];
   } else {
       [super forwardInvocation:anInvocation];
   }
}
//从某种意义上来讲，forwardInvocation:就像一个未知消息的分发中心，将这些未知的消息转发给其它对象。或者也可以像一个运输站一样将所有未知消息都发送给同一个接收对象。这取决于具体的实现。

疑惑

消息转发的意义

上一篇 下一篇

猜你喜欢

热点阅读