iOS底层原理之消息转发

2020-01-03  本文已影响0人  尘舒

简介

1.方法解析

1.1 分析
 if (resolver  &&  !triedResolver) {
        runtimeLock.unlock();
        _class_resolveMethod(cls, sel, inst);
        runtimeLock.lock();
        // Don't cache the result; we don't hold the lock so it may have 
        // changed already. Re-do the search from scratch instead.
        triedResolver = YES;
        goto retry;
    }

/***********************************************************************
* _class_resolveMethod
* Call +resolveClassMethod or +resolveInstanceMethod.
* Returns nothing; any result would be potentially out-of-date already.
* Does not check if the method already exists.
**********************************************************************/
void _class_resolveMethod(Class cls, SEL sel, id inst)
{
    if (! cls->isMetaClass()) {
        // try [cls resolveInstanceMethod:sel]
        // 调用对象方法解析
        _class_resolveInstanceMethod(cls, sel, inst);
    } 
    else {
        // try [nonMetaClass resolveClassMethod:sel]
        // and [cls resolveInstanceMethod:sel]
      // 调用类方法解析
        _class_resolveClassMethod(cls, sel, inst);
        if (!lookUpImpOrNil(cls, sel, inst, 
                            NO/*initialize*/, YES/*cache*/, NO/*resolver*/)) 
        {
            _class_resolveInstanceMethod(cls, sel, inst);
        }
    }
}

SEL SEL_resolveInstanceMethod = NULL;
SEL SEL_resolveClassMethod = NULL;

+ (BOOL)resolveClassMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
+ (BOOL)resolveInstanceMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

/***********************************************************************
* _class_resolveInstanceMethod
* Call +resolveInstanceMethod, looking for a method to be added to class cls.
* cls may be a metaclass or a non-meta class.
* Does not check if the method already exists.
**********************************************************************/
// 这里的注释很清晰，我们需要重写 resolveInstanceMethod方法，并且为cls添加一个新的方法实现
static void _class_resolveInstanceMethod(Class cls, SEL sel, id inst)
{
}
1.2 实验
        [student performSelector:@selector(sayNothing)];
        [LGStudent performSelector:@selector(sayClassNothing)];

// 打印结果
2019-12-30 17:33:14.808210+0800 LGTest[7749:256805] 错误实例方法sayNothing已经被拦截
2019-12-30 17:33:14.808864+0800 LGTest[7749:256805] 错误类方法sayClassNothing已经被拦截

- (void)unknowObjSel{
    NSLog(@"错误实例方法sayNothing已经被拦截");
}

+ (void)unknowClassSel{
    NSLog(@"错误类方法sayClassNothing已经被拦截");
}

// 类方法解析
// 注意:类方法是存在于类的元类的方法列表中
+ (BOOL)resolveClassMethod:(SEL)sel {
    if ([NSStringFromSelector(sel) isEqualToString:@"sayClassNothing"]) {
        Method toMethod = class_getClassMethod(self, @selector(unknowClassSel));
        class_addMethod(object_getClass(self), sel, method_getImplementation(toMethod), method_getTypeEncoding(toMethod));
        return YES;
    }
    
    return [super resolveClassMethod:sel];
}

// 实例方法解析
+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if ([NSStringFromSelector(sel) isEqualToString:@"sayNothing"]) {
        Method toMethod = class_getInstanceMethod(self, @selector(unknowObjSel));
        class_addMethod(self, sel, method_getImplementation(toMethod), method_getTypeEncoding(toMethod));
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

   if (resolver  &&  !triedResolver) {
        runtimeLock.unlock();
        _class_resolveMethod(cls, sel, inst);
        runtimeLock.lock();
        // Don't cache the result; we don't hold the lock so it may have 
        // changed already. Re-do the search from scratch instead.
        triedResolver = YES;
        goto retry;
    }
1.3 注意

补充： 推导引出快速转发阶段

[student performSelector:@selector(saySomething)];
image.png 
static void
log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
{
#if SUPPORT_MESSAGE_LOGGING
    if (objcMsgLogEnabled) {
        bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                      cls->nameForLogging(),
                                      implementer->nameForLogging(), 
                                      sel);
        if (!cacheIt) return;
    }
#endif
    cache_fill (cls, sel, imp, receiver);
}


// 日志打印消息发送，**/tmp/msgSends**，为日志路径
bool logMessageSend(bool isClassMethod,
                    const char *objectsClass,
                    const char *implementingClass,
                    SEL selector)
{
    char    buf[ 1024 ];

    // Create/open the log file
    if (objcMsgLogFD == (-1))
    {
        snprintf (buf, sizeof(buf), "/tmp/msgSends-%d", (int) getpid ());
        objcMsgLogFD = secure_open (buf, O_WRONLY | O_CREAT, geteuid());
        if (objcMsgLogFD < 0) {
            // no log file - disable logging
            objcMsgLogEnabled = false;
            objcMsgLogFD = -1;
            return true;
        }
    }

    // Make the log entry
    snprintf(buf, sizeof(buf), "%c %s %s %s\n",
            isClassMethod ? '+' : '-',
            objectsClass,
            implementingClass,
            sel_getName(selector));

    objcMsgLogLock.lock();
    write (objcMsgLogFD, buf, strlen(buf));
    objcMsgLogLock.unlock();

    // Tell caller to not cache the method
    return false;
}

// 控制打印开关
void instrumentObjcMessageSends(BOOL flag)
{
    bool enable = flag;

    // Shortcut NOP
    if (objcMsgLogEnabled == enable)
        return;

    // If enabling, flush all method caches so we get some traces
    if (enable)
        _objc_flush_caches(Nil);

    // Sync our log file
    if (objcMsgLogFD != -1)
        fsync (objcMsgLogFD);

    objcMsgLogEnabled = enable;
}

        instrumentObjcMessageSends(true);
        [student performSelector:@selector(saySomething)];
        instrumentObjcMessageSends(false);

- LGStudent NSObject performSelector:
+ LGStudent NSObject resolveInstanceMethod:
+ LGStudent NSObject resolveInstanceMethod:
- LGStudent NSObject forwardingTargetForSelector:
- LGStudent NSObject forwardingTargetForSelector:
- LGStudent NSObject methodSignatureForSelector:
- LGStudent NSObject methodSignatureForSelector:
- LGStudent NSObject class
- LGStudent NSObject doesNotRecognizeSelector:
- LGStudent NSObject doesNotRecognizeSelector:
- LGStudent NSObject class
- OS_xpc_serializer OS_xpc_object dealloc

2.快速转发阶段

- (id)forwardingTargetForSelector:(SEL)aSelector OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
- (id)forwardingTargetForSelector:(SEL)sel {
    return nil;
}
image.png 
// class LGStudent
- (id)forwardingTargetForSelector:(SEL)aSelector {
    if (NSSelectorFromString(@"saySomething") == aSelector) {
        return UnkonwMethodSave.new;
    }
    return [super forwardingTargetForSelector:aSelector];
}

// class UnkonwMethodSave
@implementation UnkonwMethodSave
- (void)saySomething {
    NSLog(@"UnkonwMethodSave: saySomething 被我处理了");
}
@end

// 输出
LGTest[2233:52045] UnkonwMethodSave: saySomething 被我处理了

3.常规转发阶段

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if (NSSelectorFromString(@"saySomething") == aSelector) {
        //  把不能够处理的方法，返回一个方法签名
        return [NSMethodSignature signatureWithObjCTypes:"v@:"] ;
    }
    return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
    NSLog(@"这里拦截了未知方法 method");
    // 给不同的方法，传递给不同的能够处理的类去处理
    UnkonwMethodSave *friend = UnkonwMethodSave.new;
    SEL aSelector = [anInvocation selector];
    if ([friend respondsToSelector:aSelector]) {
         // 此时  friend能够处理 方法 aSelector
         // 那么就用friend来处理
          [anInvocation invokeWithTarget:friend];
    }
    else {
          [super forwardInvocation:anInvocation];
    }
}

// 输出
LGTest[2442:64431] 这里拦截了未知方法 method
LGTest[2442:64431] UnkonwMethodSave: saySomething 被我处理了

4. 总结

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