Message forwarding mechanism (消息转发机制)

iOS的消息转发机制

iOS的消息转发机制是在消息发送给对象时，找不到对应的实例方法的情况下启动的。消息转发允许对象在运行时处理无法识别的消息，提供了一种动态的、灵活的消息处理方式。

消息转发机制主要分为三个阶段：

1. 动态方法解析
2. 快速转发
3. 标准转发

1. 动态方法解析

在这个阶段，运行时系统会询问对象是否能动态添加方法来处理未知消息。可以通过实现 +resolveInstanceMethod: 或 +resolveClassMethod: 方法来动态添加方法实现。

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if (sel == @selector(someMethod)) {
        class_addMethod([self class], sel, (IMP)someMethodImplementation, "v@:");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

void someMethodImplementation(id self, SEL _cmd) {
    NSLog(@"Dynamically added method");
}

2. 快速转发

如果动态方法解析失败，运行时会调用 -forwardingTargetForSelector: 方法，询问对象是否有其他对象可以处理该消息。通过返回另一个对象的实例，消息可以转发给该实例。

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

3. 标准转发

如果前两个阶段都失败，运行时会调用 -methodSignatureForSelector: 和 -forwardInvocation: 方法进行标准消息转发。首先，通过 -methodSignatureForSelector: 方法获取方法签名，然后在 -forwardInvocation: 方法中处理消息。

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if (aSelector == @selector(someMethod)) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];
    }
    return [super methodSignatureForSelector:aSelector];
}

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

图示

为了更好地理解这个过程，下面是一个示意图：

+----------------------+
|        Sender        |
+----------------------+
            |
            | Message: someMethod
            v
+----------------------+
|     Receiver         |
+----------------------+
|                      |
| -resolveInstanceMethod:
|     - Can add method? ----> YES (add method)
|                      |
|                      v
|  NO                  |
|                      |
| -forwardingTargetForSelector:
|     - Other object to handle? ----> YES (forward to other object)
|                      |
|                      v
|  NO                  |
|                      |
| -methodSignatureForSelector:
|     - Get method signature ----> Signature or nil
|                      |
|                      v
|  Signature           |
|                      |
| -forwardInvocation:
|     - Handle invocation ----> Forward or error
+----------------------+

示例代码

下面是一个完整的示例代码，展示如何使用消息转发机制：

#import <objc/runtime.h>

@interface SQIAlternateObject : NSObject
- (void)someMethod;
@end

@implementation SQIAlternateObject
- (void)someMethod {
    NSLog(@"AlternateObject handling someMethod");
}
@end

@interface MyObject : NSObject
@end

@implementation MyObject {
    SQIAlternateObject *alternateObject;
}

- (instancetype)init {
    self = [super init];
    if (self) {
        alternateObject = [[SQIAlternateObject alloc] init];
    }
    return self;
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if (sel == @selector(someMethod)) {
        class_addMethod([self class], sel, (IMP)someMethodImplementation, "v@:");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

void someMethodImplementation(id self, SEL _cmd) {
    NSLog(@"Dynamically added method");
}

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

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if (aSelector == @selector(someMethod)) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];
    }
    return [super methodSignatureForSelector:aSelector];
}

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

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        MyObject *obj = [[MyObject alloc] init];
        [obj someMethod];
    }
    return 0;
}

在这个示例中，我们展示了如何通过动态方法解析、快速转发和标准转发来处理 someMethod 方法。