强大的NSInvocation

前言

在消息转发中提到过NSInvocation这个类，这里说一下我所理解的NSInvocation。NSInvocation 是命令模式的一种实现，它包含选择器、方法签名、相应的参数以及目标对象。所谓的方法签名，即方法所对应的返回值类型和参数类型。当NSInvocation被调用，它会在运行时通过目标对象去寻找对应的方法，从而确保唯一性，可以用[receiver message]来解释。实际开发过程中直接创建NSInvocation的情况不多见，这些事情通常交给系统来做。比如bang的JSPatch中arm64方法替换的实现就是利用runtime消息转发最后一步中的NSInvocation实现的。

正文

基于这种命令模式，可以利用NSInvocation调用任意SEL甚至block。

SEL

- (id)performSelector:(SEL)aSelector withArguments:(NSArray *)arguments {
    
    if (aSelector == nil) return nil;
    NSMethodSignature *signature = [[self class] instanceMethodSignatureForSelector:aSelector];
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.target = self;
    invocation.selector = aSelector;
    
    // invocation 有2个隐藏参数，所以 argument 从2开始
    if ([arguments isKindOfClass:[NSArray class]]) {
        NSInteger count = MIN(arguments.count, signature.numberOfArguments - 2);
        for (int i = 0; i < count; i++) {
            const char *type = [signature getArgumentTypeAtIndex:2 + i];
            
            // 需要做参数类型判断然后解析成对应类型，这里默认所有参数均为OC对象
            if (strcmp(type, "@") == 0) {
                id argument = arguments[i];
                [invocation setArgument:&argument atIndex:2 + i];
            }
        }
    }
    
    [invocation invoke];
    
    id returnVal;
    if (strcmp(signature.methodReturnType, "@") == 0) {
        [invocation getReturnValue:&returnVal];
    }
    // 需要做返回类型判断。比如返回值为常量需要包装成对象，这里仅以最简单的`@`为例
    return returnVal;
}

与NSObject中的performSelector相比，没有了参数个数限制。

block

static id invokeBlock(id block ,NSArray *arguments) {
    if (block == nil) return nil;
    id target = [block  copy];

    const char *_Block_signature(void *);
    const char *signature = _Block_signature((__bridge void *)target);

    NSMethodSignature *methodSignature = [NSMethodSignature signatureWithObjCTypes:signature];
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
    invocation.target = target;

    // invocation 有1个隐藏参数，所以 argument 从1开始
    if ([arguments isKindOfClass:[NSArray class]]) {
        NSInteger count = MIN(arguments.count, methodSignature.numberOfArguments - 1);
        for (int i = 0; i < count; i++) {
            const char *type = [methodSignature getArgumentTypeAtIndex:1 + i];
            NSString *typeStr = [NSString stringWithUTF8String:type];
            if ([typeStr containsString:@"\""]) {
                type = [typeStr substringToIndex:1].UTF8String;
            }
            
            // 需要做参数类型判断然后解析成对应类型，这里默认所有参数均为OC对象
            if (strcmp(type, "@") == 0) {
                id argument = arguments[i];
                [invocation setArgument:&argument atIndex:1 + i];
            }
        }
    }

    [invocation invoke];

    id returnVal;
    const char *type = methodSignature.methodReturnType;
    NSString *returnType = [NSString stringWithUTF8String:type];
    if ([returnType containsString:@"\""]) {
        type = [returnType substringToIndex:1].UTF8String;
    }
    if (strcmp(type, "@") == 0) {
        [invocation getReturnValue:&returnVal];
    }
    // 需要做返回类型判断。比如返回值为常量需要包装成对象，这里仅以最简单的`@`为例
    return returnVal;
}

SEL与block比较

• invocation
  SEL既有target也有selector，block只有target
• signature
  SEL有两个隐藏参数，类型均为@ 类型为@和: ，分别对应target和selector。block有一个隐藏参数，类型为@?，对应target且block的target为他本身
• type
  以OC对象为例：SEL的type为@，block的type会跟上具体类型，如@"NSString"

再谈block

在block的invocation中有这样的代码

    const char *_Block_signature(void *);
    const char *signature = _Block_signature((__bridge void *)target);

_Block_signature其实是JavaScriptCore/ObjcRuntimeExtras.h中的私有API(这个头文件并没有公开可以戳这里查看)

既然苹果把API封了，那就自己实现咯，万能的github早有答案CTObjectiveCRuntimeAdditions
把CTBlockDescription.h和CTBlockDescription.m拖到项目中，代码这样写

static id invokeBlock(id block ,NSArray *arguments) {
    if (block == nil) return nil;
    id target = [block  copy];
    
    CTBlockDescription *ct = [[CTBlockDescription alloc] initWithBlock:target];
    NSMethodSignature *methodSignature = ct.blockSignature;
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
    invocation.target = target;
    
    // invocation 有1个隐藏参数，所以 argument 从1开始
    if ([arguments isKindOfClass:[NSArray class]]) {
        NSInteger count = MIN(arguments.count, methodSignature.numberOfArguments - 1);
        for (int i = 0; i < count; i++) {
            const char *type = [methodSignature getArgumentTypeAtIndex:1 + i];
            NSString *typeStr = [NSString stringWithUTF8String:type];
            if ([typeStr containsString:@"\""]) {
                type = [typeStr substringToIndex:1].UTF8String;
            }
            
            // 需要做参数类型判断然后解析成对应类型，这里默认所有参数均为OC对象
            if (strcmp(type, "@") == 0) {
                id argument = arguments[i];
                [invocation setArgument:&argument atIndex:1 + i];
            }
        }
    }
    
    [invocation invoke];
    
    id returnVal;
    const char *type = methodSignature.methodReturnType;
    NSString *returnType = [NSString stringWithUTF8String:type];
    if ([returnType containsString:@"\""]) {
        type = [returnType substringToIndex:1].UTF8String;
    }
    if (strcmp(type, "@") == 0) {
        [invocation getReturnValue:&returnVal];
    }
    // 需要做返回类型判断。比如返回值为常量需要包装成对象，这里仅以最简单的`@`为例
    return returnVal;
}