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NSInvocation

NSInvocation

作者: FlyOceanFish | 来源:发表于2017-09-09 13:09 被阅读41次

    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NSInvocatio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。NSInvocation与NSMethodSignature配合来完成调用。NSMethodSignature这个类的对象保存了方法的名称、参数和返回值

    • SEL
      系统NSObject自带的performSelector默认只支持一个参数,我们可以给NSObject增加一个category,增加以下方法就可以支持多个参数的调用了
    - (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;
    }
    
    ED3CFE7D-C30A-4ADD-929D-B5F7CB806E82.png FCB4F1F5-1F15-4FDC-A3AE-C1DC95AF32BF.png

    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;
    }
    
    9E8A9D90-F9A9-4DD3-AA21-9953E4953C8D.png

    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@,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;
    }
    
    17D15BD3-98AA-46E9-BAD4-2747E16DD677.png

    NSInvocation.h

    @interface NSInvocation : NSObject {
    @private
        __strong void *_frame;
        __strong void *_retdata;
        id _signature;
        id _container;
        uint8_t _retainedArgs;
        uint8_t _reserved[15];
    }
    
    // 通过NSMethodSignature对象创建NSInvocation对象,NSMethodSignature为方法签名类
    + (NSInvocation *)invocationWithMethodSignature:(NSMethodSignature *)sig;
    
    // 获取NSMethodSignature对象
    @property (readonly, retain) NSMethodSignature *methodSignature;
    
    // 保留参数,它会将传入的所有参数以及target都retain一遍
    - (void)retainArguments;
    
    // 判断参数是否还存在
    // 调用retainArguments之前,值为NO,调用之后值为YES
    @property (readonly) BOOL argumentsRetained;
    
    // 设置消息调用者,注意:target最好不要是局部变量
    @property (nullable, assign) id target;
    
    // 设置要调用的消息
    @property SEL selector;
    
    // 获取消息返回值
    - (void)getReturnValue:(void *)retLoc;
    
    // 设置消息返回值
    - (void)setReturnValue:(void *)retLoc;
    
    // 获取消息参数
    - (void)getArgument:(void *)argumentLocation atIndex:(NSInteger)idx;
    
    // 设置消息参数
    - (void)setArgument:(void *)argumentLocation atIndex:(NSInteger)idx;
    
    // 发送消息,即执行方法
    - (void)invoke;
    
    // target发送消息,既设置了target同时执行方法。这一步要注意在调用此方法之前必须设置了selector和argument
    - (void)invokeWithTarget:(id)target;
    
    @end
    

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