OC-RunTime: 消息转发之实例方法的转发流程[实例讲解]

OC-RunTime: 消息转发之实例方法的转发流程分享了消息转发的流程, 本次结合实际例子继续分析一下消息转发流程.

发送不存在的消息

在 ViewController 的 viewDidLoad 中运行 veryTestMethod 方法.

ViewController.m

static NSString * const sPerformInstanceMethodName = @"veryTestMethod";

- (void)viewDidLoad {
    
    [super viewDidLoad];
    
    // 运行实例方法
    SEL selector = NSSelectorFromString(sPerformInstanceMethodName);
    
    SuppressPerformSelectorLeakWarning(
        [self performSelector:selector withObject:nil];
    );
}

其中, SuppressPerformSelectorLeakWarning 是定义的一个宏.

#define SuppressPerformSelectorLeakWarning(Stuff) \
do { \
    _Pragma("clang diagnostic push") \
    _Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"") \
    Stuff; \
    _Pragma("clang diagnostic pop") \
} while (0)

在 ViewController 中我并没有写 veryTestMethod 这个函数, 只是借助 performSelector 动态执行, 如果编译运行直接会 crash.

可以查看 NSObject.mm 源码, 里面关于消息转发的几个重要函数都写着 _objc_fatal, 可谓是招招毙命.

紧接着, 我们可以借助 resolveInstanceMethod 来完成消息转发给 ViewController.

resolveInstanceMethod 转发

重写 NSObject 中的 resolveInstanceMethod 函数.

+resolveInstanceMethod

ViewController.m

// 记得导入 RunTime 头文件
#import <objc/runtime.h>

+ (BOOL)resolveInstanceMethod:(SEL)sel
{
    NSLog(@"---veryitman--- 1--- +resolveInstanceMethod");
    
    NSString *methodName = NSStringFromSelector(sel);
    
    if ([sPerformInstanceMethodName isEqualToString:methodName]) {
        
        SEL proxySelector = NSSelectorFromString(@"proxyMethod");
        IMP impletor = class_getMethodImplementation(self, proxySelector);
        
        // 获取实例方法
        Method method = class_getInstanceMethod(self, proxySelector);
        const char *types = method_getTypeEncoding(method);
        
        // 添加 OC 的函数
        class_addMethod([self class], sel, impletor, types);
        
        return YES;
    }
    
    return [super resolveInstanceMethod:sel];
}

ViewController 中实现的 proxyMethod

// OC 实现
- (void)proxyMethod
{
    NSLog(@"---veryitman--- -proxyMethod of instance's method for OC.");
}

在 resolveInstanceMethod 中动态添加了 veryTestMethod 方法, 并让 proxyMethod 函数来实现(IMP).

运行可以看到, 程序并没有 crash, 成功的执行了 proxyMethod.

1 2	---veryitman--- 1--- +resolveInstanceMethod ---veryitman--- -proxyMethod of instance's method for OC.

到此为止, 我们已经看到动态添加一个方法的实现并成功运行的完整例子.

接下来, 我们将转发给其他对象 MZTempObj 来执行.

自定义被转发的对象

MZTempObj.m

@implementation MZTempObj

- (void)veryTestMethod
{
    NSLog(@"---veryitman--- veryTestMethod");
}

veryTestMethod 就是我们要转发对应的消息.

消息转发实践

接下来我们把向 ViewController 发送 veryTestMethod 的消息转发给 MZTempObj 的 veryTestMethod 方法.

继续重写下面函数, 不过 resolveInstanceMethod 要稍微改造一下, 才能达到我们实践的目的.

+resolveInstanceMethod

-forwardingTargetForSelector

-methodSignatureForSelector

-forwardInvocation

-doesNotRecognizeSelector:

ViewController.m

// 记得导入 RunTime 头文件
#import <objc/runtime.h>

+ (BOOL)resolveInstanceMethod:(SEL)sel
{
    NSLog(@"---veryitman--- 1--- +resolveInstanceMethod: %@", NSStringFromSelector(sel));
    
    NSString *methodName = NSStringFromSelector(sel);
    
    // 这里故意为之, 让流程往下走
    if ([@"" isEqualToString:methodName]) {
        
        SEL proxySelector = NSSelectorFromString(@"proxyMethod");
        IMP impletor = class_getMethodImplementation(self, proxySelector);
        
        // 获取实例方法
        Method method = class_getInstanceMethod(self, proxySelector);
        const char *types = method_getTypeEncoding(method);
        
        // 添加 OC 的函数
        class_addMethod([self class], sel, impletor, types);
        
        return YES;
    }
    
    return [super resolveInstanceMethod:sel];
}

/// 转发给对应的某个对象来执行 aSelector
- (id)forwardingTargetForSelector:(SEL)aSelector
{
    NSLog(@"---veryitman--- 2--- -forwardingTargetForSelector");
    
    NSString *selectorName = NSStringFromSelector(aSelector);
    
    if ([sPerformInstanceMethodName isEqualToString:selectorName]) {
        
        // 让 MZTempObj 去执行 aSelector, 实现消息的转发
        MZTempObj *myobject = [[MZTempObj alloc] init];
        
        return myobject;
    }
    
    id obj = [super forwardingTargetForSelector:aSelector];
    
    return obj;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    NSLog(@"---veryitman--- 3--- -methodSignatureForSelector");
    
    // 找出对应的 aSelector 签名
    NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
    
    if (nil == signature) {
        
        // 是否有 aSelector
        if ([MZTempObj instancesRespondToSelector:aSelector]) {
            signature = [MZTempObj instanceMethodSignatureForSelector:aSelector];
        }
    }
        
    return signature;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    NSLog(@"---veryitman--- 4--- -forwardInvocation");
    
    if ([MZTempObj instancesRespondToSelector:anInvocation.selector]) {
        [anInvocation invokeWithTarget:[[MZTempObj alloc] init]];
    }
    else {
        [super forwardInvocation:anInvocation];
    }
}

- (void)doesNotRecognizeSelector:(SEL)aSelector
{
    NSLog(@"---veryitman--- 5--- -doesNotRecognizeSelector: %@", NSStringFromSelector(aSelector));
}

运行程序, 控制台打印结果如下:

1
2
3

---veryitman--- 1--- +resolveInstanceMethod
---veryitman--- 2--- -forwardingTargetForSelector
---veryitman--- veryTestMethod

这里对照之前的流程图是完全符合的, 那么怎么让其执行 3 和 4 呢? 很简单, 修改一下 forwardingTargetForSelector 里面的实现即可.

- (id)forwardingTargetForSelector:(SEL)aSelector
{
    NSLog(@"---veryitman--- 2--- -forwardingTargetForSelector");
    
    NSString *selectorName = NSStringFromSelector(aSelector);
    
    // 故意为之
    if ([@"" isEqualToString:selectorName]) {
        
        // 让 MZTempObj 去执行 aSelector, 实现消息的转发
        MZTempObj *myobject = [[MZTempObj alloc] init];
        
        return myobject;
    }
    
    id obj = [super forwardingTargetForSelector:aSelector];
    
    return obj;
}

再次执行看结果:

---veryitman--- 1--- +resolveInstanceMethod veryTestMethod
---veryitman--- 2--- -forwardingTargetForSelector
---veryitman--- 3--- -methodSignatureForSelector
---veryitman--- 1--- +resolveInstanceMethod: _forwardStackInvocation:
---veryitman--- 4--- -forwardInvocation
---veryitman--- veryTestMethod

注意: 这里在 3后面会多了一个 1--- resolveInstanceMethod 的打印, 是系统调用的, 此时对应的 sel 是 _forwardStackInvocation.

如果不去重写 methodSignatureForSelector 打印结果如下:

1
2
3

---veryitman--- 1--- +resolveInstanceMethod
---veryitman--- 2--- -forwardingTargetForSelector
---veryitman--- 5--- -doesNotRecognizeSelector: veryTestMethod

参考文档

1.Apple RunTime 源码 objc4-723.tar.gz

2.Message Forwarding

发送不存在的消息

resolveInstanceMethod 转发

自定义被转发的对象

消息转发实践

参考文档

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