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
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| static NSString * const sPerformInstanceMethodName = @"veryTestMethod";
- (void)viewDidLoad {
[super viewDidLoad];
SEL selector = NSSelectorFromString(sPerformInstanceMethodName);
SuppressPerformSelectorLeakWarning(
[self performSelector:selector withObject:nil];
);
}
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其中, SuppressPerformSelectorLeakWarning 是定义的一个宏.
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| #define SuppressPerformSelectorLeakWarning(Stuff) \
do { \
_Pragma("clang diagnostic push") \
_Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"") \
Stuff; \
_Pragma("clang diagnostic pop") \
} while (0)
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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
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|
#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);
Method method = class_getInstanceMethod(self, proxySelector);
const char *types = method_getTypeEncoding(method);
class_addMethod([self class], sel, impletor, types);
return YES;
}
return [super resolveInstanceMethod:sel];
}
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ViewController 中实现的 proxyMethod
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- (void)proxyMethod
{
NSLog(@"---veryitman--- -proxyMethod of instance's method for OC.");
}
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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.
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| ---veryitman--- 1--- +resolveInstanceMethod
---veryitman--- -proxyMethod of instance's method for OC.
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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
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| @implementation MZTempObj
- (void)veryTestMethod
{
NSLog(@"---veryitman--- veryTestMethod");
}
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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
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|
#import <objc/runtime.h>
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
NSLog(@"---veryitman--- 1--- +resolveInstanceMethod: %@", NSStringFromSelector(sel));
NSString *methodName = NSStringFromSelector(sel);
if ([@"" isEqualToString:methodName]) {
SEL proxySelector = NSSelectorFromString(@"proxyMethod");
IMP impletor = class_getMethodImplementation(self, proxySelector);
Method method = class_getInstanceMethod(self, proxySelector);
const char *types = method_getTypeEncoding(method);
class_addMethod([self class], sel, impletor, types);
return YES;
}
return [super resolveInstanceMethod:sel];
}
- (id)forwardingTargetForSelector:(SEL)aSelector
{
NSLog(@"---veryitman--- 2--- -forwardingTargetForSelector");
NSString *selectorName = NSStringFromSelector(aSelector);
if ([sPerformInstanceMethodName isEqualToString:selectorName]) {
MZTempObj *myobject = [[MZTempObj alloc] init];
return myobject;
}
id obj = [super forwardingTargetForSelector:aSelector];
return obj;
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
NSLog(@"---veryitman--- 3--- -methodSignatureForSelector");
NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
if (nil == signature) {
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));
}
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运行程序, 控制台打印结果如下:
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| ---veryitman--- 1--- +resolveInstanceMethod
---veryitman--- 2--- -forwardingTargetForSelector
---veryitman--- veryTestMethod
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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.
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| - (id)forwardingTargetForSelector:(SEL)aSelector
{
NSLog(@"---veryitman--- 2--- -forwardingTargetForSelector");
NSString *selectorName = NSStringFromSelector(aSelector);
if ([@"" isEqualToString:selectorName]) {
MZTempObj *myobject = [[MZTempObj alloc] init];
return myobject;
}
id obj = [super forwardingTargetForSelector:aSelector];
return obj;
}
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再次执行看结果:
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| ---veryitman--- 1--- +resolveInstanceMethod veryTestMethod
---veryitman--- 2--- -forwardingTargetForSelector
---veryitman--- 3--- -methodSignatureForSelector
---veryitman--- 1--- +resolveInstanceMethod: _forwardStackInvocation:
---veryitman--- 4--- -forwardInvocation
---veryitman--- veryTestMethod
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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:
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| ---veryitman--- 1--- +resolveInstanceMethod
---veryitman--- 2--- -forwardingTargetForSelector
---veryitman--- 5--- -doesNotRecognizeSelector: veryTestMethod
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参考文档
1.Apple RunTime 源码 objc4-723.tar.gz
2.Message Forwarding
推荐
OC-RunTime: 消息转发之实例方法的转发流程
OC-RunTime: 消息转发之类方法的转发流程
OC-RunTime: 总结消息转发中用到的知识点
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