iOS訊息轉寄學習筆記

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如果深入學習ios Runtime，不得不提到訊息轉寄，很多架構的實現都基於這一功能實現（例如JSPatch）

雖然看了很多篇關於訊息轉寄的文章，但是理解的不是很透徹，還是自己實踐一些理解能更加透徹一下。

首先我自己定義了一個MyString繼承NSString

@interface MyString : NSString@end@implementation MyString@end

然後建立一個MyString，通過performSelector調用MissMethod，MissMethod1，MissMethod2等方法。

- (void)testForward{    MyString *str = [[MyString alloc] init];    [str performSelector:@selector(MissMethod) withObject:nil];    [str performSelector:@selector(MissMethod2) withObject:nil];    [str performSelector:@selector(MissMethod3) withObject:nil];}

如果什麼都不寫，這樣肯定會crash，會出現這個錯誤[NSObject(NSObject) doesNotRecognizeSelector:]，因為MyString沒有這三個方法，而且父類也沒有。

如果沒有找到方法，系統會嘗試進行補救，看看有沒有能處理的能力，首先會調用resolveInstanceMethod這個方法，這個方法預設是返回NO，走一下步流程，如果返回YES，例如下面方法的實現，如果傳入的sel的名稱是MissMethod開頭，則認為我們的類是可以處理這個方法的。而且如果是MissMethod方法，我們就給這個類添加一個方法dynamicMethodIMP，作為MissMethod的實現。此時當外界調用MissMethod時，其實相當於調用dynamicMethodIMP

  void dynamicMethodIMP(id self, SEL _cmd) {

      NSLog(@" >> dynamicMethodIMP");

  }

 

+ (BOOL)resolveInstanceMethod:(SEL)name{    NSLog(@" >> Instance resolving %@", NSStringFromSelector(name));        NSString *selName = NSStringFromSelector(name);        if ([selName hasPrefix:@"MissMethod"]) {        if (name == @selector(MissMethod)) {            class_addMethod([self class], name, (IMP)dynamicMethodIMP, "[email protected]:");            return YES;        } else {            return NO;        }    }        return [super resolveInstanceMethod:name];}

如果resolveInstanceMethod方法返回NO了，下面怎麼辦？首先我定義了一個類MyString2作為接盤俠，實現了MissMethod1，MissMethod2方法

@interface MyString2 : NSString@end@implementation MyString2- (void)MissMethod2{    NSLog(@"MissMethod2");}- (void)MissMethod3{    NSLog(@"MissMethod3");}@end

當resolveInstanceMethod方法返回NO了，系統會嘗試調用下面方法，看看有沒有接盤俠來接這個鍋，_str2是MyString2的一個樣本，而且實現了MissMethod2方法。當MyString的樣本調用MissMethod2方法，MissMethod2->resolveInstanceMethod(NO)->forwardingTargetForSelector，返回一個接盤俠去給這個樣本發送訊息objc_sendmsg(_str2,sel)

- (id)forwardingTargetForSelector:(SEL)sel {    if(sel == @selector(MissMethod2)){        return _str2;    }    return [super forwardingTargetForSelector:sel];}

當forwardingTargetForSelector也無法處理，返回nil時，下面會走到這個方法methodSignatureForSelector，判斷sig是否為nil，如果不為nil會走forwardInvocation，最後調用forwardInvocation，如果這個方法也沒有處理，做了最後嘗試之後也就會拋出那個異常doesNotRecognizeSelector

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {        NSMethodSignature *sig;    sig = [_str2 methodSignatureForSelector:sel];    if (sig) {        return sig;    }    return [super methodSignatureForSelector:sel];}- (void)forwardInvocation:(NSInvocation *)invocation {    id target = nil;    if ([_str2 methodSignatureForSelector:[invocation selector]] ) {        target = _str2;        [invocation invokeWithTarget:target];    }}

總結一下訊息轉寄的整個流程，大致流程MissMethod->resolveInstanceMethod(NO)->forwardingTargetForSelector(nil)->methodSignatureForSelector(sig)->forwardInvocation。

下面是完整代碼實現：

////  MyString.m//  objc////  Created by zilong.li on 2017/8/17.////#import "MyString.h"#import <objc/runtime.h>@interface MyString2 : NSString@end@implementation MyString2- (void)MissMethod2{    NSLog(@"MissMethod2");}- (void)MissMethod3{    NSLog(@"MissMethod3");}@endvoid dynamicMethodIMP(id self, SEL _cmd) {    NSLog(@" >> dynamicMethodIMP");}@interface MyString (){    MyString2 *_str2;}@end@implementation MyString- (instancetype)init{    self = [super init];    if (self) {        _str2 = [[MyString2 alloc] init];    }    return self;}+ (BOOL)resolveInstanceMethod:(SEL)name{    NSLog(@" >> Instance resolving %@", NSStringFromSelector(name));        NSString *selName = NSStringFromSelector(name);        if ([selName hasPrefix:@"MissMethod"]) {        if (name == @selector(MissMethod)) {            class_addMethod([self class], name, (IMP)dynamicMethodIMP, "[email protected]:");            return YES;        } else {            return NO;        }    }        return [super resolveInstanceMethod:name];}- (id)forwardingTargetForSelector:(SEL)sel {    if(sel == @selector(MissMethod2)){        return _str2;    }    return [super forwardingTargetForSelector:sel];}- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {        NSMethodSignature *sig;    sig = [_str2 methodSignatureForSelector:sel];    if (sig) {        return sig;    }    return [super methodSignatureForSelector:sel];}- (void)forwardInvocation:(NSInvocation *)invocation {    id target = nil;    if ([_str2 methodSignatureForSelector:[invocation selector]] ) {        target = _str2;        [invocation invokeWithTarget:target];    }}@end

 

iOS訊息轉寄學習筆記