如果深入学习ios Runtime，不得不提到消息转发，很多框架的实现都基于这一功能实现（例如JSPatch）
虽然看了很多篇关于消息转发的文章，但是理解的不是很透彻，还是自己实践一些理解能更加透彻一下。
首先我自己定义了一个MyString继承NSString
@interface MyString : NSString @end @implementation MyString @end
然后创建一个MyString，通过performSelector调用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];
}
如果什么都不写，这样肯定会crash，会出现这个错误[NSObject(NSObject) doesNotRecognizeSelector:]，因为MyString没有这三个方法，而且父类也没有。
如果没有找到方法，系统会尝试进行补救，看看有没有能处理的能力，首先会调用resolveInstanceMethod这个方法，这个方法默认是返回NO，走一下步流程，如果返回YES，例如下面方法的实现，如果传入的sel的名称是MissMethod开头，则认为我们的类是可以处理这个方法的。而且如果是MissMethod方法，我们就给这个类添加一个方法dynamicMethodIMP，作为MissMethod的实现。此时当外界调用MissMethod时，其实相当于调用dynamicMethodIMP
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p.p2 { margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Menlo; color: #d12f1b }
span.s1 { color: #ba2da2 }
span.s2 { }
span.s3 { color: #000000 }
span.s4 { color: #3e1e81 }
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，下面怎么办？首先我定义了一个类MyString2作为接盘侠，实现了MissMethod1，MissMethod2方法
@interface MyString2 : NSString
@end
@implementation MyString2
- (void)MissMethod2
{
NSLog(@"MissMethod2");
}
- (void)MissMethod3
{
NSLog(@"MissMethod3");
}
@end
当resolveInstanceMethod方法返回NO了，系统会尝试调用下面方法，看看有没有接盘侠来接这个锅，_str2是MyString2的一个示例，而且实现了MissMethod2方法。当MyString的示例调用MissMethod2方法，MissMethod2->resolveInstanceMethod(NO)->forwardingTargetForSelector，返回一个接盘侠去给这个示例发送消息objc_sendmsg(_str2,sel)
- (id)forwardingTargetForSelector:(SEL)sel {
if(sel == @selector(MissMethod2)){
return _str2;
}
return [super forwardingTargetForSelector:sel];
}
当forwardingTargetForSelector也无法处理，返回nil时，下面会走到这个方法methodSignatureForSelector，判断sig是否为nil，如果不为nil会走forwardInvocation，最后调用forwardInvocation，如果这个方法也没有处理，做了最后尝试之后也就会抛出那个异常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];
}
}
总结一下消息转发的整个流程，大致流程MissMethod->resolveInstanceMethod(NO)->forwardingTargetForSelector(nil)->methodSignatureForSelector(sig)->forwardInvocation。
下面是完整代码实现：
//
// 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");
}
@end
void 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