深入浅出Cocoa之消息（二）-详解动态方法决议(Dynamic Method Resolution) 【转】

序言

一，向一个对象发送该对象无法处理的消息

@interface Foo : NSObject

-(void)Bar;

@end

@implementation Foo

-(void)Bar
{
NSLog(@" >> Bar() in Foo");
}

@end

/////////////////////////////////////////////////
#import "Foo.h"

int main (int argc, const char * argv[])
{

@autoreleasepool {

Foo * foo = [[Foo alloc] init];

[foo Bar];

[foo MissMethod];

[foo release];
}
return 0;
}

Instance method '-MissMethod' not found (return type defaults to 'id')

>> Bar() in Foo

-[Foo MissMethod]: unrecognized selector sent to instance 0x10010c840

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Foo MissMethod]: unrecognized selector sent to instance 0x10010c840'

*** Call stack at first throw:
......

terminate called after throwing an instance of 'NSException'

二，动态方法决议

+ (BOOL)resolveClassMethod:(SEL)name;
+ (BOOL)resolveInstanceMethod:(SEL)name;

//
//  Foo.m
//  DeepIntoMethod
//
//  Created by 飘飘白云 on 12-11-13.
//  Copyright (c) 2012年 kesalin@gmail.com All rights reserved.
//

#import "Foo.h"
#include <objc/runtime.h>

void dynamicMethodIMP(id self, SEL _cmd) {
NSLog(@" >> dynamicMethodIMP");
}

@implementation Foo

-(void)Bar
{
NSLog(@" >> Bar() in Foo");
}

+ (BOOL)resolveInstanceMethod:(SEL)name
{
NSLog(@" >> Instance resolving %@", NSStringFromSelector(name));

if (name == @selector(MissMethod)) {
class_addMethod([self class], name, (IMP)dynamicMethodIMP, "v@:");
return YES;
}

return [super resolveInstanceMethod:name];
}

+ (BOOL)resolveClassMethod:(SEL)name
{
NSLog(@" >> Class resolving %@", NSStringFromSelector(name));

return [super resolveClassMethod:name];
}

@end

>> Bar() in Foo.

>> Instance resolving MissMethod

>> dynamicMethodIMP called.

>> Instance resolving _doZombieMe

//class_addMethod([self class], name, (IMP)dynamicMethodIMP, "v@:");

>> Bar() in Foo.

>> Instance resolving MissMethod

+[Foo resolveInstanceMethod:MissMethod] returned YES, but no new implementation of -[Foo MissMethod] was found

>> Instance resolving _doZombieMe

 objc[1223]:+[Foo resolveInstanceMethod:MissMethod] returned YES, but no new implementation of -[Foo MissMethod] was found

-[Foo MissMethod]: unrecognized selector sent to instance 0x10010c880

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Foo MissMethod]: unrecognized selector sent to instance 0x10010c880'

*** Call stack at first throw:

......

三，源码剖析

/***********************************************************************
* _class_resolveMethod
* Call +resolveClassMethod or +resolveInstanceMethod and return
* the method added or NULL.
* Assumes the method doesn't exist already.
**********************************************************************/
__private_extern__ Method _class_resolveMethod(Class cls, SEL sel)
{
Method meth = NULL;

if (_class_isMetaClass(cls)) {
meth = _class_resolveClassMethod(cls, sel);
}
if (!meth) {
meth = _class_resolveInstanceMethod(cls, sel);
}

if (PrintResolving  &&  meth) {
_objc_inform("RESOLVE: method %c[%s %s] dynamically resolved to %p",
class_isMetaClass(cls) ? '+' : '-',
class_getName(cls), sel_getName(sel),
method_getImplementation(meth));
}

return meth;
}

/***********************************************************************
* _class_resolveInstanceMethod
* Call +resolveInstanceMethod and return the method added or NULL.
* cls should be a non-meta class.
* Assumes the method doesn't exist already.
**********************************************************************/
static Method _class_resolveInstanceMethod(Class cls, SEL sel)
{
BOOL resolved;
Method meth = NULL;

// 是否实现了 resolveInstanceMethod，如果没有返回 NULL
if (!look_up_method(((id)cls)->isa, SEL_resolveInstanceMethod,
YES /*cache*/, NO /*resolver*/))
{
return NULL;
}

// 调用 resolveInstanceMethod，并获取返回值
resolved = ((BOOL(*)(id, SEL, SEL))objc_msgSend)((id)cls, SEL_resolveInstanceMethod, sel);

if (resolved) {
// 返回值为 YES，表示 resolveInstanceMethod 声称它已经成功添加实现，则再次查找 method list
// +resolveClassMethod adds to self
meth = look_up_method(cls, sel, YES/*cache*/, NO/*resolver*/);

if (!meth) {
// resolveInstanceMethod 使诈了，它声称成功添加实现了，但实际没有，给出警告信息，并返回 NULL
// Method resolver didn't add anything?
_objc_inform("+[%s resolveInstanceMethod:%s] returned YES, but "
"no new implementation of %c[%s %s] was found",
class_getName(cls),
sel_getName(sel),
class_isMetaClass(cls) ? '+' : '-',
class_getName(cls),
sel_getName(sel));
return NULL;
}
}

// 其他情况下返回 NULL
return meth;
}

四，加入消息转发

// Proxy
@interface Proxy : NSObject

-(void)MissMethod;

@end

@implementation Proxy

-(void)MissMethod
{
NSLog(@" >> MissMethod() called in Proxy.");
}

@end

// Foo
@interface Foo : NSObject

-(void)Bar;

@end

@implementation Foo

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
SEL name = [anInvocation selector];
NSLog(@" >> forwardInvocation for selector %@", NSStringFromSelector(name));

Proxy * proxy = [[[Proxy alloc] init] autorelease];
if ([proxy respondsToSelector:name]) {
[anInvocation invokeWithTarget:proxy];
}
else {
[super forwardInvocation:anInvocation];
}
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
return [Proxy instanceMethodSignatureForSelector:aSelector];
}

-(void)Bar
{
NSLog(@">> Bar() in Foo.");
}

@end

 >> Bar() in Foo.

>> forwardInvocation for selector MissMethod

>> MissMethod() called in Proxy.

@implementation Foo

+(BOOL)resolveInstanceMethod:(SEL)name
{
NSLog(@" >> Instance resolving %@", NSStringFromSelector(name));

if (name == @selector(MissMethod)) {
class_addMethod([self class], name, (IMP)dynamicMethodIMP, "v@:");
return YES;
}

return [super resolveInstanceMethod:name];
}

+(BOOL)resolveClassMethod:(SEL)name
{
NSLog(@" >> Class resolving %@", NSStringFromSelector(name));
return [super resolveClassMethod:name];
}

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
SEL name = [anInvocation selector];
NSLog(@" >> forwardInvocation for selector %@", NSStringFromSelector(name));

Proxy * proxy = [[[Proxy alloc] init] autorelease];
if ([proxy respondsToSelector:name]) {
[anInvocation invokeWithTarget:proxy];
}
else {
[super forwardInvocation:anInvocation];
}
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
return [Proxy instanceMethodSignatureForSelector:aSelector];
}

-(void)Bar
{
NSLog(@">> Bar() in Foo.");
}

@end

 >> Bar() in Foo.

>> Instance resolving MissMethod

>> dynamicMethodIMP called.

>> Instance resolving _doZombieMe

resolveInstanceMethod:

NO

//class_addMethod([self class], name, (IMP)dynamicMethodIMP, "v@:");

 >> Bar() in Foo.

>> Instance resolving MissMethod

  objc[1618]:+[Foo resolveInstanceMethod:MissMethod] returned YES, but no new implementation of -[Foo MissMethod] was found

>> forwardInvocation for selector MissMethod

>> MissMethod() called in Proxy.

>> Instance resolving _doZombieMe

五，总结

六，引用