iOS 消息发送机制
2019-08-21 本文已影响0人
熊啊熊啊熊
通过前篇知道了,消息发送最后会来到这里。整体实现是下面这样:
IMP _class_lookupMethodAndLoadCache3(id obj, SEL sel, Class cls)
{
return lookUpImpOrForward(cls, sel, obj,
YES/*initialize*/, NO/*cache*/, YES/*resolver*/);
}
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = nil; //方法实现地址
bool triedResolver = NO; //是否尝试了方法解析
runtimeLock.assertUnlocked();
// Optimistic cache lookup
if (cache) {
imp = cache_getImp(cls, sel);
if (imp) return imp;
}
runtimeLock.lock();
checkIsKnownClass(cls); //检查是否是已知类，如果是false则会抛出错误
// 下面是对类的一个初始化
if (!cls->isRealized()) {
realizeClass(cls);
}
if (initialize && !cls->isInitialized()) {
runtimeLock.unlock();
_class_initialize (_class_getNonMetaClass(cls, inst));
runtimeLock.lock();
// If sel == initialize, _class_initialize will send +initialize and
// then the messenger will send +initialize again after this
// procedure finishes. Of course, if this is not being called
// from the messenger then it won't happen. 2778172
}
retry:
runtimeLock.assertLocked();
// Try this class's cache.
// 缓存获取imp
imp = cache_getImp(cls, sel);
if (imp) goto done;
// Try this class's method lists.
{
Method meth = getMethodNoSuper_nolock(cls, sel);
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, cls);
imp = meth->imp;
goto done;
}
}
// Try superclass caches and method lists.
{
unsigned attempts = unreasonableClassCount();
for (Class curClass = cls->superclass;
curClass != nil;
curClass = curClass->superclass)
{
// Halt if there is a cycle in the superclass chain.
if (--attempts == 0) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
imp = cache_getImp(curClass, sel);
if (imp) {
if (imp != (IMP)_objc_msgForward_impcache) {
// Found the method in a superclass. Cache it in this class.
log_and_fill_cache(cls, imp, sel, inst, curClass);
goto done;
}
else {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
}
// Superclass method list.
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
imp = meth->imp;
goto done;
}
}
}
// No implementation found. Try method resolver once.
if (resolver && !triedResolver) {
runtimeLock.unlock();
_class_resolveMethod(cls, sel, inst);
runtimeLock.lock();
// Don't cache the result; we don't hold the lock so it may have
// changed already. Re-do the search from scratch instead.
triedResolver = YES;
goto retry;
}
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlock();
return imp;
}
1.首先主要看这一段
流程：
1.先在当前类缓存中查找, 没有找到则去当前类的方法列表中查找，找到直接返回,找到将方法存储到cache中
2.如果找不到则通过isa找到父类，在父类的cache里面查找，找不到则在父类的方法列表中查找,找到直接返回,找到将方法存储到cache中
3.一直找到循环3
4.如果一直到不到则进入动态方法解析
------------------- 1. 当前类方法中查找 --------------------
// 在缓存中查找
imp = cache_getImp(cls, sel);
//如果找到就直接返回函数的IMP
if (imp) goto done;
//2.在这个类的方法列表里面寻找
// Try this class's method lists.
{
//查找方法
Method meth = getMethodNoSuper_nolock(cls, sel);
if (meth) {
//存储方法到缓存中
log_and_fill_cache(cls, meth->imp, sel, inst, cls);
imp = meth->imp;
goto done;
}
}
----------------- 2. 父类查找 --------------------
// Try superclass caches and method lists.
{
// 循环在父类里面查找
unsigned attempts = unreasonableClassCount();
for (Class curClass = cls->superclass;
curClass != nil;
curClass = curClass->superclass)
{
// Halt if there is a cycle in the superclass chain.
if (--attempts == 0) {
_objc_fatal("Memory corruption in class list.");
}
//父类的cache中查找
imp = cache_getImp(curClass, sel);
if (imp) {
if (imp != (IMP)_objc_msgForward_impcache) {
// Found the method in a superclass. Cache it in this class.
log_and_fill_cache(cls, imp, sel, inst, curClass);
goto done;
}
else {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
}
//父类方法列表中查找
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
//找到存储到当前类的cache中
log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
imp = meth->imp;
goto done;
}
}
}
动态方法解析
动态方法解析可以看到
triedResolver = YES,goto retry;发现代码还会再次走一遍消息查找上面的流程,由于triedResolver = YES所以不会再进入动态方法解析
// No implementation found. Try method resolver once.
//resolver == YES 是传参过来的
// triedResolver刚进来的时候是NO
if (resolver && !triedResolver) {
runtimeLock.unlock();
_class_resolveMethod(cls, sel, inst);
runtimeLock.lock();
// Don't cache the result; we don't hold the lock so it may have
// changed already. Re-do the search from scratch instead.
triedResolver = YES;
goto retry;
}
2.动态方法解析会调用当前类的下面的方法
在方法里面动态添加方法就不会class_addMethod，就会进入到添加方法的这个里面
//对象方法进入
+ (BOOL)resolveClassMethod:(SEL)sel {
//在这里动态添加方法
// 第一个参数是object_getClass(self)
class_addMethod(object_getClass(self), sel, (IMP)c_other, "v16@0:8");
return [super resolveClassMethod:sel];
}
//类方法
+ (BOOL)resolveInstanceMethod:(SEL)sel {
//在这里动态添加方法
return [super resolveInstanceMethod:sel];
}
3.消息转发
消息转发会进入下面三个方法:
1.判断是否调用forwardingTargetForSelector 方法，如果该方法返回消息转发的接受者A，然后对返回的消息接受者发送消息objc_msgSend(A,SEL)，如果没有返回则进行下一步
2.查看是否调用methodSignatureForSelector，如果调用,则返回NSMethodSignature这个类的对象，对象里面填写方法的参数编码(例如[a test]: v16@0:8),并且还要实现forwardInvocation这个方法，如果没有实现forwardInvocation，则跳转到最后调用doesNotRecognizeSelector报没有找到方法的错，如果实现了forwardInvocation，则不会报错，还可以拿到调用方法的接受者，参数，返回值
- (id)forwardingTargetForSelector:(SEL)aSelector {
return [super forwardingTargetForSelector:aSelector];
}
-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
return [super methodSignatureForSelector:aSelector];
}
-(void)forwardInvocation:(NSInvocation *)anInvocation {
return [super forwardInvocation:anInvocation];
}
热爱生活,记录生活!
