上节我们分析了objc_msgSend的快速查找部分（汇编部分），这一节我们来分析其慢速部分，也就是c语言部分。
方法查找流程
首先我们实现以下代码：
@interface TPerson : NSObject
- (void)sayNB;
+ (void)sayHappay;
@end
@interface TStudent : TPerson
- (void)sayHello;
+ (void)sayObjc;
@end
@interface NSObject (Category)
- (void)sayMaster;
+ (void)sayEasy;
@end
以上方法均在.m文件中实现。
一、实例方法的查找流程
当我们调用以下代码的时候：
TStudent *stu = [[TStudent alloc] init];
[stu sayHello];
[stu sayNB];
[stu sayMaster];
[stu performSelector: @selector(sayHeHe)];
我们会发现前三个方法均会调用成功，但是sayHeHe会崩溃，由此，我们可以知道：
- 自己有，可以正常调用
- 自己没有没有而父类拥有，可以去调用父类的方法，依次类推，可以去查找多级父类的方法，直至根类；
- 自己没有，父类、直至根类都没有，则会崩溃。
一、类方法的查找流程
[TStudent sayHappay];
[TStudent sayObjc];
[TStudent sayEasy];
[TStudent performSelector: @selector(sayMaster)];
[TStudent performSelector: @selector(sayNB)];
[TStudent performSelector: @selector(sayHeHe)];
运行以上代码，我们会发现前四个方法会正常执行，而sayNB、sayHeHe会崩溃，由此，可以得出：
- 自己有，可以正常调用
- 自己没有没有而父类拥有，可以去调用父类的方法，依次类推，可以去查找多级父类的方法，直至根类；
- 自己没有，父类、直至根类都没有，但是对象方法有，也可以正常调用。因为类方法存在元类中，当我们去寻找方法的时候就是在元类的父类中寻找，而根元类的父类是NSObject，由于对象方法存在类中，所以sayMaster可以正常调用。
- 自己没有，父类、直至根类都没有，则会崩溃。
接上一节objc_msgSend汇编流程结束之后，我们进入下列C方法：
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;
bool triedResolver = NO;
runtimeLock.assertUnlocked();
// Optimistic cache lookup
if (cache) {
imp = cache_getImp(cls, sel);
if (imp) return imp;
}
// runtimeLock is held during isRealized and isInitialized checking
// to prevent races against concurrent realization.
// runtimeLock is held during method search to make
// method-lookup + cache-fill atomic with respect to method addition.
// Otherwise, a category could be added but ignored indefinitely because
// the cache was re-filled with the old value after the cache flush on
// behalf of the category.
runtimeLock.lock();
checkIsKnownClass(cls);
// 准备条件，将当前类、父类的相关信息罗列出列出来
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.
imp = cache_getImp(cls, sel);
if (imp) goto done;
// Try this class's method lists.
{
// 从当前类的methodList里面去查找方法
Method meth = getMethodNoSuper_nolock(cls, sel);
if (meth) {
// 找到之后写入缓存，然后返回
log_and_fill_cache(cls, meth->imp, sel, inst, cls);
imp = meth->imp;
goto done;
}
}
// 当前类没找到
// 从父类的缓存和方法列表里面找
// Try superclass caches and method lists.
{
unsigned attempts = unreasonableClassCount();
// 循环遍历当前类的父类
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.
// 从父类的缓存中查找imp
imp = cache_getImp(curClass, sel);
if (imp) {
// 缓存命中的话，如果不是 _objc_msgForward_impcache
if (imp != (IMP)_objc_msgForward_impcache) {
// Found the method in a superclass. Cache it in this class.
// 写入当前类的缓存中，然后发返回imp
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.
// 在父类的method list 里面查找
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;
}
static void checkIsKnownClass(Class cls)
{
if (!isKnownClass(cls))
_objc_fatal("Attempt to use unknown class %p.", cls);
}
cache_getImp 则是返回汇编的查找部分，用汇编实现
STATIC_ENTRY _cache_getImp
GetClassFromIsa_p16 p0
CacheLookup GETIMP
LGetImpMiss:
mov p0, #0
ret
END_ENTRY _cache_getImp
__objc_msgForward_impcache 是汇编实现：
END_ENTRY __objc_msgForward_impcache
ENTRY __objc_msgForward
adrp x17, __objc_forward_handler@PAGE
ldr p17, [x17, __objc_forward_handler@PAGEOFF]
TailCallFunctionPointer x17
END_ENTRY __objc_msgForward
抛出异常：
objc_defaultForwardHandler(id self, SEL sel)
{
_objc_fatal("%c[%s %s]: unrecognized selector sent to instance %p "
"(no message forward handler is installed)",
class_isMetaClass(object_getClass(self)) ? '+' : '-',
object_getClassName(self), sel_getName(sel), self);
}
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
void _class_resolveMethod(Class cls, SEL sel, id inst)
{
if (! cls->isMetaClass()) {
// try [cls resolveInstanceMethod:sel]
_class_resolveInstanceMethod(cls, sel, inst);
}
else {
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
_class_resolveClassMethod(cls, sel, inst);
if (!lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
_class_resolveInstanceMethod(cls, sel, inst);
}
}
}
以上代码可以总结为以下的流程图：
image
处理unrecognized selector sent to instance报错
当我们找不到某一个方法的定义/实现的时候，系统帮我们实现了resolveInstanceMethod作为补救，我们可以在该方法里面实现对崩溃的拦截和进一步处理：
- (void)showUpdateTips {
NSLog(@"-----更新-------");
}
// 动态解析没有处理
+ (BOOL)resolveInstanceMethod:(SEL)sel {
IMP updateImp = class_getMethodImplementation(self, @selector(showUpdateTips));
Method updateMethod = class_getInstanceMethod(self, @selector(showUpdateTips));
const char *type = method_getTypeEncoding(updateMethod);
return class_addMethod(self, sel, updateImp, type);
}
总结：
objc_msgSend进入c语言部分的时候，需要先判断当前类是否是isKnownClass；如果是则继续判断是否是isRealized()，如果不是realizeClass；接着判断类是否初始化，如果没有则进行初始化操作；然后从当前类的缓存获取imp，如果缓存命中则调用imp；没有命中则去类的方法列表里寻找；如果找到就写入缓存并调用；找不到就逐层级遍历当前类的父类；在父类的中寻找和当前类中寻找的原理一样，就不一一赘述；如果在所有父类种还没找到，则调用一次_class_resolveMethod(cls, sel, inst)看看是否进行动态解析处理，然后再执行一次方法查找，如果在遍历结束还是未找到，则会抛出异常unrecognized selector sent to instance。
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