概述
接著上一篇文章，我們繼續(xù)探索消息發(fā)送的后續(xù)流程
消息轉(zhuǎn)發(fā)（報(bào)錯(cuò)）
通過(guò)上一篇文章發(fā)現(xiàn)，都找不到的時(shí)候會(huì)返回一個(gè)forward_imp，
通過(guò)源碼發(fā)現(xiàn)找到_objc_msgForward_impcache，
const IMP forward_imp = (IMP)_objc_msgForward_impcache;
源碼搜索objc_msgForward_impcache
STATIC_ENTRY __objc_msgForward_impcache
// No stret specialization.
b __objc_msgForward
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
然后會(huì)跳轉(zhuǎn)objc_msgForward
知識(shí)補(bǔ)充：adrp 通過(guò)基地址 + 偏移 獲得一個(gè)字符串(全局變量)
繼續(xù)查找objc_forward_handler
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;
這里我們看到了一個(gè)iOS開(kāi)發(fā)人員都熟知的報(bào)錯(cuò)提示 unrecognized selector sent to instance，對(duì)就是它
方法動(dòng)態(tài)解析
當(dāng)然如果imp沒(méi)有找到實(shí)現(xiàn)，系統(tǒng)還不會(huì)直接立馬報(bào)錯(cuò)，還會(huì)尋找解析的機(jī)會(huì)，繼續(xù)閱讀lookimporforward還會(huì)發(fā)現(xiàn)如下代碼：
// No implementation found. Try method resolver once.
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
注釋?zhuān)喝绻麤](méi)有實(shí)現(xiàn)，會(huì)嘗試一次方法解析
繼續(xù)查看resolveMethod_locked
static NEVER_INLINE IMP
resolveMethod_locked(id inst, SEL sel, Class cls, int behavior){
...
if (! cls->isMetaClass()) {
// try [cls resolveInstanceMethod:sel]
resolveInstanceMethod(inst, sel, cls);
}
else {
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
resolveClassMethod(inst, sel, cls);
if (!lookUpImpOrNilTryCache(inst, sel, cls)) {
resolveInstanceMethod(inst, sel, cls);
}
}
...
}
- 如果是實(shí)例方法會(huì)先調(diào)用
resolveInstanceMethod，看實(shí)例方法，有沒(méi)有實(shí)現(xiàn) - 進(jìn)入
resolveClassMethod方法，發(fā)現(xiàn)其與resolveInstanceMethod實(shí)現(xiàn)十分相似；不同地方在于resolveInstanceMethod里邊需要類(lèi)中實(shí)現(xiàn)一個(gè)類(lèi)方法resolveInstanceMethod，而resolveClassMethod是需要在元類(lèi)里實(shí)現(xiàn)一個(gè)對(duì)象方法resolveClassMethod，因?yàn)轭?lèi)方法在元類(lèi)中都是以對(duì)象方法的形式存在的；那么如何在元類(lèi)中實(shí)現(xiàn)一個(gè)對(duì)象方法呢？只需要在當(dāng)前類(lèi)中實(shí)現(xiàn)類(lèi)方法resolveClassMethod即可：
實(shí)例方法動(dòng)態(tài)實(shí)現(xiàn)：
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
if (sel == @selector(teacherSay)) {
// return YES;
IMP r_teacherSay = class_getMethodImplementation(self, @selector(r_teacherSay));
Method method = class_getInstanceMethod(self, @selector(r_teacherSay));
const char *type = method_getTypeEncoding(method);
return class_addMethod(self, sel, r_teacherSay, type);
}
return NO;
}
- (void)r_teacherSay
{
NSLog(@"%s",__func__);
}
類(lèi)方法動(dòng)態(tài)解析實(shí)現(xiàn)
+ (BOOL)resolveClassMethod:(SEL)sel
{
if (sel == @selector(sayNB)) {
IMP r_teacherSay = class_getMethodImplementation(objc_getMetaClass("QHTeacher"), @selector(r_sayNB));
Method method = class_getInstanceMethod(self, @selector(r_sayNB));
const char *type = method_getTypeEncoding(method);
return class_addMethod(objc_getMetaClass("QHTeacher"), sel, r_teacherSay, type);
}
return NO;
}
+ (void)r_sayNB
{
NSLog(@"%s",__func__);
}
通過(guò)上面的源碼我們發(fā)現(xiàn)，調(diào)用類(lèi)方法的resolveClassMethod后有調(diào)用了resolveInstanceMethod，通過(guò)這個(gè)源碼和isa分析，我們可以得出一個(gè)結(jié)論，無(wú)論是調(diào)用類(lèi)方法還是實(shí)例方法，最終都會(huì)調(diào)用resolveInstanceMethod方法。這樣我們就可以通過(guò)實(shí)現(xiàn)一個(gè)NSObject的分類(lèi)，然后通過(guò)實(shí)現(xiàn)+ (BOOL)resolveInstanceMethod:(SEL)sel，來(lái)處理所有的異常。而不用再每個(gè)類(lèi)里面都去添加。
instrumentObjcMessageSends引出方法
在lookUpImpOrForward這個(gè)方法里面找到imp后會(huì)調(diào)用log_and_fill_cache這個(gè)方法，改方法會(huì)通過(guò)objcMsgLogEnabled來(lái)控制是否打印msg日志，全局搜索objcMsgLogEnabled，這個(gè)參數(shù)會(huì)由instrumentObjcMessageSends這個(gè)外部函數(shù)來(lái)控制：
extern void instrumentObjcMessageSends(BOOL flag);
int main(int argc, const char * argv[]) {
@autoreleasepool {
QHPerson *person = [QHPerson alloc];
instrumentObjcMessageSends(YES);
[person sayHello];
instrumentObjcMessageSends(NO);
NSLog(@"Hello, World!");
}
return 0;
}
```objectivec
在看看打印的方法```logMessageSend```，
```objectivec
if (objcMsgLogFD == (-1))
{
snprintf (buf, sizeof(buf), "/tmp/msgSends-%d", (int) getpid ());
objcMsgLogFD = secure_open (buf, O_WRONLY | O_CREAT, geteuid());
if (objcMsgLogFD < 0) {
// no log file - disable logging
objcMsgLogEnabled = false;
objcMsgLogFD = -1;
return true;
}
}
發(fā)現(xiàn)日志會(huì)保存到/tmp/目錄下，如下圖：
消息快速轉(zhuǎn)發(fā)forwardingTargetForSelector
通過(guò)上面的日志發(fā)現(xiàn)調(diào)用了forwardingTargetForSelector，發(fā)現(xiàn)在源碼里面也沒(méi)有找到該方法，那么查看一下蘋(píng)果文檔
翻譯：如果一個(gè)對(duì)象實(shí)現(xiàn)（或繼承）這個(gè)方法，并返回一個(gè)非nil（非self）結(jié)果，那么返回的對(duì)象將被用作新的接收方對(duì)象，消息調(diào)度將恢復(fù)到這個(gè)新對(duì)象(顯然，如果您從這個(gè)方法返回self，代碼將落入一個(gè)無(wú)限循環(huán)。）
代碼展示：
QHPerson *p = [QHPerson alloc];
[p sayHello];
- (id)forwardingTargetForSelector:(SEL)aSelector
{
return [QHTeacher alloc];
}
將p對(duì)象里面沒(méi)有實(shí)現(xiàn)的方法轉(zhuǎn)發(fā)給QHTeacher對(duì)象
消息慢速轉(zhuǎn)發(fā)forwardingTargetForSelector
翻譯：返回一個(gè)
NSMethodSignature對(duì)象，該對(duì)象包含由給定選擇器標(biāo)識(shí)的方法的描述該方法會(huì)關(guān)聯(lián)一個(gè)
forwardInvocation方法
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
if (aSelector == @selector(sayHello)) {
return [NSMethodSignature signatureWithObjCTypes:"v@:"];
}
return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
NSLog(@"%@-%@",anInvocation.target,NSStringFromSelector(anInvocation.selector));
[anInvocation invokeWithTarget:[QHStutent alloc]];
}
forwardInvocation也可以不用處理，程序也不會(huì)出現(xiàn)崩潰，但是也耗費(fèi)程序性能，也不方便快速定位問(wèn)題
