上篇文章我们介绍到,OC方法底层通过objc_msgSend进行消息发送。依次进行缓存快速查找imp,类的方法列表慢速查找,如果仍然没有找到目标method,那么则进入消息转发流程。
1. 动态方法决议
1.1 测试代码
在父类AKPerson定义一个实例方法和一个类方法,都不实现。然后用子类AKStudent调用这两个方法。cmd + b 运行,程序崩溃'unrecognized selector sent to instance xxx'。
@interface AKPerson : NSObject
- (void)person_resolveInstanceMethed;
+ (void)person_resolveClassMethed;
@end
@implementation AKPerson
@end
@interface AKStudent : AKPerson
@end
@implementation AKStudent
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
AKStudent *student = [AKStudent alloc];
[student person_resolveInstanceMethed];
[AKStudent person_resolveClassMethed];
}
return 0;
}
根据我们上篇文章的内容,objc_msgsend(student, @selector(person_resolveInstanceMethed:))会在慢速流程中的lookUpImpOrForward方法中,遍历父类没有找到method,来到动态方法决议。
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;
}
_class_resolveMethod方法内部,会根据当前类是否为元类,来判断当前所执行的方法是对象方法还是类方法。注意此时triedResolve = NO
/***********************************************************************
* _class_resolveMethod
* Call +resolveClassMethod or +resolveInstanceMethod.
* Returns nothing; any result would be potentially out-of-date already.
* Does not check if the method already exists.
**********************************************************************/
void _class_resolveMethod(Class cls, SEL sel, id inst)
{
if (! cls->isMetaClass()) { //判断类不是元类
// try [cls resolveInstanceMethod:sel]
// 调用对象方法解析
_class_resolveInstanceMethod(cls, sel, inst);
}
else {
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
// 调用类方法解析
_class_resolveClassMethod(cls, sel, inst);
// 再次查找下方法,如果没有的话,就再转发一下resolveInstanceMethod方法
if (!lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
_class_resolveInstanceMethod(cls, sel, inst);
}
}
}
1.2 实例方法
/***********************************************************************
* _class_resolveInstanceMethod
* Call +resolveInstanceMethod, looking for a method to be added to class cls.
* cls may be a metaclass or a non-meta class.
* Does not check if the method already exists.
**********************************************************************/
static void _class_resolveInstanceMethod(Class cls, SEL sel, id inst)
{
if (! lookUpImpOrNil(cls->ISA(), SEL_resolveInstanceMethod, cls,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
// Resolver not implemented.
return;
}
//发送SEL_resolveInstanceMethod消息,
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
bool resolved = msg(cls, SEL_resolveInstanceMethod, sel);
// Cache the result (good or bad) so the resolver doesn't fire next time.
// +resolveInstanceMethod adds to self a.k.a. cls
//再次查找类中是否sel方法,因为resolveInstanceMethod方法执行后可能动态进行添加了,resolver是不要进行消息转发了
IMP imp = lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/);
if (resolved && PrintResolving) {
if (imp) {
_objc_inform("RESOLVE: method %c[%s %s] "
"dynamically resolved to %p",
cls->isMetaClass() ? '+' : '-',
cls->nameForLogging(), sel_getName(sel), imp);
}
else {
// Method resolver didn't add anything?
_objc_inform("RESOLVE: +[%s resolveInstanceMethod:%s] returned YES"
", but no new implementation of %c[%s %s] was found",
cls->nameForLogging(), sel_getName(sel),
cls->isMetaClass() ? '+' : '-',
cls->nameForLogging(), sel_getName(sel));
}
}
}
- lookUpImpOrNil检查cls中是否有SEL_resolveInstanceMethod(resolveInstanceMethod)方法。
IMP lookUpImpOrNil(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
// NO/*initialize*/, YES/*cache*/, NO/*resolver*/);
IMP imp = lookUpImpOrForward(cls, sel, inst, initialize, cache, resolver);
if (imp == _objc_msgForward_impcache) return nil;
else return imp;
}
我们的这个cls是没有这个方法的,但是其实根类NSObject已经实现了这个方法,默认返回NO。
// NSObject.mm
+ (BOOL)resolveClassMethod:(SEL)sel {
return NO;
}
+ (BOOL)resolveInstanceMethod:(SEL)sel {
return NO;
}
- 系统向当前AKPerson发送一个SEL_resolveInstanceMethod消息
- lookUpImpOrNil再次查找当前实例方法imp,找到就填充缓存,找不到就返回
- 结束动态方法决议,回到lookUpImpOrForward方法将triedResolve = YES并goto retry重新查找缓存和方法列表
动态方法决议的条件是 resolver && !triedResolver 。动态方法决议结束goto retry时,会将 triedResolve = YES,保证动态方法决议只有一次。
1.3 类方法
/***********************************************************************
* _class_resolveClassMethod
* Call +resolveClassMethod, looking for a method to be added to class cls.
* cls should be a metaclass.
* Does not check if the method already exists.
**********************************************************************/
static void _class_resolveClassMethod(Class cls, SEL sel, id inst)
{
assert(cls->isMetaClass());
// 查找下类是否实现了resolveClassMethod方法,NSObject类已经实现了
if (! lookUpImpOrNil(cls, SEL_resolveClassMethod, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
// Resolver not implemented.
return;
}
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
// 记住,此处是向元类发送resolveClassMethod消息,也就是调用resolveClassMethod方法
bool resolved = msg(_class_getNonMetaClass(cls, inst),
SEL_resolveClassMethod, sel);
// Cache the result (good or bad) so the resolver doesn`t fire next time.
// +resolveClassMethod adds to self->ISA() a.k.a. cls
IMP imp = lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/);
if (resolved && PrintResolving) {
if (imp) {
_objc_inform("RESOLVE: method %c[%s %s] "
"dynamically resolved to %p",
cls->isMetaClass() ? '+' : '-',
cls->nameForLogging(), sel_getName(sel), imp);
}
else {
// Method resolver didn`t add anything?
_objc_inform("RESOLVE: +[%s resolveClassMethod:%s] returned YES"
", but no new implementation of %c[%s %s] was found",
cls->nameForLogging(), sel_getName(sel),
cls->isMetaClass() ? '+' : '-',
cls->nameForLogging(), sel_getName(sel));
}
}
}
类方法的cls是元类。 类方法前3步与实例方法基本一致,不同的是在结束SEL_resolveClassMethod时,l ookUpImpOrNil会查找sel的imp,若有imp则退出动态方法决议,若无则进入_class_resolveInstanceMethod。
else {
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
// 调用类方法解析
_class_resolveClassMethod(cls, sel, inst);
if (!lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
_class_resolveInstanceMethod(cls, sel, inst);
}
}
为什么还要再走一遍_class_resolveInstanceMethod呢?
类方法会以实例方法的形式存在元类中。这里的cls是元类,ins是类对象。在_class_resolveInstanceMethod会沿着isa走位图((类方法 -> [元类 - 根元类 - NSObject]))遍历查找有没有以实例方法形式存储的类方法。
1.4 动态方法决议
Objective C 提供了一种名为动态方法决议的手段,使得我们能够在运行时动态地为一个 selector 提供实现。我们仅仅要实现 +resolveInstanceMethod: 或 +resolveClassMethod: 方法,通过调用执行时函数 class_addMethod为指定的 selector 提供实现就可以。这两个方法都是 NSObject 中的类方法,其原型为:
+ (BOOL)resolveClassMethod:(SEL)name;
+ (BOOL)resolveInstanceMethod:(SEL)name;
实例方法
本类重写resolveInstanceMethod添加imp
#import "AKStudent.h"
#import <objc/message.h>
@implementation LGStudent
+ (BOOL)resolveInstanceMethod:(SEL)sel{
if (sel == @selector(doSomething)) {
NSLog(@"%s - %@ 崩溃",__func__,NSStringFromSelector(sel));
IMP resoloveIMP = class_getMethodImplementation(self, @selector(doResolve));
Method resoloveMethod = class_getInstanceMethod(self, @selector(doResolve));
const char *resoloveType = method_getTypeEncoding(resoloveMethod);
return class_addMethod(self, sel, resoloveIMP, resoloveType);
}
return [super resolveInstanceMethod:sel];
}
- (void)doResolve {
NSLog(@"reslove崩溃");
}
类方法
- 类方法可以在本类重写resolveClassMethod往元类添加imp(实例方法存在类对象中,类方法存在元类对象中)
+ (BOOL)resolveClassMethod:(SEL)sel{
// 本类重写ClassMethod
if (sel == @selector(doNOImpClassMethod)) {
NSLog(@"%s - %@ 崩溃",__func__,NSStringFromSelector(sel));
IMP classIMP = class_getMethodImplementation(objc_getMetaClass(@"AKPerson"), @selector(doResolve));
Method classMethod = class_getInstanceMethod(objc_getMetaClass(@"AKPerson"), @selector(doResolve));
const char *classType = method_getTypeEncoding(classMethod);
// 类方法在元类 objc_getMetaClass("AKPerson")
return class_addMethod(objc_getMetaClass("AKPerson"), objc_getMetaClass(@"AKPerson"), classIMP, classType);
}
return [super resolveClassMethod:sel];
}
- 或者在NSObject分类重写resolveInstanceMethod添加imp(元类的方法以实例方法存储在根元类中,由于元类和根源类由系统创建无法修改,所以只能在根元类的父类NSObject中)
+ (BOOL)resolveInstanceMethod:(SEL)sel{
// NSObject分类重写InstanceMethod
if (sel == @selector(sayLove)) {
NSLog(@"%s - %@ 崩溃",__func__,NSStringFromSelector(sel));
IMP classIMP = class_getMethodImplementation(self, @selector(doResolve));
Method classMethod = class_getInstanceMethod(self, @selector(doResolve));
const char *classType = method_getTypeEncoding(classMethod);
// 类方法在元类 objc_getMetaClass("NSObject")
return class_addMethod(objc_getMetaClass("NSObject"), sel, classIMP, classType);
}
return [super resolveClassMethod:sel];
}
总结
- 实例方法可以重写resolveInstanceMethod添加imp
- 类方法可以在本类重写resolveClassMethod往元类添加imp,或者在NSObject分类重写resolveInstanceMethod添加imp
- 动态方法解析只要在任意一步lookUpImpOrNil查找到imp就不会查找下去(本类做了动态方法决议,不会走到NSObjct分类的动态方法决议)
问题来了,把所有崩溃都在NSObjct分类中处理,用前缀区分业务逻辑,是不是一劳永逸了?
这样处理耦合度很高,要做大量的判断。又或者有人已经在在NSObject之前的类已经做了动态决议。而且SDK封装起来也需要容错率。如何解决请看下面的消息转发机制。
2. 消息快速转发
当我们在动态决议阶段不做任何处理的话,如果我们调用一个不存在的方法的时候,程序会崩溃。在崩溃信息中,崩溃之前底层还调用了forwarding和_CF_forwarding_prep_0等方法。
- 在lookupimporforward方法查找流程里,当方法找到的时候都会调用log_and_fill_cache函数用来打印并缓存方法指针。
static void
log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
{
#if SUPPORT_MESSAGE_LOGGING
if (objcMsgLogEnabled) {
bool cacheIt = logMessageSend(implementer->isMetaClass(),
cls->nameForLogging(),
implementer->nameForLogging(),
sel);
if (!cacheIt) return;
}
#endif
cache_fill (cls, sel, imp, receiver);
}
- 日志会记录在/tmp/msgSends目录下,并且通过objcMsgLogEnabled变量来控制是否存储日志
bool logMessageSend(bool isClassMethod,
const char *objectsClass,
const char *implementingClass,
SEL selector)
{
char buf[ 1024 ];
// Create/open the log file--创建log日志文件
if (objcMsgLogFD == (-1))//此处为YES
{ //此处我们看到有个文件路径,猜测是日志的文件路径
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;
}
}
// Make the log entry
snprintf(buf, sizeof(buf), "%c %s %s %s\n",
isClassMethod ? '+' : '-',
objectsClass,
implementingClass,
sel_getName(selector));
objcMsgLogLock.lock();
write (objcMsgLogFD, buf, strlen(buf));
objcMsgLogLock.unlock();
// Tell caller to not cache the method
return false;
}
- 全局搜索objcMsgLogEnabled,发现instrumentObjcMessageSends可以改变objcMsgLogEnabled的值。
void instrumentObjcMessageSends(BOOL flag)
{
bool enable = flag;
// Shortcut NOP
if (objcMsgLogEnabled == enable)
return;
// If enabling, flush all method caches so we get some traces
if (enable)
_objc_flush_caches(Nil);
// Sync our log file
if (objcMsgLogFD != -1)
fsync (objcMsgLogFD);
objcMsgLogEnabled = enable;
}
- 我们可以根据以下代码来记录并查看日志
extern void instrumentObjcMessageSends(BOOL flag);
int main(int argc, const char * argv[]) {
@autoreleasepool {
AKStudent *student = [[AKStudent alloc] init];
instrumentObjcMessageSends(true);
[student doNoImpMethod];
instrumentObjcMessageSends(false);
}
}
访问msgSends文件
在动态方法决议和doesNotRecognizeSelector崩溃之间,就是消息转发流程:快速流程forwardingTargetForSelector和慢速流程methodSignatureForSelector。2.1 快速转发 - forwardingTargetForSelector
forwardingTargetForSelector在源码中只有一个声明,在方法文档中找到该方法的具体解释。
forwardingTargetForSelector会返回一个对象,将自己处理不了的消息转发给该对象,询问该对象是否能够处理该消息。
- (id)forwardingTargetForSelector:(SEL)sel {
if ([AKTeacher respondsToSelector:sel]) {
return AKTeacher.class;
}
return nil;
}
@end
2.2 慢速转发 - methodSignatureForSelector
如果快速转发还不能找到method,会来到慢速转发methodSignatureForSelector。
1. -(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
- methodSignatureForSelector会返回一个方法签名NSMethodSignature
- 根据NSMethodSignature创建NSInvocation对象。
- 将NSInvocation对象作为参数传给forwardInvocation方法.
2. -(void)forwardInvocation:(NSInvocation *)anInvocation
- forwardInvocation方法类似于将消息当做事务堆放起来,方法内部将消息给能处理该消息的对象,就算不操作也不会崩溃,这里也是防崩溃的最后处理机会。
+ (void)forwardInvocation:(NSInvocation *)invocation {
[self doesNotRecognizeSelector:(invocation ? [invocation selector] : 0)];
}
- (void)forwardInvocation:(NSInvocation *)invocation {
[self doesNotRecognizeSelector:(invocation ? [invocation selector] : 0)];
}
- 异常是在doesNotRecognizeSelector方法里面抛出的,所以我们重写forwardInvocation方法后,如果不在里面执行父类的方法,程序是不会崩溃的
// Replaced by CF (throws an NSException)
+ (void)doesNotRecognizeSelector:(SEL)sel {
_objc_fatal("+[%s %s]: unrecognized selector sent to instance %p",
class_getName(self), sel_getName(sel), self);
}
// Replaced by CF (throws an NSException)
- (void)doesNotRecognizeSelector:(SEL)sel {
_objc_fatal("-[%s %s]: unrecognized selector sent to instance %p",
object_getClassName(self), sel_getName(sel), self);
}
- 慢速转发补救
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
if (NSSelectorFromString(@"saySomething") == aSelector) {
return [NSMethodSignature signatureWithObjCTypes:"v@:"]; // 把不能够处理的方法,返回一个方法签名
}
return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
// 给不同的方法,传递给不同的能够处理的类去处理
SEL aSelector = [anInvocation selector];
if ([[AKTeacher alloc] respondsToSelector:aSelector]) {
// 此时 AKTeacher 能够处理 方法 aSelector
[anInvocation invokeWithTarget:[AKTeacher alloc]];
}
else {
[super forwardInvocation:anInvocation];
}
}
3. 消息转发总结
当开发者调用了未实现的方法,苹果提供了三个解决途径:
3.1 动态方法决议:Method resolution
- runtime调用 +resolveInstanceMethod:或者 +resolveClassMethod:,让你有机会提供一个函数实现。
- 如果你添加了函数并返回YES, 那运行时系统就会重启一次消息发送<>lookupimporforward的过程。
+ (BOOL)resolveClassMethod:(SEL)sel;
+ (BOOL)resolveInstanceMethod:(SEL)sel;
3.2 快速转发:Fast forwarding
- 如果目标对象实现了-forwardingTargetForSelector:,runtime 这时就会调用这个方法,给你把这个消息转发给其他对象的机会。
- 只要这个其他对象不是nii或者self,那运行时系统就会重启一次消息发送的过程。此时,发送消息的对象变成你返回的那个其他对象。
- (id)forwardingTargetForSelector:(SEL)aSelector;
这里叫fast,只是为了区别下一步的慢速转发。因为这一步不会创建新的对象,而Normal forwarding会创建一个NSInvocation对象。
3.3 慢速转发:Normal forwarding
这一步是runtime最后一次给你挽救的机会。
- 先调用-methodSignatureForSelector:获取方法签名并创建NSInvocation对象
- 将NSInvocation对象作为参数传给-forwardInvocation方法,在其内部做消息处理。
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector;
- (void)forwardInvocation:(NSInvocation *)anInvocation;