前言
上篇文章中，我们看到在方法的慢速查找过程中，如果在缓存和方法列表中都没有找到的话，首先会调用动态方法解析，今天我们就看动态方法解析过程。
源码
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
// 。。。。 省略部分代码
// 动态方法解析
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; // 重新查找
}
// No implementation found, and method resolver didn't help.
// 消息转发
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlock();
return imp;
}
从源码可以看出，如果动态方法解析class_resolveMethod 这个方法有实现的话，就会重新查找，继续看源码
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);
}
}
}
如果cls 不是元类，走class_resolveInstanceMethod，否则走class_resolveClassMethod这个方法
/***********************************************************************
* _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;
}
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
IMP imp = lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/);
// 。。。。省略部分代码
}
/***********************************************************************
* _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());
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;
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*/);
// 。。。。省略部分代码
}
发现两个函数实现类似
- if (! lookUpImpOrNil()这一个条件判断，主要是防止程序员写的类 不是继承自NSObject，就不需要开始动态方法解析流程了。
- (typeof(msg))objc_msgSend，一个objc_msgSend消息发送SEL_resolveInstanceMethod或者SEL_resolveClassMethod。
- 在代码的注释段我们可以看到resolveInstanceMethod或者resolveClassMethod这么两个方法，意思就是告诉我们可以自己去实现父类NSObject的方法。
- 对象方法使用的是cls，而类方法使用的是class_getNonMetaClass元类
我们继续看lookUpImpOrNil,源码：
/***********************************************************************
* lookUpImpOrNil.
* Like lookUpImpOrForward, but returns nil instead of _objc_msgForward_impcache
**********************************************************************/
IMP lookUpImpOrNil(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = lookUpImpOrForward(cls, sel, inst, initialize, cache, resolver);
if (imp == _objc_msgForward_impcache) return nil;
else return imp;
}
动态方法解析后，又回到方法慢速查找流程。
代码实现
- 首先定义一个类
@interface LGStudent : LGPerson
- (void)sayHello;
+ (void)sayObjc;
@end
@implementation LGStudent
- (void)sayHello{
NSLog(@"%s",__func__);
}
+ (void)sayObjc{
NSLog(@"%s",__func__);
}
- 添加动态解析
+ (BOOL)resolveInstanceMethod:(SEL)sel{
if (sel == @selector(saySomething)) {
NSLog(@"说话了");
IMP sayHIMP = class_getMethodImplementation(self, @selector(sayHello));
Method sayHMethod = class_getInstanceMethod(self, @selector(sayHello));
const char *sayHType = method_getTypeEncoding(sayHMethod);
return class_addMethod(self, sel, sayHIMP, sayHType);
}
return [super resolveInstanceMethod:sel];
}
- 验证
LGStudent *student = [[LGStudent alloc] init];
[student performSelector:@selector(saySomething)];
输出结果为：
2020-01-06 10:24:42.225756+0800 LGTest[3568:368647] 说话了
2020-01-06 10:24:42.226262+0800 LGTest[3568:368647] -[LGStudent sayHello]
我们在看下类方法的动态解析
+ (BOOL)resolveClassMethod:(SEL)sel{
if (sel == @selector(sayLove)) {
IMP imp = class_getMethodImplementation(objc_getMetaClass("LGStudent"), @selector(sayObjc));
Method method = class_getClassMethod(objc_getMetaClass("LGStudent"), @selector(sayObjc));
const char *types = method_getTypeEncoding(method);
return class_addMethod(objc_getMetaClass("LGStudent"), sel, imp, types);
}
return [super resolveClassMethod:sel];
}
类方法是在元类中存储的，因此，添加方法的时候需要添加到类的元类里面
验证：
[LGStudent performSelector:@selector(sayLove)];
输出内容为： 2020-01-06 10:31:54.084636+0800 LGTest[3757:389847] +[LGStudent sayObjc]
总结
- 方法动态解析分为实例方法动态解析和类方法动态解析
- 实例方法的话需要实现+ (BOOL)resolveInstanceMethod:(SEL)sel，并且把方法添加到类中
- 类方法的需要实现+ (BOOL)resolveClassMethod:(SEL)sel，并且把方法添加到元类中
- 如果实现了动态方法解析，那么会重新走方法查找（慢速）流程。
