在objc_msgSend:方法的快速查找的文章中，我们讲述了方法的快速查找流程，在快速查找的过程中，如果没有找到该方法，就会调用lookUpImpOrForward方法，那么就会进入到方法的慢速查找流程。
分析
我们先创建LYPerson类，并添加如下方法：
@interface LYPerson : NSObject
- (void)sayHello;
- (void)say666;
- (void)sayMaster;
- (void)sayGoodNight;
@end
@implementation LYPerson
- (void)sayHello{
NSLog(@"LGPerson say : Hello!!!");
}
-(void)say666 {
NSLog(@"66666");
}
-(void)sayGoodNight {
NSLog(@"good Night");
}
-(void)sayMaster {
NSLog(@"Master");
}
@end
并且给LYPerson添加一个Category，并且重写say666方法
@interface LYPerson (Extension)
@end
@implementation LYPerson (Extension)
- (void)say666 {
NSLog(@"888888888");
}
@end
我们分别调用这4种方法，并设置断点，进行跟踪分析
LYPerson *p = [LYPerson alloc] ;
[p sayMaster];
[p sayGoodNight];
[p sayHello];
[p say666];
1，在cache里面找不到目标imp时，会发送objc_msgSend_uncached，其实现使用汇编实现的：
STATIC_ENTRY __objc_msgSend_uncached
MethodTableLookup NORMAL // returns IMP in r12
bx r12
END_ENTRY __objc_msgSend_uncached
2，调用MethodTableLookup,也是用汇编语言实现的：
.macro MethodTableLookup
stmfd sp!, {r0-r3,r7,lr}
add r7, sp, #16
sub sp, #8 // align stack
FP_SAVE
// lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
.if $0 == NORMAL
// receiver already in r0
// selector already in r1
.else
mov r0, r1 // receiver
mov r1, r2 // selector
.endif
mov r2, r9 // class to search
mov r3, #3 // LOOKUP_INITIALIZE | LOOKUP_INITIALIZE
blx _lookUpImpOrForward // 1
mov r12, r0 // r12 = IMP
.if $0 == NORMAL
cmp r12, r12 // set eq for nonstret forwarding
.else
tst r12, r12 // set ne for stret forwarding
.endif
FP_RESTORE
add sp, #8 // align stack
ldmfd sp!, {r0-r3,r7,lr}
.endmacro
- 参数为
normal，就会调用_lookUpImpOrForward方法。
3，_lookUpImpOrForward的实现如下:
IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
const IMP forward_imp = (IMP)_objc_msgForward_impcache;
IMP imp = nil;
Class curClass;
runtimeLock.assertUnlocked();
// Optimistic cache lookup
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
.....
curClass = cls;
for (unsigned attempts = unreasonableClassCount();;) {
// curClass method list.
Method meth = getMethodNoSuper_nolock(curClass, sel); // 1
if (meth) {
imp = meth->imp;
goto done;
}
......
if (slowpath((curClass = curClass->superclass) == nil)) {
imp = forward_imp;
break;
}
//从父类的Cache里面进行查找
// Superclass cache.
imp = cache_getImp(curClass, sel); // 2
if (slowpath(imp == forward_imp)) {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
if (fastpath(imp)) {
// Found the method in a superclass. Cache it in this class.
goto done;
}
}
// No implementation found. Try method resolver once.
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
done:
log_and_fill_cache(cls, imp, sel, inst, curClass); // 3
runtimeLock.unlock();
done_nolock:
if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
return nil;
}
return imp;
}
- 1，先在
本类的methods()里面查找是否有目标方法。 - 2，如果在
本类没有找到，然后在其父类中先进行快速查找，然后进行慢速查找。 - 3，如果在
本类的method()中找到目标方法，则将该方法插入到cache中。
接下来，我们来详细分析getMethodNoSuper_nolock方法
getMethodNoSuper_nolock
其实现如下：
static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
// fixme nil cls?
// fixme nil sel?
auto const methods = cls->data()->methods();
for (auto mlists = methods.beginLists(),
end = methods.endLists();
mlists != end;
++mlists)
{
// getMethodNoSuper_nolock is the hottest
// caller of search_method_list, inlining it turns
// getMethodNoSuper_nolock into a frame-less function and eliminates
// any store from this codepath.
method_t *m = search_method_list_inline(*mlists, sel);
if (m) return m;
}
return nil;
}
ALWAYS_INLINE static method_t *
search_method_list_inline(const method_list_t *mlist, SEL sel)
{
int methodListIsFixedUp = mlist->isFixedUp();
int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);
if (fastpath(methodListIsFixedUp && methodListHasExpectedSize)) {
return findMethodInSortedMethodList(sel, mlist);
} else {
// Linear search of unsorted method list
for (auto& meth : *mlist) {
if (meth.name == sel) return &meth;
}
}
return nil;
}
ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list)
{
ASSERT(list);
const method_t * const first = &list->first;
const method_t *base = first;
const method_t *probe;
uintptr_t keyValue = (uintptr_t)key;
uint32_t count;
for (count = list->count; count != 0; count >>= 1) {
probe = base + (count >> 1); // 1
uintptr_t probeValue = (uintptr_t)probe->name;
if (keyValue == probeValue) {
// `probe` is a match.
// Rewind looking for the *first* occurrence of this value.
// This is required for correct category overrides.
while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
probe--;
}
return (method_t *)probe;
}
if (keyValue > probeValue) {
base = probe + 1;
count--;
}
}
return nil;
}
我们可以看出，查找逻辑在findMethodInSortedMethodList方法里面。接下来，我们来看下当前 methods的值分布：
1，查看方法的总数量，可以看出一共有 5个方法
(lldb) po list->count
5
2，查看list值
(lldb) p list
(const method_list_t *) $10 = 0x0000000100002040
(lldb) p *list
(const method_list_t) $11 = {
entsize_list_tt = {
entsizeAndFlags = 26
count = 5
first = {
name = "sayHello"
types = 0x0000000100000f9c "v16@0:8"
imp = 0x0000000100000cd0 (KCObjc`-[LYPerson sayHello])
}
}
}
3，分别查看每个方法
(lldb) p $11.get(0)
(method_t) $12 = {
name = "sayHello"
types = 0x0000000100000f9c "v16@0:8"
imp = 0x0000000100000cd0 (KCObjc`-[LYPerson sayHello])
}
(lldb) p $11.get(1)
(method_t) $13 = {
name = "sayMaster"
types = 0x0000000100000f9c "v16@0:8"
imp = 0x0000000100000d60 (KCObjc`-[LYPerson sayMaster])
}
(lldb) p $11.get(2)
(method_t) $14 = {
name = "sayGoodNight"
types = 0x0000000100000f9c "v16@0:8"
imp = 0x0000000100000d30 (KCObjc`-[LYPerson sayGoodNight])
}
(lldb) p $11.get(3)
(method_t) $15 = {
name = "say666"
types = 0x0000000100000f9c "v16@0:8"
imp = 0x0000000100000e00 (KCObjc`-[LYPerson(Extension) say666])
}
(lldb) p $11.get(4)
(method_t) $16 = {
name = "say666"
types = 0x0000000100000f9c "v16@0:8"
imp = 0x0000000100000d00 (KCObjc`-[LYPerson say666])
}
我们在这里可以看出分类的方法放到了类中同名方法的前面。
问题一 ：它是按照什么顺序进行排列的呢？？？
通过源码可知，它是通过比较 (uintptr_t)probe->name的值进行查找的，那我们来分别输出每个方法的该值。
(lldb) p (uintptr_t)$11.get(0).name
(uintptr_t) $18 = 4294971064
(lldb) p (uintptr_t)$11.get(1).name
(uintptr_t) $19 = 4294971073
(lldb) p (uintptr_t)$11.get(2).name
(uintptr_t) $20 = 4294971083
(lldb) p (uintptr_t)$11.get(3).name
(uintptr_t) $21 = 4294971096
(lldb) p (uintptr_t)$11.get(4).name
(uintptr_t) $22 = 4294971096
我们可以看出它是按照(uintptr_t)probe->name的升序进行排列的。
问题二：它是用什么算法进行查找的？？？
count >> 1 == count / 2，通过分析我们可知，它是通过二分查找算法进行查找的。
问题三：如果分类重写了方法，怎样才能保证返回的是分类里面的方法呢？
if (keyValue == probeValue) {
// `probe` is a match.
// Rewind looking for the *first* occurrence of this value.
// This is required for correct category overrides.
while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
probe--;
}
return (method_t *)probe;
}
在前面我们验证过，分类覆盖的方法会放在本类同名方法的前面。当找到目标probe时，会向前继续查找，直到找到在最前面的probe，这样，我们就确保了，找到的是分类里面的方法。
总结：
1，当在类中的cache中，找不到目标方法，会进入方法的慢速查找流程。
2，在类的methods里面进行查找时，使用的是二分查找算法。同时采用向前查找的策略，来保证找到是分类中的同名实现。
3，如果在 自身类中没有找到，会在其父类中先进行快速查找，然后进行慢速查找。直到class == nil为止，即在NSObject中也找不到（NSObject的父类为 nil）。
4，如果找到目标方法，将其插入到cache中。
其流程如下图所示：
objc_msgSend_慢速查找流程.png