简介
在前面我们知道，当我们使用xcrun 将文件编译成cpp文件的时候 就可以看到方法的本质就是消息，调用方法也就是发送消息，这就有一个很重要的函数 objc_msgSend, 下面我们就来看看 消息发送的实现
从下面的注释，我们可以知道，objc_msgSend 就是发送一个消息给类的实例对象
可以从note 处看到 那么多方法都是基于 objc_msgSend实现的
/**
* Sends a message with a simple return value to an instance of a class.
*
* @param self A pointer to the instance of the class that is to receive the message.
* @param op The selector of the method that handles the message.
* @param ...
* A variable argument list containing the arguments to the method.
*
* @return The return value of the method.
*
* @note When it encounters a method call, the compiler generates a call to one of the
* functions \c objc_msgSend, \c objc_msgSend_stret, \c objc_msgSendSuper, or \c objc_msgSendSuper_stret.
* Messages sent to an object’s superclass (using the \c super keyword) are sent using \c objc_msgSendSuper;
* other messages are sent using \c objc_msgSend. Methods that have data structures as return values
* are sent using \c objc_msgSendSuper_stret and \c objc_msgSend_stret.
*/
OBJC_EXPORT id _Nullable
objc_msgSend(id _Nullable self, SEL _Nonnull op, ...)
OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0);
一 方法的快速查找
1. objc_msgSend 的具体实现
我们在前面的编译的源码中，全局搜索 objc_msgSend ,在objc-msg-arm64.s 文件中 我们可以看到这样一段代码
objc_msgSend 函数的是用汇编实现的，
ENTRY _objc_msgSend // ENTRY 表示函数的入口，是一个格式
UNWIND _objc_msgSend, NoFrame
cmp p0, #0 // nil check and tagged pointer check
#if SUPPORT_TAGGED_POINTERS //在arm64-asm.h 可以看到 当arm64架构时是true
b.le LNilOrTagged // (MSB tagged pointer looks negative)
#else
b.eq LReturnZero
#endif
// 根据isa， 获取类对象
ldr p13, [x0] // p13 = isa
GetClassFromIsa_p16 p13 // p16 = class
LGetIsaDone:
// calls imp or objc_msgSend_uncached -- 调用imp方法实现，或者调用objc_msgSend_uncached
CacheLookup NORMAL, _objc_msgSend
#if SUPPORT_TAGGED_POINTERS
LNilOrTagged:
b.eq LReturnZero // nil check
// tagged
adrp x10, _objc_debug_taggedpointer_classes@PAGE
add x10, x10, _objc_debug_taggedpointer_classes@PAGEOFF
ubfx x11, x0, #60, #4
ldr x16, [x10, x11, LSL #3]
adrp x10, _OBJC_CLASS_$___NSUnrecognizedTaggedPointer@PAGE
add x10, x10, _OBJC_CLASS_$___NSUnrecognizedTaggedPointer@PAGEOFF
cmp x10, x16
b.ne LGetIsaDone
// ext tagged
adrp x10, _objc_debug_taggedpointer_ext_classes@PAGE
add x10, x10, _objc_debug_taggedpointer_ext_classes@PAGEOFF
ubfx x11, x0, #52, #8
ldr x16, [x10, x11, LSL #3]
b LGetIsaDone
// SUPPORT_TAGGED_POINTERS
#endif
LReturnZero:
// x0 is already zero
mov x1, #0
movi d0, #0
movi d1, #0
movi d2, #0
movi d3, #0
ret
END_ENTRY _objc_msgSend
2.CacheLookup 实现
我们详细看一下 CacheLookup 怎么定义的
从下面的注释中我们可以看到，这个方法的含义是 从类的缓存中获取方法的imp
.macro CacheLookup
//
// Restart protocol:
//
// As soon as we're past the LLookupStart$1 label we may have loaded
// an invalid cache pointer or mask.
//
// When task_restartable_ranges_synchronize() is called,
// (or when a signal hits us) before we're past LLookupEnd$1,
// then our PC will be reset to LLookupRecover$1 which forcefully
// jumps to the cache-miss codepath which have the following
// requirements:
//
// GETIMP:
// The cache-miss is just returning NULL (setting x0 to 0)
//
// NORMAL and LOOKUP:
// - x0 contains the receiver
// - x1 contains the selector
// - x16 contains the isa
// - other registers are set as per calling conventions
//
LLookupStart$1:
// 注释
// 通过isa 偏移拿到类的方法缓存中的buckets、以及occupied和mask
// p1 = SEL, p16 = isa
ldr p11, [x16, #CACHE] // p11 = mask|buckets
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
and p10, p11, #0x0000ffffffffffff // p10 = buckets
and p12, p1, p11, LSR #48 // x12 = _cmd & mask
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
and p10, p11, #~0xf // p10 = buckets
and p11, p11, #0xf // p11 = maskShift
mov p12, #0xffff
lsr p11, p12, p11 // p11 = mask = 0xffff >> p11
and p12, p1, p11 // x12 = _cmd & mask
#else
#error Unsupported cache mask storage for ARM64.
#endif
add p12, p10, p12, LSL #(1+PTRSHIFT)
// p12 = buckets + ((_cmd & mask) << (1+PTRSHIFT))
// 在 bucket 中查找方法，也就是在类的方法缓存中查找方法，找到就返回
// 也就是 CacheHit 在上面，可以看到方法实现 ，也就是方法签名，并且调用方法
ldp p17, p9, [x12] // {imp, sel} = *bucket
1: cmp p9, p1 // if (bucket->sel != _cmd)
b.ne 2f // scan more
CacheHit $0 // call or return imp
// 方法没有在缓存中找到，此时调用 CheckMiss 函数， 在CheckMiss函数实现中，又调用了 __objc_msgSend_uncached
2: // not hit: p12 = not-hit bucket
CheckMiss $0 // miss if bucket->sel == 0
cmp p12, p10 // wrap if bucket == buckets
b.eq 3f
ldp p17, p9, [x12, #-BUCKET_SIZE]! // {imp, sel} = *--bucket
b 1b // loop
3: // wrap: p12 = first bucket, w11 = mask
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
add p12, p12, p11, LSR #(48 - (1+PTRSHIFT))
// p12 = buckets + (mask << 1+PTRSHIFT)
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_LOW_4
add p12, p12, p11, LSL #(1+PTRSHIFT)
// p12 = buckets + (mask << 1+PTRSHIFT)
#else
#error Unsupported cache mask storage for ARM64.
#endif
// Clone scanning loop to miss instead of hang when cache is corrupt.
// The slow path may detect any corruption and halt later.
ldp p17, p9, [x12] // {imp, sel} = *bucket
1: cmp p9, p1 // if (bucket->sel != _cmd)
b.ne 2f // scan more
CacheHit $0 // call or return imp
2: // not hit: p12 = not-hit bucket
CheckMiss $0 // miss if bucket->sel == 0
cmp p12, p10 // wrap if bucket == buckets
b.eq 3f
ldp p17, p9, [x12, #-BUCKET_SIZE]! // {imp, sel} = *--bucket
b 1b // loop
LLookupEnd$1:
LLookupRecover$1:
3: // double wrap
JumpMiss $0
.endmacro
这个方法的主体就是查找 cache_t，并且查找cache_t中是否有传进来的方法。 如果缓存中存在就执行 CacheHit,不存在就执行 CheckMiss
3. CacheHit
- 由于之前是normal 查找，所以就直接签名并返回了imp，
/********************************************************************
*
* CacheLookup NORMAL|GETIMP|LOOKUP <function>
*
* Locate the implementation for a selector in a class method cache.
*
* When this is used in a function that doesn't hold the runtime lock,
* this represents the critical section that may access dead memory.
* If the kernel causes one of these functions to go down the recovery
* path, we pretend the lookup failed by jumping the JumpMiss branch.
*
* Takes:
* x1 = selector
* x16 = class to be searched
*
* Kills:
* x9,x10,x11,x12, x17
*
* On exit: (found) calls or returns IMP
* with x16 = class, x17 = IMP
* (not found) jumps to LCacheMiss
*
********************************************************************/
#define NORMAL 0
#define GETIMP 1
#define LOOKUP 2
// CacheHit: x17 = cached IMP, x12 = address of cached IMP, x1 = SEL, x16 = isa
.macro CacheHit
.if $0 == NORMAL
TailCallCachedImp x17, x12, x1, x16 // authenticate and call imp
.elseif $0 == GETIMP
mov p0, p17
cbz p0, 9f // don't ptrauth a nil imp
AuthAndResignAsIMP x0, x12, x1, x16 // authenticate imp and re-sign as IMP
9: ret // return IMP
.elseif $0 == LOOKUP
// No nil check for ptrauth: the caller would crash anyway when they
// jump to a nil IMP. We don't care if that jump also fails ptrauth.
AuthAndResignAsIMP x17, x12, x1, x16 // authenticate imp and re-sign as IMP
ret // return imp via x17
.else
.abort oops
.endif
.endmacro
4. CheckMiss
可知，如果方法没有在缓存中找到，就执行了CheckMiss，之前的类型是 NORMAl，也就执行了 __objc_msgSend_uncached
.macro CheckMiss
// miss if bucket->sel == 0
.if $0 == GETIMP
cbz p9, LGetImpMiss
.elseif $0 == NORMAL
cbz p9, __objc_msgSend_uncached
.elseif $0 == LOOKUP
cbz p9, __objc_msgLookup_uncached
.else
.abort oops
.endif
.endmacro
二 方法的慢速查找
1. __objc_msgSend_uncached
全局搜索 ，可以看到这样一段代码， 可以看到函数内部 中有有一段说明 这不是一个C函数方法，可以在类中搜索，也就是MethodTableLookup
STATIC_ENTRY __objc_msgSend_uncached
UNWIND __objc_msgSend_uncached, FrameWithNoSaves
// THIS IS NOT A CALLABLE C FUNCTION
// Out-of-band p16 is the class to search
MethodTableLookup
TailCallFunctionPointer x17
END_ENTRY __objc_msgSend_uncached
2. MethodTableLookup
可以看到 内部调用了 lookUpImpOrForward 函数， 参数是 obj，sel，class 等
.macro MethodTableLookup
// push frame
SignLR
stp fp, lr, [sp, #-16]!
mov fp, sp
// save parameter registers: x0..x8, q0..q7
sub sp, sp, #(10*8 + 8*16)
stp q0, q1, [sp, #(0*16)]
stp q2, q3, [sp, #(2*16)]
stp q4, q5, [sp, #(4*16)]
stp q6, q7, [sp, #(6*16)]
stp x0, x1, [sp, #(8*16+0*8)]
stp x2, x3, [sp, #(8*16+2*8)]
stp x4, x5, [sp, #(8*16+4*8)]
stp x6, x7, [sp, #(8*16+6*8)]
str x8, [sp, #(8*16+8*8)]
// lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
// receiver and selector already in x0 and x1
mov x2, x16
mov x3, #3
bl _lookUpImpOrForward
// IMP in x0
mov x17, x0
// restore registers and return
ldp q0, q1, [sp, #(0*16)]
ldp q2, q3, [sp, #(2*16)]
ldp q4, q5, [sp, #(4*16)]
ldp q6, q7, [sp, #(6*16)]
ldp x0, x1, [sp, #(8*16+0*8)]
ldp x2, x3, [sp, #(8*16+2*8)]
ldp x4, x5, [sp, #(8*16+4*8)]
ldp x6, x7, [sp, #(8*16+6*8)]
ldr x8, [sp, #(8*16+8*8)]
mov sp, fp
ldp fp, lr, [sp], #16
AuthenticateLR
.endmacro
3. lookUpImpOrForward
最终我们在 objc-runtime-new.mm 文件中找到函数实现
/***********************************************************************
* lookUpImpOrForward.
* The standard IMP lookup.
* Without LOOKUP_INITIALIZE: tries to avoid +initialize (but sometimes fails)
* Without LOOKUP_CACHE: skips optimistic unlocked lookup (but uses cache elsewhere)
* Most callers should use LOOKUP_INITIALIZE and LOOKUP_CACHE
* inst is an instance of cls or a subclass thereof, or nil if none is known.
* If cls is an un-initialized metaclass then a non-nil inst is faster.
* May return _objc_msgForward_impcache. IMPs destined for external use
* must be converted to _objc_msgForward or _objc_msgForward_stret.
* If you don't want forwarding at all, use LOOKUP_NIL.
**********************************************************************/
IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
const IMP forward_imp = (IMP)_objc_msgForward_impcache;
IMP imp = nil;
Class curClass;
// 防止线程并发操作 加锁
runtimeLock.assertUnlocked();
// 判断缓存中是否存在，存在就返回imp
// Optimistic cache lookup
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
// 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();
// We don't want people to be able to craft a binary blob that looks like
// a class but really isn't one and do a CFI attack.
//
// To make these harder we want to make sure this is a class that was
// either built into the binary or legitimately registered through
// objc_duplicateClass, objc_initializeClassPair or objc_allocateClassPair.
//
// TODO: this check is quite costly during process startup.
//判断该类是否是已知类
checkIsKnownClass(cls);
// //判断当前类是否加载到内存中（isa和rw信息是否存在）。
if (slowpath(!cls->isRealized())) {
//如果不存在进行类的加载，并且会递归加载所有父类、元类。为了确定类的父类、isa链
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}
//判断类是否初始化，同样是递归判断所有
if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
// runtimeLock may have been dropped but is now locked again
// 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
}
runtimeLock.assertLocked();
curClass = cls;
// The code used to lookpu the class's cache again right after
// we take the lock but for the vast majority of the cases
// evidence shows this is a miss most of the time, hence a time loss.
//
// The only codepath calling into this without having performed some
// kind of cache lookup is class_getInstanceMethod().
// 遍历查询
for (unsigned attempts = unreasonableClassCount();;) {
// curClass method list.
//使用二分法在当前类的方法列表methodList中查找该方法
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
imp = meth->imp;
goto done;
}
//将当前类切换为父类， 如果当前类的父类是nil，也就是在NSObject中都没有找到则直接跳出循环
if (slowpath((curClass = curClass->superclass) == nil)) {
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = forward_imp;
break;
}
// Halt if there is a cycle in the superclass chain.
// 父类链中存在循环，停止
if (slowpath(--attempts == 0)) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
//在父类的方法缓存中进行查找
imp = cache_getImp(curClass, sel);
//如果父类没找到,首次会进入动态方法解析
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;
}
// 如果方法找到了 就将其加入缓存
if (fastpath(imp)) {
// Found the method in a superclass. Cache it in this class.
goto done;
}
}
// No implementation found. Try method resolver once.
// 没有找到方法实现，调用一次方法动态解析
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);
runtimeLock.unlock();
done_nolock:
if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
return nil;
}
return imp;
}
4. 方法最后一次补救机会
我们可以看到，当系统没有找到imp 实现的时候， 就会跳出for循环，进入这里的动态尝试，这是慢速查找的补救措施，
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
这段代码只执行一次,
第一进入时：behavior=3, LOOKUP_RESOLVER = 2，执行与计算behavior & LOOKUP_RESOLVER = 2，此时 if 成立，
然后behavior等于LOOKUP_RESOLVER的反，
也就是 下一次计算时是 一个数的反 与上这个数，结果一定为0，所以这个这个方法只会走一次
如果动态解析查找失， 就会 抛出forward_imp异常 ，就crash了
补充 --- 动态方法决议 解析
resolveMethod_locked
接下看我们就看看 resolveMethod_locked 函数的执行过程
static NEVER_INLINE IMP
resolveMethod_locked(id inst, SEL sel, Class cls, int behavior)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
runtimeLock.unlock();
// 动态方法决议： 重新开始查询
if (! cls->isMetaClass()) { // 对象查询，类查询
// try [cls resolveInstanceMethod:sel]
resolveInstanceMethod(inst, sel, cls);
}
else { // 元类查询
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
resolveClassMethod(inst, sel, cls);
if (!lookUpImpOrNil(inst, sel, cls)) {
resolveInstanceMethod(inst, sel, cls);
}
}
// chances are that calling the resolver have populated the cache
// so attempt using it
// 再次调用
return lookUpImpOrForward(inst, sel, cls, behavior | LOOKUP_CACHE);
}
- 函数入参
inst: 表示真正持有sel方法的类(对象方法存储在类中，类方法 存放在 元类中)
sel: 方法名
cls: 方法接收的类，也就是调用方法的类
behavior: 影响方法的调用
- 方法解析
- 加锁 -> 判定 类是否加载 -> 解锁
- 判定类是否是元类，如果不是元类。就代表方法是对象方法，所以进入对象方法查询
- 如果是类方法，就调用
resolveClassMethod查询类方法，然后lookUpImpOrNil检查是否找到imp， 如果没有找到就调用resolveInstanceMethod
意思就是，如果类方法沿着元类继承链中查找，直到找到NSObject 元类中都没有找到，接着就是NSObject本类中，其中存储的是对象方法，所以需要使用resolveInstanceMethod在查找一次
resolveInstanceMethod
static void resolveInstanceMethod(id inst, SEL sel, Class cls)
{
runtimeLock.assertUnlocked();
ASSERT(cls->isRealized());
// 获取 resolveInstanceMethod 的方法名 SEL
SEL resolve_sel = @selector(resolveInstanceMethod:);
// 检查 类对象的元类中是否有 resolveInstanceMethod 方法，如果没有就 return; 但是在根元类中默认实现了该方法, 所以不会 return
if (!lookUpImpOrNil(cls, resolve_sel, cls->ISA())) {
// Resolver not implemented.
return;
}
// 消息发送 调用一次 resolveInstanceMethod 方法
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
bool resolved = msg(cls, resolve_sel, sel);
// 再次查询一次 imp, 意思就是 如果上面的 resolveInstanceMethod 方法 实现了 sel, 我们就能拿到 imp,并将其写入 cls 的缓存
IMP imp = lookUpImpOrNil(inst, sel, cls);
// 日志收集
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
/* method lookup */
enum {
LOOKUP_INITIALIZE = 1,
LOOKUP_RESOLVER = 2,
LOOKUP_CACHE = 4,
LOOKUP_NIL = 8,
};
static inline IMP
lookUpImpOrNil(id obj, SEL sel, Class cls, int behavior = 0)
{
// behavior = 0， LOOKUP_CACHE = 4， LOOKUP_NIL = 8
return lookUpImpOrForward(obj, sel, cls, behavior | LOOKUP_CACHE | LOOKUP_NIL);
}
内部继续调用了
lookUpImpOrForward，然后behavior = 0|4|8 = 12
此时进入 lookUpImpOrForward 函数中, 以下条件就会成立， 就会在缓存中查找一次
fastpath(behavior & LOOKUP_CACHE) = 12 & 4 = 4
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
接着 (slowpath(behavior & LOOKUP_RESOLVER)) = 12 & 2 = 0 条件不成立，就不会进入动态决议方法
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
所以 lookUpImpOrNil中的 lookUpImpOrForward会循环遍历cls继承链的所有类的cache和methodList来寻找imp
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