1
- (void)callNoIMPSel {
2
    NSLog(@"调用了没IMP的SEL");
3
}
4

5
+ (BOOL)resolveInstanceMethod:(SEL)sel {
6
    if (sel == @selector(noIMP)) {
7
        IMP safeIMP = class_getMethodImplementation(self, @selector(callNoIMPSel));
8
        Method method =  class_getInstanceMethod(self, @selector(callNoIMPSel));
9
        const char *type = method_getTypeEncoding(method);
10
        NSLog(@"对%@进行动态决议", NSStringFromSelector(sel));
11
        return class_addMethod(self, sel, safeIMP, type);
12
    }
13
    return [super resolveInstanceMethod:sel];
14
}

1
对noIMP进行动态决议
2
调用了没IMP的SEL

1
对noIMP进行动态决议
2
调用了没IMP的SEL
3
调用了没IMP的SEL

1
static void resolveInstanceMethod(id inst, SEL sel, Class cls)
2
{
3
    ...
4
    SEL resolve_sel = @selector(resolveInstanceMethod:);
5

6
    ...
7

8
    BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
9
    // 直接调用 objc_msgSend 调用 resolveInstanceMethod:
10
    bool resolved = msg(cls, resolve_sel, sel);
11

12
    // 动态决议处理后来到这里
13
    // Cache the result (good or bad) so the resolver doesn't fire next time.
14
    // +resolveInstanceMethod adds to self a.k.a. cls
15
    IMP imp = lookUpImpOrNilTryCache(inst, sel, cls);
16

17
        if (resolved  &&  PrintResolving) {
18
            if (imp) {
19
                ...
20
            }
21
            else {
22
                // Method resolver didn't add anything?
23
                ...
24
            }
25
        }
26
}

1
static IMP _lookUpImpTryCache(id inst, SEL sel, Class cls, int behavior)
2
{
3
    ...
4

5
    IMP imp = cache_getImp(cls, sel);
6
    if (imp != NULL) goto done;
7
#if CONFIG_USE_PREOPT_CACHES
8
    。。。
9
#endif
10
    if (slowpath(imp == NULL)) {
11
        // 再查找
12
        return lookUpImpOrForward(inst, sel, cls, behavior);
13
    }
14

15
done:
16
    if ((behavior & LOOKUP_NIL) && imp == (IMP)_objc_msgForward_impcache) {
17
        // 需要转发
18
        return nil;
19
    }
20
    return imp;
21
}

1
+ (BOOL)resolveClassMethod:(SEL)sel {
2
    if (sel == @selector(noClassIMP)) {
3
        IMP safeIMP = class_getMethodImplementation(self, @selector(callClassNoIMPSel));
4
        Method method =  class_getClassMethod(self, @selector(callClassNoIMPSel));
5
        const char *type = method_getTypeEncoding(method);
6
        NSLog(@"对%@进行动态决议（类方法）", NSStringFromSelector(sel));
7
        return class_addMethod(self, sel, safeIMP, type);
8
    }
9
    return [super resolveClassMethod:sel];
10
}

1
+ (BOOL)resolveClassMethod:(SEL)sel {
2
    if (sel == @selector(noClassIMP)) {
3
        Class meta = objc_getMetaClass("RYModel");
4
        IMP safeIMP = class_getMethodImplementation(meta, @selector(callClassNoIMPSel));
5
        Method method =  class_getClassMethod(meta, @selector(callClassNoIMPSel));
6
        const char *type = method_getTypeEncoding(method);
7
        NSLog(@"对%@进行动态决议（类方法）", NSStringFromSelector(sel));
8
        return class_addMethod(meta, sel, safeIMP, type);
9
    }
10
    return [super resolveClassMethod:sel];
11
}

1
对noClassIMP进行动态决议（类方法）
2
调用了Class没IMP的SEL

1
// try [nonMetaClass resolveClassMethod:sel]
2
// and [cls resolveInstanceMethod:sel]
3
resolveClassMethod(inst, sel, cls);
4
if (!lookUpImpOrNilTryCache(inst, sel, cls)) {
5
    resolveInstanceMethod(inst, sel, cls);
6
}

1
STATIC_ENTRY __objc_msgForward_impcache
2

3
// No stret specialization.
4
b  __objc_msgForward
5

6
END_ENTRY __objc_msgForward_impcache

1
ENTRY __objc_msgForward
2

3
adrp  x17, __objc_forward_handler@PAGE
4
ldr  p17, [x17, __objc_forward_handler@PAGEOFF]
5
TailCallFunctionPointer x17
6

7
END_ENTRY __objc_msgForward

1
#if !__OBJC2__
2

3
...
4

5
#else
6
    // Default forward handler halts the process.
7
    __attribute__((noreturn, cold)) void
8
    objc_defaultForwardHandler(id self, SEL sel)
9
    {
10
        _objc_fatal("%c[%s %s]: unrecognized selector sent to instance %p "
11
                    "(no message forward handler is installed)",
12
                    class_isMetaClass(object_getClass(self)) ? '+' : '-',
13
                    object_getClassName(self), sel_getName(sel), self);
14
    }
15
    void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
16

17
    #if SUPPORT_STRET (ARM64 = 0)
18
    ...
19
    #endif
20

21
#endif
22

23
void objc_setForwardHandler(void *fwd, void *fwd_stret)
24
{
25
    _objc_forward_handler = fwd;
26
#if SUPPORT_STRET
27
    _objc_forward_stret_handler = fwd_stret;
28
#endif
29
}

1
*** First throw call stack:
2
(
3
    ...
4
  3   CoreFoundation                      0x00007fff203f790b ___forwarding___ + 1448
5
  4   CoreFoundation                      0x00007fff203f72d8 _CF_forwarding_prep_0 + 120
6
    ...
7
)

1
// 一次动态决议
2
+[RYModel resolveInstanceMethod:]--noIMP
3

4
// 打印堆栈信息
5
(lldb) bt
6
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 6.1
7
  * frame #0: 0x000000010031682a libobjc.A.dylib`lookUpImpOrForward(inst=0x0000000000000000, sel="noIMP", cls=RYModel, behavior=2) at objc-runtime-new.mm:6536:9
8
    frame #1: 0x00000001002edcb9 libobjc.A.dylib`class_getInstanceMethod(cls=RYModel, sel="noIMP") at objc-runtime-new.mm:6192:5
9
    frame #2: 0x00007fff2040d653 CoreFoundation`__methodDescriptionForSelector + 276
10
    frame #3: 0x00007fff20426fa0 CoreFoundation`-[NSObject(NSObject) methodSignatureForSelector:] + 30
11
    frame #4: 0x00007fff203f74ef CoreFoundation`___forwarding___ + 396
12
    frame #5: 0x00007fff203f72d8 CoreFoundation`_CF_forwarding_prep_0 + 120
13
    frame #6: 0x00000001000034d0 KCObjcBuild`main + 64
14
    frame #7: 0x00007fff20337f5d libdyld.dylib`start + 1
15

16
// 打印变量信息
17
(lldb) po inst
18
 nil
19
(lldb) po sel
20
"noIMP"
21

22
(lldb) po cls
23
objc[3185]: mutex incorrectly locked
24
objc[3185]: mutex incorrectly locked
25
RYModel
26

27
(lldb) po behavior
28
2