Runtime - Method

参考文档

• Type Encodings

---

Method

struct method_t {
    SEL name;
    const char *types;
    IMP imp;

    struct SortBySELAddress :
        public std::binary_function<const method_t&,
                                    const method_t&, bool>
    {
        bool operator() (const method_t& lhs,
                         const method_t& rhs)
        { return lhs.name < rhs.name; }
    };
};

源码阅读

1. 寻找 IMP（函数指针）

IMP lookUpImpOrForward(Class cls, SEL sel, id inst, 
                       bool initialize, bool cache, bool resolver)
{
    Class curClass;
    IMP imp = nil;
    Method meth;
    bool triedResolver = NO;

    runtimeLock.assertUnlocked();

    // 获取缓存中的 imp
    if (cache) {
        imp = cache_getImp(cls, sel);
        if (imp) return imp;
    }

    // realizedClass if !cls->isRealized()
    if (!cls->isRealized()) {
        rwlock_writer_t lock(runtimeLock);
        realizeClass(cls);
    }

    // initialize if !cls->isInitialized()
    if (initialize  &&  !cls->isInitialized()) {
        _class_initialize (_class_getNonMetaClass(cls, inst));
    }

 retry:
    runtimeLock.read();

    // Ignore GC selectors
    if (ignoreSelector(sel)) {
        imp = _objc_ignored_method;
        cache_fill(cls, sel, imp, inst);
        goto done;
    }

    // Try this class's cache.
    imp = cache_getImp(cls, sel);
    if (imp) goto done;

    // 获取 class 中的 method_list
    meth = getMethodNoSuper_nolock(cls, sel);
    if (meth) {
        log_and_fill_cache(cls, meth->imp, sel, inst, cls);
        imp = meth->imp;
        goto done;
    }

    // Loop 直到 superclass = nil
    curClass = cls;
    while ((curClass = curClass->superclass)) {

        // Superclass cache.
        imp = cache_getImp(curClass, sel);
        if (imp) {
            if (imp != (IMP)_objc_msgForward_impcache) {
                log_and_fill_cache(cls, imp, sel, inst, curClass);
                goto done;
            }
            else {
                // Found a forward:: entry in a superclass.
                // Stop searching, but don't cache yet; call method 
                // resolver for this class first.
                break;
            }
        }

        // 获取 Superclass 的 method_list
        meth = getMethodNoSuper_nolock(curClass, sel);
        if (meth) {
            log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
            imp = meth->imp;
            goto done;
        }
    }

    // 依旧没有找到则，尝试 resolver method
    if (resolver  &&  !triedResolver) {
        runtimeLock.unlockRead();
        _class_resolveMethod(cls, sel, inst);
        triedResolver = YES;
        goto retry;
    }

    // 方法没有实现且不支持 resolver，尝试开始转发
    imp = (IMP)_objc_msgForward_impcache;
    cache_fill(cls, sel, imp, inst);

 done:
    runtimeLock.unlockRead();
    assert(!(ignoreSelector(sel)  &&  imp != (IMP)&_objc_ignored_method));
    assert(imp != _objc_msgSend_uncached_impcache);
    return imp;
}

• 第一步: 从 class cache 中找 IMP
• 第二步: 从 objc_class->class_rw_t->methods 中找 IMP
• 第三步: Loop (curClass = curClass->superclass) 直到 superClass = nil， 从 curClass cache 中找 IMP，如果找不到，从 curClass->class_rw_t->methods 中找 IMP
• 第四步: 如果寻找不到 IMP，返回 _objc_msgSend_uncached_impcache 转发函数指针

2. Method->types 类型编码

编译器对方法的返回值和参数进行编码，例如:

char *buf1 = @encode(int **);

type encodings	描述
c	A char
i	An int
s	A short
l	A longl is treated as a 32-bit quantity on 64-bit programs.
q	A long long
C	An unsigned char
I	An unsigned int
S	An unsigned short
L	An unsigned long
Q	An unsigned long long
f	A float
d	A double
B	A C++ bool or a C99 _Bool
v	A void
*	A character string (char *)
@	An object (whether statically typed or typed id)
#	A class object (Class)
:	A method selector (SEL)
[array type]	An array
{name=type...}	A structure
(name=type...)	A union
bnum	A bit field of num bits
^type	A pointer to type
?	An unknown type (among other things, this code is used for function pointers)

type encodings	关键字
r	const
n	in
N	inout
o	out
O	bycopy
R	byref
V	oneway