2021年07月03日阅读 54

iOS底层 - objc_msgSend(下)

上一篇文章中我们探索了消息的快速查找(缓存)，本文我们探索慢速查找。

汇编探索回到`C++`探索

上一篇我们已经知道当快速查找找不到的时候会执行到__objc_msgSend_uncached方法，我们先看一下这个方法的源码：

	STATIC_ENTRY __objc_msgSend_uncached
	UNWIND __objc_msgSend_uncached, FrameWithNoSaves
	// THIS IS NOT A CALLABLE C FUNCTION
	// Out-of-band p15 is the class to search
	// imp
	//方法表查找
	MethodTableLookup
	//调用返回 x17
	TailCallFunctionPointer x17
	END_ENTRY __objc_msgSend_uncached
	
	.macro MethodTableLookup
	///
	SAVE_REGS MSGSEND
	// lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
	// receiver and selector already in x0 and x1
	mov	x2, x16
	mov	x3, #3
	//方法查找
	bl	_lookUpImpOrForward

	// IMP in x0 --
	//将查找到的方法传给 x17
	mov	x17, x0
	RESTORE_REGS MSGSEND
.endmacro
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因为x17是反回值，mov x17, x0是将查找结果传给x17,必然前面的代码是查找过程，或者通过名字我们也很容易猜到_lookUpImpOrForward就是具体查找的方法。我们在objc源码中全局搜索_lookUpImpOrForward,发现并没有搜索到定义方法的代码。因为汇编的方法相比c++会有前缀_，所以我们尝试去掉下划线看有没有对应c++的方法，在objc-runtime-new.mm文件的6400行，找到了方法的定义。

lookUpImpOrForward

NEVER_INLINE
IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
    ///省略代码
    //判断类是否已加载
    checkIsKnownClass(cls);
    ///加载类 本文不探讨
    cls = realizeAndInitializeIfNeeded_locked(inst, cls, behavior & LOOKUP_INITIALIZE);
    // runtimeLock may have been dropped but is now locked again
    runtimeLock.assertLocked();
    curClass = cls;
    // The code used to lookup 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().
    ///循环查找方法实现
    for (unsigned attempts = unreasonableClassCount();;) {
        if (curClass->cache.isConstantOptimizedCache(/* strict */true)) {
#if CONFIG_USE_PREOPT_CACHES
            ///共享缓存查找
            imp = cache_getImp(curClass, sel);
            if (imp) goto done_unlock;
            curClass = curClass->cache.preoptFallbackClass();
#endif
        } else {
            // curClass method list.
            // 二分法 查找当前类的方法列表
            Method meth = getMethodNoSuper_nolock(curClass, sel);
            if (meth) {
                ///找到方法 结束循环跳转到done:
                imp = meth->imp(false);
                goto done;
            }
            if (slowpath((curClass = curClass->getSuperclass()) == nil)) {
                // No implementation found, and method resolver didn't help.
                // Use forwarding.
              	/// 找到根类也没找到方法实现  进行下一步操作forward_imp 下一篇探索
                imp = forward_imp;
                break;
            }
        }
        // Halt if there is a cycle in the superclass chain.
        if (slowpath(--attempts == 0)) {
            _objc_fatal("Memory corruption in class list.");
        }
        // Superclass cache.
      	// 查找父类缓存
        imp = cache_getImp(curClass, sel);
        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)) {
            //找到done
            // Found the method in a superclass. Cache it in this class.
            goto done;
        }
      	//父类缓存没找到继续for循环 现在curClass实际是superclass
    }
    // No implementation found. Try method resolver once.
    if (slowpath(behavior & LOOKUP_RESOLVER)) {
        behavior ^= LOOKUP_RESOLVER;
        return resolveMethod_locked(inst, sel, cls, behavior);
    }
 done:
    if (fastpath((behavior & LOOKUP_NOCACHE) == 0)) {
#if CONFIG_USE_PREOPT_CACHES
        while (cls->cache.isConstantOptimizedCache(/* strict */true)) {
            cls = cls->cache.preoptFallbackClass();
        }
#endif
        //查找成功 添加到缓存
        log_and_fill_cache(cls, imp, sel, inst, curClass);
    }
 done_unlock:
    runtimeLock.unlock();
    if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
        return nil;
    }
    return imp;
}
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看代码和注释，我们可以清楚的看到方法实现的递归查找流程

查找共享缓存是否有实现
查找当前类是否有方法，如果有查找有没有分类方法有实现，如果分类有实现就返回分类的实现即imp,如果分类没有实现返回查找到的实现imp
如果当前类没有实现方法，继续查找父类的缓存是否有实现
如果父类缓存有实现，则到done:插入查找的类（注意不是父类）的缓存
如果父类没有缓存实现，就继续for循环查找父类的方法列表
最后如果找到根类还是没找到，就执行imp = forward_imp;也就是动态方法解析，动态方法解析我们下一篇探索。

二分法查找

getMethodNoSuper_nolock一步步跟进查找，我们可以看到查找遍历方法列表的方法是findMethodInSortedMethodList,它巧妙的使用了二分查找的方式遍历方法列表，源码如下

ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list, const getNameFunc &getName)
{
    ASSERT(list);

    auto first = list->begin();
    auto base = first;
    decltype(first) probe;
    uintptr_t keyValue = (uintptr_t)key;
    uint32_t count;
    for (count = list->count; count != 0; count >>= 1) {
        probe = base + (count >> 1);
        uintptr_t probeValue = (uintptr_t)getName(probe);
        //当前sel==要查找的sel
        if (keyValue == probeValue) {
            // `probe` is a match.
            // Rewind looking for the *first* occurrence of this value.
            // This is required for correct category overrides.
            // 可能有同名方法 分类里的实现优先
            while (probe > first && keyValue == (uintptr_t)getName((probe - 1))) {
                probe--;
            }
            return &*probe;
        }

        if (keyValue > probeValue) {
            base = probe + 1;
            count--;
        }
    }
    return nil;
}
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