OC底层探究之alloc流程-一一网

Setp1 –> 源码下载

分析alloc流程，源码必不可少，通过源码我们可以逐步查看调用流程。
- 官方源码，选择macOS ->11.3 -> objc4-824进行下载
- 大神编译好的源码

Setp2 –> alloc方法调试

方法1：下符号断点,
首先断点在alloc行, 同时下符号断点alloc（开始设置为disable，否则会先断住NSObject的alloc），commond + setp into调试获取

方法2：通过汇编调试
开启汇编调试模式

control + step into(单步调试)，确定下步调用函数objc_alloc

下符号断点

再次运行程序，又可获得一个符号断点_objc_rootAllocWithZone，依次类推获取整个调用流程

Step3 –> 源码分析

alloc入口

+ (id)alloc {
    return _objc_rootAlloc(self);
}
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点击_objc_rootAlloc

id _objc_rootAlloc(Class cls)
{
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}
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点击callAlloc

static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__ //判断是不是 objc2.0版本
    if (slowpath(checkNil && !cls)) return nil;
    //判断该类是否实现自自定义的 +allocWithZone，没有则进入if条件句
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        return _objc_rootAllocWithZone(cls, nil);
    }
#endif

    // No shortcuts available.
    if (allocWithZone) {
        return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
    }
    return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}
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点击_objc_rootAllocWithZone

NEVER_INLINE
id
_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
    // allocWithZone under __OBJC2__ ignores the zone parameter
    return _class_createInstanceFromZone(cls, 0, nil,
                                         OBJECT_CONSTRUCT_CALL_BADALLOC);
}
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点击_class_createInstanceFromZone，进入核心代码，三处核心点

static ALWAYS_INLINE id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
                              int construct_flags = OBJECT_CONSTRUCT_NONE,
                              bool cxxConstruct = true,
                              size_t *outAllocatedSize = nil)
{
    ASSERT(cls->isRealized());

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    size_t size;

    size = cls->instanceSize(extraBytes);//核心1：编译时期确定了成员变量，计算大小，为开辟内存做准备
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
        obj = (id)calloc(1, size);//核心2：开辟内存空间，返回指针
    }
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    if (!zone && fast) {
        obj->initInstanceIsa(cls, hasCxxDtor);//核心3：初始化指针，和类关联
    } else {
        // Use raw pointer isa on the assumption that they might be
        // doing something weird with the zone or RR.
        obj->initIsa(cls);//初始化isa指针和类关联
    }

    if (fastpath(!hasCxxCtor)) {
        return obj;
    }

    construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
    return object_cxxConstructFromClass(obj, cls, construct_flags);
}
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