alloc流程

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1.探索方式

1.objc源码库直接调试

objc源码地址:

opensource.apple.com

opensource.apple.com/tarballs/

2.符号断点法

3.菜单栏 Debug->Debug Workflow->Always Show Disassembly 显示汇编代码,然后Control + Step into

image.png

2.查看alloc流程

下载完objc4-818源码, 流程图如下:

graph TD
alloc --> _objc_rootAlloc --> callAlloc--> _objc_rootAllocWithZone--> _class_createInstanceFromZone

源码如下:

1.alloc

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

2._objc_rootAlloc

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

3.callAlloc

static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__
    if (slowpath(checkNil && !cls)) return nil;
    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));
}

4._objc_rootAllocWithZone

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);
}

5._class_createInstanceFromZone

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);
    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);
    }
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    if (!zone && fast) {
        obj->initInstanceIsa(cls, hasCxxDtor);
    } else {
        // Use raw pointer isa on the assumption that they might be
        // doing something weird with the zone or RR.
        obj->initIsa(cls);
    }

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

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

_class_createInstanceFromZone 方法中有核心三个方法需要实现

  • cls->instanceSize : 计算内存大小
  • (id)calloc(1, size) : 开辟内存,返回地址指针
  • obj->initInstanceIsa :初始化指针,和类关联起来

补充内存字节对齐的原则,主要有以下三点

数据成员对齐规则: struct 或者 union 的数据成员,第一个数据成员放在offset为0的地方,以后每个数据成员存储的起始位置要从该成员大小或者成员的子成员大小(只要该成员有子成员,比如数据、结构体等)的整数倍开始(例如int在32位机中是4字节,则要从4的整数倍地址开始存储)

数据成员为结构体: 如果一个结构里有某些结构体成员,则结构体成员要从其内部最大元素大小的整数倍地址开始存储(例如:struct a里面存有struct b,b里面有char、int、double等元素,则b应该从8的整数倍开始存储)

结构体的整体对齐规则: 结构体的总大小,即sizeof的结果,必须是其内部做大成员的整数倍,不足的要补齐