iOS 類的結(jié)構(gòu)分析

1. 類的初探

在isa結(jié)構(gòu)解析中，自定義LSPerson 類繼承自NSObject,重寫成C++代碼如下

struct LSPerson_IMPL {
    struct NSObject_IMPL NSObject_IVARS;
};

struct NSObject_IMPL結(jié)構(gòu)體定義如下

struct NSObject_IMPL {
    Class isa;
};

typedef struct objc_class *Class;
Class為指向結(jié)構(gòu)體struct objc_class的指針佛嬉，所以繼承自NSObject的對(duì)象中都有isa成員變量。那么NSObject的isa來自何處呢定页？

objc_class源碼如下

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags

    class_rw_t *data() const {
        return bits.data();
    }

    ...省略的函數(shù)
}

由此可以知道struct objc_class繼承自struct objc_object

struct objc_object源碼如下

struct objc_object {
private:
    isa_t isa;

public:
    ...省略的函數(shù)
}

總結(jié)：在OC層梦湘，對(duì)象的isa指向?qū)ο蟮念愃澳龋瑢?duì)象的類繼承自NSObject魁兼，NSObject是以結(jié)構(gòu)體struct objc_class為模版創(chuàng)建的婉徘；在C++層，struct objc_class繼承自struct objc_object咐汞，struct objc_object內(nèi)部有isa成員變量盖呼。

2. 類的結(jié)構(gòu)分析

由結(jié)構(gòu)體struct objc_class源碼可知，類的信息存儲(chǔ)在class_data_bits_t bits;中化撕。
為了取到class_data_bits_t bits;几晤，需要在結(jié)構(gòu)體struct objc_class進(jìn)行地址偏移計(jì)算。

Class ISA; //結(jié)構(gòu)體指針類型 8字節(jié)
Class superclass;//結(jié)構(gòu)體指針類型 8字節(jié)
cache_t cache;

cache_t源碼

struct cache_t {
#if CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_OUTLINED
    explicit_atomic<struct bucket_t *> _buckets;
    explicit_atomic<mask_t> _mask;
#elif CACHE_MASK_STORAGE == CACHE_MASK_STORAGE_HIGH_16
    explicit_atomic<uintptr_t> _maskAndBuckets;
    mask_t _mask_unused;

    ...其他全局變量

#if __LP64__
    uint16_t _flags;
#endif
    uint16_t _occupied;

    ...其他方法

cache_t結(jié)構(gòu)體計(jì)算：explicit_atomic<uintptr_t> _maskAndBuckets; long類型 8字節(jié)植阴； mask_t _mask_unused; int類型 4字節(jié)蟹瘾； uint16_t _flags; short類型 2字節(jié)；uint16_t _occupied; short類型 2字節(jié)掠手。根據(jù)內(nèi)存對(duì)齊原則cache_t大小為16字節(jié)热芹。故為了取到class_data_bits_t bits;，需要在結(jié)構(gòu)體struct objc_class對(duì)首地址進(jìn)行偏移32字節(jié)惨撇。

探索流程：

//獲取LSPerson類首地址
(lldb) p/x LSPerson.class
(Class) $0 = 0x00000001000021c8 LSPerson

//獲取類isa中對(duì)內(nèi)存信息
(lldb) x/4gx 0x00000001000021c8
(lldb) x/4gx 0x00000001000021c8
0x1000021c8: 0x00000001000021a0 0x00007fff944ae118
0x1000021d8: 0x00007fff6cd18140 0x0000801000000000

//根據(jù)地址偏移0x1000021c8 -> 0x1000021d8 獲取bits信息
(lldb) p (class_data_bits_t *)0x1000021d8
(class_data_bits_t *) $1 = 0x00000001000021e8

//根據(jù)struct objc_class中data()函數(shù)獲取class_rw_t
(lldb) p $1->data()
(class_rw_t *) $2 = 0x00000001010224b0

(lldb) p *($2)
(class_rw_t) $4 = {
  flags = 2148007936
  witness = 0
  ro_or_rw_ext = {
    std::__1::atomic<unsigned long> = 4294975744
  }
  firstSubclass = nil
  nextSiblingClass = NSUUID
}

(lldb) p $4.methods()      //打印出對(duì)象方法列表
(lldb) p $4.properties()    //打印出屬性列表
(lldb) p $4.protocols()     //打印出協(xié)議列表

class_rw_t源碼

struct class_rw_t {
    // Be warned that Symbolication knows the layout of this structure.
    uint32_t flags;
    uint16_t witness;
#if SUPPORT_INDEXED_ISA
    uint16_t index;
#endif

    explicit_atomic<uintptr_t> ro_or_rw_ext;

    Class firstSubclass;
    Class nextSiblingClass;

private: 
    ...省去其他函數(shù)

public: 
    ...省去其他函數(shù)
    const class_ro_t *ro() const {
        auto v = get_ro_or_rwe();
        if (slowpath(v.is<class_rw_ext_t *>())) {
            return v.get<class_rw_ext_t *>()->ro;
        }
        return v.get<const class_ro_t *>();
    } 

    const method_array_t methods() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>()->methods;
        } else {
            return method_array_t{v.get<const class_ro_t *>()->baseMethods()};
        }
    }

    const property_array_t properties() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>()->properties;
        } else {
            return property_array_t{v.get<const class_ro_t *>()->baseProperties};
        }
    }

    const protocol_array_t protocols() const {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>()->protocols;
        } else {
            return protocol_array_t{v.get<const class_ro_t *>()->baseProtocols};
        }
    }
};

根據(jù)已獲取的class_rw_t，調(diào)用結(jié)構(gòu)體內(nèi)部函數(shù)methods()府寒、properties()魁衙、protocols()即可獲取類的方法列表报腔，屬性列表以及協(xié)議列表。

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