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说一下 Runtime 的方法缓存？存储的形式、数据结构以及查找的过程？ #59

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tbfungeek opened this issue Feb 9, 2020 · 0 comments

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tbfungeek commented Feb 9, 2020

objc_class 结构体中包含了方法缓存的存储信息cache_t类型的cache属性。

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
};

cache_t结构如下所示：

struct cache_t {
    struct bucket_t *_buckets; // 散列表
    mask_t _mask; //散列表的长度 - 1
    mask_t _occupied; // 已经缓存的方法数量
}

里面最关键的属性为_buckets它实际上是一个增量扩展的哈希表结构，哈希表内部存储的是一个个bucket_t:

struct bucket_t {
private:
    MethodCacheIMP _imp;
    cache_key_t _key;
}

每个bucket_t 包含了key以及对应的函数实体地址IMP.每次查找方法的时候会将_mask与传入的key进行&操作后，作为索引值，如果发生冲突cache_next方法将索引值减1继续搜索。

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