docs: fishhook 底层原理

Chapter1 - iOS/1.87.md

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+# Objective-C 底层
+
+1. Objective-C 中对象、类主要是基于 C/C++ 中的结构体实现的。
+
+  方法一：
+
+  可以用 clang 验证。`clang -rewrite-objc main.m -o main.cpp`
+  转到指定平台代码。`xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp`
+
+  ``` 
+  struct NSObject_IMPL {
+  	Class isa;
+  };
+  
+  /// An opaque type that represents an Objective-C class.
+  typedef struct objc_class *Class;
+  ```
+
+  所以 Class 是指向结构体的指针，所以在 64 位系统中占据8个字节，32位系统占据4个字节。
+
+  NSObject 就是结构体占据8个字节。
+
+  ![image-20200726130625217](/Users/lbp/Library/Application Support/typora-user-images/image-20200726130625217.png)
+
+  ![image-20200726125926892](/Users/lbp/Library/Application Support/typora-user-images/image-20200726125926892.png)
+
+  发现 `class_getInstanceSize` 和 `malloc_size` 结果不一致。
+
+  ```c
+  // Class's ivar size rounded up to a pointer-size boundary.
+  uint32_t alignedInstanceSize() const {
+  	return word_align(unalignedInstanceSize());
+  }
+  ```
+
+  `class_getInstanceSize` 返回的是类的实例对象的成员变量的大小（四舍五入等于指针指向对象的大小，所以不精确）
+
+  ```c
+  extern size_t malloc_size(const void *ptr);
+      /* Returns size of given ptr */
+  ```
+
+  `malloc_size` 返回的就是指针指向的对象大小.
+
+  ![image-20200726130920234](/Users/lbp/Library/Application Support/typora-user-images/image-20200726130920234.png)
+
+  结论：
+
+  - 当某个类继承自 NSObject 的时候，如果没有其他属性，则这个类占据16个字节。`class_getInstanceSize` 占据 8 ， `malloc_size` 占据 16
+  - 当某个类继承自 NSObject 的时候，如果有其他属性，则这个类占据16个字节。`class_getInstanceSize` 占据 16 ， `malloc_size` 占据 16
+
+  方法二： 从源代码角度出发验证(从上大下)
+
+  ```c++
+  // NSObject.mm
+  // Replaced by ObjectAlloc
+  + (id)allocWithZone:(struct _NSZone *)zone {
+      return _objc_rootAllocWithZone(self, (malloc_zone_t *)zone);
+  }
+  
+  // objc-class-old.mm
+  id
+  _objc_rootAllocWithZone(Class cls, malloc_zone_t *zone)
+  {
+      id obj;
+  
+      if (fastpath(!zone)) {
+          obj = class_createInstance(cls, 0);
+      } else {
+          obj = class_createInstanceFromZone(cls, 0, zone);
+      }
+  
+      if (slowpath(!obj)) obj = _objc_callBadAllocHandler(cls);
+      return obj;
+  }
+  
+  // objc-class-old.mm
+  /***********************************************************************
+  * _class_createInstance.  Allocate an instance of the specified
+  * class with the specified number of bytes for indexed variables, in
+  * the default zone, using _class_createInstanceFromZone.
+  **********************************************************************/
+  static id _class_createInstance(Class cls, size_t extraBytes)
+  {
+      return _class_createInstanceFromZone (cls, extraBytes, nil);
+  }
+  
+  
+  
+  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);
+  }
+  
+  // objc-runtime-new.h
+  // Class's ivar size rounded up to a pointer-size boundary.
+  uint32_t  alignedInstanceSize() const {
+    return word_align(unalignedInstanceSize());
+  }
+  
+  // objc-runtime-new.h
+  size_t instanceSize(size_t extraBytes) const {
+    if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
+      return cache.fastInstanceSize(extraBytes);
+    }
+  
+    size_t size = alignedInstanceSize() + extraBytes;
+    // CF requires all objects be at least 16 bytes.
+    if (size < 16) size = 16;
+    return size;
+  }
+  ```
+
+  `CF requires all objects be at least 16 bytes.` 系统为 NSObject 对象分配了至少16个字节大小的空间，但是它使用了 8 个字节大小的空间用来存放 ivars（64位系统）
+
+2. 某个类继承自 NSObject 的情况，内存如何分配