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Setter和Getter原理

Posted on Edited on In Disqus:

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当我们在@property上加上strong和copy关键字以后，编译器都为我们做了什么？

1. 实例代码

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@interface HRObject : NSObject
@property (nonatomic, strong) NSString *strongName;
@property (nonatomic, copy) NSString *weakName;
@end

@implementation HRObject
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        NSLog(@"Hello, World!");
        
        HRObject *object = [HRObject new];
        object.strongName = @"strong";
        object.weakName = @"weak";
    }
    return 0;
}
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xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o arm64.cpp

2. 编译后源码

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int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_kw_5dr8tfr96kn9f2mf2nh0111r0000gp_T_main_c597e0_mi_0);

        HRObject *object = ((HRObject *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("HRObject"), sel_registerName("new"));
        ((void (*)(id, SEL, NSString *))(void *)objc_msgSend)((id)object, sel_registerName("setStrongName:"), (NSString *)&__NSConstantStringImpl__var_folders_kw_5dr8tfr96kn9f2mf2nh0111r0000gp_T_main_c597e0_mi_1);
        ((void (*)(id, SEL, NSString *))(void *)objc_msgSend)((id)object, sel_registerName("setWeakName:"), (NSString *)&__NSConstantStringImpl__var_folders_kw_5dr8tfr96kn9f2mf2nh0111r0000gp_T_main_c597e0_mi_2);

    }
    return 0;
}

可以看到我们在使用点语法的时候，最终是生成了相应的set方法setStrongName:和setWeakName:，找到这两个方法的实现，_INSTANCE_METHODS_HRObject给所有参数都申请了Setter和Getter方法。而object.weakName = @"weak";最终调用的是_I_HRObject_setWeakName_方法。

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static struct /*_method_list_t*/ {
	unsigned int entsize;  // sizeof(struct _objc_method)
	unsigned int method_count;
	struct _objc_method method_list[8];
} _OBJC_$_INSTANCE_METHODS_HRObject __attribute__ ((used, section ("__DATA,__objc_const"))) = {
	sizeof(_objc_method),
	8,
	{{(struct objc_selector *)"strongName", "@16@0:8", (void *)_I_HRObject_strongName},
	{(struct objc_selector *)"setStrongName:", "v24@0:8@16", (void *)_I_HRObject_setStrongName_},
	{(struct objc_selector *)"weakName", "@16@0:8", (void *)_I_HRObject_weakName},
	{(struct objc_selector *)"setWeakName:", "v24@0:8@16", (void *)_I_HRObject_setWeakName_},
	{(struct objc_selector *)"strongName", "@16@0:8", (void *)_I_HRObject_strongName},
	{(struct objc_selector *)"setStrongName:", "v24@0:8@16", (void *)_I_HRObject_setStrongName_},
	{(struct objc_selector *)"weakName", "@16@0:8", (void *)_I_HRObject_weakName},
	{(struct objc_selector *)"setWeakName:", "v24@0:8@16", (void *)_I_HRObject_setWeakName_}}
};

_I_HRObject_setWeakName_最终调用的是objc_setProperty

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static void _I_HRObject_setWeakName_(HRObject * self, SEL _cmd, NSString *weakName) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct HRObject, _weakName), (id)weakName, 0, 1); }

3.Runtime源码

申明

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/* Properties */

//🏆Getter方法
// Read or write an object property. Not all object properties use these.
id _Nullable objc_getProperty(id _Nullable self, SEL _Nonnull _cmd,
                 ptrdiff_t offset, BOOL atomic);

//🏆Setter方法
void objc_setProperty(id _Nullable self, SEL _Nonnull _cmd, ptrdiff_t offset,
                 id _Nullable newValue, BOOL atomic, signed char shouldCopy);

void objc_setProperty_atomic(id _Nullable self, SEL _Nonnull _cmd,
                        id _Nullable newValue, ptrdiff_t offset);

void objc_setProperty_nonatomic(id _Nullable self, SEL _Nonnull _cmd,
                           id _Nullable newValue, ptrdiff_t offset);
                           
void objc_setProperty_atomic_copy(id _Nullable self, SEL _Nonnull _cmd,
                             id _Nullable newValue, ptrdiff_t offset);

void objc_setProperty_nonatomic_copy(id _Nullable self, SEL _Nonnull _cmd,
                                id _Nullable newValue, ptrdiff_t offset);

// Read or write a non-object property. Not all uses are C structs, 
// and not all C struct properties use this.
void objc_copyStruct(void * _Nonnull dest, const void * _Nonnull src,
                ptrdiff_t size, BOOL atomic, BOOL hasStrong);

// Perform a copy of a C++ object using striped locks. Used by non-POD C++ typed atomic properties.
void objc_copyCppObjectAtomic(void * _Nonnull dest, const void * _Nonnull src,
                         void (* _Nonnull copyHelper)
                           (void * _Nonnull dest, const void * _Nonnull source))

Getter

Getter方法的逻辑比较简单，判断是否有加锁，没有的话，直接返回变量的值，有的话则先加锁。

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id objc_getProperty(id self, SEL _cmd, ptrdiff_t offset, BOOL atomic) {
    if (offset == 0) {
        return object_getClass(self);
    }

    // Retain release world
    id *slot = (id*) ((char*)self + offset);
    if (!atomic) return *slot;
    
    // Atomic retain release world
    spinlock_t& slotlock = PropertyLocks[slot];
    slotlock.lock();
    id value = objc_retain(*slot);
    slotlock.unlock();
    
    // for performance, we (safely) issue the autorelease OUTSIDE of the spinlock.
    return objc_autoreleaseReturnValue(value);
}

Setter

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void objc_setProperty_atomic(id self, SEL _cmd, id newValue, ptrdiff_t offset) {
    reallySetProperty(self, _cmd, newValue, offset, true, false, false);
}

void objc_setProperty_nonatomic(id self, SEL _cmd, id newValue, ptrdiff_t offset) {
    reallySetProperty(self, _cmd, newValue, offset, false, false, false);
}

void objc_setProperty_atomic_copy(id self, SEL _cmd, id newValue, ptrdiff_t offset) {
    reallySetProperty(self, _cmd, newValue, offset, true, true, false);
}

void objc_setProperty_nonatomic_copy(id self, SEL _cmd, id newValue, ptrdiff_t offset) {
    reallySetProperty(self, _cmd, newValue, offset, false, true, false);
}
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void objc_setProperty(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, signed char shouldCopy) {
    bool copy = (shouldCopy && shouldCopy != MUTABLE_COPY);
    bool mutableCopy = (shouldCopy == MUTABLE_COPY);
    reallySetProperty(self, _cmd, newValue, offset, atomic, copy, mutableCopy);
}
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static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy) {
    if (offset == 0) {
        object_setClass(self, newValue);
        return;
    }

    id oldValue;
    id *slot = (id*) ((char*)self + offset);

    if (copy) {
        newValue = [newValue copyWithZone:nil];
    } else if (mutableCopy) {
        newValue = [newValue mutableCopyWithZone:nil];
    } else {
        if (*slot == newValue) return;
        newValue = objc_retain(newValue);
    }

    if (!atomic) {
        oldValue = *slot;
        *slot = newValue;
    } else {
        spinlock_t& slotlock = PropertyLocks[slot];
        slotlock.lock();
        oldValue = *slot;
        *slot = newValue;        
        slotlock.unlock();
    }

    objc_release(oldValue);
}

4. 成员变量的Offset是怎么计算的？

The sequence of operations can be illustrated in the following way:

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1. 0
2. ((TYPE *)0)
3. ( ((TYPE *)0)->MEMBER )
4. &( ((TYPE *)0)->MEMBER )
5. ( (size_t) &( ((TYPE *)0)->MEMBER )
  1. 定义数字0。
  2. 将数字0转换成指向struct Type的指针。也就是把Type的起始位置转变成0。
  3. 获取结构体Type的变量MEMBER。
  4. 但是并没有使用MEMBER的值，使用的是MEMBER的内存地址。
  5. 最终又把地址转换成了size_t的类型。

因为这个结构体的起始位置被指定为0，所以MEMBER的地址转换成数字的时候，就是MEMBER基于struct的偏移量。

这个特定代码无害的原因是没有写入，甚至没有访问过内存位置。 一切都只涉及指向这些位置（而不是它们的内容）和数字的指针。 所有这些都保存在机器寄存器或通常的本地堆栈中。

How to understand “((size_t) &((TYPE *)0)->MEMBER)”?