深入探究属性 - Lab.Lawliet

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深入探究属性

2021-08-01

2026-10-12

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@interface RYModel : NSObject
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@property (nonatomic, copy) NSString *name;
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@property (nonatomic, assign) NSInteger age;
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@property (nonatomic, assign) float height;
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@property (nonatomic, strong) RYModel *father;
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@property (nonatomic, assign) BOOL isBoy;
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- (void)dosomething;
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- (void)dosomethingWith:(NSString *)title;
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+ (void)classDoSomething;
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- (NSString *)sayMyName;
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@end

一、成员变量和属性#

我们知道，属性会生成带下划线的成员变量和对应的set、get方法。我们将上面用到的RYModel，编译成C++看下。

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static NSString * _I_RYModel_name(RYModel * self, SEL _cmd) { return (*(NSString **)((char *)self + OBJC_IVAR_$_RYModel$_name)); }
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extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);
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static void _I_RYModel_setName_(RYModel * self, SEL _cmd, NSString *name) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct RYModel, _name), (id)name, 0, 1); }
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static NSInteger _I_RYModel_age(RYModel * self, SEL _cmd) { return (*(NSInteger *)((char *)self + OBJC_IVAR_$_RYModel$_age)); }
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static void _I_RYModel_setAge_(RYModel * self, SEL _cmd, NSInteger age) { (*(NSInteger *)((char *)self + OBJC_IVAR_$_RYModel$_age)) = age; }
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static float _I_RYModel_height(RYModel * self, SEL _cmd) { return (*(float *)((char *)self + OBJC_IVAR_$_RYModel$_height)); }
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static void _I_RYModel_setHeight_(RYModel * self, SEL _cmd, float height) { (*(float *)((char *)self + OBJC_IVAR_$_RYModel$_height)) = height; }
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static RYModel * _I_RYModel_father(RYModel * self, SEL _cmd) { return (*(RYModel **)((char *)self + OBJC_IVAR_$_RYModel$_father)); }
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static void _I_RYModel_setFather_(RYModel * self, SEL _cmd, RYModel *father) { (*(RYModel **)((char *)self + OBJC_IVAR_$_RYModel$_father)) = father; }
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static BOOL _I_RYModel_isBoy(RYModel * self, SEL _cmd) { return (*(BOOL *)((char *)self + OBJC_IVAR_$_RYModel$_isBoy)); }
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static void _I_RYModel_setIsBoy_(RYModel * self, SEL _cmd, BOOL isBoy) { (*(BOOL *)((char *)self + OBJC_IVAR_$_RYModel$_isBoy)) = isBoy; }

8.1 copy 修饰符#

name 属性用的是 copy 来修饰的。很容易发现它的set方法明显有不同。

他用的是 objc_setProperty ，而其他的都是直接通过内存平移对成员变量进行赋值操作。

二、objc_setProperty#

我们看下 objc_setProperty 的源码：

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/// 对象 sel 偏移量 值 原子性 是否copy
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void objc_setProperty(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, signed char shouldCopy)
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{
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    // 是否是 Copy
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    bool copy = (shouldCopy && shouldCopy != MUTABLE_COPY);
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    // 是否是 可变的Copy
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    bool mutableCopy = (shouldCopy == MUTABLE_COPY);
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    reallySetProperty(self, _cmd, newValue, offset, atomic, copy, mutableCopy);
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}

发现他实际调用了 reallySetProperty

2.1 reallySetProperty#

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static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
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{
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    if (offset == 0) {
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        // 修改 ISA
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        object_setClass(self, newValue);
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        return;
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    }
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    id oldValue;
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    // 定位（首地址+偏移量）
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    id *slot = (id*) ((char*)self + offset);
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    // Copy相关操作
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    if (copy) {
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        newValue = [newValue copyWithZone:nil];
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    } else if (mutableCopy) {
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        newValue = [newValue mutableCopyWithZone:nil];
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    } else {
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        if (*slot == newValue) return;// 相同的话不做操作
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        // 引用计数+1
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        newValue = objc_retain(newValue);
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    }
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    // 原子性相关
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    if (!atomic) {
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        oldValue = *slot;
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        *slot = newValue;
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    } else {
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      // 加锁
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        spinlock_t& slotlock = PropertyLocks[slot];
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        slotlock.lock();
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        oldValue = *slot;
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        *slot = newValue;
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        slotlock.unlock();
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    }
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    // release 旧值 引用计数 -1
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    objc_release(oldValue);
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}

三、 Set 流程图#

Set方法

四、Type Encodings 类型编码#

我们会看到这样一些 objc_selector，其中有 v24@0:8@16 类似这样的一些看不懂的字符串。他们叫类型编码

具体可以对照官方文档这里就不展开聊了

以 setName 为例分析一次：

v	24	@	0	:	8	@	16
返回值类型-void	占用大小-24	参数-id	从偏移量0开始	SEL	从偏移量8开始	参数-id	从偏移量16开始

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static struct /*_method_list_t*/ {
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  unsigned int entsize;  // sizeof(struct _objc_method)
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  unsigned int method_count;
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  struct _objc_method method_list[21];
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} _OBJC_$_INSTANCE_METHODS_RYModel __attribute__ ((used, section ("__DATA,__objc_const"))) = {
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  sizeof(_objc_method),
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  21,
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  {{(struct objc_selector *)"ry_protocolDoSomething", "v16@0:8", (void *)_I_RYModel_ry_protocolDoSomething},
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  {(struct objc_selector *)"name", "@16@0:8", (void *)_I_RYModel_name},
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  {(struct objc_selector *)"setName:", "v24@0:8@16", (void *)_I_RYModel_setName_},
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  {(struct objc_selector *)"age", "q16@0:8", (void *)_I_RYModel_age},
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  {(struct objc_selector *)"setAge:", "v24@0:8q16", (void *)_I_RYModel_setAge_},
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  {(struct objc_selector *)"height", "f16@0:8", (void *)_I_RYModel_height},
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  {(struct objc_selector *)"setHeight:", "v20@0:8f16", (void *)_I_RYModel_setHeight_},
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  {(struct objc_selector *)"father", "@16@0:8", (void *)_I_RYModel_father},
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  {(struct objc_selector *)"setFather:", "v24@0:8@16", (void *)_I_RYModel_setFather_},
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  {(struct objc_selector *)"isBoy", "c16@0:8", (void *)_I_RYModel_isBoy},
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  {(struct objc_selector *)"setIsBoy:", "v20@0:8c16", (void *)_I_RYModel_setIsBoy_},
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    ...
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  {(struct objc_selector *)"isBoy", "c16@0:8", (void *)_I_RYModel_isBoy},
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  {(struct objc_selector *)"setIsBoy:", "v20@0:8c16", (void *)_I_RYModel_setIsBoy_}}
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};

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Class-cache_t

2.1 reallySetProperty

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三、 Set 流程图

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四、Type Encodings 类型编码