Runtime--Instance Variables
2016-09-19 17:28
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简介
Instance Variables(以下称实例变量)指在一个对象生存时存在并且保存值的变量,其内存申请和释放和对象的创建 (alloc)和销毁(dealloc)同步进行。比如在header文件中
@interface Data : NSObject { //Instance Variables 区 } @end
或者在implementation文件中
#import "Data.h" @interface Data () { //Instance Variables 区 } @end @implementation Data @end
标识的区域都可以声明属性变量。
与Runtime
注:主要介绍ARC环境下Instance Variables(以下简称Ivar)在Runtime的操作。Ivar的官方声明
/// An opaque type that represents an instance variable. typedef struct objc_ivar *Ivar;
相关的操作有
//获取Ivar名称 const char *ivar_getName(Ivar v) ; //获取@encode() const char *ivar_getTypeEncoding(Ivar v); //获取内存偏移量 ptrdiff_t ivar_getOffset(Ivar v);
下边通过简单的代码示例展示相关用法,首先声明一个简单的类
@interface Data : NSObject { NSString *_firstName; NSString *_firstName2; } @property (nonatomic,copy)NSString *secondName; @property (nonatomic,copy)NSString *secondName2; @end
获取Ivar
获取指定的Ivar
//获取实例变量 Ivar var_Instance = class_getInstanceVariable([Data class], "_firstName"); const char *name_Instance = ivar_getName(var_Instance); NSLog(@"var_Instance:%@",[NSString stringWithUTF8String:name_Instance]); //输出 var_Instance:_firstName
Ivar相关属性的获取
const char *name_var = ivar_getName(var_Instance);//名称 const char *type_var = ivar_getTypeEncoding(var_Instance);//有对照表 ptrdiff_t offset_var = ivar_getOffset(var_Instance);//获取变量内存偏移量 NSLog(@"Ivar Name:%@ TypeEncoding:%@ Offset:%td",[NSString stringWithUTF8String:name_var],[NSString stringWithUTF8String:type_var],offset_var); //输出 Ivar Name:_firstName TypeEncoding:@"NSString" Offset:8
关于输出结果说明:
1、Name:_firstName同Ivar的名称一样
2、TypeEncoding:@”NSString”:声明的变量类型,同@encode()命令的结果;Type Encodings
3、Offset:8 在32和64位下不同,具体值跟指针变量在当前系统下占用空间有关,可以通过sizeof(<#expression-or-type#>)查看验证
获取Ivar列表
Ivar *class_copyIvarList(Class cls, unsigned int *outCount)
示例如下
/* 获取实例变量列表 */ unsigned int count = 0; Ivar *list = class_copyIvarList([Data class], &count); for (int i = 0; i < count ; i ++) { Ivar var = list[i]; const char *name_var = ivar_getName(var); const char *type_var = ivar_getTypeEncoding(var);//有对照表 ptrdiff_t offset_var = ivar_getOffset(var);//获取变量内存偏移量 NSLog(@"Ivar Name:%@ TypeEncoding:%@ Offset:%td",[NSString stringWithUTF8String:name_var],[NSString stringWithUTF8String:type_var],offset_var); } free(list);//必须free //输出 Ivar Name:_firstName TypeEncoding:@"NSString" Offset:8 Ivar Name:_firstName2 TypeEncoding:@"NSString" Offset:16 Ivar Name:_secondName TypeEncoding:@"NSString" Offset:24 Ivar Name:_secondName2 TypeEncoding:@"NSString" Offset:32
提示:在header和implementation文件中声明的Ivar,以上获取方法均可以正常获取。
Ivar Value
对于Ivar的值操作无法以下两种://获取 id object_getIvar(id obj, Ivar ivar) ; //设置 void object_setIvar(id obj, Ivar ivar, id value);
示例代码如下
Data *data = [[Data alloc] init]; data.secondName = @"secondName";//只赋值一个 //获取 Ivar var_1 = class_getInstanceVariable([Data class], "_secondName"); id value_1 = object_getIvar(data, var_1); NSLog(@"value_1:%@",value_1); //设置 NSLog(@"设置前:%@",data.secondName2); Ivar var_2 = class_getInstanceVariable([Data class], "_secondName2"); object_setIvar(data, var_2, @"secondName2"); id value = object_getIvar(data, var_2); NSLog(@"var_2:%@",value); NSLog(@"设置后:%@",data.secondName2); //输出 value_1:secondName 设置前:(null) var_2:secondName2 设置后:secondName2
添加Ivar
/** * Adds a new instance variable to a class. * * @return 如果成功返回YES,否则NO (比如Ivar已存在) * * @note 只能在objc_allocateClassPair 和 before objc_registerClassPair之间调用; * @note 不支持已有类添加Ivar。 * @note 不支持metaclass. * @note size参数 : sizeof(pointer_type) * @note alignment参数 : log2(sizeof(pointer_type)) * @note types参数 :@encode(type) */ BOOL class_addIvar(Class cls, const char *name, size_t size, uint8_t alignment, const char *types)
示例代码
Class newClass = objc_allocateClassPair([NSObject class], "Data3", 0); class_addIvar(newClass, "add", sizeof(NSString*), log2(sizeof(NSString*)), @encode(NSString)); objc_registerClassPair(newClass); unsigned int count = 0; Ivar *list = class_copyIvarList(objc_getClass("Data3"), &count); for (int i = 0; i < count ; i ++) { Ivar var = list[i]; const char *name_var = ivar_getName(var); const char *type_var = ivar_getTypeEncoding(var);//有对照表 ptrdiff_t offset_var = ivar_getOffset(var);//获取变量内存偏移量 NSLog(@"Ivar Name:%@ TypeEncoding:%@ Offset:%td",[NSString stringWithUTF8String:name_var],[NSString stringWithUTF8String:type_var],offset_var); } free(list); //输出 Ivar Name:add TypeEncoding:{NSString=#} Offset:8
此部分更深层的内容参考此文章和此问答。
此文章的其它参考资料Objective-C Runtime。
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