@property属性的实质是什么?

作者: heweiming 分类: iOS基础 发布时间: 2017-02-23 18:44
属性的实质,虽然一般不需要知道,但是知道之后,你可以做更多的东西,也可以防止出现某些问题。
@property = ivar + getter + setter;

实例变量+get方法+set方法,也就是说使用@property 系统会自动生成setter和getter方法;

一丶验证:

//添加一个属性

@interface Research001 ()
@property (nonatomic, copy) NSString *name;
@end

利用class_copyPropertyList 查看类的所有属性

unsigned int count;
objc_property_t *propertyList = class_copyPropertyList([self class], &count);
for (unsigned int i = 0; i< count; i++)
{
    const char *name = property_getName(propertyList[i]);
    NSLog(@"__%@",[NSString stringWithUTF8String:name]);
    objc_property_t property = propertyList[i];
    const char *a = property_getAttributes(property);
    NSLog(@"属性信息__%@",[NSString stringWithUTF8String:a]);
}

结果:

__name
属性信息__T@"NSString",C,N,V_name

再利用class_copyMethodList查看方法列表

u_int methodCount;
NSMutableArray *methodList = [NSMutableArray array];
Method *methods = class_copyMethodList([self class], &methodCount);
for (int i = 0; i < methodCount; i++)
{
    SEL name = method_getName(methods[i]);
    NSString *strName = [NSString stringWithCString:sel_getName(name) encoding:NSUTF8StringEncoding];
    [methodList addObject:strName];
}
free(methods);

NSLog(@"方法列表:%@",methodList);

结果:

方法列表:    (
    ".cxx_destruct",
    "name",
    "viewDidLoad",
    "setName:",
)

二丶分析

属性信息__T@"NSString",C,N,V_name

这一堆信息是什么?
翻阅Objective-C Runtime Programming Guide

Declared property type encodings

得出结论:

//T@"NSString",C,N,V_name
//T 类型
//C copy
//N nonatomic
//V 实例变量
"name", "setName:", 可以从方法列表中,得出生成了set/get方法;

三丶runtime 实现

思路利用runtime.h文件的2个函数完成操作;

/** 
 * Adds a property to a class.
 * 
 * @param cls 修改的类
 * @param name 属性名字
 * @param attributes 属性数组
 * @param attributeCount 属性数组数量
 * @return y 成功,n失败
 */
OBJC_EXPORT BOOL class_addProperty(Class cls, const char *name, const objc_property_attribute_t *attributes, unsigned int attributeCount)
    OBJC_AVAILABLE(10.7, 4.3, 9.0, 1.0);
/** 
 * Adds a new method to a class with a given name and implementation.
 * 
 * @param cls The class to which to add a method.//添加方法名字
 * @param name A selector that specifies the name of the method being added.//方法名称
 * @param imp A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.//方法的实现必须至少2个参数,self 和 _cmd
 * @param types An array of characters that describe the types of the arguments to the method. //描述
 * 
 * @return YES if the method was added successfully, otherwise NO   //y成功,n失败
 *  (for example, the class already contains a method implementation with that name).
 *
 * @note class_addMethod will add an override of a superclass's implementation, //会覆盖superclass 的实现;
 *  but will not replace an existing implementation in this class. //已经存在的不会替换
 *  To change an existing implementation, use method_setImplementation.//想要改变,使用method_setImplementation方法
 */
OBJC_EXPORT BOOL class_addMethod(Class cls, SEL name, IMP imp, 
                                 const char *types) 
    OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0);

我们要生成一个myName的属性;结果要跟@property 一样;

第一步:生成属性

objc_property_attribute_t type = { "T", "@\"NSString\"" };
objc_property_attribute_t ownership = { "C", "" }; // C = copy
objc_property_attribute_t nonatomic = { "N", "" }; //nonatomic
objc_property_attribute_t backingivar  = { "V", "_myName" };//V 实例变量
objc_property_attribute_t attrs[] = { type, ownership,nonatomic, backingivar };
class_addProperty([self class], "myName", attrs, 4);

第二步:生成方法:

NSString *myNameGetter(id self, SEL _cmd) {
    Ivar ivar = class_getInstanceVariable([self class], "_privateMyName");
    return object_getIvar(self, ivar);
}
void myNameSetter(id self, SEL _cmd, NSString *newName) {
    Ivar ivar = class_getInstanceVariable([self class], "_privateMyName");
    id oldName = object_getIvar(self, ivar);
    if (oldName != newName) object_setIvar(self, ivar, [newName copy]);
}//其中 “v@:” 表示返回值和参数

if(class_addMethod([self class],  NSSelectorFromString(@"myName"), (IMP)myNameGetter, "@@:"))
{
    NSLog(@"myName get 方法添加成功");
}
else
{
    NSLog(@"myName get 方法添加失败");
}

if(class_addMethod([self class], NSSelectorFromString(@"setMyName:"), (IMP)myNameSetter, "v@:@"))
{
    NSLog(@"myName set 方法添加成功");
}
else
{
    NSLog(@"myName set 方法添加失败");
}

打印结果:

方法列表:    (
    "setMyName:",
    "myName",
    "touchesBegan:withEvent:",
    "viewDidLoad",
    ".cxx_destruct",
    "setName:",
    "name",

四丶扩展

cxx_destruct这个原来是C++跟C的东西,在ARC的里面会出现,是用来实现了自动内存的释放,如果想详细了解底层也可以看一下这篇文章:ARC下dealloc过程及.cxx_destruct的探究

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
   //偷偷调用一下
    NSLog(@"%@",[self performSelector:NSSelectorFromString(@".cxx_destruct")]); ;
}

都是空的;
ZBResearchDemo[9629:166638] (null)
ZBResearchDemo[9629:166638] (null)
ZBResearchDemo[9629:166638] (null)
1.只有在ARC下这个方法才会出现(试验代码的情况下)
2.只有当前类拥有实例变量时(不论是不是用property)这个方法才会出现,且父类的实例变量不会导致子类拥有这个方法
3.出现这个方法和变量是否被赋值,赋值成什么没有关系

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