@property 的本质是什么？
@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
Paste_Image.png
得出:
//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",
!!!cxx_destruct 是什么东西???哪来的
四丶扩展
cxx_destruct 字面意思是自毁方法?
- (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)
根据这篇博客:http://blog.jobbole.com/65028/
可以窥探一二
.cxx_destruct方法原本是为了C++对象析构的，ARC借用了这个方法插入代码实现了自动内存释放的工作
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
