• @public、@protected、@private的使用

　　　　在OC中声明一个类的时候，可以使用上面 @public、@protected、@private三个关键字声明实例的权限，例如下面的代码：

#import <Foundation/Foundation.h>@interface Person : NSObject{@public    NSString *_height;@protected    NSString *_weight;@private    NSString *_wife;}@end@interface Person (){    @public    NSString *_name;    @protected    NSString *_sex;    @private    NSString *_phoneNO;}- (void)publicMethod;@end@implementation Person- (instancetype)init{    if(self = [super init])    {        _height = @"1.8m";        _weight = @"65Kg";        _wife = @"none";        _name = @"zhangSan";        _sex  = @"man";        _phoneNO = @"1122334455";    }    return self;}- (void)publicMethod{    }@end

　　在.h文件中定义了三个实例，这三个实例的权限和public、protected、private相对应，对于一个Person类型的对象p，_name在任何地方都是可以访问的，_sex只能在Person

的子类中访问，_wife只能自己内部访问。在.m文件中定义的三个实例也与public、protected、private相对应，需要注意的是，在其它文件中定义的Person类型的对象p，都不能访问到这三个实例，只有在.m文件内部，public、protected、private才会起到对应的作用。实际上OC中实例的权限控制只是编译时的一种权限限制，在OC runtime的类结构中，实例变量是不存在权限控制的，同样类方法、实例方法也不存在权限控制。

• 对于KVC来说，无论实例是哪种权限都是可以轻松访问到的，如下面的代码：

•  @interface Student : Person@end@implementation Student+ (void)testProtected{    Person *p = [Person new];    NSLog(@"%@",p->_height);    NSLog(@"%@",p->_weight);}- (NSString *)description{    Person *p = [Person new];    NSLog(@"%@",p->_height);    NSLog(@"%@",p->_weight);//    NSLog(@"%@",p->_wife);//    [p publicMethod];//    [self publicMethod];    //return [NSString stringWithFormat:@"name == %@\nsex == %@\nphone == %@",_name, _sex,_phoneNO]//    return [NSString stringWithFormat:@"name == %@\nsex == %@",_name, _sex];//    return [NSString stringWithFormat:@"%@ , %@ , %@",_height , _weight ,_wife];    return [NSString stringWithFormat:@"%@ , %@",_height , _weight];    }@end

  KVC访问：

• #import "Other.h"#import "Person.h"@interface Other : NSObject- (void)testProtected;- (void)testKVC;@end@implementation Other- (void)testProtected{    Person *p = [Person new];    NSLog(@"%@",p->_height);//    NSLog(@"%@",p->_weight);}- (void)testKVC{    Person *p = [Person new];    NSLog(@"%@",[p valueForKey:@"_name"]);    NSLog(@"%@",[p valueForKey:@"_sex"]);    NSLog(@"%@",[p valueForKey:@"_phoneNO"]);    NSLog(@"%@",[p valueForKey:@"_wife"]);}@end

  实际KVC读取一个实例变量：

• - (id)valueForKey: (NSString*)key{    if (!key) {        id value =http://www.mamicode.com/ [self valueForUndefinedKey:nil];        return value;    }    const char *keyCString = [key UTF8String];    SEL sel = sel_getUid(keyCString);    // FIXME: getKey, _getKey, isKey, _isKey are missing    if ([self respondsToSelector:sel]) {        id value =http://www.mamicode.com/ [self _wrapReturnValueForSelector:sel];        return value;    }    size_t keyCStringLength = strlen(keyCString);    char *selBuffer = __builtin_alloca(keyCStringLength + 5);    char *keyname = __builtin_alloca(keyCStringLength + 1);    strcpy(keyname, keyCString);    #define TRY_FORMAT(format)\            sprintf(selBuffer, format, keyname);            sel = sel_getUid(selBuffer);            if ([self respondsToSelector:sel]) {                id value =http://www.mamicode.com/ [self _wrapReturnValueForSelector:sel];                return value;            }    TRY_FORMAT("_%s");    keyname[0] = toupper(keyname[0]);    TRY_FORMAT("is%s");    TRY_FORMAT("_is%s");//    TRY_FORMAT("get%s");//    TRY_FORMAT("_get%s");    #undef TRY_FORMAT    if ([isa accessInstanceVariablesDirectly]) {        sprintf(selBuffer, "_%s", keyCString);        sel = sel_getUid(selBuffer);        if ([self respondsToSelector:sel]) {            id value =http://www.mamicode.com/ [self _wrapReturnValueForSelector:sel];            return value;        }        Ivar ivar = class_getInstanceVariable(isa, selBuffer);        if (!ivar) {            ivar = class_getInstanceVariable(isa, keyCString);        }        if (ivar) {            id value = http://www.mamicode.com/[self _wrapValue:(void*)self + ivar_getOffset(ivar) ofType:ivar_getTypeEncoding(ivar)];            return value;        }    }    id value =http://www.mamicode.com/ [self valueForUndefinedKey:key];    return value;}

   

IOS中实例的权限控制