IOS中实例的权限控制

• @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 = [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 = [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 = [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 = [self _wrapReturnValueForSelector:sel];
              return value;
          }
  
          Ivar ivar = class_getInstanceVariable(isa, selBuffer);
          if (!ivar) {
              ivar = class_getInstanceVariable(isa, keyCString);
          }
  
          if (ivar) {
              id value = [self _wrapValue:(void*)self + ivar_getOffset(ivar) ofType:ivar_getTypeEncoding(ivar)];
              return value;
          }
  
      }
  
      id value = [self valueForUndefinedKey:key];
      return value;
  }