IOS KVC 筆記

2014-07-09 Kommentare IOS

KVC可以存取純量(scalar，我的感覺就是基礎型別，如：(int, bool)以及結構(struct)。

如同KVC存取物件一樣實現setter(-set<Key>:) & getter(-<key>) 方法。

但是透過KVC去存取的話，必需進行打包和解包的動作。

純量 VS NSNumber，結構 VS NSValue。

例如：

  • 透過KVC存值給int型別必須透過NSNumber去存取

    [self setValue:@100 forKey:@"x"]

  • 透過KVC去取int型別的值也是回傳NSNumber

    NSNumber *number = [self valueForKey:@"x"]

在我們想要透過KVC存取純量及結構的同時，我們也必須實現setNilValueForKey:方法，因為我們有可能透過KVC setter存入一個nil進去。
純量存取
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@interface MyClass
@property (nonatomic) float f;
@end

@implementation MyClass
-(void)test{
	_f = 100;								// f = 100
	[self valueForKey:@"f"];			// 100
	[self setValue:@200 forKey:@"f"];	// f = 200

	// will run setNilValueForKey
	[self setValue:nil forKey:@"f"];	// f = 5

	// compile fail
	//self.f = nil;
}

-(void)setNilValueForKey:(NSString *)key{
	if([key isEqualToString:@"f"]){
  		_f = 5;
		//[self setValue:@5 forKey:@"f"];
	}else{
		[super setNilValueForKey:theKey];
	}
}

結構存取
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typedef struct {
    float x, y, z;
} ThreeFloats;

@interface MyClass{

}
@property (nonatomic) ThreeFloats threeFloats;
@end

@implementation MyClass

@synthesize threeFloats = _threeFloats;

-(void)test{
	  _threeFloats.x=11;
    _threeFloats.y=22;
    _threeFloats.z=33;

    NSValue *gValue = [self valueForKey:@"threeFloats"];
    ThreeFloats f3;
    [gValue getValue:&f3];

    f3.x = 100;
    NSValue *sValue = [NSValue value:&f3 withObjCType:@encode(ThreeFloats)];
    [self setValue:sValue forKey:@"threeFloats"];
}

- (void)setThreeFloats:(ThreeFloats)threeFloats{
    _threeFloats = threeFloats;
}

- (ThreeFloats)threeFloats{
    return _threeFloats;
}
@end

詭異點(小細節？？)

這篇有提到說請確保與KVC相容 Ensuring KVC Compliance

  • Implement a method named -<key>, -is<Key>
  • or have an instance variable <key> or _<key>
  • If the property is mutable, then it should also implement -set<Key>:

getter依序註解getFf,ff,isFf。

setter依序註解setFf:。

KVC getter & setter test
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@interface MyClass
@property (nonatomic,getter=issFf,setter=ssetFf:) float ff;
@end

@implementation MyClass

@synthesize ff = _ff;

-(void)test{
  NSNumber *fff = [self valueForKeyPath:@"ff"];
   [self setValue:@2000 forKey:@"ff"];	}

-(void)setFf:(float)value{
    _ff = value;
}

-(void)ssetFf:(float)value{
    _ff = value;
}

-(float)getFf{
    return _ff+1;
}

-(float)ff{
    return _ff+2;
}

-(float)isFf{
    return _ff+3;
}

-(float)issFf{
    return _ff+4;
}
@end

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#IOS #KVC