UIButton自定义属性

Question

我要给UIButton自定义一个属性，我这样做的

MyUIButton.h

@interface MyUIButton : UIButton{
    NSString *idx;
}
@property (nonatomic,retain) NSString *idx;
@end

MyUIButton.m

@implementation MyUIButton
@synthesize idx;
@end

#import "MyUIButton.h"
    MyUIButton *btn = ((MyUIButton *)[MyUIButton buttonWithType:UIButtonTypeRoundedRect]);
    btn.idx = @"abcd";

然后报错了：

-[UIRoundedRectButton setIdx:]: unrecognized selector sent to instance 0x816b2a0

难道这样扩展属性不对么？

Answer 1

你代码中虽继承了UIbutton重写了init，但是未重写buttonWithType:，所以在调用[MyUIButton buttonWithType:UIButtonTypeRoundedRect]时实际上调用了父类的buttonWithType:，父类的buttonWithType:调用了某种UIButton的init。

为什么我说是某种UIButton？因为UIButton的buttonWithType:可以生成不同类型的对象，这些对象都是UIButton的子类。(当然不可能生成MyUIButton类型的对象，也就无法响应setIdx:方法)

实际上，UIButton是一种聚类，你不能直接继承它。应当增加扩展，使用运行时增加关联对象。注意.m中引入了#import <objc/runtime.h>
：

@interface UIButton (IdxProperty)
@property (nonatomic,retain) NSString *idx;
@end

#import <objc/runtime.h>
@implementation MyUIButton
@dynamic idx;
@end

- (NSString *)idx
{
    NSString *idx = objc_getAssociatedObject(self, @"kUIButtonIdxKey");
    return idx;
}

- (void)setIdx:(NSString *)idx
{
    objc_setAssociatedObject(self, @"kUIButtonIdxKey", idx, OBJC_ASSOCIATION_RETAIN);
}

更干净的写法是给@"kUIButtonIdxKey"加个宏。此处我写的有点dirty
楼主，还是那句话，加强下面向对象的学习

Answer 2

-(id)buttonWithType:(UIButtonType)type
{
  [super buttonWithType:type]; 
   self.idx = @"abcd";
}