UISlider添加单击事件

原生UISlider用作进度条时没有提供单击事件的接口，通过继承在子类中实现单击事件又有杀鸡用牛刀的感觉，于是考虑在Category中实现。
创建UISlider+touch.h和UISlider+touch.m文件，添加单击事件方法:

@interface UISlider (touch)
// 单击手势
- (void)addTapGestureWithTarget: (id)target
action: (SEL)action;
@end

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实现原理是为UISlider对象添加UITapGestureRecognizer，当触发tap事件时利用传入的target和action回调单击事件。

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在Category添加方法很简单，但是要扩展属性以保存传入的target和action两个参数就需要利用associative机制了（‘associative’机制参考文章）。使用它我们要在.m文件中引入#import <objc/runtime.h>

这里我们主要用到以下两个方法:

objc_setAssociatedObject:
// 添加扩展属性
void objc_setAssociatedObject(id object, void *key, id value, objc_AssociationPolicy policy)
objc_getAssociatedObject
// 获取扩展属性
id objc_getAssociatedObject(id object, void *key)

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通过objc_setAssociatedObject方法我们可以为UISlider扩展两个属性来接收传入的target和action。

通过objc_getAssociatedObject方法就可以在接收到UITapGestureRecognizer单击手势时获取target和action,利用[target performSelector:action withObject:self]方法实现单击事件的回调。
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下面看下.m中的具体实现：

addTapGestureWithTarget方法为UISlider对象添加UITapGestureRecognizer手势，并为它扩展了三个属性。

static char *gestureKey;
static char *targetKey;
static char *actionStringKey;
@implementation UISlider (touch)
- (void)addTapGestureWithTarget: (id)target action: (SEL)action
{
id gesture = objc_getAssociatedObject(self, &gestureKey);
if (!gesture) {
objc_setAssociatedObject(self, &targetKey, target, OBJC_ASSOCIATION_ASSIGN);
objc_setAssociatedObject(self, &actionStringKey, NSStringFromSelector(action), OBJC_ASSOCIATION_RETAIN);
UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc]initWithTarget:self action: @selector(tap:)];
tap.numberOfTapsRequired = 1;
tap.numberOfTouchesRequired = 1;
[self addGestureRecognizer:tap];
objc_setAssociatedObject(self, &gestureKey, tap, OBJC_ASSOCIATION_RETAIN);
}
}

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tap:方法中根据点击的位置更新UISlider对象的value，同时从扩展属性中获取target和action回调单击事件。

- (void)tap: (UITapGestureRecognizer *)sender
{
if (sender.state == UIGestureRecognizerStateEnded) {
CGPoint location = [sender locationInView:self];
float x = location.x;
float r = x/self.frame.size.width;
float value = (self.maximumValue-self.minimumValue) * r;
[self setValue:value animated:YES];
id target = objc_getAssociatedObject(self, &targetKey);
if (target) {
NSString *actionStr = objc_getAssociatedObject(self, &actionStringKey);
SEL action = NSSelectorFromString(actionStr);
[target performSelector:action withObject:self];
}
}
}

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dealloc方法中移除UITapGestureRecognizer

- (void)dealloc
{
UITapGestureRecognizer *tap;
id gesture = objc_getAssociatedObject(self, &gestureKey);
if (gesture) {
tap = (UITapGestureRecognizer*)gesture;
[self removeGestureRecognizer:tap];
}
}
@end

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这样我们为UISlider添加单击事件，只需要将Category文件拷入项目import引入就OK了。

原文链接：http://www.cocoachina.com/bbs/read.php?tid=145660&fpage=3

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