UIButton根据不同的状态设置layer

这需要用到KVO，监听button的highlighted属性的变化，在监听回调里根据监听到得属性值设置layer

    设置监听如下

[button  addObserver:self forKeyPath:@"highlighted" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL];
监听回调如下
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {    UIButton * bt = (UIButton* )object;
     if([keyPath isEqualToString:@"highlighted"])
    {
     if(bt.state == UIControlStateNormal) {
        bt.layer.bordColor  = [UIColor redColor].CGColor;
     } else{
        bt.layer.bordColor  = [UIColor black].CGColor;
   }
}