学步园

先在.h里面定义一个变量CGPoint startPoint；然后在.m里面加入下面两个方法：

- (void)touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event
{
    //保存触摸起始点位置
    CGPoint point=[[touches anyObject] locationInView:self];
    startPoint=point;
    
    //该view置于最前
    [[self superview] bringSubviewToFront:self];
}

- (void)touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event
{
    //计算位移=当前位置-起始位置
    CGPoint point=[[touches anyObject] locationInView:self];
    float dx=point.x-startPoint.x;
    float dy=point.y-startPoint.y;
    
    //计算移动后的view中心点
    CGPoint newcenter=CGPointMake(self.center.x+dx,self.center.y+dy);
    
    
    /*限制用户不可将视图托出屏幕*/
    float halfx=CGRectGetMidX(self.bounds);
    //x坐标左边界
    newcenter.x=MAX(halfx,newcenter.x);
    //x坐标右边界
    newcenter.x=MIN(self.superview.bounds.size.width-halfx,newcenter.x);
    
    //y坐标同理
    float halfy=CGRectGetMidY(self.bounds);
    newcenter.y=MAX(halfy,newcenter.y);
    newcenter.y=MIN(self.superview.bounds.size.height-halfy,newcenter.y);
    
    //移动view
    self.center=newcenter;
}