这可以帮助您按照优先级组织子视图。
继承自 ULPriorityView 的视图实例在最顶层的 most priorityLevel。
可以设置视图的优先级，优先级越高越靠上。（考上是指在（view.subViews中的index值））
//如果priorityLevel设置值相等，则谁后加到view上谁靠上。（和普通view特性一致）

• (void)testSameLevel{
  UIView *subView1 = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 30, 30)];
  subView1.priorityLevel = 1;
  subView1.backgroundColor = [UIColor redColor];
  [view addSubview:subView1];
  
  UIView *subView2 = [[UIView alloc] initWithFrame:CGRectMake(20, 20, 30, 30)];
  subView2.priorityLevel = 1;
  subView2.backgroundColor = [UIColor blueColor];
  [view addSubview:subView2];
  
  //此时subView1的index = 0，subView2的index = 1
  
  [view bringSubviewToFront:subView1];
  
  //此时subview1的index = 1，subview2的index = 0
  }

//如果priorityLevel设置值不相等，值越大越靠上（不管谁先加到view上）。

• (void)testDiffentLevel{
  UIView *subView2 = [[UIView alloc] initWithFrame:CGRectMake(20, 20, 30, 30)];
  subView2.priorityLevel = 2;
  subView2.backgroundColor = [UIColor blueColor];
  [view addSubview:subView2];
  
  UIView *subView1 = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 30, 30)];
  subView1.priorityLevel = 1;
  subView1.backgroundColor = [UIColor redColor];
  [view addSubview:subView1];
  
  //此时subview1的index = 0，subView2的index = 1。尽管subview2先addsubView
  
  [view bringSubviewToFront:subView1];
  
  //但由于subView2的priorityLevel较大，所以调用这个方法后subView1的index仍然为0，subview2的index还是1。尽管subview2先addsubView.
  //bringSubviewToFront,sendsubviewToBack只针对相同的priorityLevel调用有效
  

}