如何实现浮动在内容视图上的覆盖按钮

in some iPhone apps i saw a button which was floating over the content view, eg. in the app EyeEm. When the user is scrolling the content, the button remains where it is and is still an interaction element.

我如何实现这个？

我的方法是：

• 创建包含内容的视图
• 在上面放一个按钮
• 但是如何让按钮浮动呢？

edit:

浮动似乎是默认行为。有趣的是addSubview and insertSubview放置按钮时具有相同的行为...两者都浮动在内容上。

- (void)addOverlayButton {
UIButton *oButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[oButton addTarget:self
           action:@selector(aMethod:)
           forControlEvents:UIControlEventTouchDown];
[oButton setTitle:@"Show View" forState:UIControlStateNormal];
oButton.frame = CGRectMake(80.0, 210.0, 160.0, 40.0);
[self.view addSubview:oButton];
//[self.view insertSubview:oButton aboveSubview:_scrollView];  // same result as addSubview. 
// Both solutions let the button float over the content. 
}

将子视图添加到窗口的内容视图中insertSubview:aboveSubview:方法以按钮和滚动视图作为参数，但要小心：如果两个同级视图都具有透明度，则生成的绘图行为是未定义的。