在tableView、scrollView、collectionView上层添加一个固定的UIButton,不随其滚动

- (void)viewWillAppear:(BOOL)animated {

    [super viewWillAppear:animated];

    //1 弹出半透明的view

    //获取window

    UIWindow *window = [UIApplication sharedApplication].keyWindow;

    UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(ScreenW-55,ScreenH-49-55,40,40)];

    self.button = button;

    button.layer.cornerRadius = 20;

    button.layer.masksToBounds = YES;

    [button setBackgroundImage:[UIImage imageNamed:@"button_add"]  forState:UIControlStateNormal];

    [window addSubview:button];

}

- (void)viewDidDisappear:(BOOL)animated {

    [super viewDidDisappear:animated];

    [self.button removeFromSuperview];

}