详解ios中scrollView上使用masonry

_scroll_Bg = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 100, SCREEN_W, 200)];
  _scroll_Bg.pagingEnabled = YES;
  _scroll_Bg.delegate = self;
  _scroll_Bg.backgroundColor = [UIColor redColor];
  [self.view addSubview:_scroll_Bg];

UIView *bgView = [[UIView alloc] init];
  bgView.backgroundColor = [UIColor blueColor];
  [_scroll_Bg addSubview:bgView];
  [bgView mas_makeConstraints:^(MASConstraintMaker *make) {
    make.top.left.bottom.and.right.equalTo(_scroll_Bg).with.insets(UIEdgeInsetsZero);
    make.width.equalTo(_scroll_Bg);
  }];

UIView *childV = [[UIView alloc] init];
  childV.backgroundColor = [UIColor cyanColor];
  [bgView addSubview:childV];
  [childV mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.right.mas_equalTo(0);
    make.top.mas_equalTo(250);
    make.height.mas_equalTo(1000);
  }];

[bgView mas_makeConstraints:^(MASConstraintMaker *make) {
    make.bottom.equalTo(childV.mas_bottom);
  }];