UIScrollView使用AutoLayout注意点

在UIScrollView添加多个subview，但是subview一直不能显示出来。
约束如下:

 [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[bgScrollView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(bgScrollView)]];
 [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[bgScrollView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(bgScrollView)]];

 [bgScrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[topBannerScrollView]|" options:0 metrics:nil views:@{@"topBannerScrollView":self.topBannerScrollView}]];
 [bgScrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[classityCollectionView]|" options:0 metrics:nil views:@{@"classityCollectionView":self.classityCollectionView}]];
 [bgScrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[topBannerScrollView(==200)][classityCollectionView(==70)]" options:kNilOptions metrics:0 views:@{@"topBannerScrollView":self.topBannerScrollView, @"classityCollectionView":self.classityCollectionView}]];

观察它的图层显示Scrollable content size is ambiguous。

问题在于用autolayout给UIScrollView设约束没有设置它的contentsize而是根据subview的大小来自适应的，而上面代码subview的宽度却又是根据bgScrollView的边界来确定的，两者互相矛盾，所以必须先设置好subview的宽度：

 [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[bgScrollView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(bgScrollView)]];
 [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[bgScrollView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(bgScrollView)]];
   
  for (UIView *view in @[self.topBannerScrollView, self.classityCollectionView ])
  {
      [bgScrollView addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:bgScrollView attribute:NSLayoutAttributeWidth multiplier:1 constant:0]];
  }
  [bgScrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[topBannerScrollView]" options:0 metrics:nil views:@{@"topBannerScrollView":self.topBannerScrollView}]];
  [bgScrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[classityCollectionView]" options:0 metrics:nil views:@{@"classityCollectionView":self.classityCollectionView}]];
  [bgScrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[topBannerScrollView(==200)][classityCollectionView(==70)]" options:kNilOptions metrics:0 views:@{@"topBannerScrollView":self.topBannerScrollView, @"classityCollectionView":self.classityCollectionView}]];

同样约束subview的宽度和bgScrollView相同，但@"H:|[subview]|"˙这样约束是有问题的（设置subview高度同理）。

以上，记开发过程中遇到的好几次坑~