UIScrollView with sticky footer UIView and dynamic

问题:

Challenge time!

Imagine we have 2 content views:

1. UIView with dynamically height content (expandable UITextView) = RED
2. UIView as a footer = BLUE

This content is inside a UIScrollView = GEEN

How should I structure and handle the constraints with auto-layout to archive all the following cases?

I am thinking next basic structure to start with:

- UIScrollView (with always bounce vertically)
    - UIView - Container
       - UIView - DynamicHeightContent
       - UIView - Sticky Footer

Keyboard handling should be done by code watching notifications UIKeyboardWillShowNotification and UIKeyboardWillHideNotification. We can chose to set the keyboard's end frame height to Container UIView bottom pin constraint or to the UIScrollView bottom contentInset.

Now, the tricky part is the sticky footer.

• How we make sure the sticky footer UIView stays at the bottom if there is more screen available than the whole Container View?
• How do we know the available screen space when the keyboard is shown/hidden? we'll surely need it.
• Is is it right this structure I purpose?

Thank you.

回答1:

When the text content of the UITextView is relatively short, the content view's subviews (i.e., the text view and footer) will not be able to dictate the size of their content view through constraints. That's because when the text content is short, the content view's size will need to be determined by the scroll view's size.

Update: The latter paragraph is untrue. You could install a fixed-height constraint either on the content view itself or somewhere in the content view's view hierarchy. The fixed-height constraint's constant could be set in code to reflect the height of the scroll view. The latter paragraph also reflects a fallacy in thinking. In a pure Auto Layout approach, the content view's subviews don't need to dictate the scroll view's contentSize; instead, it's the content view itself that ultimately must dictate the contentSize.

Regardless, I decided to go with Apple's so-called "mixed approach" for using Auto Layout with UIScrollView (see Apple's Technical Note: https://developer.apple.com/library/ios/technotes/tn2154/_index.html)

Some iOS technical writers, like Erica Sadun, prefer using the mixed approach in pretty much all situations ("iOS Auto Layout Demystified", 2nd Ed.).

In the mixed approach, the content view's frame and the scroll view's content size are explicitly set in code.

Here's the GitHub repo I created for this challenge: https://github.com/bilobatum/StickyFooterAutoLayoutChallenge. It's a working solution complete with animation of layout changes. It works on different sized devices. For simplicity, I disabled rotation to landscape.

For those who don't want to download and run the GitHub project, I have included some highlights below (for the complete implementation, you'll have to look at the GitHub project):

The content view is orange, the text view is gray, and the sticky footer is blue. The text is visible behind the status bar while scrolling. I don't actually like that, but it's fine for a demo.

The only view instantiated in storyboard is the scroll view, which is full-screen (i.e., underlaps status bar).

For testing purposes, I attached a double tap gesture recognizer to the blue footer for the purpose of dismissing the keyboard.

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.scrollView.alwaysBounceVertical = YES;

    [self.scrollView addSubview:self.contentView];
    [self.contentView addSubview:self.textView];
    [self.contentView addSubview:self.stickyFooterView];

    [self configureConstraintsForContentViewSubviews];

    // Apple's mixed (a.k.a. hybrid) approach to laying out a scroll view with Auto Layout: explicitly set content view's frame and scroll view's contentSize (see Apple's Technical Note TN2154: https://developer.apple.com/library/ios/technotes/tn2154/_index.html)
    CGFloat textViewHeight = [self calculateHeightForTextViewWithString:self.textView.text];
    CGFloat contentViewHeight = [self calculateHeightForContentViewWithTextViewHeight:textViewHeight];
    // scroll view is fullscreen in storyboard; i.e., it's final on-screen geometries will be the same as the view controller's main view; unfortunately, the scroll view's final on-screen geometries are not available in viewDidLoad
    CGSize scrollViewSize = self.view.bounds.size;

    if (contentViewHeight < scrollViewSize.height) {
        self.contentView.frame = CGRectMake(0, 0, scrollViewSize.width, scrollViewSize.height);
    } else {
        self.contentView.frame = CGRectMake(0, 0, scrollViewSize.width, contentViewHeight);
    }

    self.scrollView.contentSize = self.contentView.bounds.size;
}

- (void)configureConstraintsForContentViewSubviews
{
    assert(_textView && _stickyFooterView); // for debugging

    // note: there is no constraint between the subviews along the vertical axis; the amount of vertical space between the subviews is determined by the content view's height

    NSString *format = @"H:|-(space)-[textView]-(space)-|";
    [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:format options:0 metrics:@{@"space": @(SIDE_MARGIN)} views:@{@"textView": _textView}]];

    format = @"H:|-(space)-[footer]-(space)-|";
    [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:format options:0 metrics:@{@"space": @(SIDE_MARGIN)} views:@{@"footer": _stickyFooterView}]];

    format = @"V:|-(space)-[textView]";
    [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:format options:0 metrics:@{@"space": @(TOP_MARGIN)} views:@{@"textView": _textView}]];

    format = @"V:[footer(height)]-(space)-|";
    [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:format options:0 metrics:@{@"space": @(BOTTOM_MARGIN), @"height": @(FOOTER_HEIGHT)} views:@{@"footer": _stickyFooterView}]];

    // a UITextView does not have an intrinsic content size; will need to install an explicit height constraint based on the size of the text; when the text is modified, this height constraint's constant will need to be updated
    CGFloat textViewHeight = [self calculateHeightForTextViewWithString:self.textView.text];

    self.textViewHeightConstraint = [NSLayoutConstraint constraintWithItem:self.textView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:0 multiplier:1.0f constant:textViewHeight];

    [self.textView addConstraint:self.textViewHeightConstraint];
}

- (void)keyboardUp:(NSNotification *)notification
{
    // when the keyboard appears, extraneous vertical space between the subviews is eliminated–if necessary; i.e., vertical space between the subviews is reduced to the minimum if this space is not already at the minimum

    NSDictionary *info = [notification userInfo];
    CGRect keyboardRect = [[info objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
    keyboardRect = [self.view convertRect:keyboardRect fromView:nil];
    double duration = [[info objectForKey:UIKeyboardAnimationDurationUserInfoKey] doubleValue];

    CGFloat contentViewHeight = [self calculateHeightForContentViewWithTextViewHeight:self.textView.bounds.size.height];
    CGSize scrollViewSize = self.scrollView.bounds.size;

    [UIView animateWithDuration:duration animations:^{

        self.contentView.frame = CGRectMake(0, 0, scrollViewSize.width, contentViewHeight);
        self.scrollView.contentSize = self.contentView.bounds.size;
        UIEdgeInsets insets = UIEdgeInsetsMake(0, 0, keyboardRect.size.height, 0);
        self.scrollView.contentInset = insets;
        self.scrollView.scrollIndicatorInsets = insets;

        [self.view layoutIfNeeded];

    } completion:^(BOOL finished) {

        [self scrollToCaret];
    }];
}

Although the Auto Layout component of this demo app took some time, I spent almost as much time on scrolling issues related to a UITextView being nested inside of a UIScrollView.

回答2:

Instead of using a UIScrollView you would very likely be better off with a UITableView. It also might be better to not using auto-layout. At least, I've found it better to not use it for these sorts of manipulations.

Look into the following:

• UITextView textViewDidChange
  • Change the size of the text view using sizeThatFits (limiting width and using FLT_MAX for height). Change the frame, not the contentSize.
  • Call UITableView beginUpdates/endUpdates to update the table view
  • Scroll to the cursor
• UIKeyboardWillShowNotification notification
  • On NSNotification that comes through, you can call userInfo (a Dictionary), and the key UIKeyboardFrameBeginUserInfoKey. Reduce the frame of the table view based on the height of the size of the keyboard.
  • Scroll to cursor again (since the layouts will have all changed)
• UIKeyboardWillHideNotification notification
  • The same as the show notification, just opposite (increasing the table view height)

To have the footer view stick to the bottom, you could add an intermediate cell to the table view, and have it change size depending on the size of the text and whether the keyboard is visible.

The above will definitely require some extra manipulation on your part - I don't fully understand all of your cases, but it should definitely get you started.

回答3:

If I understand whole task, my solution is put "red" and "blue" views to one container view, and in the moment when you know size of dynamic content (red) you can calculate size of container and set scrollView content size. Later, on keyboard events you can adjust white space between content and footer views