Oh, Pivots in UWP, you so crazy.

You may have heard that I'm writing a Markdown editor, MarkUpDown. It can have several files open at once, of course, and I'm using a UWP Pivot control to handle navigation.

As I got closer to releasing, it hit me that Ctrl-W, though common, isn't the most discoverable way to close those PivotItems. And now that I'm maintaining state between starts, so that whatever tabs you have open when you close the app are back on restart, closing each file "on purpose" is a lot more important than when they all closed on each restart.

That is, it looks like I need a UI for closeable tabs.

Not as easily said as done. You can customize the Header of the PivotItem pretty easily. In fact, a Header can be any object. You create a custom UserControl, throw in your favorite UI widgets, and you're golden.

In that UserControl, I put a TextArea and an icon of some sort, here a box with an X in it donated from Material Design Icons... Took a little experimenting to figure out exactly how to put text and an icon into the header presentably, though. I initially used a ViewBox, which seemed to scale the Path style image I'm using fairly well, no resizing necessary, but the ViewBox hated responding to clicks (or, in this case, "Taps" and "Presses"). Apparently you can manually set something called HandledEventsToo to make it catch events that are already caught by something with higher priority, but that strange Boolean reminds me of CSS' !important a little too much.

I replaced the ViewBox with a Button to be conventional, and then placed the Path style icon in there. I've got decent click event handling now, though it was difficult (well, at least "non-trivial") to get the icon to display centered. I went through the old process of using WPF Geometry Transformer to "Tansform" [sic] the Path into something smaller, and poof... -ish.


    <StackPanel Orientation="Horizontal">
        <TextBlock Margin="0,0,0,0" Name="txtHeader" FontSize="18">Test</TextBlock>
        <Button Background="White" Click="Button_Click">
                <PathIcon Data="M13.3000001907349,2.09999990463257L3.5,2.09999990463257 2.51005053520203,2.51005053520203 2.09999990463257,3.5 2.09999990463257,13.3000001907349 2.51005053520203,14.2899494171143 3.5,14.6999998092651 13.3000001907349,14.6999998092651 14.2899494171143,14.2899494171143 14.6999998092651,13.3000001907349 14.6999998092651,3.5 14.2899494171143,2.51005053520203 13.3000001907349,2.09999990463257 M13.3000001907349,13.3000001907349L3.5,13.3000001907349 3.5,3.5 13.3000001907349,3.5 13.3000001907349,13.3000001907349 M11.8999996185303,5.87999963760376L9.3799991607666,8.39999961853027 11.8999996185303,10.9200000762939 10.9200000762939,11.8999996185303 8.39999961853027,9.3799991607666 5.87999963760376,11.8999996185303 4.90000009536743,10.9200000762939 7.42000007629395,8.39999961853027 4.90000009536743,5.87999963760376 5.87999963760376,4.90000009536743 8.39999961853027,7.42000007629395 10.9200000762939,4.90000009536743 11.8999996185303,5.87999963760376z" />

That's fun, isn't it?

example of two closeable tabs

That looks okay, but there are still two problems.

  1. How do I set the Label (now placed in txtHeader) of my UserControl in XAML?
  2. How I do I remove the PivotItem once its button is clicked.

Okay, three problems -- I really should change the icon to a greyed-out version when the PivotItem's not active. That looks awful. Mark that down for version two.

Accessing a property in custom XAML from C

I found the answer to the first pretty easily: You have to use a Dependency Property, of course!

Dependency property values are not stored as fields on the class, they are stored by the xaml framework, and are referenced using a key, which is retrieved when the property is registered with the Windows Runtime property system by calling the DependencyProperty.Register method.

But wait! There's more!

Defining a dependency property can be thought of as a set of concepts. These concepts are not necessarily procedural steps, because several concepts can be addressed in a single line of code in the implementation. This list gives just a quick overview. We'll explain each concept in more detail later in this topic, and we'll show you example code in several languages.

  • Register the property name with the property system (call Register), specifying an owner type and the type of the property value.
  • There's a required parameter for Register that expects property metadata. Specify null for this, or if you want property-changed behavior, or a metadata-based default value that can be restored by calling ClearValue, specify an instance of PropertyMetadata.
  • Define a DependencyProperty identifier as a public static readonly property member on the owner type.
  • Define a wrapper property, following the property accessor model that's used in the language you are implementing. The wrapper property name should match the name string that you used in Register. Implement the get and set accessors to connect the wrapper with the dependency property that it wraps, by calling GetValue and SetValue and passing your own property's identifier as a parameter.
  • (Optional) Place attributes such as ContentPropertyAttribute on the wrapper.

Note If you are defining a custom attached property, you generally omit the wrapper. Instead, you write a different style of accessor that a XAML processor can use. See Custom attached properties.

... [Oh, but there's more... -mfn] ...

In some scenarios, you are defining dependency properties for objects that are used on more than one UI thread. This might be the case if you are defining a data object that is used by multiple apps, or a control that you use in more than one app. You can enable the exchange of the object between different UI threads by providing a CreateDefaultValueCallback implementation rather than a default value instance, which is tied to the thread that registered the property.

Tis a silly place

I feel like the Knights of the Round Table after they've considered whether to go to Camelot. Okay, nevermind. 'Tis a silly place. I won't make these new headers XAML-friendly for "version mvp". We're going to set the Label somewhere outside of XAML. Moving on to #2...

Removing a PivotItem on custom UI click

My first bright idea, which I've used before, is to take the sender, cast to the appropriate widget, find its Parent, and just slowly climb my way back up the UI hierarchy until I find what I need. That is, I'll drill up, like a jQuery closest, until I have the PivotItem, then the Pivot, that I need. Then I can remove the PivotItem from the Pivot's Items collection. Easy, right? The best part is that this normally doesn't change, and you can have the PivotItem remove itself without any serious hooking in with what's around it. (I could probably do this more impressively with reflection, and essentially hook an extension method up for UIElement that parallels the jQuery closest method I references earlier.)

But crimminy. Here's the drill-up code I ended up with, originally in the UserControl's Button's Click event. Remember that this is just exploratory code at this point.

Button btn = (Button)sender;

var spam = (StackPanel)(btn.Parent);

var header = (CloseableTabHeader)(spam.Parent);
var headerItem = (PivotHeaderItem)(header.Parent);

PivotHeaderPanel headerPanel = (PivotHeaderPanel)(headerItem.Parent);
Grid weirdPiGrid = (Grid)headerPanel.Parent;

ContentControl contentControl = (ContentControl)weirdPiGrid.Parent;
Grid anotherGrid = (Grid)contentControl.Parent;
PivotPanel pivotPanel = (PivotPanel)anotherGrid.Parent;
ScrollViewer scrollViewer = (ScrollViewer)pivotPanel.Parent;
Grid yaGrid = (Grid)scrollViewer.Parent;
Grid yaGrid2 = (Grid)yaGrid.Parent;

That's interesting, right? Not only is it much more convoluted a design than I would've expected, we also never get a PivotItem or a Pivot -- yaGrid2 is the "root element" of the page, I think. And the stuff we do get, like PivotPanel, come with warnings like this:

PivotPanel isn't typically used for general XAML UI composition.

/sigh This was easy, wasn't it? My UI traversal has me lost in the funhouse rather than in a cleanly executed UI widget hierarchy that follows its own mental model. Feels like I went out of the back door and I'm seeing the outside of the house isn't finished quite yet. Unfortunately, I get the impression that UWP is held together by chicken wire and duct tape a lot more than I'm used to seeing in a Microsoft UI toolkit.

There's probably some easy way to save what I'm trying to do from the fire, but forget it. For version mvp, I'm changing the CloseableHeader constructor from this...

public CloseableTabHeader(string headerText) : base()
    txtHeader.Text = headerText;

... to this...

public PivotItem itemParent = null;
public Pivot pivotParent = null;

public CloseableTabHeader(string headerText, PivotItem itemParent, Pivot pivotParent) : base()
    txtHeader.Text = headerText;
    this.itemParent = itemParent;
    this.pivotParent = pivotParent;

I think you see where I'm going. Yes, I feel a little dirty too. Now I can remove it easily in the Button's Click event:

private void Button_Click(object sender, RoutedEventArgs e)
    if (null != this.itemParent && null != this.pivotParent)

Hrm. Perhaps "parentPivot" is better than "pivotParent", and same with itemParent. In any event, you want to bubble up to the closest PivotItem and Pivot, and this makes that stupidly straightforward, though I do wonder how much Dispose-ing I should be doing.

I don't love that set-up, and I really dislike the way the buttons' bounds "highlight" simply when you mouse over the icon, like this:

highlighted close button

... but overall, that's a workable solution that will make discovery for tab closure much more straightforward for non-keyboard addicts.

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