VirtualizingGrid

Remarks

VirtualizingPanel's bargain in two dimensions. The items are the caller's and the tiles are a pool; this control knows how many items there are and how big a tile is, and nothing about what an item is. Forty thousand of them is forty thousand of the caller's own objects and about sixty elements.

⚠ Its own control rather than a mode on the panel, and the reason is one number. A list's row occupies the full width, so item n is at n × height and nothing else is needed. A grid's item is at a position that depends on Columns, which is a function of the measured width — so the same resize that changes the viewport also changes which item is where, and the content height with it. Folding that into a control whose entire premise is "row n is at n × height" would make every line of it conditional.

⚠ Tiles are a fixed size, from --tile-width and --tile-height. The same reason a row has a fixed height: knowing where item 40 000 is without having measured the 39 999 before it is what makes this arithmetic instead of a walk. A grid of tiles that sized themselves to their captions would also be one whose columns moved as you typed in a search box, which is a grid you cannot aim at.

⚠ Nothing has to call Realise. It runs on LayoutFinished, which is the only place that knows how wide the viewport ended up — and for a grid that matters more than for a list, because the width is what decides the layout rather than merely how much of it is visible.

Fields and properties (13)

• public const int Overscan

  How many lines of tiles are realised above and below the viewport.

• protected override string TagName

  The element name this type answers to when a caller does not choose one.

• protected override bool AcceptsFocus

  Whether the focus can rest on this kind of control at all.

• public ScrollView Scroller

  The scroller the tiles live in.

• public IReadOnlyList<UiElement> Tiles

  The tiles that exist as elements, in pool order rather than in item order.

• public int FirstItem

  Which item the first tile of the pool is showing.

• public int Columns

  How many tiles fit across, as of the last realise.

• public float TileWidth

  How wide a tile is, from --tile-width.

• public float TileHeight

  How tall a tile is, from --tile-height.

• public int Count
• public Func<VirtualizingGrid, UiElement>? CreateTile

  Makes a tile element. Called only when the pool has to grow.

• public Action<UiElement, int>? BindTile

  Puts an item's data on a tile.

• public static readonly UiPropertyKey CountProperty

  Identity for Count.

Methods (6)

• protected override void OnCreated()

  Builds whatever this element is made of, once, as it joins a document.

• protected override void OnRemoved()

  Called once, as the element leaves the document.

• public UiElement? TileOf(int item)

  The element showing an item, or null if it is not realised.

• public void ScrollIntoView(int item)

  Scrolls until an item is inside the viewport.

• public void Realise()

  Makes sure there is a tile for every visible one, and binds them.

• public void ClearCount()

  Unsets Count, so it takes its default or its ancestor's value again.

Used by (2)

• VirtualizingGridTestsVixen.Ui.Controls.Tests
• AssetGridVixen.Editor.App