Data-bound windows

WindowDescriptor.DataSource holds the data that a window shows. The window sets it as the UI Toolkit dataSource of its content. Bindings in the content resolve against it, also after the window moves into a tab group or a dock.

Bind in UXML with <Bindings> and data-source-path, or in C# with SetBinding.
Plain C# objects work. The binding system polls them each frame. For many windows, implement INotifyBindablePropertyChanged or IDataSourceViewHashProvider on the data so that bindings update only on change.
Public fields bind by default. A property needs [CreateProperty].
Toolbox charts bind too. Use the C# property name: <ui:DataBinding property="Value" data-source-path="Temperature" />. The bindable properties are Gauge.Value, MinValue and MaxValue, ProgressRing.Value, PieSlice.Label and Value, PieChart.CenterLabel, and GraphSeries.Label and Data. Data takes a float[] or a List<float>. The charts redraw only when a bound value changes.
Set DataSource again to show other data in the same window. The window raises DataSourceChanged. Use it for state that bindings do not cover. For the title, set TitleFromData.
Layouts save window ids, not data. A window keeps its DataSource across close, reopen and layout load, because the workspace keeps the descriptor.

One content template, many windows:

<!-- Resources/UI/ItemDetail.uxml -->
<ui:UXML xmlns:ui="UnityEngine.UIElements">
  <ui:Label>
    <Bindings>
      <ui:DataBinding property="text" data-source-path="Name" binding-mode="ToTarget" />
    </Bindings>
  </ui:Label>
</ui:UXML>

public class Item
{
    public string Id = string.Empty;
    public string Name = string.Empty;
}

public static class ItemWindows
{
    private static readonly VisualTreeAsset Template = Resources.Load<VisualTreeAsset>("UI/ItemDetail");

    // Opens the item's window, or focuses it when it is already registered
    public static void Show(Workspace workspace, Item item)
    {
        var id = $"Item_{item.Id}";
        if (workspace.GetWindow(id) == null)
        {
            WindowDescriptor.Builder()
                .WithId(id)
                .WithTitle(item.Name)
                .WithContent(Template.Instantiate())
                .WithDataSource(item)
                .BuildAndRegister(workspace);
        }
        workspace.OpenWindow(id);
    }

    // Queries use GetAllWindows
    public static WindowDescriptor? Find(Workspace workspace, Item item) =>
        workspace.GetAllWindows().FirstOrDefault(w => w.DataSource == item);

    public static IEnumerable<WindowDescriptor> All(Workspace workspace) =>
        workspace.GetAllWindows().Where(w => w.DataSource is Item);
}

Binding converters

ToolboxConverters registers converter groups for conversions that Unity does not have. A binding opts in by the group id, in UXML or in UI Builder:

<ui:VisualElement>
  <Bindings>
    <ui:DataBinding property="style.display" data-source-path="IsAlive" binding-mode="ToTarget" source-to-ui-converters="tb:visible" />
  </Bindings>
</ui:VisualElement>
<ui:RadioButtonGroup choices="Easy,Normal,Hard">
  <Bindings>
    <ui:DataBinding property="value" data-source-path="Difficulty" binding-mode="TwoWay"
                    source-to-ui-converters="tb:enum-int" ui-to-source-converters="tb:enum-int" />
  </Bindings>
</ui:RadioButtonGroup>

A group has no parameters, so the id says what it does. A group matches the declared type of the source exactly, with no base types. Groups do not chain.

Group	Source	UI	Result
`tb:visible`, `tb:hidden`	bool	`DisplayStyle`, `style.display`	True shows (`Flex`) or hides (`None`). `tb:hidden` inverts.
`tb:visibility`, `tb:invisibility`	bool	`Visibility`, `style.visibility`	Same, but the element keeps its space.
`tb:not`	bool	bool	Inverts, both ways.
`tb:has-value`	string, object, a class	bool	False for null, a destroyed `UnityEngine.Object` or an empty string.
`tb:percent`	float, double 0 to 1	`Length`, `StyleLength`	0.25 gives 25%.
`tb:hex`	Color	string	`#RRGGBB`, or `#RRGGBBAA` when not opaque. Both ways; text that is not a color gives clear. Use `is-delayed="true"` on the text field.
`tb:display-name`	an enum	string	The `[InspectorName]`, or the name with spaces: `MaxHealth` gives "Max Health".
`tb:f0`, `tb:f1`, `tb:f2`	int, long, float, double	string	0, 1 or 2 decimals.
`tb:n0`	int, long, float, double	string	Thousands separators: "1,234,567".
`tb:percent-text`	int, long, float, double	string	0.256 gives "26%".
`tb:currency`	int, long, float, double	string	Money in the current culture.
`tb:bytes`	int, long, float, double	string	1536000 gives "1.5 MB" (1024 based).
`tb:duration`	seconds (int, long, float, double), TimeSpan	string	5400 gives "1:30:00"; 90 gives "1:30".
`tb:date`, `tb:time`, `tb:datetime`	DateTime	string	Short date, short time, or both.
`tb:upper`, `tb:lower`	string	string	Upper or lower case.
`tb:xy`, `tb:xz`, `tb:yz`	Vector2, Vector3, Vector2Int, Vector3Int	Vector3, Vector2	The id names the axes of the Vector3. `tb:xz`: (x, y) gives (x, 0, y), and (x, y, z) gives (x, z). Both ways.
`tb:enum-int`	an enum	int	The int value, both ways. For `RadioButtonGroup.value` and `DropdownField.index`.

All text groups use the current culture. The groups register when the player starts and when the editor loads. Startup adds tb:enum-int and tb:display-name for each public enum, and tb:has-value for each public class, in UIToolbox.Runtime and in the assemblies that reference it. For other types, call ToolboxConverters.RegisterEnum<KeyCode>() or ToolboxConverters.RegisterHasValue<MyType>() before the binding updates.

#Data-bound windows

#Binding converters

Data-bound windows

Binding converters