BbResizablePanelGroup: no way to observe the panel sizes a drag produced

[mathewtaylor]

Split out of #473, where @SimonLiebers-Dev noted in passing that BbResizablePanelGroup exposes "no OnResize, no size binding, no throttle interval and no way to opt out of the per-frame callback".

#473 removes the need for the last two — the drag no longer round-trips per frame, so there is nothing left to throttle or opt out of. But it also makes the first one matter more than it did before: component state is now deliberately stale for the duration of a drag and is committed once on pointerup, so there is a defined sync point and still no way for a consumer to observe it.

Today the sizes a user drags to are unobservable from C#. There is no way to persist a layout across sessions, react to a panel crossing a threshold, or even read the current split.

Proposal

An event callback on BbResizablePanelGroup, carrying the panel sizes as percentages in declared order.

Following the shape the library already uses for exactly this event — BbDashboardGrid.OnWidgetResizeEnd with WidgetResizeEventArgs — rather than inventing a new one.

Suggested name OnResizeEnd rather than OnResize: it fires once when the drag completes, and a name implying continuous firing would invite consumers to expect the per-frame behaviour #473 exists to remove.

Notes

• Should fire only when the sizes actually changed — a press-and-release on the handle with no movement is not a resize.
• Should carry the clamped sizes the component committed, not the raw values JS proposed.
• Needs the usual public-API companions: live demo example, CodeExamples/.../*.txt snippet, API-reference entry, API-surface snapshot.

Depends on #473 (PR #474), which introduces the drag-end commit this hangs off.

[mathewtaylor]

Implemented in #476, now merged to develop.

Shipped as OnResizeEnd carrying a PanelResizeEventArgs, following BbDashboardGrid.OnWidgetResizeEnd rather than introducing a second convention for the same event shape.

<BbResizablePanelGroup OnResizeEnd="HandleResizeEnd"> ... </BbResizablePanelGroup>

private void HandleResizeEnd(PanelResizeEventArgs args) => lastSizes = args.Sizes;

Both notes from the issue are honoured: it fires only when the sizes actually changed (press-and-release on a handle raises nothing), and it reports the clamped sizes the component committed rather than the raw values JS proposed. The fallback interop path raises it exactly once as well, so consumers cannot tell the two paths apart.

Implemented as a second [JSInvokable] — CommitPanelSizes — rather than a flag on UpdatePanelSizes, which leaves the existing signature untouched.

[mathewtaylor]

Released in BlazorBlueprint.Components 3.16.0, live on NuGet now.

BbResizablePanelGroup.OnResizeEnd fires once when a drag finishes, with the resulting panel sizes.

dotnet add package BlazorBlueprint.Components --version 3.16.0

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