RFC: Drag and Drop API #5863

dweymouth · 2025-07-15T16:01:24Z

Description:

RfC / Draft of API to allow drag and drop between CanvasObjects

For #142

Example use cases

Dragging to reorder items in a list (with possible multi-selection)

The list item widget (each individual row) implements DragSource
When a selected list item row is dragged, it returns one DragItem for each selected row
The list itself (extended) implements DropTarget, and as DropDragged is invoked, it draws a horizontal line to indicate where the items would be inserted if they were to be dropped
Custom mime types and URI schemes can be defined by the app to represent to itself the different types of content it supports drag-and-drop for

Dragging DocTabs items from one DocTabs widget to another (eg multi-pane text editor)

The tab itself can implement DragSource
When dragged over a potential DropTarget (e.g. another DocTabs) the tab item can gray itself out in response to DropPending
The DocTabs implements DropTarget and communicates with the tabitems renderer to render a horizontal line in between adjacent tabs where the new document would be inserted.

Dragging items from one top-level Fyne window to another window in the same app (process)

The API should support this, just need to handle it in the driver

Open questions

Should DropPending also pass the DropTarget as a parameter? I can see that it may be useful, for example, in a multi pane text editor app to know if a tab would be dropped in the same DocTabs (e.g. simply reordering the tabs) or a different DocTabs (moving from one pane to another)
How to handle drop from outside of the Fyne app onto a widget
How to handle drag from within a Fyne window and drop outside the window? (e.g. for browsers or text editors where dragging a tab outside the window opens a new window

Checklist:

Tests included.
Lint and formatter run with no errors.
Tests all pass.

Where applicable:

Public APIs match existing style and have Since: line.
Any breaking changes have a deprecation path or have been discussed.
Check for binary size increases when importing new modules.

coveralls · 2025-07-15T16:09:03Z

coverage: 62.391%. remained the same
when pulling 38e6ba8 on dweymouth:feature/drag-and-drop
into 43c35cb on fyne-io:develop.

andydotxyz · 2025-07-21T22:40:09Z

canvasobject.go

+	URI() URI
+}
+
+type DragItemCursor interface {


I think Cursor is a desktop extension, so a DragCursor should be too.
What might be good to handle at this level would be the drag preview which would appear next to the cursor and floating above the content, where desired?

andydotxyz · 2025-07-21T22:41:45Z

canvasobject.go

+// onto another CanvasObject which implements DropTarget.
+type DragSource interface {
+	// DragSourceBegin is invoked when a drag and drop is initiated on this DragSource.
+	// It should return the DragItems it is sourcing, and optionally a non-nil DragItemCursor


Let's not bake cursors into the main abstraction. Perhaps the DragItem could optionally implement DragItemCursor? Or even just Cursorable?

andydotxyz · 2025-07-21T22:42:41Z

canvasobject.go

+
+	// DragSourceDragged is invoked when a drag and drop continues within this DragSource
+	// and has not yet entered over a potential DropTarget.
+	DragSourceDragged(*PointEvent)


I don't understand what this adds? Isn't it just the same as Hoverable except the widget has initiated a drag? (which it already knows)

andydotxyz · 2025-07-21T22:44:17Z

canvasobject.go

+	// which MAY be a subset of the items returned by DragSourceBegin.
+	// Implementations may wish to use this hook to update rendering to
+	// highlight the item(s) which may be about to be dropped.
+	DropPending([]DragItem)


Do you have a concrete example of this in use? I'm not entirely sure that it will be supported at the OS level...

andydotxyz · 2025-07-21T22:45:18Z

canvasobject.go

+	// that DropTarget, but the drag still remains in progress.
+	// Implementations may wish to cancel any visual changes made in response
+	// to the corresponding DropPending invocation.
+	DropUnpending()


If the above is not needed then neither is this. Feels cleaner without - but I didn't quite understand the use-case

andydotxyz · 2025-07-21T22:45:36Z

canvasobject.go

+	// accepted by a DropTarget.
+	DropCanceled()
+
+	// DropCanceled is invoked when a drag and drop ends with the content


typo, just FYI :)

andydotxyz · 2025-07-21T22:47:12Z

canvasobject.go

+	// the in-progress drag and drop operation.
+	// If both the DragSource and this DropTarget return a cursor, the DropTarget
+	// cursor will be used to represent the drag and drop while it is dragged over this target.
+	DropBegin(*PointEvent, []DragItem) (accept []DragItem, cursor DragItemCursor)


As above I don't think that cursor implementation is needed at this level - we already support that through Cursorable in the desktop extension.

In addition to that I'm not sure that the implication in the comments that the cursor of the initialing drag would remain in use throughout the operation necessarily stands (normally).

andydotxyz · 2025-07-21T22:48:49Z

canvasobject.go

+	// (resulting in a DropBegin invocation) continues to be dragged within this DropTarget.
+	// Implementations may wish to update rendering to show where the dragged content
+	// would land if dropped.
+	DropDragged(*PointEvent)


Is this and DropEnd needed? we already handle dragging in a non dnd operation, so if the position matters it can still be discovered...

andydotxyz · 2025-07-21T22:49:43Z

canvasobject.go

+	DropEnd()
+
+	// Dropped is invoked when a drag and drop ends by being dropped on this DropTarget.
+	Dropped(*PointEvent)


Isn't the []DragItem more important than the *PointEvent?

andydotxyz

Thanks for all of this, a great bit of work towards fleshing it out.
I worry however that it is almost a super-set of DnD + cursor management + draggable, so have notes inline about how it could be trimmed down.

It looks like it could support the OS level integrations but the docs write very specifically about dragging to (or from) other CanvasObjects - but probably we don't want to limit to that?

dweymouth and others added 2 commits July 15, 2025 09:00

draft Drag and Drop API

fb1dae4

Update canvasobject.go

8000a49

Update canvasobject.go

38e6ba8

andydotxyz reviewed Jul 21, 2025

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Description:

Example use cases

Dragging to reorder items in a list (with possible multi-selection)

Dragging DocTabs items from one DocTabs widget to another (eg multi-pane text editor)

Dragging items from one top-level Fyne window to another window in the same app (process)

Open questions

Checklist:

Where applicable:

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coveralls commented Jul 15, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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dweymouth commented Jul 15, 2025 •

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coveralls commented Jul 15, 2025 •

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