Drag and drop

Drag(value) makes an element the source of a value dragged within the window: once the pointer pressing it moves a few DIPs, a translucent copy of the element follows the pointer, and the press is no click. ui.Drop[T](e) makes e take values of type T, returning the one dropped on it, and ui.DragOver[T](e) the one over it, for showing it would take it:

for _, task := range app.todo {
	ui.Row(c).Key(task.ID).Drag(task).Children(func() {
		ui.Text(c, task.Name)
	})
}

done := ui.Column(c).Grow(1).Padding(12).Radius(8).Border(1, t.Border)
if _, over := ui.DragOver[*Task](done); over {
	done.Border(2, t.Accent)
}
if task, ok := ui.Drop[*Task](done); ok {
	task.Done = true
}

• Targets. The innermost element under the pointer that takes the value's type gets it, so a target inside another that takes other types does not block it.
• Canceling. Escape gives up a drag.
• Scrolling. Scroll containers scroll as a drag nears their top or bottom, faster nearer to the edge.
• Dragging several. The copy shows how many values a drag holds, beside the pointer, as when the rows chosen in a list are dragged together.

Dragging reports that an element is dragging its value, to dim it where it was.

Reordering

Lists, tables and grid views reorder their rows by dragging with ListState.Reorder and GridState.Reorder: the rows chosen move together when the row dragged is one of them, the list shows where they would go with a line, and the rows go where the pointer let them go.

app.list.Reorder = func(rows []int, to int) {
	app.items = move(app.items, rows, to) // before row to, len for the end
}

See List and Grid view.

Files dragged from other apps are dropped files.