55 lines
2.6 KiB
Markdown
55 lines
2.6 KiB
Markdown
Title: Drag-and-Drop in GTK
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Drag-and-Drop (DND) is a user interaction pattern where users drag a UI element
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from one place to another, either inside a single application or between
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different application windows.
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When the element is 'dropped', data is transferred from the source to the
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destination, according to the drag action that is negotiated between both
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sides. Most commonly, that is a _copy_, but it can also be a _move_ or a
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_link_, depending on the kind of data and how the drag operation has been
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set up.
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This chapter gives an overview over how Drag-and-Drop is handled with event
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controllers in GTK.
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## Drag sources
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To make data available via DND, you create a [class@Gtk.DragSource]. Drag sources
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are event controllers, which initiate a Drag-and-Drop operation when the user clicks
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and drags the widget.
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A drag source can be set up ahead of time, with the desired drag action(s) and the data
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to be transferred. But it is also possible to provide the data when a drag operation
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is about to begin, by connecting to the [signal@Gtk.DragSource::prepare] signal.
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The GtkDragSource emits the [signal@Gtk.DragSource::drag-begin] signal when the DND
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operation starts, and the [signal@Gtk.DragSource::drag-end] signal when it is done.
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But it is not normally necessary to handle these signals. One case in which a ::drag-end
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handler is necessary is to implement `GDK_ACTION_MOVE`.
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## Drop targets
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To receive data via DND, you create a [class@Gtk.DropTarget], and tell it what kind of
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data to accept. You need to connect to the [signal@Gtk.DropTarget::drop] signal to receive
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the data when a DND operation occurs.
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While a DND operation is ongoing, GTK provides updates when the pointer moves over
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the widget to which the drop target is associated. The [signal@Gtk.DropTarget::enter],
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[signal@Gtk.DropTarget::leave] and [signal@Gtk.DropTarget::motion] signals get emitted
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for this purpose.
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GtkDropTarget provides a simple API, and only provides the data when it has been completely
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transferred. If you need to handle the data transfer yourself (for example to provide progress
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information during the transfer), you can use the more complicated [class@Gtk.DropTargetAsync].
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## Other considerations
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It is sometimes necessary to update the UI of the destination while a DND operation is ongoing,
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say to scroll or expand a view, or to switch pages. Typically, such UI changes are triggered
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by hovering over the widget in question.
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[class@Gtk.DropControllerMotion] is an event controller that can help with implementing such
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behaviors. It is very similar to [class@Gtk.EventControllerMotion], but provides events during
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a DND operation.
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