Like all modern GUI toolkits, GtkAda has a full support for drag-and-drop operations. This is a mechanism for interactively transferring data between two widgets, either in the same application or in two different applications. The user clicks on a widget (called a "drag source"), and, while keeping the mouse button pressed, moves it to another widget, where the mouse button is released (this other widget is called a "drop site"). As a result, and if both widgets can handle the same type of data, some data is either copied or moved to this new widget.
This is a very intuitive way, in some cases, to enhance the usability of your application, although you should carefully consider whether this should be used or not.
GtkAda supports several drag-and-drop protocols, so as to be able to communicate with the maximum number of applications. These protocols are Xdnd and Motif.
Below is a summary of what is needed to add drag-and-drop capabilities to your application. We highly recommend that you look at, and understand, the example in testgtk (create_dnd.adb), before using these features in your own application.
See also the package Gtk.Selection, that contains some lower subprograms and data types that are used when implementing drag-and-drop.
Defining a widget as a possible drag source
You need to call Source_Set, specifying which mouse buttons can activate the drag, which types of data will be given, and which kind of action will be performed. You then need to connect to the signal "drag_data_get", that will be emitted when the user has dropped the item and GtkAda needs to find the data. You must call Selection_Data_Set in the handler to set the actual data. You can also connect the widget to "drag_data_delete", which will be called whenever the data set for the selection is no longer required and should be deleted. The signal will be emitted only if the drop site requests it, or if the selected action for the drag-and-drop operation was Action_Move. It will not be called automatically for an Action_Copy. Note that the callback might be called several times, if for instance this was an Action_Move, and the drop site requires explicitly to delete the data in its call to Finish.
Defining a widget as a possible drop site
You need to call Dest_Set, specifying which types of Data are accepted by the widget, which actions are recognized, and whether you accept drops from external applications. You also need to connect to "drag_data_received", that will be emitted when the user has dropped some data on the widget. The handler should call Finish, to warn the source widget that the drag and drop operation is finished, and whether it was successful or not.
<c_version>2.16.6</c_version> <group>Inter-Process communication</group> <testgtk>create_dnd.adb</testgtk>
function Check_Threshold
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Start_X : Gint;
Start_Y : Gint;
Current_X : Gint;
Current_Y : Gint) return Boolean
Checks to see if a mouse drag starting at (Start_X, Start_Y) and ending at (Current_X, Current_Y) has passed the GTK drag threshhold, and thus should trigger the beginning of a drag-and-drop operation. Return True if the drag threshold has been passed.
procedure Dest_Add_Image_Targets
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Add the image/text/URI targets supported by Gtk_Selection to the target list of the drag destination. The targets are added with Info = 0. If you need another value, use Gtk.Selection.Target_List_Add_*_Targets, and Dest_Set_Target_List
procedure Dest_Add_Text_Targets
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Add the image/text/URI targets supported by Gtk_Selection to the target list of the drag destination. The targets are added with Info = 0. If you need another value, use Gtk.Selection.Target_List_Add_*_Targets, and Dest_Set_Target_List
procedure Dest_Add_Uri_Targets
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Add the image/text/URI targets supported by Gtk_Selection to the target list of the drag destination. The targets are added with Info = 0. If you need another value, use Gtk.Selection.Target_List_Add_*_Targets, and Dest_Set_Target_List
Dest_Default_All : constant Dest_Defaults;
If set, specifies that all default actions should be taken.
Dest_Default_Drop : constant Dest_Defaults;
If set for a widget, when a drop occurs, GtkAda+ will check if the drag matches this widget's list of possible targets and actions. If so, GtkAda will call Get_Data on behalf of the widget. Whether or not the drop is succesful, GtkAda will call Drag_Finish. If the action was a move, then if the drag was succesful, then True will be passed for the delete parameter to Finish.
Dest_Default_Highlight : constant Dest_Defaults;
If set for a widget, GtkAda will draw a highlight on this widget as long as a drag is over this widget and the wiget drag format and action is acceptable.
Dest_Default_Motion : constant Dest_Defaults;
If set for a widget, GtkAda, during a drag over this widget will check if the drag matches this widget's list of possible targets and actions. gdk_drag_status is called as appropriate.
type Dest_Defaults is new Integer;
Specify the various types of action that will be taken on behalf of the user for a drag destination site.
function Dest_Find_Target
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Context : Gdk.Drag_Contexts.Drag_Context;
Target_List : Gtk.Target_List.Gtk_Target_List) return Gdk.Types.Gdk_Atom
Looks for a match between the targets set for context and the Target_List, returning the first matching target, otherwise returning GDK_NONE. Target_List should usually be the return value from Dest_Get_Target_List, but some widgets may have different valid targets for different parts of the widget; in that case, they will have to implement a drag_motion handler that passes the correct target list to this function.
function Dest_Get_Target_List
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
return Gtk.Target_List.Gtk_Target_List
Sets the target types that this widget can accept from drag-and-drop. The widget must first be made into a drag destination with Dest_Set.
function Dest_Get_Track_Motion
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
return Boolean
Control whether the widget emits drag-motion and drag-leave events regardless of the targets and the Dest_Default_Motion flag.
This may be used when a widget wants to do generic actions regardless of the targets that the source offers.
Dest_No_Default : constant Dest_Defaults;
No default behavior is provided for the drop site, this is your own responsabily. You need to handler the "drag_drop" signal yourself.
procedure Dest_Set
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Flags : Dest_Defaults := Dest_No_Default;
Targets : Target_Entry_Array := No_Target_Entry;
Actions : Drag_Action := Action_Any)
Set a widget as a potential drop destination.
Flags specifies what action GtkAda should take on behalf of a widget for drops onto that widget. The Targets and Actions fields are used only if Dest_Default_Motion or Dest_Default_Drop are given.
Targets indicates the drop types that Widget accepts. If no item from Targets matches the list of targets emitted by the source (as set in Source_Set), then the drop will be considered illegal and refused.
Actions is a bitmask of possible actions for a drop onto Widget. At least of the actions must be in common with what was set for the source in Source_Set, or the drop is considered illegal.
procedure Dest_Set_Proxy
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Proxy_Window : Gdk.Gdk_Window;
Protocol : Drag_Protocol;
Use_Coordinates : Boolean)
Set this widget as a proxy for drops to another window. All drag events on Widget will be forwarded to Proxy_Window. Protocol is the drag protocol that Proxy_Window accepts. You can use Gdk.Drag.Get_Protocol to determine this. If Use_Coordinates is True, send the same coordinates to the destination because it is an embedded subwindow.
procedure Dest_Set_Target_List
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Target_List : Gtk.Target_List.Gtk_Target_List)
Sets the target types that this widget can accept from drag-and-drop. The widget must first be made into a drag destination with Dest_Set.
procedure Dest_Set_Track_Motion
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Track_Motion : Boolean)
Control whether the widget emits drag-motion and drag-leave events regardless of the targets and the Dest_Default_Motion flag.
This may be used when a widget wants to do generic actions regardless of the targets that the source offers.
procedure Dest_Unset
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Clear information about a drop destination set with Dest_Set. The widget will no longer receive notification of drags.
function Drag_Begin
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Targets : Gtk_Target_List;
Actions : Drag_Action;
Button : Gint;
Event : Gdk.Event.Gdk_Event) return Drag_Context
Initiate a drag on the source side. The function only needs to be used when the application is starting drags itself, and is not needed when Source_Set is used. Targets is the list of targets (data formats) in which the source can provide the data. Actions is a bitmask of the allowed drag actions for this drag. Button is the button the user clicked to start the drag. Event is the event that triggered the start of the drag.
procedure Finish
(Context : Drag_Context;
Success : Boolean;
Del : Boolean;
Time : Guint32 := 0)
Inform the drag source that the drop is finished, and that the data of the drag will no longer be required. Success should indicate whether the drop was successful. Del should be set to True if the source should delete the original data (this should be True for a move).
procedure Get_Data
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Context : Drag_Context;
Target : Gdk.Types.Gdk_Atom;
Time : Guint32 := 0)
Get the data associated with a drag. When the data is received or the retrieval fails, GtkAda will emit a "drag_data_received" signal. Failure of the retrieval is indicated by the length field of the selection_data signal parameter being negative. However, when Get_Data is called implicitely because the Drag_Default_Drop was set, then the widget will not receive notification of failed drops.
Target is the target (form of the data) to retrieve. Time is a timestamp to retrive the data, and will be given to "drag_data_motion" or "drag_data_drop" signals.
function Get_Source_Widget
(Context : Drag_Context) return Gtk.Widget.Gtk_Widget
Determine the source widget for a drag. If the drag is occuring within a single application, this function returns the source widget. Otherwise, it returns null.
procedure Highlight (Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Draw a highlight around a widget.
procedure Set_Icon_Default (Context : Drag_Context)
Set the icon for a particular drag to the default icon. This must be called with a context for the source side of a drag.
procedure Set_Icon_Name
(Context : Drag_Context;
Icon_Name : String;
Hot_X : Gint;
Hot_Y : Gint)
Sets the icon for a given drag from a named themed icon. See the docs for Gtk_Icon_Theme for more details. Note that the size of the icon depends on the icon theme (the icon is loaded at the symbolic size GTK_ICON_SIZE_DND), thus Hot_X and Hot_Y have to be used with care.
procedure Set_Icon_Pixbuf
(Context : Drag_Context;
Pixbuf : Gdk.Pixbuf.Gdk_Pixbuf;
Hot_X : Gint;
Hot_Y : Gint)
Sets Pixbuf as the icon for a given drag.
the context for a drag. (This must be called with a context for the source side of a drag)
the Gdk_Pixbuf to use as the drag icon.
the X offset within the pixbuf of the hotspot.
the Y offset within the pixbuf of the hotspot.
procedure Set_Icon_Stock
(Context : Drag_Context;
Stock_Id : String;
Hot_X : Gint;
Hot_Y : Gint)
Sets the icon for a given drag from a stock ID
the context for a drag. (This must be called with a context for the source side of a drag)
the ID of the stock icon to use for the drag.
the X offset within the icon of the hotspot.
the Y offset within the icon of the hotspot.
procedure Set_Icon_Widget
(Context : Drag_Context;
Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Hot_X : Gint;
Hot_Y : Gint)
Change the icon for a drag. GtkAda will not destroy the icon, so if you don't want it to persist, you should connect to the "drag_end" signal and destroy it yourself. Context is the reference to the current drag operation. Widget is the toplevel window to use as an icon. (Hot_X, Hot_Y) is the coordinates of the hot point (that will be just under the mouse) within Widget.
procedure Source_Add_Image_Targets
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Add the writable image/text/URI targets supported by Gtk_Selection to the target list of the drag source. The targets are added with Info = 0. If you need another value, use Gtk.Selection.Target_List_Add_*_Targets, and Source_Set_Target_List
a #GtkWidget that's is a drag source
procedure Source_Add_Text_Targets
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Add the writable image/text/URI targets supported by Gtk_Selection to the target list of the drag source. The targets are added with Info = 0. If you need another value, use Gtk.Selection.Target_List_Add_*_Targets, and Source_Set_Target_List
a #GtkWidget that's is a drag source
procedure Source_Add_Uri_Targets
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Add the writable image/text/URI targets supported by Gtk_Selection to the target list of the drag source. The targets are added with Info = 0. If you need another value, use Gtk.Selection.Target_List_Add_*_Targets, and Source_Set_Target_List
a #GtkWidget that's is a drag source
function Source_Get_Target_List
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
return Gtk.Target_List.Gtk_Target_List
Changes the target types that this widget offers for drag-and-drop. The widget must first be made into a drag source with Source_Set.
procedure Source_Set
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Start_Button_Mask : Gdk.Types.Gdk_Modifier_Type;
Targets : Target_Entry_Array := No_Target_Entry;
Actions : Drag_Action)
Set up a widget so that GtkAda will start a drag operation when the user clicks and drags on the widget. The widget must have a window.
Targets is the list of targets that the drag can provide. The first possible target accepted by the drop site will be used. For instance, if Targets contains "text/plain" and "text/url", and the drop site only accepts "text/url", this will be the one used. However, if the drop site also accepts "text/plain", the latter will be prefered.
Widget needs to be able to convert the data to any of the types in Target, as any of them might be requested by the drop site.
Actions is a list of possible actions for drags from Widget. At least one of the actions must be in common with the drop site for the drag-and-drop operation to succeed.
procedure Source_Set_Icon_Name
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Icon_Name : String)
Set the icon that will be used for drags from a particular widget. GtkAda retains a reference count for the arguments, and will release them when they are no longer needed.
procedure Source_Set_Icon_Pixbuf
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Pixbuf : Gdk.Pixbuf.Gdk_Pixbuf)
Set the icon that will be used for drags from a particular widget. GtkAda retains a reference count for the arguments, and will release them when they are no longer needed.
procedure Source_Set_Icon_Stock
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Stock_Id : String)
Set the icon that will be used for drags from a particular widget. GtkAda retains a reference count for the arguments, and will release them when they are no longer needed.
procedure Source_Set_Target_List
(Widget : access Gtk.Widget.Gtk_Widget_Record'Class;
Target_List : Gtk.Target_List.Gtk_Target_List)
Changes the target types that this widget offers for drag-and-drop. The widget must first be made into a drag source with Source_Set.
procedure Source_Unset (Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Undo the effects of Source_Set
procedure Unhighlight (Widget : access Gtk.Widget.Gtk_Widget_Record'Class)
Remove a highlight set by Highlight.