GtkAda Reference Manual - Package Gtk.Dnd

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# Package Gtk.Dnd

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.

- Gtk.Dnd Signals
- Gtk.Dnd Types
- Gtk.Dnd Subprograms

## Signals

- "drag_begin" procedure Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context);
  A new drag-and-drop operation has just been started from Widget. This callback can be used for instance to modify the visual aspect of the widget, so as to give a visual clue as to what widget is the source.
- "drag_data_delete" procedure Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context);
  This handler is called whenever the drop site of a drag-and-drop operation has decided that the data should be deleted, or automaticallyif the selected action was Action_Move. Widget is the drag source.
- "drag_data_get" procedure Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context;
  Data : Selection_Data;
  Info : Guint;
  Time : Guint);
  This should be connected to every drag source. This is used to request the actual data to be transfered to the drop site once the drop has been done. Info is the type of the expected Data, and is in fact the third field of the Target_Entry record, whose value you have define yourself. Data should be modified to include a pointer or a copy of the data, through Selection_Data_Set.
- "drag_data_received" procedure Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context;
  X : Gint;
  Y : Gint;
  Data : Selection_Data;
  Info : Guint;
  Time : Guint);
  This signal should be connected to every drop site. The handler is called every time some new data has been dropped onto Widget. (X, Y) are the mouse coordinates, relative to the widget's window, where the data was dropped. Info is the type of the data, has set in the third field of the Target_Entry record, and Data contains a pointer to the actual data.
- "drag_drop" function Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context;
  X : Gint;
  Y : Gint;
  Time : Guint)
  return Boolean;
  This is called whenever a drop is about to be performed on the widget. Note that this is called even if no common target type has been found between the drag source and the drop site. Thus, you will need to analyze the result of Get_Targets (Context) to find the possible targets. The data is sent separately through the "drag_data_received" signal, and might not even be available when "drag_drop" is emitted. This signal is mostly used if you have chosen not to use any of the default behavior when calling Dest_Set. Otherwise, everything is already handled directly by GtkAda. This handler should return True if Widget acknowledges that it is a possible drop site for the particular targets provided by the drag source.
- "drag_end" procedure Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context);
  The drag-and-drop operation that was started from the widget has been completed, and the standard set of the widget can be restored.
- "drag_leave" procedure Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context;
  Time : Guint);
  Signal emitted whenever a drag-and-drop operation is being performed, and the mouse has just left the area covered by a widget on the screen. This can be used to restore the default visual aspect of the widget. This is also emitted when the drop has been performed on the widget.
- "drag_motion" function Handler (Widget : access Gtk_Widget_Record'Class;
  Context : Drag_Context;
  X : Gint;
  Y : Gint;
  Time : Guint)
  return Boolean;
  This is called every time the user is doing a dnd operation, and the mouse is currently over Widget (but not released yet). This can be used to change the visual aspect of Widget to provide visual clues to the user. The "opposite" signal is drag_leave. The return value is ignored if Dest_Default_Motion was set when Source_Set was called. This handler should return True if Widget acknowledges that it is a possible drop site for the particular targets provided by the drag source.

## Types

    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.

## Subprograms

### Setting up a widget as a destination
    procedure Dest_Set
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class;
       Flags              :        Dest_Defaults
                           := Dest_No_Default;
       Targets            :        Target_Entry_Array
                           := Any_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.Window.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_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 Dest_Find_Target
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class;
       Context            :        Drag_Context;
       Target_List        :        Gtk.Selection.Target_List)
       return Gdk.Types.Gdk_Atom;
    function Dest_Get_Target_List
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class)
       return Target_List;
    procedure Dest_Set_Target_List
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class;
       Target_List        :        Gtk.Selection.Target_List);
### Setting up a widget as a source
    procedure Source_Set
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class;
       Start_Button_Mask  :        Gdk.Types.Gdk_Modifier_Type;
       Targets            :        Target_Entry_Array;
       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, it 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_Unset
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class);
| Undo the effects of Source_Set.
### The drag-and-drop operation
    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 Unhighlight
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class);
| Remove a highlight set by Highlight.
    function Drag_Begin
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class;
       Targets            :        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.
    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.
### Icons
    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 Set_Icon_Pixmap
      (Context            :        Drag_Context;
       Colormap           :        Gdk.Color.Gdk_Colormap;
       Pixmap             :        Gdk.Pixmap.Gdk_Pixmap;
       Mask               :        Gdk.Bitmap.Gdk_Bitmap;
       Hot_X              :        Gint;
       Hot_Y              :        Gint);
| Sets a given pixmap as the icon for a given drag. GtkAda retains a reference count for the arguments, and will release them when they are no longer needed. (Hot_X, Hot_Y) is the coordinates of the hotspot within Pixmap.
    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_Default_Icon
      (Colormap           :        Gdk.Color.Gdk_Colormap;
       Pixmap             :        Gdk.Pixmap.Gdk_Pixmap;
       Mask               :        Gdk.Bitmap.Gdk_Bitmap;
       Hot_X              :        Gint;
       Hot_Y              :        Gint);
| Change the default drag icon. GtkAda retains a reference count for the arguments, and will release them when they are no longer needed. This procedure is deprecated.
    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. Context: the context for a drag. (This must be called with a context for the source side of a drag) Pixbuf: the Gdk_Pixbuf to use as the drag icon. Hot_x: the X offset within the pixbuf of the hotspot. Hot_y: 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 Context: the context for a drag. (This must be called with a context for the source side of a drag) Stock: the ID of the stock icon to use for the drag. Hot_x: the X offset within the icon of the hotspot. Hot_y: the Y offset within the icon of the hotspot.
    procedure Source_Set_Icon
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class;
       Colormap           :        Gdk.Color.Gdk_Colormap;
       Pixmap             :        Gdk.Pixmap.Gdk_Pixmap;
       Mask               :        Gdk.Bitmap.Gdk_Bitmap);
| 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);
    procedure Source_Set_Icon_Stock
      (Widget             : access Gtk.Widget.Gtk_Widget_Record'Class;
       Stock_Id           :        String);

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