# 4.4 - Drag and Drop

[*Drag and drop in
general*](https://web.archive.org/web/2004if_/http://developer.apple.com/documentation/Cocoa/TasksAndConcepts/ProgrammingTopics/DragandDrop/index.html)
is easy, but even easier in `NSTableView`. Here is an article about it:
[*Drag and Drop
Destinations*](http://cocoadevcentral.com/articles/000056.php).
Basically, drag and drop is copy and paste with graphic interactivity.
Therefore, it involves the pasteboard. I'll add the fuction of drag and
drop in this tutorial so that you can drag and drop in-between different
document. `NSTableView` has its built-in drag and drop support. Therefore,
it is sort of different from drag and drop in general. Here is the
[*drag and drop in `NSTableView` from
Cocoa*](https://web.archive.org/web/2004if_/http://developer.apple.com/documentation/Cocoa/TasksAndConcepts/ProgrammingTopics/TableView/Tasks/UsingDragAndDrop.html).

Firstly, I need to set up the dragging source, the source you can drag
something out. I need to register the table as dragging source, and
implement one method.

`Document.m:`

```objc
- (void) windowControllerDidLoadNib: (NSWindowController*) controller {
    NSTableColumn *column;
    NSArray *columns = [tableView tableColumns];

    column = [columns objectAtIndex: 0];
    [column setWidth: 100];
    [column setEditable: YES];
    [column setResizable: YES];
    [column setIdentifier: @"date"];
    [[column headerCell] setStringValue: @"Date"];

    column = [columns objectAtIndex: 1];
    [column setWidth: 100];
    [column setEditable: YES];
    [column setResizable: YES];
    [column setIdentifier: @"item"];
    [[column headerCell] setStringValue: @"Item"];

    column = [[NSTableColumn alloc] initWithIdentifier: @"amount"];
    [column setWidth: 100];
    [column setEditable: YES];
    [column setResizable: YES];
    [[column headerCell] setStringValue: @"Amount"];
    [tableView addTableColumn: column];
    RELEASE(column);

    [tableView sizeLastColumnToFit];
    [tableView setAutoresizesAllColumnsToFit: YES];
    
    [tableView registerForDraggedTypes: [NSArray arrayWithObjects: NSGeneralPboardType, nil]];
}

- (BOOL) tableView: (NSTableView*) view
         writeRows: (NSArray *) rows
      toPasteboard: (NSPasteboard *) pboard
{
    id object = [records objectAtIndex: [[rows lastObject] intValue]];
    NSData *data = [NSArchiver archivedDataWithRootObject: object];

    [pboard declareTypes: [NSArray arrayWithObject: @"NSGeneralPboardType"]
                                             owner: nil];
    [pboard setData: data forType: @"NSGeneralPboardType"];
    return YES;
}
```

In method -windowControllerDidLoadNib:, `NSTableView` register what kind
of pasteboard it will use. Once it registers, it is ready to drag and
drop. The [*usage of
pasteboard*](https://web.archive.org/web/2004if_/http://developer.apple.com/documentation/Cocoa/TasksAndConcepts/ProgrammingTopics/CopyandPaste/index.html)
is not covered in this tutorial. Basically, you set the type of
pasteboard, put data in, and take it out.

Method -tableView:writeRows:toPasteboard will be called when user try to
drag some rows out of the table, It is the methods in the data source of
table, not in the delegate of table. Therefore, in this method, I only
need to put the data into pasteboard. Since this table only allow single
selection, I only need to handle the data in one row, which is a
NSDictionary. I archive NSDictionary into NSData, put it into pasteboard
of NSGeneralPboardType, then return YES. Now, you can drag the row out
of the table.

`NSTableView` will handle all the graphic interactivity. When users drop
the data into another document, I need to implement two methods.

`Document.m:`

```objc
- (NSDragOperation) tableView: (NSTableView*) view
                 validateDrop: (id<NSDraggingInfo>) info
                  proposedRow: (int) row
        proposedDropOperation: (NSTableViewDropOperation) operation
{
    if (row > [records count])
    return NSDragOperationNone;

    if (nil == [info draggingSource]) { // From other application
        return NSDragOperationNone;
    }
    else if (tableView == [info draggingSource]) { // From self
        return NSDragOperationNone;
    }
    else { // From other documents 
        [view setDropRow: row dropOperation: NSTableViewDropAbove];
        return NSDragOperationCopy;
    }
}

- (BOOL) tableView: (NSTableView*) view
        acceptDrop: (id<NSDraggingInfo>) info
               row: (int) row
     dropOperation: (NSTableViewDropOperation) operation
{
    NSPasteboard *pboard = [info draggingPasteboard];
    NSData *data = [pboard dataForType: @"NSGeneralPboardType"];

    if (row > [records count]) {
        return NO;
    }

    if (nil == [info draggingSource]) { // From other application
        return NO;
    }
    else if (tableView == [info draggingSource]) { // From self
        return NO;
    }
    else { // From other documents
        id object = [NSUnarchiver unarchiveObjectWithData: data];
        [records insertObject: object atIndex: row];
        [tableView reloadData];

        return YES;
    }
    return NO;
}
```

When users hold the mouse button and move above the table, the table
will keep receiving the call for method
-tableView:validateDrop:proposedRow:ProposedDropOperation:. The only
thing I need to do in this method is to examine the proposed drop and
return the action I can accept. If the source is not from other
documents, return NSDragOperationNone to indicate that this table
doesn't accept drop. Otherwise, return NSDragOperationCopy to indicate
that this table accept copy. In table, you can drop between rows or over
rows. That's NSTableViewDropOperation means. I only want to insert the
data. When it is proposed to drop, I change the propose by
-setDropRow:dropOperation:. By this way, I can change all the drop
in-between rows, not over rows. So this method gives the table chances
to change its interface when mouse is moving above it.

Once users release the mouse button, method
-tableView:acceptDrop:row:dropOperation: will be called. I get the
dropped data from this method. Extract the data from pbastebord,
unarchive into NSDictionary, and add into records. Don't forget to
reload the table to reflect the change.

`NSTableView` offers the basic fuction of drag and drop, which is useful
for general applications. If you want more support for drag and drop,
you have to subclass `NSTableView` and write your own using the general
drag and drop support of GNUstep. For example, `NSTableView` doesn't
support drag and drop from other application. If you want to do that,
you have to subclass `NSTableView`. I'll talk about the drag and drop in
general later.

Here is the source code:
[DragDropInTable-src.tar.gz](http://gnustep.made-it.com/GSPT/DragDropInTable/DragDropInTable-src.tar.gz)