Integrating with Drag and Drop

This chapter describes the drag-and-drop user model and the Common Desktop Environment drag-and-drop convenience application program interface (API), and describes how to use drag and drop.

• Summary

• Drag-and-Drop User Model

• Drag-and-Drop Convenience API

• Drag-and-Drop Transaction

• Integration Action Plan

• API Overview

• How Drag Sources Are Used

• How Drop Zones Are Used

Summary

The Common Desktop Environment contains an application program interface (API) for drag and drop that is layered on top of Motif to provide convenient, consistent, and interoperable drag and drop across the desktop. The Common Desktop Environment drag-and-drop API makes it easier for developers to implement drag and drop. With drag and drop, users can manipulate objects on the screen directly by grabbing them, dragging them around the display, and dropping them on other objects to perform a transfer of data.

Text, files, and buffers are the three categories of data that are used with the Common Desktop Environment drag-and-drop API. Text is defined, in this context, as any user-visible text such as text in type-in fields. A file is a container of data that resides in the file system. Each file also has a format that describes its contents. Buffers are data contained in memory. Typically, each buffer also has a format that describes its contents.

Library and Header Files

To use drag and drop, you need to link to the DtSvc library. The header file is Dt/Dnd.h.

Demo Program

A demo program containing an example of drag and drop is in /usr/dt/examples/dtdnd.

Using Drag and Drop

To Integrate with Drag and Drop

To integrate your application with drag and drop, follow these steps:

1. Include Dt/Dnd.h.

2. Link to libDtsvc.

3. As recipient:

   1. Register as a drop zone using DtDndDropRegister.

   2. Optionally, write a drop animate callback.

   3. Write a transfer callback.

4. As source:

   1. Recognize user action (possibly requiring a modification of translation tables) and call DtDndDragStart.

   2. Write a convert callback.

   3. Write a drag finish callback.

Drag-and-Drop User Model

This section describes the user model behind drag and drop to help you design an application that is consistent with the rest of the desktop and users' expectations.

See the Common Desktop Environment: Style Guide and Certification Checklist for more information about the drag-and-drop user model and for guidelines for the visual appearance of drag-and-drop elements.

When drag and drop is available for all applications on the desktop, the system is more predictable to the user and is, therefore, easier to use and to learn. Users leverage their learning across more applications by using skills that they already know. In addition, many users prefer drag and drop to using menus.

In this chapter, the term drop zone is used to describe places where users can drop something. Drop zones are usually represented by a control or icon graphic; for example, a trash icon or a type-in field graphic. The term drop target is used to describe the rectangular area that represents the drop zone.

Drag and Drop Capability

With the Drag and Drop capability, users can select and manipulate objects represented as icons.

Note - Drag and drop is an accelerator to functionality that is accessible through other user interface controls supported within your application. However, not all users are able to take advantage of drag and drop. Do not support any basic operations solely through drag and drop. Any basic function that your application supports through drag and drop should also be supported by menus, buttons, or dialog boxes.

Drag Icons

When users select and manipulate icons using drag and drop, they expect the graphic icon that represents the item being dragged to remain consistent from the selection through the drag and drop. If the user selects a message icon in the File Manager and starts to drag it, the source portion of the drag icon is represented by that message icon. Providing this kind of consistency makes drag and drop more predictable to the user. Where the destination application uses icons, the icon shown should, in most cases, be the same one that was selected and then dragged and dropped. This behavior is not, however, always appropriate for all applications. Dragging text is an exception. A text drag icon is used instead of dragging the selected text.

Both the source and destination applications specify the visual appearance of drag icons. You are responsible for ensuring that an application has a consistent and appropriate icon to drag. Although the drag-and-drop library provides default icons, it is a good idea for you to specify your own for each application. Most often, you should use the data-typing database to obtain the icon associated with the type data represented by the icon. See Chapter 9, Accessing the Data-Typing Database.

When users start a drag without selecting an icon, it is appropriate for you to provide a relevant drag icon. For example, in an appointment editor, the user can select an appointment out of a scrolling list--which may or may not show icons. You should use an appointment icon as the source indicator. The destination application (for example, a File Manager) should display the same appointment icon.

Parts of the Drag Icon

The drag icon changes appearance to provide drag-over feedback when the user moves it over potential drop zones.

The drag icon has three parts that combine to provide the drag-over feedback:

• A state indicator

• An operation indicator

• A source indicator

The state indicator is a pointer used for positioning combined with a valid or invalid drop zone indicator. The valid state indicator is an arrow pointer. The pointer has a hot spot so users can position it in a predictable manner. The invalid state indicator--a circle with a diagonal line--is displayed when users have positioned the cursor over an invalid drop zone.

The operation indicator gives users feedback on what operation is occurring during the drag; either move, copy, or link. Because most drags are moves, users are given additional feedback when they perform the less-frequent copy or link operations.

Note - The operation feedback is drawn on top of the state and source feedback. This behavior is consistent with Motif drag-and-drop behavior.

The user can choose the drag operation move, copy, or link by pressing and holding certain keys during a drag, as shown in Table 5-1.

Table 5-1 Keys Used to Modify a Drag Operation

The source application can force a copy, as in the case of the read-only File Manager window. When the user chooses an operation, the drop zone must match that operation for the drop to succeed; otherwise, the drop zone is invalid. In other words, if the user chooses a copy by holding down the Control key, and then drags the drag icon over the trash icon, the drag icon should show the trash icon as an invalid drop zone and any drop should fail, because copying to the trash is not allowed.

The source indicator represents the selection (or the item being dragged). The source indicator varies depending on whether the selection represents single or multiple items and what kind of item the selection represents.