C H A P T E R 9

Drag and Drop

Single-Gesture Selection and Dragging

Because dragging is defined as moving the mouse while the mouse button is held down, a mouse-down event must occur before dragging can take place. A selection can be made as a result of this mouse-down event, just before the user starts dragging. For example, the user can select and drag a folder icon in a single gesture; the user does not have to click the folder icon first, release the mouse button, and then press again to begin dragging the icon. Your application should ensure that implicit selection occurs, when appropriate, when the user starts dragging.

Single-gesture selection and dragging is possible only when the process of selecting an item does not require dragging. Range-selection operations—such as selecting text or multiple graphic objects—don’t lend themselves to single-gesture selection and dragging because the range-selection operation itself requires dragging.

Background Selections

When a window containing a highlighted selection becomes inactive, your application should maintain the selection so that users can drag previously selected data from inactive windows to the active window.

Background selections are not required if the dragged item is discrete—for example, an icon or graphical object—because implicit selection can occur when an item is dragged. However, items selected only by range-selection operations—for example, text or a group of icons—must have a background selection to allow the user to drag these items out of inactive windows. Whenever an inactive window is made key, the background selection, if any, becomes highlighted as a normal selection.

Drag Feedback

Your application should provide drag feedback as soon as the user drags an item at least three pixels. If a user holds the mouse button down on an object or selected text, it should become draggable immediately and stay draggable as long as the mouse remains down. Typically, applications have to provide an image to drag and have to handle the receiver frame. In Aqua, dragged items are transparent.

Note: Proxy icons are not immediately draggable. Since the proxy icon is in the title bar, where a user often clicks to initiate moving a window, a proxy icon requires a user to hold the mouse button down briefly before it becomes draggable.

Destination Feedback

If the user drags an item to a destination in your application, your application provides feedback that indicates whether it will accept that item. Destination feedback should not occur simply because your application is “drag-aware”; rather, it should depend on the destination’s ability to accept the type of data contained in the dragged item. For example, a text entry field that accepts only text should not be highlighted when the dragged item is a graphic.

2008-06-09 | © 1992, 2001-2003, 2008 Apple Inc. All Rights Reserved.

C H A P T E R 9

Drag and Drop

Use cursors to indicate what result letting go of the mouse will have. For example, if you are dragging an icon out of a toolbar, show the poof cursor when the user has the icon outside of the toolbar to indicate that if they let go of it there, the item will disappear. Other cursors that provide useful feedback during a drag operation include the alias, copy, and not allowed cursors. See “Cursors” (page 157) for more information on the cursors available in Mac OS X.

Carbon: The actual appearance of destination feedback depends on the type of destination. The Drag Manager provides some utilities for simple highlighting; if your application needs more complex highlighting, you must provide your own highlighting utilities.

Windows

The valid destination region of a document window is usually the window’s content area minus the title bar and areas used for controls (such as scroll bars, resize controls, tool palettes, rulers, and placards). When there are multiple destination regions within a window, only one destination region is highlighted at a time.

When the user drags an acceptable item from one destination region to another, your application highlights the destination region as soon as the pointer enters it and removes the highlighting when the pointer leaves the region.

If a drag-and-drop operation takes place entirely within one destination region (moving a document icon to a different location in the same folder window, for example), don’t highlight the destination region, to avoid distracting the user. However, if the user drags an item completely out of a destination region and then drags the same item back to the same destination region, the destination region should be highlighted.

You can provide more specific destination feedback within a larger destination region. For example, when the user drags text from one document window to another, the destination window should display an insertion point where the dragged text would go if the user releases the mouse button.

In many situations, highlighting a more narrowly defined area of a window is more appropriate than highlighting the entire content region; examples are spreadsheets, text boxes, fill-in forms, and panes. In these cases, the destination region must be tailored to more precisely indicate the specific destination.

Text

While the user is dragging an item to a text area, an insertion indicator (a vertical bar) should appear in the text where the dragged item would be inserted if the user releases the mouse button.

Lists

An insertion indicator should appear in a list where a dragged item would be inserted if the user releases the mouse button. For example, when a user drags an item into the Sidebar of the Finder, a horizontal insertion indicator appears.

2008-06-09 | © 1992, 2001-2003, 2008 Apple Inc. All Rights Reserved.