TREESCRL Scrolls in a Tree Control During Drag-and-Drop

A TreeView control is one capable of displaying items in a tree-like hierarchical fashion. The control supports drag-and-drop. When an item is dragged to the top or bottom of the control, the control does not automatically scroll vertically.

Treescrl.exe shows how to add scrolling functionality to a tree control during drag-and-drop.

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TREESCRL uses the CTreeView view class that encapsulates the functionality of a view and a tree control. The sample's drag-and-drop functionality is similar to the one demonstrated in CMNCTRLS, a sample shipped with the Visual C++ product. When TREESCRL is built and run, an SDI window with a CTreeView comes up. When you grab a tree control item (parent or child), and drag it above/below the top/bottom of the view window, the tree control starts scrolling upward/downward.

The idea behind TREESCRL is to install a timer when a drag-and-drop operation starts. In the handler for the WM_TIMER message, it is first determined if scrolling is needed. If it is, then the appropriate scroll message is sent to the control. When the drag-and-drop operation terminates, the timer is killed. The sample shows how to draw the drag image and how to select the drop site during the dragging phase.

The tree control can scroll in different modes. During the drag phase, when the mouse cursor is close to the top or the bottom border of the view, scrolling is slow. The control scrolls at this slow speed when the horizontal distance between the mouse and the corresponding view border (top/bottom) is less than the width of a horizontal scrollbar. Scrolling speeds up when the mouse is taken further away from either border.

SLOWSCROLL_FREQUENCY is a macro symbol in the view's .cpp file. This symbol determines the relative speeds of normal and slow scrolling. Normal scrolling occurs approximately every 75 milliseconds (the frequency of the timer). In the sample, SLOWSCROLL_FREQUENCY is defined to be three (3), which means slow scrolling is three times slower than normal scrolling. To change the slow scrolling speed, modify this value to a positive integer.