Editors and editing

Because of the virtual nature of Virtual Treeview editing becomes a difficult issue. Read here what needs to be considered and where you can hook in to allow any editor for a node.

Generally it cannot be said what data a user will edit when he or she edits a node. In the case of the string tree it becomes a lot easier to decide because we have, as the name implies, strings and captions to edit. But this is only a special case and the underlaying edit principle must be flexible to allow editing various different data of a node, including several items instead of only single ones. 

Since you cannot generally tell what will be edited the used solution does not assume anything. Instead it delegates the entire process to the application or derived trees via the IVTEditLink class. This interface defines some necessary methods which allow interaction between the tree and the editor but the actual editor implementation is up to the edit link (which can of course delegate this task to even another instance like the application). The edit link is responsible for everything including to hide and show the editor, reading the old values of a node and setting the new values etc. The tree only signals some general states like the edit start, end or cancellation. 

Editing starts with the protected DoEdit method which may be triggered by the edit timer (which in turn is triggered by clicking again on the focused node), by pressing F2 or by calling EditNode. DoEdit creates an editor (actually only the edit link) via the virtual CreateEditor method which should be overridden by descendant trees to return a valid edit link (as TVirtualStringTree does). Otherwise the method will query the application for an editor link. Actual editing starts only if CreateEditor returns a valid edit link. 

After the tree received a valid edit link it initiates communication by calling PrepareEdit which can be used by the link to retrieve the values to be edited using the given node and column. If the edit link returns True in this call another call is initiated by the tree telling the link where to place the editor using the SetBounds method. Finally the tree calls BeginEdit to actually start the edit operation. From now on the edit link is responsible for any further action including passing on key presses like VK_UP and VK_DOWN to select a new node to edit etc. The link must also be aware that editing might be stopped at any time by EndEdit or CancelEdit. Otherwise however the edit link (and its editor(s)) is completely autonomous and can use whatever it considers as being appropriate for the editing task. It isn't even limited to use an in-place editor. 

With the class TStringEditLink you will find a sample implementation used in the string tree to edit single node captions. By examining the used editor (a normal TEdit control) you will find some things which should be handled the same or in a similar way to make editing smooth. 

Starting with version 3.8 Virtual Treeview allows to use the TNT controls suite from Troy Wolbrink, which allow to edit node captions with Unicode content. Download the latest package and add its path after installation to your project. Enable the TntSupport compiler switch by changing it from {.$define TntSupport} to {$define TntSupport} and recompile.

Inner fundamentals