Drag'n drop and clipboard handling

Virtual Treeview behaves also well when it comes to data exchange with other applications or structural manipulations using the mouse. In both cases the prerred method is using OLE. Read here why and what's behind it.

One important aspect for system integration under Windows is the ability to use OLE (object linking and embedding) to transfer data from and to other applications. Unfortunately this is a dark chapter in Delphi's feature list because there has never been support for either OLE drag'n drop or OLE clipboard handling (until Delphi 6 at least). Instead a proprietary mechanism had been invented which is not at all compatible with the rest of the system. 

Virtual Treeview supports both kinds of drag'n drop (VCL and OLE) and tries to present a single interface to the application. This means that those (already existing) events which can be reused are used in the process (like OnStartDrag and OnEndDrag). Other events however differ significantly from the VCL variants because of the additional information available during OLE drag'n drop. These events are OnDragOver and OnDragDrop. Read there for a detail description of the parameters. Since in a VCL drag'n drop operation the source is always known as being a VCL control it is relatively easy to determine the participants. This however is not very data-oriented and OLE drag'n drop focuses exactly on this issue. In such an operation a so called data object is passed to the receiver which is a COM interface (IDataObject) and can be used to retrieve the dragged data in various formats. 

To accept OLE drag'n drop an application has basically the same steps to perform as always used for VCL drag'n drop plus some extra work to handle the different data coming in during the drop event. Usually there is an event handler for OnDragOver which tells not only whether dropping is allowed on a particular position but also which effect should then take place. Allowed effects are copy, move and link. This is the first new aspect which is not possible with VCL drag'n drop. As always the real work must be done in the drop event and Virtual Treeview supports processing its own native data format (which is a stream of chunks to represent the tree structure) by a special method called ProcessDrop. Note that this method can only be used for the internal format and does not process other formats like text or images. From this information you can easily conclude that a lot of other formats can be passed around with the mighty OLE drag'n drop mechanism. It is however out of the scope of this help to describe how this mechanism works or to give an overview of possible data formats. Please read the Win32 SDK documentation as it comes with your Delphi copy or browse the MSDN online documents at MSDN online for a detailed description. The only interesting aspect you should keep in mind at the moment is that the data object used in a drag'n drop operation is the same as used for OLE clipboard data. Hence you can share code for handling of both and you don't have to learn different ways or data structures. 

The typical approach to determine how to handle data during the drop event in Virtual Treeview is as follows: 

• If the given data object is nil then the source of the drag operation is the VCL and you have to figure out yourself what and how to process the drop. The other parameters contain also mostly useful data (Effects is set to default values however). Read more details at OnDragDrop.
• With a valid data object you know OLE data is being passed. Check the source parameter to learn whether a Virtual Treeview is the source or something else. Although further processing can successfully be done without this information it is still useful if you want to optimize data transition and source as well as target tree are in the same process (in which case source memory can be accessed from the target tree).
• Loop through the given formats list to find a format you can handle. Since it is recommended to sort this list so that preferred formats come first you can simply accept the first format you find in the array which you are able to handle. With a Virtual Treeview as source usually already the second entry represents the native format (the first is a special reference format which is not useful for an application) and can be passed to ProcessDrop. The native format is registered as CF_VIRTUALTREE while other typical formats include CF_TEXT or CF_HDROP. Note that, because Virtual Treeview is already OLE drag'n drop aware, you do not need to register its window for accepting file drops. If the user drops files onto a Virtual Treeview window you will get the CF_HDROP format in the format list passed to OnDragDrop.
• Depending on the data formats you might want to take various actions. For the native tree format you will likely want ProcessDrop to handle the data. If you made sure source and target tree are in the same application (process) you can even omit the entire handling and simply call MoveTo or CopyTo.
• If you do not call any tree method or handle the dropped data somehow yourself nothing will happen. No data will be added.

Inner fundamentals