A vast majority of successful applications use the tree control to display a variety of information to the user. The tree control has become so ubiquitous that the users are very familiar with its operations. One of the common problems with the tree control is saving the state between sessions. This article presents a class
TBTreeStateMgr that makes it effortless to save and restore the state of multiple trees.
What is a Tree State ?
The tree state consists of:
- the expanded state, which nodes are expanded and which are collapsed
- the selection state, which nodes are selected
- the visibility state, which nodes are currently visible
TBTreeStateMgr only stores the expanded state.
The following are the highlights of
- Can handle multiple tree instances
- Each tree instance must be assigned a unique tree ID
- Uses XML to store/load the tree states
- All methods in
TBTreeStateMgr are declared
static. This is due to the fact that a single instance of
TBTreeStateMgr can handle storage of a number of tree states.
TBTreeStateMgr is threadsafe
- Uses COM Structured Storage to store individual XML files
- Include the following header file in your project:
- Ensure that
OleInitialize is called in the
- Place this code in the
InitInstance function of your MFC application:
- Place this code in the
- To save the tree state, simply call:
BOOL TBTreeStateMgr::SaveTreeState(LPCTSTR lpszTreeContextName,CTreeCtrl * pTreeCtrl);
Note: This function is typically called before quitting the window. For example: by processing the
TBTreeStateMgr::SaveTreeState(_T("My Tree1 View"),&wnd_TreeCtrl);
- To restore the tree instance to its saved state, simply call:
BOOL TBTreeStateMgr::LoadTreeState(LPCTSTR lpszTreeContextName,CTreeCtrl * pTreeCtrl);
Note: This function is typically called just after initializing the tree control with the data. For example: in the
TBTreeStateMgr::LoadTreeState(_T("My Tree1 View"),&wnd_TreeCtrl);
- [OPTIONAL] Change the storage file.
TBTreeStateMgr by default stores the Structured Storage file in the current directory under the file name "CTLStateStg.ctss", if you want to specify a different directory or path, use the
To reset all tree states, simply delete the "CtlStateStg.ctss" file. This file will be automatically generated if it is not found.
This section describes in short some of the implementation choices I had to make. I hope my use of XML and
IStorage/IStream in this project will demonstrate how powerful these tools are. Structured Storage is in fact used by MSWord. The DOC format is actually a
IStorage compatible file.
There are many mechanisms to store and retrieve tree states. A popular approach seems to be to store the tree state in the registry. For document/view applications that use the tree control, some developers choose to store the latest tree state as part of the document. So each document will be able to restore its view to the state when it was last closed. The registry method is not secure, safe nor scalable. Most projects do not have the luxury of modifying the document file to accommodate storing the latest state. In any case, it is a bad idea to store the view state along with the document object.
While I was evaluating techniques to store the tree control state, I stumbled upon XML. XML is also highly tree structured. If we could somehow transfer the tree state to an XML document efficiently and back, we could have a really cool solution. The MSXML parser is highly efficient and widely available with IE5 deployments. The bulk of the code in
TBTreeStateMgr is devoted to translating the tree state to XML and back. The XML DTD for this project is:
<!ELEMENT node (#PCDATA)>
The next major problem was: How to store multiple trees?
If you open the CtlState.ctss using the "Docfile Viewer", we can see the following structure:
As the above figure shows, the DOCfile stores each XML document in a separate stream. The reason for this architecture is: Any large scale application would have to deal with multiple trees (or) with multiple users view of the same tree. An application may also have trees in the views of multiple documents. For example: Each book document can have a tree view showing the table of contents. If we created a different XML Document for each tree instance, we may end up with hundreds of files. Another reassuring factor was Structured Storage is used extensively by MS in its own products (MSWord/Excel store DOC,XLS files in Structured Storage).
This can be enhanced to support storage of any state. Just change the XML DTD to add elements of your own liking. For example: this can be enhanced to save/restore state of Toolbars, Windows, Splitter Windows, List, Grid.
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