Undo Mechanism

This page documents a simple global non-contextual undo/redo mechanism.

Mechanism

Database client keeps a global list of undoable/redoable operations. Operations are added to undo list during commit. Each commit creates a change set to server which defines the changes to resource values and statements. Change set also contains metadata for interpreting the change. The metadata format is defined by client and the server can not read or interpret it. Each operation has a unique change set identifier. Sequential operations are tagged as combined by giving them the same operation id. All operations with same id will be treated as single undoable/redoable operation.

Combining write requests

Currently there is only one way to combine several database write requests into a single operation.

Normally an operation is created out of each separate write transaction. This happens when write transactions are initiated by invoking Session.async/asyncRequest. To combine a separate write transaction into a single operation, the write request must be initiated from within the write transaction using any WriteGraph.async/asyncRequest interface methods. The database client will group all write transaction change sets initiated like this into one operation.

Undo and redo handling in the Simantics workbench

If org.eclipse.ui.edit.undo and org.eclipse.ui.edit.redo commands are bound to handlers Session{Undo,Redo}Handler then the undo/redo mechanism is activated. The handlers undo/redo one operation from the global undo/redo lists with each button press. It is desirable that each developer who develops requests that modify the graph comment their changes as shown by thefollowing example.

<syntaxhighlight lang="java">

   session.sync(new WriteRequest() {
       @Override
       public void perform(WriteGraph graph) throws DatabaseException {
           // Do your modifications.
           Layer0 b = Layer0.getInstance(graph);
           Resource s = graph.newResource();
           graph.claim(s, b.InstanceOf, b.Entity);
           
           // Add comment to change set.
           CommentMetadata cm = graph.getMetadata(CommentMetadata.class);
           cm.add("My comment.");
           graph.addMetadata(cm);
       }
   });

</syntaxhighlight>

You can test your operations using the following procedure:

• Do the operation.
• Check from Teamwork / Graph History view which change sets have been created and that they are commented properly.

Related Documents

• Undo and Redo - internals of the mechanism.