Undo Mechanism

This page documents a mechanism for defining context specific undo/redo operations.

Mechanism

Client keeps context specifc lists of undoable/redoable operations. Operations are added to undo list during commit if committed request has an undo context. 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 unique change set identifier. Sequential operations can be tagged as combined by giving them the same operation id. All operations with same id will be treated as single undoable/redoable operation.

Procedure

Create context

    Session session = getSession();
    UndoContext uctx = new UndoContextEx();

Create request with context

    session.syncRequest(new UndoWriteRequest(uctx, true) {
        @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." + cu);/
            graph.addMetadata(cm);
        }
    });

Use undo and redo operations

   UndoRedoSupport support = session.getService(UndoRedoSupport.class);
   uctx.undo(support);
   uctx.redo(support);

Diagram undo and redo handling

Each diagram viewer has its own undo context which can be get with getAdapter call. Crtl-z and crtl-y keys are mapped to call DiagramUndo/RedoHandler which uses getAdapter call to get the undo context and then calls undo/redo operation. Each developer who develops diagram(s)) must check that wanted operations are undoable for the diagram undo to be consistent. This can be done by the following procedure:

• Do the operation.
• Check from local history view which change sets have been created.
• Check from undo view that change sets are combined to operations and added to the undo context correctly.