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 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. Database client groups all change sets created within one write request as one operation.

Undo and redo handling

If crtl-z and crtl-y keys are mapped to call DiagramUndo/RedoHandler 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 them as the following example shows.

    session.syncRequest(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);
        }
    });

This can be done by the following procedure:

• Do the operation.
• Check from graph history view which change sets have been created and that they are commented properly.

Related Documents

• Undo and Redo - internals of the mechanism.