Diagram state management concerns how the state of a diagram is managed by the UI and other objects that need access to it.
The state of a diagram and the operations available to manipulate it are available via a layered architecture, wherein a simple diagram data model is aggregated by objects of increasing complexity. The important principle guiding this design is that each diagram tab manages a single diagram, which is supported by stateful wrappers that always remain consistent with it.
The objects involved in this design are:
Diagram: The simple data model for a diagram: nodes and edges.DiagramRenderer: A wrapper that can compute the geometry of the diagram. This wrapper has additional state to cache computations and support global path planning of edges.DiagramBuilder: Wrapper around aDiagramRendererthat provides business rules for modifying a diagram while preserving its validity.DiagramCanvas: The area of the GUI where the diagram is visually displayed.DiagramTab: The top-level GUI component that hosts the diagram canvas.
To keep the diagram state management principled, references to the diagram should be obtained via the highest-level wrapper available and not aliased. Each class in the design provides getter methods for obtaining the lower wrappers in the layered architecture. This design feature is illustrated in the following diagram.
For example, within DiagramCanvas, the diagram is obtained by calling aDiagramBuilder.diagram(), and similarly for obtaining the renderer.