Responding to and Propagating Changes
When an element is created, deleted or updated, you can write code that propagates the change to other parts of the model, or to external resources such as files, databases, or other components.
In This Section
As a guideline, consider these techniques in the following order:
Technique |
Scenarios |
For more information |
|---|---|---|
Define a Calculated domain property. |
A domain property whose value is calculated from other properties in the model. For example, a price that is the sum of prices of related elements. |
|
Define a Custom Storage domain property. |
A domain property stored in other parts of the model or externally. For example, you could parse an expression string into a tree in the model. |
|
Override change handlers such as OnValueChanging and OnDeleting |
Keep different elements in sync, and keep external values in sync with the model. Constrain values to defined ranges. Called immediately before and after property value and other changes. You can terminate the change by throwing an exception. |
|
Rules |
You can define rules that are queued for execution just before the end of a transaction in which a change has happened. They are not executed on Undo or Redo. Use them to keep one part of the store in synch with another. |
|
Store Events |
The modeling store provides notifications of events such as adding or deleting an element or link, or changing the value of a property. The event is also executed on Undo and Redo. Use store events to update values that are not in the store. |
|
.NET Events |
Shapes have event handlers that respond to mouse clicks and other gestures. You have to register for these events for each object. Registration is typically done in an override of InitializeInstanceResources, and must be done for each element. These events usually occur outside a transaction. |
|
Bounds Rules |
A bounds rule is used specifically to constrain the bounds of a shape. |
|
Selection rules |
Selection rules specifically constrain what the user can select. |
|
OnAssocatedPropertyChanged |
Indicate the model elements’ states using features of shapes and connectors such as shadow, arrowheads, color, and line widths and style. |
Comparing Rules and Store Events
Change notifiers, rules, and events are run when changes occur in a model.
Rules are usually applied at the end transaction in which the change has occurred, and events are applied after changes in a transaction are committed.
Use store events to synchronize the model with objects outside the Store, and rules to maintain consistency within the Store.
Creating Custom Rules You create a custom rule as a derived class from an abstract rule. You must also notify the framework about the custom rule. For more information, see Rules Propagate Changes Within the Model.
Subscribing to Events Before you can subscribe to an event, create an event handler and delegate. Then use the EventManagerDirectoryproperty to subscribe to the event. For more information, see Event Handlers Propagate Changes Outside the Model.
Undoing Changes When you undo a transaction, events are raised, but rules are not applied. If a rule changes a value and you undo that change, the value is reset to the original value during the undo action. When an event is raised, you must manually change the value back to its original value. To learn more about transactons and undo, see How to: Use Transactions to Update the Model.
Passing Event Arguments to Rules and Events Both events and rules are passed an EventArgs parameter that has information about how the model changed.
See Also
Concepts
How to: Intercept a Click on a Shape or Decorator