Undo Manager

Undo is a required feature in modern applications. Gaphor is no exception. Having an undo function in place means you can change the model and easily revert to an older state.

Overview of Transactions

The recording and playback of changes in Gaphor is handled by the the Undo Manager. The Undo Manager works transactionally. A transaction must succeed or fail as a complete unit. If the transaction fails in the middle, it is rolled back. In Gaphor this is achieved by the transaction module, which provides a context manager Transaction and a decorator called @transactional.

When transactions take place, they emit event notifications on the key transaction milestones so that other services can make use of the events. The event notifications are for the begin of the transaction, and the commit of the transaction if it is successful or the rollback of the transaction if it fails.

Start of a Transaction

1. A Transaction object is created.

2. TransactionBegin event is emitted.

3. The UndoManager instantiates a new ActionStack which is the transaction object, and adds the undo action to the stack.

Nested transactions are supported to allow a transaction to be added inside of another transaction that is already in progress.

Successful Transaction

1. A TransactionCommit event is emitted

2. The UndoManager closes and stores the transaction.

Failed Transaction

1. A TransactionRollback event is emitted.

2. The UndoManager plays back all the recorded actions, but does not store it.

References

• A Framework for Undoing Actions in Collaborative Systems

• Undoing Actions in Collaborative Work: Framework and Experience

• Implementing a Selective Undo Framework in Python