Support undo/redo stack within the core package

[gohabereg]

Problem statement

Undo/redo stack is currently implemented within Collaboration Manager. It sounded logical at the start — manager is responsible for creation and transformation of the operations on the model. Undo is basically applying a reverse operation. However, this brings a major issue — the core functionality is being handled essentially by a plugin.

Proposed solution

To support undo/redo stack core only needs to know how to inverse an operation and then apply it back on the model. It doesn't need to know about remote operations, their transformation and so on.

I see two options:

1. Create operations in the model package (we can call them "Transactions") and emit them as event detail of the model updates events. (We can move Operation code to the sdk package for example)
   a. Core would receive operations from the model and handle operations inversion and undo/redo stack
   b. Collaboration Manager would receive operations from the model and handle their transformations and communication with the OT server
2. Do not move the operations, but implement similar interface within the core with inversion as only available transformation. Model would stay the same

I believe the second option is better as changing the model events interface would require massive refactoring. Moreover, within the core we don't need to create an operation entity, it only needs to be able to store undo stack, inverse the update and apply it back.

In both cases, when undo operation is applied on the model, Collaboration Manager would just receive a regular model update and create a new operation.

Let's discuss

[ilyamore88]

To support undo/redo stack core only needs to know how to inverse an operation and then apply it back on the model. It doesn't need to know about remote operations, their transformation and so on.

If I understand you correctly, the solution with undo/redo in the core module works for both single/collaborative editing. But if this is collaborative, there are corner cases where people work on nearby text parts. How do you plan to update local transactions in undo/redo in this case?

[gohabereg]

You right, stack should be transformed too. Maybe we can implement the stack separately and add it to core. And if collaboration manager is in use, use the stack from collaboration manager.

The idea I had is in case undo/redo is implemented in the core, collaboration manager could be a plugin. So 3rd party dev could implement its own version not relying on OT for example.

We could actually transform the stack based on the model updates. So it's independent from the collaborative capabilities implementation

[e11sy]

sounds good with the idea of moving single user undo/redo logic into the core and allowing to overwrite this undo/redo management by plugin (since we want CollaborationManager to be a plugin)

[gohabereg]

Implemented here #145