# qooxdoo

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# Undo and Redo

## Limitations

The implementation of undo/redo in the HtmlArea has some limitations you should be aware of. It is possible to `undo` all of your steps but `redo` is only possible when no other action occured between the undo and the redo action. If you undo several steps and e.g. enter some text you can not execute redo anymore.

Note

If you use the Undo/Redo functionality you have to make sure you are not manipulating the content of the HtmlArea by using the `innerHTML` property of an element.

This will break Undo/Redo functionality!

## Implementation: Description on a high-level

The implementation is split up into two different approaches.

For Internet Explorer the `execCommand` approach can't be used anymore. The internal undo / redo stack gets broken on every DOM manipulation. So, if any qooxdoo decorator is used this approach is a dead end. Instead an own implementation using `innerHTML` is used for IE browsers.

For all other browsers the base of the Undo/Redo functionality is to use the `execCommand` method to manipulate the content whenever possible. Each change which is performed with a call of `execCommand` is easy to undo/redo. For any manipulation which cannot be achieved using the built-in `execCommand` a special implementation for each browser is necessary (e.g. changing the background color of the whole document).

### Using the Decorator Pattern

To easily integrate the undo/redo management with the commands of the HtmlArea the `UndoManager` class is a decorator of the `CommandManager` class. It takes the method calls from the HtmlArea class, collects the info for undo the action and calls the decorated `commandManager` class to actually perform the requested action. This keeps both implementations clean and separated.

### Tracking changes using stacks

Two stacks keep track of the changes which are done to the content: an undo stack and the corresponding redo stack. Currently each stack holds four different types of changes:

- Command
- Content-block
- Custom
- Internal

Each entry in the stacks is represented by an object which holds additional info (the type above is among this info).

#### Command

Every change which is performed with the `execCommand` method is equipped with this type. These changes are the easiest to track and to undo/redo.

#### Content-block

Each keypress event is observed to determine changes in the content and to mark a set of content changes as an own block which is capable for an undo/redo step. For example IE and Gecko do both recognize text changes as a content block if the text changes occured between two calls of `execCommand`.

#### Custom

These changes are the ones which cannot be handled with the built-in `execCommand` method. For example changing the background color of the whole document is a custom undo/redo step which needs to be handled in a special way by each browser.

#### Internal

These steps are included to keep the stacks intact if the user e.g. resizes an images with the handles provided by the browsers. It is possible to undo/redo these internal changes with the common `execCommand` method. The primary task here is to record these changes and add them to the stack(s).