Oct 27, 2010
I'm writing a bitmap editor where I use the Command Pattern to represent actions that will transform the document. I keep all the commands executed so far in a list and, to implement undo, I restore the document to its initial state and then replay all but the last command.
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I would like my undo/redo system to have the following feature: When the user closes the editor and returns, the document, including the available undo and redo commands, should be restored to the state it was in when the user left.
I'm implementing this for Android where your application can be given very little notice before it will be cleared from memory if e.g. the user gets a phone call. Also, some of my commands are e.g. a list of all the x,y co-ord the user painted on so these might take a few moments to save to disk.
My current idea is as follows:
When a new action is performed, the command object is added to a list S for commands that need to be saved to disk.
A background thread is used that will continually take commands from list S and save them to disk. The postfix of the filenames used will be numbered in sequence. For example, if the user filled the screen then drew 2 circles, the command files might be called FillCommand1.cmd, DrawCircleCommand2.cmd, DrawCircleCommand3.cmd.
Periodically, we save a "checkpoint" command whose purpose is to store the full document state so that, even if one of the .cmd files is corrupted, we can restore a recent version of the document.
When the user exits the app, the background thread attempts to finish up saving all the commands it can (but it might get killed).
On startup, we look for the most recent .cmd file that represents a checkpoint that we can load successfully. All the .cmd files we can load after this (i.e. some files might be corrupt) go in the redo command list, all the .cmd files we can load between the first checkpoint loaded and the oldest checkpoint we can load go in the undo list.
I want the undo limit to be about 20 or 30 commands back so I need extra logic for discarding commands, deleting .cmd files and I've got to worry about multi-threading behaviour. This system seems pretty complex and will need a lot of testing to make sure it doesn't go wrong.
Is there anything in Java or Android than can help make this easier? Am I reinventing the wheel anywhere? Maybe a database would be better?