Android FingerPaint Undo/Redo implementation

Question

I\'m working on a test project which is something similar to FingerPaint example in Android SDK Demos. I was trying to implement undo/redo functionality in my p

Answer 1

One way to implement a do/redo functionality is to encapsulate a method call and all info needed for the call in an object so that you can store it and call it later - the Command Pattern.

In this pattern, each action has its own object: DrawCircleCommand, DrawPathCommand, FillColorCommand, etc. In each object the draw() method is implemented in a unique way but is always called as draw(Canvas canvas) which allows the CommandManager to iterate through the commands. To undo you iterate over the objects calling the undo() method;

Each command object implements an Interface

public interface IDrawCommand {  
  public void draw(Canvas canvas);  
  public void undo();  
}  

An object would look like:

public class DrawPathCommand implements IDrawCommand{  
  public Path path;  
  public Paint paint;  

  public void setPath(path){
    this.path = path
  }

  public void draw(Canvas canvas) {  
    canvas.drawPath( path, paint );  
  }  

  public void undo() {  
   //some  action to remove the path
 }  

}

Your commands are added to the CommandManager:

mCommandManager.addCommand(IDrawCommand command)

and to undo a command you just call:

mCommandManager.undo();

The CommandManager stores the commands in a data structure e.g. list that allows it to iterative over the command objects.

You can find a complete tutorial here on how to implement the Command Pattern with do/undo for Canvas drawing on Android.

Here is a another tutorial on how implement the Command Pattern in Java;