命令模式

public class DrawInvoker {

    //绘制列表
    private List<DrawPath> drawList = Collections.synchronizedList(new ArrayList<DrawPath>());

    //重做列表
    private List<DrawPath> redoList = Collections.synchronizedList(new ArrayList<DrawPath>());

    //添加一条命令
    public void add(DrawPath command) {
        redoList.clear();
        drawList.add(command);
    }

    //撤销操作
    public void undo() {
        if(drawList.size() >0 ) {
            DrawPath undo = drawList.get(drawList.size() - 1);
            drawList.remove(drawList.size() - 1); // 把drawList中的最后一条命令删除掉.
            undo.undo();
            redoList.add(undo);
        }
    }
}