命令模式和OnPaint事件问题

时间:2011-06-14 15:46:32

标签: c# design-patterns

我正在尝试使用撤消功能将Command Pattern改编为简单的绘图应用程序。我在撤消操作时遇到了OnPaint事件。这是代码:

[已解决]帖子末尾的详细信息

interface ICommand {
    void Execute();
    void UnExecute();
}

class DrawLineCommand : ICommand {
    private SimpleImage simpleImage;
    private Image prevImage;
    public DrawLineCommand(SimpleImage simpleImage) {
        this.simpleImage = simpleImage;
        this.prevImage = simpleImage.Image;
    }
    public void Execute() {
        simpleImage.DrawLine();
    }
    public void UnExecute() {
        simpleImage.Image = prevImage;
    }
}

class CommandManager {
    private Stack undoStack = new Stack();
    public void ExecuteCommand(ICommand command) {
        command.Execute();
        undoStack.Push(command);
    }
    public void UnExecuteCommand() {
        if (undoStack.Count > 0) {
            ICommand command = (ICommand)undoStack.Pop();
            command.UnExecute();
        }
    }
}

class SimpleImage {
    private Point startPoint;
    private Point endPoint;
    private PictureBox pictureBox;
    public SimpleImage(PictureBox pictureBox) {
        this.pictureBox = pictureBox;
        pictureBox.Paint += new PaintEventHandler(pictureBox_Paint);
    }
    void pictureBox_Paint(object sender, PaintEventArgs e) {
        // this code shows the line during drawing
        // this code is under "if operation == drawLine" block
        Graphics graphics = e.Graphics;
        graphics.DrawLine(Pens.Red, startPoint, endPoint);

        // how can i refresh picturebox after undo operation?
        // "if operation == undo" then ??
    }
    public void DrawLine() {
        // this code actually saves finally drawn line
        Image img = Image;
        Graphics graphics = Graphics.FromImage(img);
        graphics.DrawLine(Pens.Red, startPoint, endPoint);
        Image = img;
    }

    public void Invalidate() {
        pictureBox.Invalidate();
    }
    public Image Image {
        get { return pictureBox.Image; }
        set { pictureBox.Image = value; }
    }
    public Point StartPoint {
        get { return startPoint; }
        set { startPoint = value; }
    }
    public Point EndPoint {
        get { return endPoint; }
        set { endPoint = value; }
    }
}

public partial class FormMain : Form {
    private PictureBox pictureBox;
    private SimpleImage simpleImage;
    private CommandManager commandManager;
    public FormMain() {
        InitializeComponent();
        simpleImage = new SimpleImage(this.pictureBox);
        commandManager = new CommandManager();
    }
    void pictureBox_MouseDown(object sender, MouseEventArgs e) {
        if (e.Button != MouseButtons.Left)
            return;

        simpleImage.StartPoint = e.Location;
    }
    void pictureBox_MouseMove(object sender, MouseEventArgs e) {
        if (e.Button != MouseButtons.Left)
            return;

        simpleImage.EndPoint = e.Location;
        simpleImage.Invalidate();
    }
    void pictureBox_MouseUp(object sender, MouseEventArgs e) {
        simpleImage.Invalidate();
        commandManager.ExecuteCommand(new DrawLineCommand(simpleImage));
    }
}

它实际上绘制了一条线,它执行命令并将其推送到堆栈上。我无法实现UNDO的工作。我的意思是。逐步调试我看到对象从堆栈中弹出,然后执行OnPaint。但实际上没有显示“上一个”图像。

我已阅读过很多网站,而且我还有一个代码项目网站/文章的示例应用程序。它使用TextBox和Bold / Italicize操作提供相同的方法。它像地狱一样工作。唯一的区别是这种残忍的OnPaint方法..

提前感谢任何建议!

[编辑] 匆忙我忘记了将一个引用类型分配给另一个不是复制它(创建独立对象),改变了这个:

this.prevImage = simpleImage.Image;
在很少的地方解决了这个问题。现在一切都有效..

2 个答案:

答案 0 :(得分:1)

这里的要点不是直接在画布上绘画,而是有一个代表你的绘画的数据结构。然后,您将向此绘制对象添加一条线,并且画布的主循环将从数据结构中绘制相应的图形。那么你的do / undo方法只需要操作数据结构,而不是绘画。

你需要这样的东西:

interface IPaintable // intarface for Lines, Text, Circles, ...
{
    void OnPaint(Image i); // does the painting
}

interface IPaintableCommand // interface for commands 
{
    void Do(ICollection<IPaintable> painting); // adds line/text/circle to painting
    void Undo(ICollection<IPaintable> painting);  // removes line/text/circle from painting    
}

您的主应用程序只会保留一个List,并在命令更改绘图集时重新绘制画布。

答案 1 :(得分:1)

看起来您遇到aliasing问题。在DrawLineCommand中,您在操作之前引入对图像的引用并将其存储为:

this.prevImage = simpleImage.Image;

您现在有两个对同一对象的引用。绘图线操作发生在同一图像上操作:

Image img = Image; // Now a third reference to the same image object
Graphics graphics = Graphics.FromImage(img);
graphics.DrawLine(Pens.Red, startPoint, endPoint);
Image = img; // and you set the Image reference back to the same object

以上使img成为对Image的不必要引用。但是,您仍然在命令中有另一个对Image的引用。垃圾收集器运行后,您将返回到对同一图像对象的引用。撤消然后执行以下操作:

simpleImage.Image = prevImage;

在这里你没有改变Image,只是让Image引用了它所引用的同一个对象。

虽然我非常同意m0sa,但在这种情况下,修复是在创建命令时使prevImage成为原始图像的 COPY 。对于以下内容,我假设Image.Clone()已实现,但我自己从未尝试过:

this.prevImage = simpleImage.Image.Clone();

注意:如果使用此方法,您可能会因大图像或许多命令而快速耗尽内存。