将事件附加到类

Question

我正在尝试将一个事件注册到 MainPage.xaml.cs如下所示：

public partial class MainPage : PhoneApplicationPage
{
public static ICanvas CurrentCanvas;

private void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
    //HERE I TRY TO REGISTER FOR MY EVENT
    ((WP8Canvas)CurrentCanvas).Redraw += WP8EventHandler_RedrawCanvas;
    //HERE I TRY TO REGISTER FOR MY EVENT/
    System.Threading.ThreadStart start = new System.Threading.ThreadStart(launchProcessA);
    System.Threading.Thread t = new System.Threading.Thread(start);
    t.Name = "ProcessA Thread";
    t.Start();
    }
    /// <summary>
    /// Listen to WP8Canvas repaint() methods
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    public void WP8EventHandler_RedrawCanvas(object sender, EventArgs e)
    {
        wpCanvas.InvalidateArrange();
        Debug.WriteLine("Redraw Canvas Event"); 
    }
}

WP8帆布.cs

public class WP8Canvas : WP8Displayable, ICanvas
{
    public Canvas canvas ;
    public event EventHandler Redraw;
    protected object reference;

    public WP8Canvas(object reference)
    {
        this.reference = reference;
        MainPage.CurrentCanvas = this;
        Debug.WriteLine("WP8Canvas instance");
    }
    public void repaint()
    {            
        RedrawCanvas();
    }
    /// <summary>
    /// Raise event if repaint() method is hit
    /// </summary
    private void RedrawCanvas()
    {
        //Null check makes sure the main page is attached to the event
        if (this.Redraw != null)
            this.Redraw(new object(), new EventArgs());
    }
}

在我的评论之间//这里我尝试注册我的事件我尝试注册一个非实例化对象 WP8Canvas，但当然会发生空异常。这是我的线程t控制WP8Canvas对象的创建，但这发生在运行时，我不知道什么时候。

问：如何将我的WP8Canvas重绘事件注册到主页而不会遇到空指针异常。

Answer 1

如果您无法控制 launchProcessA ，那么达到此结果的一种方法是使用属性：

public WP8Canvas CurrentWP8Canvas
{
    get
    {
        return this.CurrentCanvas as WP8Canvas;
    }
    set
    {
        this.CurrentCanvas = value;
        value.Redraw += WP8EventHandler_RedrawCanvas;
    }    
}

然后，您只需更改代码，以便线程分配CurrentWP8Canvas属性而不是CurrentCanvas

编辑：如果需要它是静态的，一种方法可能是将事件处理程序存储在临时静态变量中：

public static EventHandler RedrawCanvas { get; set; }

然后从 MainPage 的构造函数中设置它：

RedrawCanvas = WP8EventHandler_RedrawCanvas;

最后，将 CurrentWP8Canvas 声明为静态，并分配您存储的事件处理程序：

public static WP8Canvas CurrentWP8Canvas
{
    get
    {
        return CurrentCanvas as WP8Canvas;
    }
    set
    {
        CurrentCanvas = value;
        value.Redraw += RedrawCanvas;
    }    
}

它应该有效。但是，由于多种原因，这是非常错误的：线程同步问题，内存泄漏的风险......在这一点上，你应该得出结论，你已经走到了死胡同，回溯，并考虑重新设计你的架构。