Question

手势有问题。当我编译应用程序时，我遇到了错误：

System.NullReferenceException：未将对象引用设置为对象的实例。

在这一行：

 view.GestureRecognizers.Add(panGesture);

我的整个代码：

    private ISwipeCallBack mISwipeCallback;
    private double translatedX = 0, translatedY = 0;

    public SwipeListener(View view , ISwipeCallBack iSwipeCallBack)
    {

        mISwipeCallback = iSwipeCallBack;
        var panGesture = new PanGestureRecognizer();
        panGesture.PanUpdated += OnPanUpdated;
        view.GestureRecognizers.Add(panGesture);
    }

    void OnPanUpdated(object sender, PanUpdatedEventArgs e)
    {

        View Content = (View)sender;

        switch (e.StatusType)
        {

            case GestureStatus.Running:

                try
                {
                    translatedX = e.TotalX;
                    translatedY = e.TotalY;
                }
                catch (Exception err)
                {
                    System.Diagnostics.Debug.WriteLine("" + err.Message);
                }
                break;

            case GestureStatus.Completed:

                System.Diagnostics.Debug.WriteLine("translatedX : " + translatedX);
                System.Diagnostics.Debug.WriteLine("translatedY : " + translatedY);

                if (translatedX < 0 && Math.Abs(translatedX) > Math.Abs(translatedY))
                {
                    mISwipeCallback.onLeftSwipe(Content);
                }
                else if (translatedX > 0 && translatedX > Math.Abs(translatedY))
                {
                    mISwipeCallback.onRightSwipe(Content);
                }
                else if (translatedY < 0 && Math.Abs(translatedY) > Math.Abs(translatedX))
                {
                    mISwipeCallback.onTopSwipe(Content);
                }
                else if (translatedY > 0 && translatedY > Math.Abs(translatedX))
                {
                    mISwipeCallback.onBottomSwipe(Content);
                }
                else
                {
                    mISwipeCallback.onNothingSwiped(Content);
                }

                break;

        }
    }

 <StackLayout
    >

    <Label 
    x:Name="lbl_swipe"
    Text="Welcome to Xamarin Forms! Drag it" 
    WidthRequest="300" 
    HeightRequest="200" 
    BackgroundColor="Yellow"
    TextColor="Black"
    FontSize="20"
    VerticalOptions="Center" 
    HorizontalOptions="Center">
    </Label>




    <Label 
    x:Name="lbl_result"
    WidthRequest="300" 
    HeightRequest="100" 
    BackgroundColor="Purple"
    TextColor="White"
    FontSize="20"
    VerticalOptions="End" 
    HorizontalOptions="Center">
    </Label>

</StackLayout>

的MainPage

public partial class Testowy : ContentPage, ISwipeCallBack
{
    public Testowy()
    {
        SwipeListener swipeListener = new SwipeListener(lbl_swipe, this);
    }

    public void onBottomSwipe(View view)
    {
        if (view == lbl_swipe)
        {
            lbl_result.Text = "OnBottomSwipe";
        }
    }

    public void onLeftSwipe(View view)
    {
        if (view == lbl_swipe)
        {
            lbl_result.Text = "onLeftSwipe";
        }
    }

    public void onNothingSwiped(View view)
    {
        if (view == lbl_swipe)
        {
            lbl_result.Text = "onNothingSwiped";
        }
    }

    public void onRightSwipe(View view)
    {
        if (view == lbl_swipe)
        {
            lbl_result.Text = "onRightSwipe";
        }
    }

    public void onTopSwipe(View view)
    {
        if (view == lbl_swipe)
        {
            lbl_result.Text = "onTopSwipe";
        }
    }
}

Answer 1

您错过了InitializeComponent方法 - 该方法应初始化lbl_result或lbl_swipe等引用。

public Testowy()
{
    InitializeComponent(); //<---- add this line here.
    SwipeListener swipeListener = new SwipeListener(lbl_swipe, this);
}

Android上的手势识别器Xamarn.Forms System.NullReferenceException

1 个答案: