来自另一个片段的 FragmentTransaction 调用使 activity 变量为空

Question

背景使用 ValueAnimator 和自定义 ValueAnimator.Update 方法制作动画。在 activity onCreate 方法中，使用 FragmentTransaction 第一个片段调用。在第一个片段中，另一个 FragmetTranstaction 基于用户按钮点击调用。在第一笔交易中一切正常，但在第二笔交易中，ValueAnimator.Update 中使用的 activity 变量变为空。问题出在哪里？

Activity onCreate:

    private const int TO_LEFT = 0;
    private const int TO_RIGHT = 1;
    private const long DURATION = 15000;

    private int bgAnimDir = TO_LEFT;
    private RectF mDisplayRect = new RectF();
    private Matrix matrix = new Matrix();
    private ValueAnimator bgAnimator;

    [BindView(Resource.Id.background)]
    ImageView background;
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.Main);
        Cheeseknife.Bind(this);

        background.Post(() =>
        {
            animate();
        });


        fragmentTransaction = FragmentManager.BeginTransaction();
        fragmentTransaction.Add(Resource.Id.home_container, new Home(), "Home");
        fragmentTransaction.Commit();
    }

背景变量和矩阵变量均引发空指针异常

animate()函数：

    private void animate()
    {
        updateDisplayRect();
        if (bgAnimDir == TO_LEFT)
            animate(mDisplayRect.Left, mDisplayRect.Left - (mDisplayRect.Right - background.Width));
        else
            animate(mDisplayRect.Left, 0.0f);
    }

    private void animate(float from, float to)
    {
        bgAnimator = ValueAnimator.OfFloat(from, to);
        bgAnimator.Update +=
            (object sender, ValueAnimator.AnimatorUpdateEventArgs e) =>
            {
                float value = (float)e.Animation.AnimatedValue;
                matrix.Reset();
                matrix.PostTranslate(value, 0);
                background.ImageMatrix = matrix;
            };
        bgAnimator.SetDuration(DURATION);
        bgAnimator.AnimationEnd +=
            (object sender, EventArgs e) =>
            {
                bgAnimDir = bgAnimDir == TO_LEFT ? TO_RIGHT : TO_LEFT;
                animate();
            };
        bgAnimator.Start();
    }

bgAnimator.Update 委托

中引发的异常

Answer 1

我使用 try 和 catch 解决了这个问题。

这是我的新动画函数：

    private void animate(float from, float to)
    {
        bgAnimator = ValueAnimator.OfFloat(from, to);
        bgAnimator.Update +=
            (object sender, ValueAnimator.AnimatorUpdateEventArgs e) =>
            {
                try
                {
                    float value = (float)e.Animation.AnimatedValue;
                    matrix.Reset();
                    matrix.PostTranslate(value, 0);
                    background.ImageMatrix = matrix;
                }
                catch
                {
                    background = (ImageView)FindViewById(Resource.Id.background);
                }
            };
        bgAnimator.SetDuration(DURATION);
        bgAnimator.AnimationEnd +=
            (object sender, EventArgs e) =>
            {
                bgAnimDir = bgAnimDir == TO_LEFT ? TO_RIGHT : TO_LEFT;
                animate();
            };
        bgAnimator.Start();
    }

    private void updateDisplayRect()
    {
        try
        {
            mDisplayRect.Set(0, 0, background.Drawable.IntrinsicWidth, background.Drawable.IntrinsicHeight);
            matrix.MapRect(mDisplayRect);
        }
        catch
        {
            background = (ImageView)FindViewById(Resource.Id.background);
        }
    }

只需要在需要时填充空变量即可！

但是正如你所猜到的，这并不是一个简单的回答来解释为什么后台在这个过程中变为空！

来自另一个片段的 FragmentTransaction 调用使 activity 变量为空

FragmentTransaction call from another fragment makes activity variables null

android

android-fragments

xamarin

fragmenttransaction