每次滚动时都会调用 PagerAdapter 实例化项目

Question

我有一个使用 Coverflow library 进行寻呼机的活动。但是我看到一个问题，每次滚动时，都会在限制中的所有项目上再次调用 instantiateItem（在这种情况下为 3，因为屏幕外限制设置为 1）。我的寻呼机只有 2 个项目，因此所有这些项目都应该在每个项目的实例化项目调用中加载（这意味着无论我滚动多少次，总共需要 2 次调用），而不是每次滚动时调用 2 次。这使得滚动不平滑和滞后，这根本不漂亮。我试图将此与使用寻呼机设置一个全新的项目并在我的设备上运行它进行比较，并且滚动流畅。

知道我在搞砸什么吗？

我看到的日志是（你可以在下面的适配器代码中看到日志行）：

I/MyViewAdapter.class: [Wed Jan 08]>>>> Instantiate item in my view adapter called. Position: 0
I/MyViewAdapter.class: [Wed Jan 08]>>>> Instantiate item in my view adapter ended. Position: 0
I/MyViewAdapter.class: [Wed Jan 08]>>>> Instantiate item in my view adapter called. Position: 1
I/MyViewAdapter.class: [Wed Jan 08]>>>> Instantiate item in my view adapter ended. Position: 1

XML如下：

MyActivity.xml

...
<FrameLayout
        android:id="@+id/frame"
        android:layout_width="match_parent"
        android:layout_height="270dp"
        android:background="@android:color/transparent">
    <me.crosswall.lib.coverflow.core.PagerContainer
        android:id="@+id/pager_container"
        android:layout_width="match_parent"
        android:layout_height="275dp">

            <androidx.viewpager.widget.ViewPager
                android:id="@+id/overlap_pager"
                android:layout_width="250dp"
                android:layout_height="match_parent"
                android:layout_gravity="center"
                android:overScrollMode="never"/>

    </me.crosswall.lib.coverflow.core.PagerContainer>
</FrameLayout>
...

MyViewItem.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/linearlayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    android:background="@android:color/transparent">

    <androidx.cardview.widget.CardView
        android:id="@+id/cardview"
        android:layout_width="250dp"
        android:layout_height="250dp"
        android:layout_centerInParent="true"
        app:cardCornerRadius="4dp"
        app:cardElevation="2dp"
        app:cardUseCompatPadding="false"
        app:cardPreventCornerOverlap="true"
        android:background="@android:color/transparent"
        app:cardBackgroundColor="@android:color/transparent">

        <FrameLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center">

            <de.hdodenhof.circleimageview.CircleImageView
                xmlns:app="http://schemas.android.com/apk/res-auto"
                android:id="@+id/profilePicture"
                android:layout_width="240dp"
                android:layout_height="240dp"
                android:layout_centerHorizontal="true"
                android:layout_gravity="center_horizontal"
                android:layout_marginBottom="8dp"
                android:scaleType="centerCrop"
                app:civ_border_color="#fff"
                app:civ_border_width="2dip"
                android:alpha="0.5"
                />

            <ProgressBar
                android:id="@+id/progressbar"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:progressBackgroundTint="@color/colorPrimary"
                android:progressTint="@color/colorPrimary"
                android:layout_centerInParent="true"
                android:layout_gravity="center"
                />

        </FrameLayout>

    </androidx.cardview.widget.CardView>

</LinearLayout>

在活动中，我在onCreate 中加载寻呼机，如下所示：

MyActivity.java

        PagerContainer container = (PagerContainer) findViewById(R.id.pager_container);
        container.setOverlapEnabled(true);
        pager = container.getViewPager();
        pager.setAdapter(new MyViewAdapter(MyActivity.this, models, firstPosition));
        pager.setOffscreenPageLimit(2);

        // This is related to layout of the pager container: https://github.com/crosswall/Android-Coverflow
        CoverFlow coverFlow = new CoverFlow.Builder()
                .with(pager)
                .scale(0.3f)
                .pagerMargin(getResources().getDimensionPixelSize(R.dimen.pager_margin))
                .spaceSize(0f)
                .build();

        pager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                Log.i(TAG, String.format("onPageSelected: (prevPosition, position) = (%d, %d)", prevPosition, position));
                ImageView imageView = pager.findViewWithTag(pagerViewTagForPosition(position));
                imageView.setAlpha(1f);

                ImageView imageViewPrev;
                if (prevPosition == -1) {
                    imageViewPrev = pager.findViewWithTag(pagerViewTagForPosition(firstPosition));
                } else {
                    imageViewPrev = pager.findViewWithTag(pagerViewTagForPosition(prevPosition));
                }
                imageViewPrev.setAlpha(0.5f);
                prevPosition = position;
                // Update background and animation objects
            }
        });

在适配器中： MyViewAdapter.java

    @Override
    public Object instantiateItem(ViewGroup container, final int position) {
        Log.i(TAG, String.format("[%s]>>>> Instantiate item in my view adapter called. Position: %d", new Date(), position));
        View view = LayoutInflater.from(context).inflate(R.layout.my_view,null);
        final CircleImageView circleImageView = (CircleImageView) view.findViewById(R.id.profilePicture);
        final ProgressBar progressbar = (ProgressBar) view.findViewById(R.id.progressbar);

        circleImageView.setTag("tag_" + position);

        Picasso.get().load(models.get(position)).into(circleImageView, new Callback() {
                @Override
                public void onSuccess() {
                    progressbar.setVisibility(View.GONE);
                }

                @Override
                public void onError(Exception e) {
                    Log.e(TAG, String.format("Failed to image. Error: %s", e));
                }
            });
        }

        container.addView(view);

        Log.i(TAG, String.format("[%s]>>>> Instantiate item in my view adapter ended. Position: %d", new Date(), position));
        return view;
    }


    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((View)object);
    }

    @Override
    public int getCount() {
        return models.size();
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view.equals(object);
    }

Answer 1

发现我正在使用的库was calling instantiate item for on page selection everytime。所以解决方案是编写我自己的 PagerContainer 扩展库中的一个（因为库确实使视图看起来很漂亮），并覆盖 onPageSelected 方法：

public class MyViewContainer extends PagerContainer {

    public MyViewContainer(Context context) {
        super(context);
        init();
    }

    public MyViewContainer(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public MyViewContainer(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init() {
        //Disable clipping of children so non-selected pages are visible
        setClipChildren(false);

        //Child clipping doesn't work with hardware acceleration in Android 3.x/4.x
        //You need to set this value here if using hardware acceleration in an
        // application targeted at these releases.
        setLayerType(View.LAYER_TYPE_SOFTWARE, null);
    }

    @Override
    public void onPageSelected(int position) {
        // Override the page selected from the library since that one calls instantiate item again.
    }
}

在 XML 文件中也使用您自己的容器。应该就是这样。