PagerAdapter实例化项目在每次滚动时都被调用

Question

我使用寻呼机的Coverflow library进行活动。但是我看到一个问题，每次滚动时，都会在限制内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，因此对于每个项目，所有这些都应该在一次调用中加载到InstantiateItem（这意味着无论我滚动多少次，总计2个调用），而不是每次滚动都加载2个调用。这使滚动条变得不平滑和不整齐，这一点都不漂亮。我试图将其与设置带有寻呼机的全新项目并将其在我的设备上运行进行比较，然后滚动流畅。

知道我搞砸了吗？

我看到的日志是（您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>

并且在活动中，我将寻呼机加载到onCreate中，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
            }
        });

并且在适配器中：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was calling instantiate item for on page selection everytime的库。因此，解决方案是只编写我自己的PagerContainer来从库中扩展一个（因为该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.
    }
}

也请在XML文件中使用您自己的容器。就是这样。