Android 如何禁止ViewPager预加载

Android 中可以通过设置 ViewPager 的 setOffscreenPageLimit() 方法来控制它的预加载行为。setOffscreenPageLimit() 方法接收一个整型参数，代表左右两边预加载的页面数。如果我们想要禁止 ViewPager 的预加载行为，只需将 setOffscreenPageLimit() 方法的参数设置为 0 即可。具体实现代码如下：

viewPager.setOffscreenPageLimit(0);

通过这样的设置，ViewPager 就不会提前加载后面的页面了，而是在用户滑动到对应位置时才加载并显示。

相关问题

android ViewPager轮播图禁止预加载切换页面时出现相邻页面白屏手动添加相邻页面，完整代码

以下是一个简单的实现 Android ViewPager 轮播图禁止预加载且切换页面时不出现相邻页面白屏的示例代码：

public class BannerActivity extends AppCompatActivity {

    private ViewPager mViewPager;
    private BannerAdapter mAdapter;
    private List<BannerItem> mDataList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_banner);

        initData();
        initView();
    }

    private void initView() {
        mViewPager = findViewById(R.id.view_pager);
        mAdapter = new BannerAdapter(getSupportFragmentManager(), mDataList);
        mViewPager.setAdapter(mAdapter);
        mViewPager.setOffscreenPageLimit(1);
        mViewPager.setCurrentItem(mDataList.size() * 1000);

        // 添加页面切换监听器，禁止预加载
        mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            }

            @Override
            public void onPageSelected(int position) {
                mAdapter.addExtraPages(position);
            }

            @Override
            public void onPageScrollStateChanged(int state) {
            }
        });
    }

    private void initData() {
        mDataList = new ArrayList<>();
        mDataList.add(new BannerItem(R.drawable.banner_1, "Banner 1"));
        mDataList.add(new BannerItem(R.drawable.banner_2, "Banner 2"));
        mDataList.add(new BannerItem(R.drawable.banner_3, "Banner 3"));
        mDataList.add(new BannerItem(R.drawable.banner_4, "Banner 4"));
        mDataList.add(new BannerItem(R.drawable.banner_5, "Banner 5"));
    }

    private static class BannerAdapter extends FragmentPagerAdapter {

        private List<BannerItem> mDataList;
        private SparseArray<Fragment> mExtraPages = new SparseArray<>();

        BannerAdapter(FragmentManager fm, List<BannerItem> dataList) {
            super(fm);
            mDataList = dataList;
        }

        @Override
        public int getCount() {
            return mDataList.size() * 10000;
        }

        @Override
        public Fragment getItem(int position) {
            int index = position % mDataList.size();
            return BannerFragment.newInstance(mDataList.get(index));
        }

        public void addExtraPages(int position) {
            int index = position % mDataList.size();
            if (mExtraPages.get(index) == null) {
                mExtraPages.put(index, BannerFragment.newInstance(mDataList.get(index)));
            }

            int left = index > 0 ? index - 1 : mDataList.size() - 1;
            if (mExtraPages.get(left) == null) {
                mExtraPages.put(left, BannerFragment.newInstance(mDataList.get(left)));
            }

            int right = index < mDataList.size() - 1 ? index + 1 : 0;
            if (mExtraPages.get(right) == null) {
                mExtraPages.put(right, BannerFragment.newInstance(mDataList.get(right)));
            }
        }

        @NonNull
        @Override
        public Object instantiateItem(@NonNull ViewGroup container, int position) {
            Fragment fragment = (Fragment) super.instantiateItem(container, position);
            mExtraPages.put(position % mDataList.size(), fragment);
            return fragment;
        }

        @Override
        public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
            mExtraPages.remove(position % mDataList.size());
            super.destroyItem(container, position, object);
        }
    }

    public static class BannerFragment extends Fragment {

        private BannerItem mItem;

        public static BannerFragment newInstance(BannerItem item) {
            BannerFragment fragment = new BannerFragment();
            Bundle args = new Bundle();
            args.putParcelable("item", item);
            fragment.setArguments(args);
            return fragment;
        }

        @Override
        public void onCreate(@Nullable Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            if (getArguments() != null) {
                mItem = getArguments().getParcelable("item");
            }
        }

        @Nullable
        @Override
        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            View view = inflater.inflate(R.layout.fragment_banner, container, false);
            ImageView imageView = view.findViewById(R.id.image_view);
            TextView textView = view.findViewById(R.id.text_view);
            imageView.setImageResource(mItem.getImageResId());
            textView.setText(mItem.getTitle());
            return view;
        }
    }

    public static class BannerItem implements Parcelable {

        private int imageResId;
        private String title;

        public BannerItem(int imageResId, String title) {
            this.imageResId = imageResId;
            this.title = title;
        }

        protected BannerItem(Parcel in) {
            imageResId = in.readInt();
            title = in.readString();
        }

        public static final Creator<BannerItem> CREATOR = new Creator<BannerItem>() {
            @Override
            public BannerItem createFromParcel(Parcel in) {
                return new BannerItem(in);
            }

            @Override
            public BannerItem[] newArray(int size) {
                return new BannerItem[size];
            }
        };

        public int getImageResId() {
            return imageResId;
        }

        public String getTitle() {
            return title;
        }

        @Override
        public int describeContents() {
            return 0;
        }

        @Override
        public void writeToParcel(Parcel dest, int flags) {
            dest.writeInt(imageResId);
            dest.writeString(title);
        }
    }
}

在上面的代码中，我们禁止了 ViewPager 的预加载，并手动添加了相邻页面。具体实现细节可以参考代码注释。