Google 致力于为黑人社区推动种族平等。查看具体举措

使用 ViewPager 在 Fragment 之间滑动

屏幕滑动是两个完整屏幕之间的切换,在设置向导或幻灯片等界面中很常见。本节课向您介绍如何使用支持库提供的 ViewPager 创建屏幕滑动。ViewPager 对象可自动为屏幕滑动添加动画。下面是一个屏幕滑动的效果,是从一个屏幕的内容切换到下一个屏幕的内容:

屏幕滑动动画
 

ViewPager 是 AndroidX 的一部分。如需了解详情,请参阅使用 AndroidX

创建视图

创建一个稍后将用于 Fragment 内容的布局文件。您还需要为 Fragment 的内容定义一个字符串。以下示例包含一个显示了一些文本的文本视图:

    <!-- fragment_screen_slide_page.xml -->
    <ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <TextView style="?android:textAppearanceMedium"
            android:padding="16dp"
            android:lineSpacingMultiplier="1.2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/lorem_ipsum" />
    </ScrollView>
    

创建 Fragment

创建一个 Fragment 类,以返回您刚刚在 onCreateView() 方法中创建的布局。然后,每当您需要向用户显示新页面时,便可以在父 Activity 中创建此 Fragment 的实例:

Kotlin

    import android.support.v4.app.Fragment

    class ScreenSlidePageFragment : Fragment() {

        override fun onCreateView(
                inflater: LayoutInflater,
                container: ViewGroup?,
                savedInstanceState: Bundle?
        ): View = inflater.inflate(R.layout.fragment_screen_slide_page, container, false)
    }
    

Java

    import android.support.v4.app.Fragment;
    ...
    public class ScreenSlidePageFragment extends Fragment {

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            ViewGroup rootView = (ViewGroup) inflater.inflate(
                    R.layout.fragment_screen_slide_page, container, false);

            return rootView;
        }
    }
    

添加 ViewPager

ViewPager 对象具有内置滑动手势来切换页面,并且默认显示屏幕滑动动画,因此您无需创建自己的动画。ViewPager 使用 PagerAdapter 对象提供要显示的新页面,因此 PagerAdapter 会使用您之前创建的 Fragment 类。

首先,创建一个包含 ViewPager 的布局:

    <!-- activity_screen_slide.xml -->
    <android.support.v4.view.ViewPager
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
    

创建可执行以下操作的 Activity:

  • 将内容视图设置为包含 ViewPager 的布局。
  • 创建扩展 FragmentStatePagerAdapter 抽象类的类,并实现 getItem() 方法以提供 ScreenSlidePageFragment 实例作为新页面。页导航控件适配器还要求您实现 getCount() 方法,该方法返回适配器将创建的页数(示例中为 5 页)。
  • PagerAdapterViewPager 挂钩。

Kotlin

    import android.support.v4.app.Fragment
    import android.support.v4.app.FragmentManager
    ...
    /**
     * The number of pages (wizard steps) to show in this demo.
     */
    private const val NUM_PAGES = 5

    class ScreenSlidePagerActivity : FragmentActivity() {

        /**
         * The pager widget, which handles animation and allows swiping horizontally to access previous
         * and next wizard steps.
         */
        private lateinit var mPager: ViewPager

        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_screen_slide)

            // Instantiate a ViewPager and a PagerAdapter.
            mPager = findViewById(R.id.pager)

            // The pager adapter, which provides the pages to the view pager widget.
            val pagerAdapter = ScreenSlidePagerAdapter(supportFragmentManager)
            mPager.adapter = pagerAdapter
        }

        override fun onBackPressed() {
            if (mPager.currentItem == 0) {
                // If the user is currently looking at the first step, allow the system to handle the
                // Back button. This calls finish() on this activity and pops the back stack.
                super.onBackPressed()
            } else {
                // Otherwise, select the previous step.
                mPager.currentItem = mPager.currentItem - 1
            }
        }

        /**
         * A simple pager adapter that represents 5 ScreenSlidePageFragment objects, in
         * sequence.
         */
        private inner class ScreenSlidePagerAdapter(fm: FragmentManager) : FragmentStatePagerAdapter(fm) {
            override fun getCount(): Int = NUM_PAGES

            override fun getItem(position: Int): Fragment = ScreenSlidePageFragment()
        }
    }
    

Java

    import android.support.v4.app.Fragment;
    import android.support.v4.app.FragmentManager;
    ...
    public class ScreenSlidePagerActivity extends FragmentActivity {
        /**
         * The number of pages (wizard steps) to show in this demo.
         */
        private static final int NUM_PAGES = 5;

        /**
         * The pager widget, which handles animation and allows swiping horizontally to access previous
         * and next wizard steps.
         */
        private ViewPager mPager;

        /**
         * The pager adapter, which provides the pages to the view pager widget.
         */
        private PagerAdapter pagerAdapter;

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

            // Instantiate a ViewPager and a PagerAdapter.
            mPager = (ViewPager) findViewById(R.id.pager);
            pagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
            mPager.setAdapter(pagerAdapter);
        }

        @Override
        public void onBackPressed() {
            if (mPager.getCurrentItem() == 0) {
                // If the user is currently looking at the first step, allow the system to handle the
                // Back button. This calls finish() on this activity and pops the back stack.
                super.onBackPressed();
            } else {
                // Otherwise, select the previous step.
                mPager.setCurrentItem(mPager.getCurrentItem() - 1);
            }
        }

        /**
         * A simple pager adapter that represents 5 ScreenSlidePageFragment objects, in
         * sequence.
         */
        private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
            public ScreenSlidePagerAdapter(FragmentManager fm) {
                super(fm);
            }

            @Override
            public Fragment getItem(int position) {
                return new ScreenSlidePageFragment();
            }

            @Override
            public int getCount() {
                return NUM_PAGES;
            }
        }
    }
    

使用 PageTransformer 自定义动画

要显示非默认屏幕幻灯片动画,请实现 ViewPager.PageTransformer 接口并将其提供给 viewpager。该接口只会公开一个方法 transformPage()。在屏幕切换的每个点,系统会针对每个可见页面(通常只有一个可见页面)和刚刚离开屏幕的相邻页面调用此方法。 例如,如果第 3 页可见且用户拖向第 4 页,则在手势的每个步骤,会针对第 2 页、第 3 页和第 4 页调用 transformPage()

然后在 transformPage() 实现中,您可以创建自定义滑动动画,方法是从 transformPage()方法的 position 参数获取页面在屏幕上的位置,然后根据该位置确定需要转换哪些页面。

position 参数表示指定页面相对于屏幕中心的位置。 它是一个动态属性,会随着用户滚动浏览页面而变化。当页面填满整个屏幕时,其位置值为 0。 当页面刚刚离开屏幕右侧时,其位置值为 1。如果用户在第一页和第二页之间滚动到一半,则第一页的位置为 -0.5,第二页的位置为 0.5。根据页面在屏幕上的位置,您可以使用 setAlpha()setTranslationX()setScaleY() 之类的方法设置页面属性,从而创建自定义滑动动画。

如果您具有 PageTransformer 实现,通过您的实现调用 setPageTransformer() 来应用自定义动画。例如,如果您有一个名为 ZoomOutPageTransformerPageTransformer,则可以按如下所示设置自定义动画:

Kotlin

    val mPager: ViewPager = findViewById(R.id.pager)
    ...
    mPager.setPageTransformer(true, ZoomOutPageTransformer())
    

Java

    ViewPager mPager = (ViewPager) findViewById(R.id.pager);
    ...
    mPager.setPageTransformer(true, new ZoomOutPageTransformer());
    

有关 PageTransformer 的示例和视频,请参见缩小页面转换器深度页面转换器部分。

缩小页面转换器

在相邻页面之间滚动时,该页面转换器会使页面收缩并淡出。随着页面越来越靠近中心,页面会恢复到正常大小并淡入。

ZoomOutPageTransformer 示例
 

Kotlin

    private const val MIN_SCALE = 0.85f
    private const val MIN_ALPHA = 0.5f

    class ZoomOutPageTransformer : ViewPager.PageTransformer {

        override fun transformPage(view: View, position: Float) {
            view.apply {
                val pageWidth = width
                val pageHeight = height
                when {
                    position < -1 -> { // [-Infinity,-1)
                        // This page is way off-screen to the left.
                        alpha = 0f
                    }
                    position <= 1 -> { // [-1,1]
                        // Modify the default slide transition to shrink the page as well
                        val scaleFactor = Math.max(MIN_SCALE, 1 - Math.abs(position))
                        val vertMargin = pageHeight * (1 - scaleFactor) / 2
                        val horzMargin = pageWidth * (1 - scaleFactor) / 2
                        translationX = if (position < 0) {
                            horzMargin - vertMargin / 2
                        } else {
                            horzMargin + vertMargin / 2
                        }

                        // Scale the page down (between MIN_SCALE and 1)
                        scaleX = scaleFactor
                        scaleY = scaleFactor

                        // Fade the page relative to its size.
                        alpha = (MIN_ALPHA +
                                (((scaleFactor - MIN_SCALE) / (1 - MIN_SCALE)) * (1 - MIN_ALPHA)))
                    }
                    else -> { // (1,+Infinity]
                        // This page is way off-screen to the right.
                        alpha = 0f
                    }
                }
            }
        }
    }
    

Java

    public class ZoomOutPageTransformer implements ViewPager.PageTransformer {
        private static final float MIN_SCALE = 0.85f;
        private static final float MIN_ALPHA = 0.5f;

        public void transformPage(View view, float position) {
            int pageWidth = view.getWidth();
            int pageHeight = view.getHeight();

            if (position < -1) { // [-Infinity,-1)
                // This page is way off-screen to the left.
                view.setAlpha(0f);

            } else if (position <= 1) { // [-1,1]
                // Modify the default slide transition to shrink the page as well
                float scaleFactor = Math.max(MIN_SCALE, 1 - Math.abs(position));
                float vertMargin = pageHeight * (1 - scaleFactor) / 2;
                float horzMargin = pageWidth * (1 - scaleFactor) / 2;
                if (position < 0) {
                    view.setTranslationX(horzMargin - vertMargin / 2);
                } else {
                    view.setTranslationX(-horzMargin + vertMargin / 2);
                }

                // Scale the page down (between MIN_SCALE and 1)
                view.setScaleX(scaleFactor);
                view.setScaleY(scaleFactor);

                // Fade the page relative to its size.
                view.setAlpha(MIN_ALPHA +
                        (scaleFactor - MIN_SCALE) /
                        (1 - MIN_SCALE) * (1 - MIN_ALPHA));

            } else { // (1,+Infinity]
                // This page is way off-screen to the right.
                view.setAlpha(0f);
            }
        }
    }
    

深度页面转换器

此页面转换器使用默认的滑动动画向左滑动页面,而使用“深度”动画向右滑动页面。此深度动画会使页面淡出,并将其线性地缩小。

DepthPageTransformer 示例
 

在深度动画期间,默认动画(屏幕滑动)仍然会发生,因此您必须使用负 X 平移抵消屏幕滑动。例如:

Kotlin

    view.translationX = -1 * view.width * position
    

Java

    view.setTranslationX(-1 * view.getWidth() * position);
    

以下示例展示了如何抵消运行中的页面转换器中的默认屏幕滑动动画:

Kotlin

    private const val MIN_SCALE = 0.75f

    class DepthPageTransformer : ViewPager.PageTransformer {

        override fun transformPage(view: View, position: Float) {
            view.apply {
                val pageWidth = width
                when {
                    position < -1 -> { // [-Infinity,-1)
                        // This page is way off-screen to the left.
                        alpha = 0f
                    }
                    position <= 0 -> { // [-1,0]
                        // Use the default slide transition when moving to the left page
                        alpha = 1f
                        translationX = 0f
                        scaleX = 1f
                        scaleY = 1f
                    }
                    position <= 1 -> { // (0,1]
                        // Fade the page out.
                        alpha = 1 - position

                        // Counteract the default slide transition
                        translationX = pageWidth * -position

                        // Scale the page down (between MIN_SCALE and 1)
                        val scaleFactor = (MIN_SCALE + (1 - MIN_SCALE) * (1 - Math.abs(position)))
                        scaleX = scaleFactor
                        scaleY = scaleFactor
                    }
                    else -> { // (1,+Infinity]
                        // This page is way off-screen to the right.
                        alpha = 0f
                    }
                }
            }
        }
    }
    

Java

    public class DepthPageTransformer implements ViewPager.PageTransformer {
        private static final float MIN_SCALE = 0.75f;

        public void transformPage(View view, float position) {
            int pageWidth = view.getWidth();

            if (position < -1) { // [-Infinity,-1)
                // This page is way off-screen to the left.
                view.setAlpha(0f);

            } else if (position <= 0) { // [-1,0]
                // Use the default slide transition when moving to the left page
                view.setAlpha(1f);
                view.setTranslationX(0f);
                view.setScaleX(1f);
                view.setScaleY(1f);

            } else if (position <= 1) { // (0,1]
                // Fade the page out.
                view.setAlpha(1 - position);

                // Counteract the default slide transition
                view.setTranslationX(pageWidth * -position);

                // Scale the page down (between MIN_SCALE and 1)
                float scaleFactor = MIN_SCALE
                        + (1 - MIN_SCALE) * (1 - Math.abs(position));
                view.setScaleX(scaleFactor);
                view.setScaleY(scaleFactor);

            } else { // (1,+Infinity]
                // This page is way off-screen to the right.
                view.setAlpha(0f);
            }
        }
    }