使用 ViewPager 在片段之间滑动

尝试 Compose 方式
Jetpack Compose 是 Android 推荐的 UI 工具包。了解如何在 Compose 中使用 Pager。
Pager →

屏幕滑动是在一个完整屏幕到另一个完整屏幕之间的过渡，在设置向导或幻灯片等 UI 中很常见。本课程将向您展示如何使用 ViewPager（由 支持库 提供）进行屏幕滑动。ViewPager 对象可以自动为屏幕滑动设置动画。以下是屏幕滑动从一个内容屏幕过渡到下一个屏幕的外观

屏幕滑动动画
 

ViewPager 是 AndroidX 的一部分。有关更多信息，请参阅 使用 AndroidX。

创建视图

创建一个布局文件，您稍后将用于片段的内容。您还需要为片段的内容定义一个字符串。以下示例包含一个文本视图以显示一些文本

<!-- 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>

创建片段

创建一个 Fragment 类，该类在 onCreateView() 方法中返回您刚刚创建的布局。然后，您可以在父活动中随时创建此片段的实例，以向用户显示新页面

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 对象内置了滑动手势以切换页面，并且默认情况下会显示屏幕滑动动画，因此您无需创建自己的动画。ViewPager 使用 PagerAdapter 对象作为要显示的新页面的供应，因此 PagerAdapter 将使用您之前创建的片段类。

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" />

创建一个执行以下操作的活动

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

要显示与默认屏幕滑动动画不同的动画，请实现 ViewPager.PageTransformer 接口并将其提供给视图分页器。该接口公开了一个方法，transformPage()。在屏幕过渡的每个点，此方法都会为每个可见页面（通常只有一个可见页面）以及屏幕外的相邻页面调用一次。例如，如果页面三可见且用户向页面四拖动，则 transformPage() 会在手势的每个步骤中为页面二、三和四调用。

在 transformPage() 的实现中，您可以通过根据页面在屏幕上的位置（从 position 参数中获取 transformPage() 方法中获得）确定哪些页面需要转换来创建自定义滑动动画。

该 position 参数指示给定页面相对于屏幕中心的位置。它是一个动态属性，会随着用户在页面之间滚动而发生变化。当页面充满屏幕时，其位置值为 0。当页面绘制在屏幕右侧时，其位置值为 1。如果用户在页面一和页面二之间滚动一半，则页面一的位置为 -0.5，页面二的位置为 0.5。根据页面在屏幕上的位置，您可以通过使用 setAlpha()、setTranslationX() 或 setScaleY() 等方法设置页面属性来创建自定义滑动动画。

当您拥有 PageTransformer 的实现时，请使用您的实现调用 setPageTransformer() 以应用您的自定义动画。例如，如果您有一个名为 ZoomOutPageTransformer 的 PageTransformer，则可以像这样设置您的自定义动画

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());

请参阅 缩放页面转换器 和 深度页面转换器 部分，了解 PageTransformer 的示例和视频。

缩放页面转换器

此页面转换器在滚动相邻页面之间时会缩小和淡出页面。随着页面越来越靠近中心，它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滑动动画将页面滑动到左侧，同时使用“深度”动画将页面滑动到右侧。此深度动画会淡出页面，并将其线性缩小。

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 翻译抵消屏幕滑动。例如

Kotlin

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

Java

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

以下示例显示如何在工作页面转换器中抵消默认屏幕滑动动画

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);
        }
    }
}