屏幕滑动是在一个完整屏幕和另一个屏幕之间进行的转场,在设置向导或幻灯片等 UI 中很常见。本课程介绍如何使用支持库提供的 ViewPager
进行屏幕滑动。 ViewPager
对象可以自动为屏幕滑动添加动画。以下是内容从一个屏幕转场到下一个屏幕的屏幕滑动示例
ViewPager
是 AndroidX 的一部分。如需了解详情,请参阅使用 AndroidX。
创建视图
创建稍后用于 Fragment 内容的布局文件。您还需要为 Fragment 的内容定义一个字符串。以下示例包含一个文本视图用于显示一些文本
<!-- 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 Fragment
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,只要需要向用户显示新页面,您就可以在父 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
对象内置了滑动页面转场手势,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
对象作为显示新页面的来源,因此 PagerAdapter
将使用您之前创建的 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" />
创建一个执行以下操作的 Activity
- 将内容视图设置为包含
ViewPager
的布局。 - 创建一个类,该类扩展抽象类
FragmentStatePagerAdapter
并实现getItem()
方法以提供ScreenSlidePageFragment
的实例作为新页面。页面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面数量(示例中为五个)。 - 将
PagerAdapter
连接到ViewPager
。
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寻呼机。该接口公开了一个方法 transformPage()
。在屏幕转场的每个点,此方法会为每个可见页面(通常只有一个可见页面)和刚好离开屏幕的相邻页面调用一次。例如,如果页面三可见,用户拖动到页面四,则在手势的每个步骤中,都会为页面二、三、四调用 transformPage()
。
在 transformPage()
的实现中,您可以根据页面在屏幕上的位置(通过 transformPage()
方法的 position
参数获取)来确定哪些页面需要转换,从而创建自定义滑动动画。
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,随着用户滑动页面而改变。当页面填充屏幕时,其 position
值为 0
。当页面刚好绘制在屏幕右侧时,其 position
值为 1
。如果用户滑动到页面一和页面二之间的一半位置,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,您可以通过设置页面属性(例如使用 setAlpha()
、setTranslationX()
或 setScaleY()
等方法)来创建自定义滑动动画。
实现 PageTransformer
后,使用您的实现调用 setPageTransformer()
以应用自定义动画。例如,如果您有一个名为 ZoomOutPageTransformer
的 PageTransformer
,您可以像这样设置自定义动画
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());
请参阅放大缩小页面转换器和深度页面转换器部分,了解 PageTransformer
的示例和视频。
放大缩小页面转换器
此页面转换器会在相邻页面之间滚动时缩小并淡出页面。当页面靠近中心时,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
此页面转换器对向左滑动的页面使用默认滑动动画,而对向右滑动的页面使用“深度”动画。这种深度动画会使页面淡出并线性缩小。
DepthPageTransformer
示例在深度动画期间,默认动画(屏幕滑动)仍然会发生,因此您必须通过负 X 轴平移来抵消屏幕滑动。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在有效的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); } } }