螢幕投影片會在整個畫面間轉換，常見於設定精靈或投影播放等使用者介面中。本課程說明如何使用支援資料庫提供的 ViewPager 來輕觸螢幕。ViewPager 物件可自動為螢幕投影片加上動畫效果。以下是從一個內容畫面轉換到下一個畫面的螢幕投影片外觀：
畫面投影片動畫
ViewPager 是 AndroidX 的一部分。詳情請參閱「使用 AndroidX」一文。
建立檢視畫面
建立版面配置檔案，稍後將用於片段的內容。此外，您還必須為片段內容定義字串。以下範例包含可顯示部分文字的文字檢視區塊：
<!-- 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>
建立片段
建立 Fragment 類別，以傳回您剛在 onCreateView() 方法中建立的版面配置。然後，如果您需要向使用者顯示新頁面，就能在父項活動中建立這個片段的執行個體：
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;
}
}
新增 ViewPager
ViewPager 物件內建滑動手勢來切換頁面，且預設會顯示螢幕投影片動畫，因此您不需要自行建立動畫。ViewPager 會使用 PagerAdapter 物件做為要顯示的新頁面，因此 PagerAdapter 會使用您先前建立的片段類別。
首先，請建立包含 ViewPager 的版面配置：
<!-- 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" />
建立可執行下列操作的活動：
- 將內容檢視畫面設為含有
ViewPager的版面配置。 - 建立可擴充
FragmentStatePagerAdapter抽象類別的類別，並實作getItem()方法，以提供ScreenSlidePageFragment的執行個體做為新頁面。此外，呼叫器轉接程式也必須實作getCount()方法，藉此傳回轉接程式會建立的網頁數量 (在本例中為五個)。 - 將
PagerAdapter連接到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 自訂動畫
如要顯示預設畫面投影片動畫以外的其他動畫，請實作 ViewPager.PageTransformer 介面，並將其提供給檢視區塊分頁器。介面會顯示單一方法 transformPage()。在畫面轉換的每個時間點，這個方法都會針對每個可見頁面 (通常只會顯示一個頁面) 以及位於畫面外的頁面呼叫一次。舉例來說，如果顯示第 3 頁，且使用者朝向第 4 頁拖曳，那麼在手勢的每個步驟中，系統會呼叫第 2、3 和 4 頁的 transformPage()。
在 transformPage() 實作中，您可以根據頁面位置 (從 transformPage() 方法的 position 參數取得) 決定要轉換哪些頁面，藉此建立自訂投影片動畫。
position 參數代表指定頁面相對於畫面中央的位置。這種動態屬性會隨使用者捲動網頁而變更。當網頁填滿整個畫面時，其位置值為 0。當頁面剛繪製在畫面右側時，其位置值為 1。如果使用者捲動網頁一到兩頁之間，半數為 -0.5，第 2 頁的排名則為 0.5。根據頁面在畫面上的位置，您可以使用 setAlpha()、setTranslationX() 或 setScaleY() 等方法設定頁面屬性，建立自訂投影片動畫。
實作 PageTransformer 時，請在實作方式呼叫 setPageTransformer() 以套用自訂動畫。舉例來說，如果您有名為 ZoomOutPageTransformer 的 PageTransformer，可以按照下列方式設定自訂動畫：
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());
如需 PageTransformer 的範例和影片，請參閱「縮放頁面轉換器」和「深度頁面轉換器」一節。
縮小頁面轉換器
這個頁面轉換器在捲動瀏覽相鄰頁面時，會縮小並淡出頁面。隨著頁面靠近中心點，會逐漸縮減回正常大小並逐漸淡入。
ZoomOutPageTransformer 範例
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);
}
}
}
深度頁面轉換器
此頁面轉換器使用預設的投影片動畫，將頁面向左滑動，同時使用「深度」動畫來往右滑動頁面。這段深度動畫會將頁面淡出，再以線性比例縮小。
DepthPageTransformer 範例
在深度動畫期間，系統仍會顯示預設動畫 (螢幕投影片)，因此您必須以負 X 軸平移螢幕滑動。例如：
Kotlin
view.translationX = -1 * view.width * position
Java
view.setTranslationX(-1 * view.getWidth() * position);
以下範例說明如何在工作頁面轉換器中，反制預設畫面投影片動畫：
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);
}
}
}