屏幕滑动就是在两个或多个满屏页面间相互移动，主要用于新手引导或幻灯片。这节课我们会介绍如何用ViewPager实现屏幕间的滑动切换，其中也会介绍到创建Fragment，和利用PageTransformer自定义动画样式。
准备开始
首先为fragment创建一个content布局：
<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>
创建一个Fragment
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;
}
}
添加一个ViewPager
ViewPager已经实现了利用手势切换页面，并且有一个默认的动画，你根本不需要做什么。ViewPager使用PagerAdapter来填充其中的pager，所以你需要创建一个PagerAdapter并hooks up it to ViewPager。
<!-- 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" />
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 mPagerAdapter;
@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);
mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
mPager.setAdapter(mPagerAdapter);
}
@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
想要实现一个不同于默认动画的屏幕切换动画就需要实现ViewPager.PageTransformer接口，这个接口只有一个方法transformPage()。这个方法会在屏幕切换的时候被回调，其回调的内容会是当前显示屏幕和其邻屏幕，比如当我们要从page3滑动到page4的时候，page2、page3、page4都会调用其对应的transformPage()。
position参数表明了这个page处于当前屏幕中心的相对位置，这是一个动态的值，会随用户手指移动而改变。当这个page充满屏幕的时候position的值是0；当这个page在屏幕的右侧(屏幕右外边)的值是1；当用户滑动page到一半的时候，一个page的position值是－0.5，另一个是0.5；你可以用setAlpha()、setTranslationX()或setSaleY()来自定义切换动画。
你可以创建一个ZoomOutPageTransformer并向下面一样引用：
ViewPager mPager = (ViewPager) findViewById(R.id.pager); ... mPager.setPageTransformer(true, new ZoomOutPageTransformer());
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(0);
} 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(0);
}
}
}
如果想要查看源码，可以访问：http://developer.android.com/training/animation/screen-slide.html