ViewPager切換動畫PageTransformer的使用

Android從3.0開始添加了屬性動畫後，諸多難以實作的動畫都可以輕松解決了，v4包下的ViewPager控件當然也不例外，相對于非常平庸的預設切換動畫，Google官方給我們展示了兩個動畫例子：DepthPageTransformer和ZoomOutPageTransformer，比較炫。

實際上我們也可以通過實作ViewPager.PageTransformer接口來做出完全不同的切換動畫效果。實作PageTransformer接口就隻要實作一個方法：transformPage(View view,float position);該方法的兩個參數尤為重要，了解它是我們自定義ViewPager動畫的重點。

transformPage(View view,float position);

第一個參數view顧名思義就是我們需要滑動的那個View。

第二個參數position，這個float類型的參數并不是我們平常所了解的位置，而是表示滑動的View的一種狀态，比如：當滑動到正全屏時，position是0，而向左滑動，使得右邊剛好有一部被進入螢幕時，position是1，如果前一頁和下一頁基本各在螢幕占一半時，前一頁的position是-0.5，後一頁的posiotn是0.5，是以根據position的值我們就可以自行設定需要的alpha，ScaleX/Y,translationX/Y等參數值。

下面，我們就來看看Google官方提供的兩種動畫效果的源碼及對應的動畫，了解了原理自定義動畫可以仿照源碼了：

DepthPageTransformer:

import android.annotation.SuppressLint;
import android.support.v4.view.ViewPager.PageTransformer;
import android.view.View;
 
public class DepthPageTransformer implements PageTransformer {
    private static float MIN_SCALE = 0.75f;
 
    @SuppressLint("NewApi")
    @Override
    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(0);
        } else if (position <= 0) { // [-1,0]
            // Use the default slide transition when
            // moving to the left page
            view.setAlpha(1);
            view.setTranslationX(0);
            view.setScaleX(1);
            view.setScaleY(1);
        } 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(0);
 
        }
    }
 
}
           

ZoomOutPageTransformer：

import android.support.v4.view.ViewPager.PageTransformer;
import android.view.View;
 
public class ZoomOutPageTransformer implements PageTransformer {
    private static float MIN_SCALE = 0.85f;
 
    private static float MIN_ALPHA = 0.5f;
 
    @Override
    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);
        }
    }
}
           

下面我們來看看怎麼使用ViewPager，讓它加入動畫。

其實就是通過mViewPager.setPageTransformer(true, new DepthPageTransformer());來設定。

布局：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</RelativeLayout>
           

代碼：

public class MainActivity extends ActionBarActivity {
    private int[] imageId = new int[]{R.mipmap.bg,R.mipmap.one, R.mipmap.two, R.mipmap.three};
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ViewPager mViewPager = (ViewPager) findViewById(R.id.viewpager);
        mViewPager.setAdapter(new ViewPagerAdapter(this,imageId));
//      mViewPager.setPageTransformer(true, new ZoomOutPageTransformer());
        mViewPager.setPageTransformer(true,new DepthPageTransformer());
    }
 
    class ViewPagerAdapter extends PagerAdapter {
        private ArrayList<ImageView> mImages ;
        private int[] imagesId;
        private Context context;
        public ViewPagerAdapter(Context context,int[] resourceId){
            this.context = context;
            this.imagesId = resourceId;
            mImages = new ArrayList<>();
        }
        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            ImageView imageView = new ImageView(context);
            imageView.setImageResource(imagesId[position]);
            imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
            container.addView(imageView);
            mImages.add(imageView);
            return imageView;
        }
 
        @Override
        public void destroyItem(ViewGroup container, int position, Object object) {
//            super.destroyItem(container, position, object);//這行要去除，否則會報錯
            container.removeView(mImages.get(position));
        }
 
        @Override
        public int getCount() {
            return imagesId.length;
        }
 
        @Override
        public boolean isViewFromObject(View view, Object object) {
            return view==object;
        }
    }
}
           

另外說明一點：

關于ViewPager的擴充卡，有兩種實作：

直接繼承PagerAdapter，即：

class ViewPagerAdapter extends PagerAdapter {
        private ArrayList<ImageView> mImages ;
        private int[] imagesId;
        private Context context;
        public ViewPagerAdapter(Context context,int[] resourceId){
            this.context = context;
            this.imagesId = resourceId;
            mImages = new ArrayList<>();
        }
        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            ImageView imageView = new ImageView(context);
            imageView.setImageResource(imagesId[position]);
            imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
            container.addView(imageView);
            mImages.add(imageView);
            return imageView;
        }
 
        @Override
        public void destroyItem(ViewGroup container, int position, Object object) {
//            super.destroyItem(container, position, object);//這行要去除，否則會報錯
            container.removeView(mImages.get(position));
        }
 
        @Override
        public int getCount() {
            return imagesId.length;
        }
 
        @Override
        public boolean isViewFromObject(View view, Object object) {
            return view==object;
        }
           

繼承至FragmentPagerAdapter：即

class FragmentViewPagerAdapter extends FragmentPagerAdapter{
        private final List<Fragment> mFragments = new ArrayList<>();//添加的Fragment的集合
        public FragmentViewPagerAdapter(FragmentManager fm) {
            super(fm);
        }
        public void addFragment(Fragment fragment) {
            mFragments.add(fragment);
        }
        @Override
        public Fragment getItem(int position) {
            //得到對應position的Fragment
            return mFragments.get(position);
        }
 
        @Override
        public int getCount() {
            //傳回Fragment的數量
            return mFragments.size();
        }
    }