android千变万化的ViewPager之二 _viewPager

眼前多少难甘事，自古男儿当自强。这篇文章主要讲述android千变万化的ViewPager之二相关的知识，希望能为你提供帮助。
之一已经说了，怎么实现图片的切换。这里我说下怎么给切换的动画添加动画，对！你没听错，我们要给ViewPager加动画了。
首先贴上源代码代码

1 package com.example.android_viewpager; 2 3 import java.util.ArrayList; 4 import java.util.List; 5 6 import android.app.Activity; 7 import android.os.Bundle; 8 import android.support.v4.view.PagerAdapter; 9 import android.support.v4.view.ViewPager; 10 import android.view.View; 11 import android.view.ViewGroup; 12 import android.widget.ImageView; 13 import android.widget.ImageView.ScaleType; 14 15 public class MainActivity extends Activity { 16private ViewPager viewpager = null; 17private int[] res = new int[] { R.drawable.guide_image1, R.drawable.guide_image2, R.drawable.guide_image3 }; 18private List< ImageView> list = new ArrayList< ImageView> (); 19protected void onCreate(Bundle savedInstanceState) { 20super.onCreate(savedInstanceState); 21setContentView(R.layout.activity_main); 22viewpager = (ViewPager) findViewById(R.id.viewpager); 23viewpager.setPageTransformer(true, new DepthPageTransformer()); 24viewpager.setAdapter(new PagerAdapter() { 25@Override 26public Object instantiateItem(ViewGroup container, int position) { 27ImageView imageview = new ImageView(MainActivity.this); 28imageview.setImageResource(res[position]); 29imageview.setScaleType(ScaleType.CENTER_CROP); 30container.addView(imageview); 31list.add(imageview); 32return imageview; 33} 34@Override 35public void destroyItem(ViewGroup container, int position, Object object) { 36// TODO Auto-generated method stub 37container.removeView(list.get(position)); 38} 39public boolean isViewFromObject(View arg0, Object arg1) { 40// TODO Auto-generated method stub 41return arg0 == arg1; 42} 43 44@Override 45public int getCount() { 46// TODO Auto-generated method stub 47return res.length; 48} 49}); 50} 51 }

看上面的代码，我们会发现比原来的代码多了一行，这就是会ViewPager设置了动画，但是那个莫名其妙的类从哪里来的？
从API中可以看出setPageTransformer方法原型是：setPageTransformer(boolean reverseDrawingOrder, ViewPager.PageTransformer transformer)；第一个参数是布尔类型，就不用说了，而第二个参数是啥？PageTransformer类型参数？
我来解释下PageTransformer类，它是一个接口，专门用来给ViewPager添加动画的。它里面只有一个抽象方法public void transformPage(View view, float position)。我们通常操作这个方法来给ViewPager添加动画。
然后，我们来看看怎么怎么使用PageTransformer类。首先我们使用的它的子类，而且是自定义的子类，为什么？因为它没有系统的子类。那怎么创建它的子类呢？我从安卓官方的API上抄来了两个代码，供大家参考
1.DepthPageTransformer类

1 private static final float MIN_SCALE = 0.75f; 2 3public void transformPage(View view, float position) { 4int pageWidth = view.getWidth(); 5 6if (position < -1) { // [-Infinity,-1) 7// This page is way off-screen to the left. 8view.setAlpha(0); 9 10} else if (position < = 0) { // [-1,0] 11// Use the default slide transition when moving to the left page 12view.setAlpha(1); 13view.setTranslationX(0); 14view.setScaleX(1); 15view.setScaleY(1); 16 17} else if (position < = 1) { // (0,1] 18// Fade the page out. 19view.setAlpha(1 - position); 20 21// Counteract the default slide transition 22view.setTranslationX(pageWidth * -position); 23 24// Scale the page down (between MIN_SCALE and 1) 25float scaleFactor = MIN_SCALE 26+ (1 - MIN_SCALE) * (1 - Math.abs(position)); 27view.setScaleX(scaleFactor); 28view.setScaleY(scaleFactor); 29 30} else { // (1,+Infinity] 31// This page is way off-screen to the right. 32view.setAlpha(0); 33} 34}

2.ZoomOutPageTransformer类

1 private static final float MIN_SCALE = 0.85f; 2private static final float MIN_ALPHA = 0.5f; 3 4public void transformPage(View view, float position) { 5int pageWidth = view.getWidth(); 6int pageHeight = view.getHeight(); 7 8if (position < -1) { // [-Infinity,-1) 9// This page is way off-screen to the left. 10view.setAlpha(0); 11 12} else if (position < = 1) { // [-1,1] 13// Modify the default slide transition to shrink the page as well 14float scaleFactor = Math.max(MIN_SCALE, 1 - Math.abs(position)); 15float vertMargin = pageHeight * (1 - scaleFactor) / 2; 16float horzMargin = pageWidth * (1 - scaleFactor) / 2; 17if (position < 0) { 18view.setTranslationX(horzMargin - vertMargin / 2); 19} else { 20view.setTranslationX(-horzMargin + vertMargin / 2); 21} 22 23// Scale the page down (between MIN_SCALE and 1) 24view.setScaleX(scaleFactor); 25view.setScaleY(scaleFactor); 26 27// Fade the page relative to its size. 28view.setAlpha(MIN_ALPHA + 29(scaleFactor - MIN_SCALE) / 30(1 - MIN_SCALE) * (1 - MIN_ALPHA)); 31 32} else { // (1,+Infinity] 33// This page is way off-screen to the right. 34view.setAlpha(0); 35} 36}

【android千变万化的ViewPager之二】我们可以通过这两个代码来给ViewPager加上动画。