转载请注明出处：http://blog.csdn.net/allen315410/article/details/44200623
ViewPager作为Android最常用的的组件之一，相信大家在项目中会频繁的使用到的，例如利用ViewPager制作引导页、轮播图，甚至做整个app的表现层的框架等等。
但是在Android 3.0（API 11）以下的ViewPager是比较死板的，不支持动画特效的，这也就让ViewPager在切换的时候达不到很好的用户体验，下面就是Android3.0以下不添加动画的ViewPager的实现代码以及效果演示：
public class MainActivity extends Activity {
private ViewPager mViewPager;
private int[] imgRes = new int[] { R.drawable.guide_image1, R.drawable.guide_image2, R.drawable.guide_image3 };
private List<ImageView> imgList = new ArrayList<ImageView>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.activity_main);
mViewPager = (ViewPager) findViewById(R.id.viewpager);
mViewPager.setAdapter(new PagerAdapter() {
@Override
public boolean isViewFromObject(View arg0, Object arg1) {
return arg0 == arg1;
}
@Override
public int getCount() {
return imgRes.length;
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
ImageView mImageView = new ImageView(MainActivity.this);
mImageView.setBackgroundResource(imgRes[position]);
mImageView.setScaleType(ScaleType.CENTER_CROP);
imgList.add(mImageView);
container.addView(mImageView);
return mImageView;
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
container.removeView(imgList.get(position));
}
});
}
}上面是最简单的ViewPager使用的Demo，运行如下，看起来很普通很死板：
public void setPageTransformer(boolean reverseDrawingOrder, PageTransformer transformer){...}其中第一个参数boolean类型设置true就好，第二个参数PageTransformer就是我们自定义好的动画效果：mViewPager = (ViewPager) findViewById(R.id.viewpager); mViewPager.setPageTransformer(true, new ZoomOutPageTransformer());其中ZoomOutPageTransformer的代码来自于google的training文档中，英文好的朋友可以直接进入文档查看，链接是
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);
}
}
}另外Google文档中还提供了另外一个动画的实现方式，我暂且把源码附在下面：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(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);
}
}
}两种方式的所实现的效果如下所示，一是ZoomOutPageTransformer，二是DepthPageTransformer
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) {
// view.setAlpha(0);
ViewHelper.setAlpha(view, 0);
} else if (position <= 0) {
// view.setAlpha(1);
ViewHelper.setAlpha(view, 1);
// view.setTranslationX(0);
ViewHelper.setTranslationX(view, 0);
// view.setScaleX(1);
ViewHelper.setScaleX(view, 1);
// view.setScaleY(1);
ViewHelper.setScaleY(view, 1);
} else if (position <= 1) {
// view.setAlpha(1 - position);
ViewHelper.setAlpha(view, 1 - position);
// view.setTranslationX(pageWidth * -position);
ViewHelper.setTranslationX(view, pageWidth * -position);
float scaleFactor = MIN_SCALE + (1 - MIN_SCALE) * (1 - Math.abs(position));
// view.setScaleX(scaleFactor);
ViewHelper.setScaleX(view, scaleFactor);
// view.setScaleY(scaleFactor);
ViewHelper.setScaleY(view, scaleFactor);
} else {
// view.setAlpha(0);
ViewHelper.setAlpha(view, 0);
}
}
}为了好理解，我没有将Google提供的源码删除，而是注释掉了，方便大家进行比较阅读，然而即使这样使用了NineOldAndroids对我们的动画源码进行了改造，当我们打开一个Android2.3的模拟器运行一下的时候发现，没有起到效果，也就是这个动画效果并没有执行，在模拟器上运行的ViewPager滑动的效果还是原始默认的左右来回切换的动画，这又是怎么回事呢？为止，我们需要打开ViewPager的源码进行阅读一下了，既然是在mViewPager.setPageTransformer(true,
new DepthPageTransformer());这句代码没有起到效果，那么我们就点进去查看一下setPageTransformer这个方法的源码，源码如下：public void setPageTransformer(boolean reverseDrawingOrder, PageTransformer transformer){
if (Build.VERSION.SDK_INT >= 11) {
boolean hasTransformer = transformer != null;
boolean needsPopulate = hasTransformer != (this.mPageTransformer != null);
this.mPageTransformer = transformer;
setChildrenDrawingOrderEnabledCompat(hasTransformer);
if (hasTransformer) {
this.mDrawingOrder = (reverseDrawingOrder ? 2 : 1);
} else {
this.mDrawingOrder = 0;
}
if (needsPopulate)
populate();
}
}好了，源码一目了然，我们分析到在ViewPager中的setPageTransFormer这个方法中，首先判断了一下当前设备的Build.VERSION_SDK_INT>=11，也就是说当前设备是Android3.0以上系统的话，这个方法体执行没问题，但是若是3.0一下，那就抱歉，无法执行了。知道这个原因之后我们就需要解决这个问题了，我在这里修改了一下ViewPager的源码，修改ViewPager的源码之前，读者可以先去下载一份ViewPager的源码，然后拷贝到工程中，重新命名一下，将里面的if判断语句给删除了。以下就是我修改后的部分源码，命名为ViewPagerCompat：public void setPageTransformer(boolean reverseDrawingOrder, ViewPager.PageTransformer transformer) {
// if (Build.VERSION.SDK_INT >= 11) {
final boolean hasTransformer = transformer != null;
final boolean needsPopulate = hasTransformer != (mPageTransformer != null);
mPageTransformer = transformer;
setChildrenDrawingOrderEnabledCompat(hasTransformer);
if (hasTransformer) {
mDrawingOrder = reverseDrawingOrder ? DRAW_ORDER_REVERSE : DRAW_ORDER_FORWARD;
} else {
mDrawingOrder = DRAW_ORDER_DEFAULT;
}
if (needsPopulate)
populate();
// }
}好，源码修改完毕，将您的源码中的ViewPager替换成这个修改后的源码ViewPagerCompat，那么ViewPager切换动画的效果就出现了。public void transformPage(View view, float position) {
...
Log.i("TAG", "view = " + view + ",position = " + position);
...
}如上所示，我先在这个方法中打印出方法中的参数view对象和position的值，然后我们来看一下LOG的输出：
/**
* ViewPager自定义旋转动画
*
* @author Vincent
*
*/
public class RotateDownTransformer implements PageTransformer {
// 旋转的最大角度为20度
private static final float MAX_ROTATE = 20.0f;
// 旋转过程中的角度
private float currentRotate;
@Override
public void transformPage(View view, float position) {
int pageWidth = view.getWidth();
Log.i("TAG", "view = " + view + ",position = " + position);
if (position < -1) {
ViewHelper.setRotation(view, 0);
} else if (position <= 0) {
// position范围[-1.0,0.0],此时A页动画移出屏幕
currentRotate = position * MAX_ROTATE;
// 设置当前页的旋转中心点，横坐标是屏幕宽度的1/2,纵坐标为屏幕的高度
ViewHelper.setPivotX(view, pageWidth / 2);
ViewHelper.setPivotY(view, view.getHeight());
ViewHelper.setRotation(view, currentRotate);
} else if (position <= 1) {
// position范围(0.0,1.0],此时B页动画移到屏幕
currentRotate = position * MAX_ROTATE;
// 设置当前页的旋转中心点，横坐标是屏幕宽度的1/2,纵坐标为屏幕的高度
ViewHelper.setPivotX(view, pageWidth / 2);
ViewHelper.setPivotY(view, view.getHeight());
ViewHelper.setRotation(view, currentRotate);
} else {
ViewHelper.setRotation(view, 0);
}
}
}
Android为ViewPager增加切换动画——使用属性动画
原文：http://blog.csdn.net/allen315410/article/details/44200623