我们在使用 ViewPager 时，如果需要修改滑动的动画效果，可以通过 ViewPager 提供的 setPageTransformer 方法去设置一个转换器。
首先看一下这个方法的函数签名：
/**
* Set a PageTransformer that will be called for each attached page whenever
* the scroll position is changed. This allows the application to apply custom property
* transformations to each page, overriding the default sliding look and feel.
*
* Note: Prior to Android 3.0 the property animation APIs did not exist.
* As a result, setting a PageTransformer prior to Android 3.0 (API 11) will have no effect.
*
* @param reverseDrawingOrder true if the supplied PageTransformer requires page views
* to be drawn from last to first instead of first to last.
* @param transformer PageTransformer that will modify each page's animation properties
*/
public void setPageTransformer(boolean reverseDrawingOrder, PageTransformer transformer)
我实在没试出来第一个参数设成 true 或 false 有什么区别，有知道的童鞋给我说一下。。。T_T
所以，我们需要一个实现了 PageTransformer 接口的实例化对象：
/**
* A PageTransformer is invoked whenever a visible/attached page is scrolled.
* This offers an opportunity for the application to apply a custom transformation
* to the page views using animation properties.
*
* As property animation is only supported as of Android 3.0 and forward,
* setting a PageTransformer on a ViewPager on earlier platform versions will
* be ignored.
*/
public interface PageTransformer {
/**
* Apply a property transformation to the given page.
*
* @param page Apply the transformation to this page
* @param position Position of page relative to the current front-and-center
* position of the pager. 0 is front and center. 1 is one full
* page position to the right, and -1 is one page position to the left.
*/
public void transformPage(View page, float position);
}
transformPage 的 page 参数代表 adapter 中已经实例化的 view (默认加载当前页及左右两页)，position 指 view 向左或向右的偏移量，有效区间为[-1, 1]。
假设当前页为 page2，它的 position 为：
position 对应位置
意思就是 page2 向左滑动，令 page3 逐渐显示的时候，position 会逐渐减小到 -1，如果向右滑动，则增加到 1。
实现某些应用里的左右拖动效果：
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public void transformPage(View page, float position) {
if (position < -1 || position > 1) return;
int pageWidth = page.getWidth();
page.setTranslationX(position < 0 ? 0 : pageWidth * -position);
}
仿 Gallery 的效果：
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首先，ViewPager 所在的布局要写成这样
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:clipChildren="false"
tools:context="xyz.berial.pagetransformer.MainActivity">
<android.support.v4.view.ViewPager
android:id="@+id/view_pager"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_marginBottom="32dp"
android:layout_marginLeft="40dp"
android:layout_marginRight="40dp"
android:layout_marginTop="32dp"
android:clipChildren="false"/>
</LinearLayout>
关键所在是 ViewPager 及其父标签都要加上 android:clipChildren="false"。然后给 viewpager 设置 page 间的间距
int pageMargin = (int) 16, getResources().getDisplayMetrics()));
viewPager.setPageMargin(pageMargin);
transformPage 方法里：
public void transformPage(View page, float position) {
if (position < -1 || position > 1) {
page.setScaleX(0.9F);
page.setScaleY(0.9F);
} else {
float scaleFactor = Math.max(0.9F, 1 - Math.abs(position));
page.setScaleX(scaleFactor);
page.setScaleY(scaleFactor);
}
}
PS：当界面上能够显示去左右两个 page 时，position < -1 和 position > 1 也是有效的区间，所以要设置成默认的缩放大小。
仿 Gallery 效果视差效果
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public void transformPage(View page, float position) {
if (position < -1 || position > 1) return;
final Random random = new Random();
// v4 包中的 fragment 会在最外层套上一层 FrameLayout, 所以如果用这个,
// child 就是 page.xml 里的外层 Layout
// 所以, 有时为了减少布局嵌套, 并且不准备兼容低版本, 可以考虑重新写一个 FragmentPagerAdapter,
// 把 v4 包下的 Fragment 相关类 改成 android.app 包下的
// final ViewGroup group = (ViewGroup) page;
final ViewGroup group = (ViewGroup) page.findViewById(R.id.root_layout);
final int childCount = group.getChildCount();
for (int i = 0; i < childCount; i++) {
View view = group.getChildAt(i);
float factor;
if (view.getTag() != null) {
factor = (float) view.getTag();
} else {
factor = random.nextFloat();
view.setTag(factor);
}
view.setTranslationX(Math.abs(view.getWidth() * position * factor));
}
}