ViewPager在开发中的使用频率非常的高，所以在此做个总结。主要包括以下几方面：
- ViewPager的简介和作用
- ViewPager的适配器
- ViewPager的翻页动画
- 简化ViewPager的使用
- ViewPager结合第三方库实现小圆点指示器效果
- ViewPager结合design库实现tab切换
- 基于ViewPager实现广告轮播控件
按照惯例，先上个效果图
demo.gif
基础篇
1.ViewPager的简介和作用
ViewPager是android扩展包v4包中的类，这个类可以让用户左右切换当前的view
1）ViewPager类直接继承了ViewGroup类，所有它是一个容器类，可以在其中添加其他的view类。
2）ViewPager类需要一个PagerAdapter适配器类给它提供数据。
3）ViewPager经常和Fragment一起使用，并且提供了专门的FragmentPagerAdapter和FragmentStatePagerAdapter类供Fragment中的ViewPager使用。
2.ViewPager的适配器
简介中提到了PagerAdapter，和ListView等控件使用一样,需要ViewPager设置PagerAdapter来完成页面和数据的绑定，这个PagerAdapter是一个基类适配器，我们经常用它来实现app引导图，它的子类有FragmentPagerAdapter和FragmentStatePagerAdapter,这两个子类适配器用于和Fragment一起使用，在安卓应用中它们就像listview一样出现的频繁。
实现一个最基本的PagerAdapter,《必须实现四个方法》，在代码里有注释
public class AdapterViewpager extends PagerAdapter {
private List<View> mViewList;
public AdapterViewpager(List<View> mViewList) {
this.mViewList = mViewList;
}
@Override
public int getCount() {//必须实现
return mViewList.size();
}
@Override
public boolean isViewFromObject(View view, Object object) {//必须实现
return view == object;
}
@Override
public Object instantiateItem(ViewGroup container, int position) {//必须实现，实例化
container.addView(mViewList.get(position));
return mViewList.get(position);
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {//必须实现，销毁
container.removeView(mViewList.get(position));
}
}
实现一个最基本的FragmentPagerAdapter
public class AdapterFragment extends FragmentPagerAdapter {
private List<Fragment> mFragments;
public AdapterFragment(FragmentManager fm, List<Fragment> mFragments) {
super(fm);
this.mFragments = mFragments;
}
@Override
public Fragment getItem(int position) {//必须实现
return mFragments.get(position);
}
@Override
public int getCount() {//必须实现
return mFragments.size();
}
@Override
public CharSequence getPageTitle(int position) {//选择性实现
return mFragments.get(position).getClass().getSimpleName();
}
}
FragmentStatePagerAdapter的实现和FragmentPagerAdapter的实现一样就不在写了
3个适配器的基本实现讲完了是不是很简单，那他们的区别是什么呢？
PagerAdapter是基类适配器是一个通用的ViewPager适配器，相比PagerAdapter，FragmentPagerAdapter和FragmentStatePagerAdapter更专注于每一页是Fragment的情况，而这两个子类适配器使用情况也是有区别的。FragmentPagerAdapter适用于页面比较少的情况，FragmentStatePagerAdapter适用于页面比较多的情况。为什么？简单分析下两个适配器的源码就可以知道了。
-
FragmentStatePagerAdapter
@Override public Object instantiateItem(ViewGroup container, int position) { // If we already have this item instantiated, there is nothing // to do. This can happen when we are restoring the entire pager // from its saved state, where the fragment manager has already // taken care of restoring the fragments we previously had instantiated. if (mFragments.size() > position) { Fragment f = mFragments.get(position);//fragment被释放后这里得到的null值 if (f != null) { return f; } } if (mCurTransaction == null) { mCurTransaction = mFragmentManager.beginTransaction(); } Fragment fragment = getItem(position);//fragment被释放后或者是初次进入页面拿到新的Fragment实例 if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment); if (mSavedState.size() > position) { Fragment.SavedState fss = mSavedState.get(position); if (fss != null) { fragment.setInitialSavedState(fss); } } while (mFragments.size() <= position) { mFragments.add(null); } fragment.setMenuVisibility(false); fragment.setUserVisibleHint(false); mFragments.set(position, fragment); mCurTransaction.add(container.getId(), fragment);//新的Fragment实例 是add上去的 return fragment; } @Override public void destroyItem(ViewGroup container, int position, Object object) { Fragment fragment = (Fragment) object; if (mCurTransaction == null) { mCurTransaction = mFragmentManager.beginTransaction(); } if (DEBUG) Log.v(TAG, "Removing item #" + position + ": f=" + object + " v=" + ((Fragment)object).getView()); while (mSavedState.size() <= position) { mSavedState.add(null); } mSavedState.set(position, fragment.isAdded() ? mFragmentManager.saveFragmentInstanceState(fragment) : null); mFragments.set(position, null);//真正释放了fragment实例 mCurTransaction.remove(fragment); } -
FragmentPagerAdapter
@Override public Object instantiateItem(ViewGroup container, int position) { if (mCurTransaction == null) { mCurTransaction = mFragmentManager.beginTransaction(); } final long itemId = getItemId(position); // Do we already have this fragment? String name = makeFragmentName(container.getId(), itemId); Fragment fragment = mFragmentManager.findFragmentByTag(name); if (fragment != null) { if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment); mCurTransaction.attach(fragment);//因为fragment实例没有被真正释放，所以可以直接attach效率高 } else { fragment = getItem(position);//初始化页面的时候拿到fragment的实例 if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment); mCurTransaction.add(container.getId(), fragment, makeFragmentName(container.getId(), itemId));//add上去 } if (fragment != mCurrentPrimaryItem) { fragment.setMenuVisibility(false); fragment.setUserVisibleHint(false); } return fragment; } @Override public void destroyItem(ViewGroup container, int position, Object object) { if (mCurTransaction == null) { mCurTransaction = mFragmentManager.beginTransaction(); } if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object + " v=" + ((Fragment)object).getView()); mCurTransaction.detach((Fragment)object);//并没有真正释放fragment对象只是detach }
从源码中我们可以看出FragmentStatePagerAdapter中fragment实例在destroyItem的时候被真正释放，所以FragmentStatePagerAdapter省内存。FragmentPagerAdapter中的fragment实例在destroyItem的时候并没有真正释放fragment对象只是detach，所以FragmentPagerAdapter消耗更多的内存，带来的好处就是效率更高一些。所以得出这样的结论：FragmentPagerAdapter适用于页面比较少的情况，FragmentStatePagerAdapter适用于页面比较多的情况，因此不同的场合选择合适的适配器才是正确的做法
3.ViewPager的翻页动画
为ViewPager设置适配器后，就可以正常使用了，接下来我们为ViewPager增加翻页动画，毕竟人的审美会疲劳，加上一些动画交互会提高不少逼格~~，ViewPager提供了PageTransformer接口用于实现翻页动画。
官方提供了PageTransformer的实现例子。
public class DepthPageTransformer implements ViewPager.PageTransformer {
private static final float MIN_SCALE = 0.75f;
public void transformPage(View view, float position) {
Log.d("DepthPageTransformer", view.getTag() + " , " + 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);
}
}
}
public class ZoomOutPageTransformer implements ViewPager.PageTransformer {
private static final float MIN_SCALE = 0.85f;
private static final float MIN_ALPHA = 0.5f;
@SuppressLint("NewApi")
public void transformPage(View view, float position) {
int pageWidth = view.getWidth();
int pageHeight = view.getHeight();
Log.e("TAG", view + " , " + position + "");
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);
}
}
}
实现翻页动画的关键就是重写transformPage方法，方法里有两个参数view和position,理解这两个参数非常重要。假设有三个页面view1，view2，view3从左至右在viewPager中显示
- 往左滑动时：view1，view2，view3的position都是不断变小的。
view1的position: 0 → -1 → 负无穷大
view2的position: 1 → 0 → -1
view3的position: 1 → 0 - 往右滑动时：view1，view2，view3的position都是不断变大的。
view1的position: -1 → 0
view2的position: -1 → 0 → 1
view3的position: 0 → 1→ 正无穷大
当position是正负无穷大时view就离开屏幕视野了。因此最核心的控制逻辑是在[-1,0]和(0,1]这两个区间，通过设置透明度，平移，旋转，缩放等动画组合可以实现各式各样的页面变化效果。
4.简化ViewPager的使用
不会偷懒的程序猿不是好程序员
这里只是做了最简单的封装，可以根据需要调整
- PagerAdapter简化
public class QuickPageAdapter<T extends View> extends PagerAdapter { private List<T> mList; public QuickPageAdapter(List<T> mList) { this.mList = mList; } @Override public int getCount() { return mList.size(); } @Override public boolean isViewFromObject(View view, Object object) { return object == view; } @Override public Object instantiateItem(ViewGroup container, int position) { container.addView(mList.get(position)); return mList.get(position); } @Override public void destroyItem(ViewGroup container, int position, Object object) { container.removeView(mList.get(position)); }
}
使用它，这样不用每次都写个适配器
List<View> views = new ArrayList<>();
...
mViewPager.setAdapter(new QuickPageAdapter<View>(views));
- FragmentPagerAdapter简化
```java
public class QuickFragmentPageAdapter<T extends Fragment> extends FragmentPagerAdapter {
private List<T> mList;
private String[] mStrings;
/**
* @param fm
* @param list
* @param titles PageTitles
*/
public QuickFragmentPageAdapter(FragmentManager fm, List<T> list, String[] titles) {
super(fm);
mList = list;
mStrings = titles;
}
@Override
public Fragment getItem(int position) {
return mList.get(position);
}
@Override
public int getCount() {
return mList.size();
}
@Override
public CharSequence getPageTitle(int position) {
return mStrings == null ? super.getPageTitle(position) : mStrings[position];
}
}
FragmentStatePagerAdapter封装类似FragmentPagerAdapter就不写了，基本使用讲完了。
5.补充一个知识点
mViewPager.setOffscreenPageLimit()//这个方法是用来控制fragment不重新走生命周期的个数的，打个比方一共4个fragment页面，如果mViewPager.setOffscreenPageLimit(3)，那么所有的fragment都只走一次生命周期，如果是mViewPager.setOffscreenPageLimit(2)，那么其中有一个fragment会在切换的时候重新走一遍生命周期，FragmentStatePagerAdapter和FragmentPagerAdapter都是这样，但是FragmentPagerAdapter设置setOffscreenPageLimit不影响fragment缓存的个数,而FragmentStatePagerAdapter缓存的fragment实例个数就是setOffscreenPageLimit设置的值+1。另外setOffscreenPageLimit的缺省值是1，设置0是无效的会被强制赋值成1。
private static final int DEFAULT_OFFSCREEN_PAGES = 1;
public void setOffscreenPageLimit(int limit) {
if (limit < DEFAULT_OFFSCREEN_PAGES) {
Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " +
DEFAULT_OFFSCREEN_PAGES);
limit = DEFAULT_OFFSCREEN_PAGES;//强制赋值为1
}
if (limit != mOffscreenPageLimit) {
mOffscreenPageLimit = limit;
populate();
}
}
提高篇
1.ViewPager结合第三方库实现小圆点指示器效果
https://github.com/ongakuer/CircleIndicator
screenshot.gif
使用看官方文档很简单。
看一下实现思路
public void setViewPager(ViewPager viewPager) {
mViewpager = viewPager;
if (mViewpager != null && mViewpager.getAdapter() != null) {
mLastPosition = -1;
createIndicators();
mViewpager.removeOnPageChangeListener(mInternalPageChangeListener);
mViewpager.addOnPageChangeListener(mInternalPageChangeListener);//绑定上内部实现的PageChangeListener
mInternalPageChangeListener.onPageSelected(mViewpager.getCurrentItem());
}
}
private final OnPageChangeListener mInternalPageChangeListener = new OnPageChangeListener() {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
}
@Override public void onPageSelected(int position) {//这里是动画的核心
if (mViewpager.getAdapter() == null || mViewpager.getAdapter().getCount() <= 0) {
return;
}
if (mAnimatorIn.isRunning()) {
mAnimatorIn.end();
mAnimatorIn.cancel();
}
if (mAnimatorOut.isRunning()) {
mAnimatorOut.end();
mAnimatorOut.cancel();
}
View currentIndicator;
if (mLastPosition >= 0 && (currentIndicator = getChildAt(mLastPosition)) != null) {//页面离开屏幕时指示器动画
currentIndicator.setBackgroundResource(mIndicatorUnselectedBackgroundResId);
mAnimatorIn.setTarget(currentIndicator);
mAnimatorIn.start();
}
View selectedIndicator = getChildAt(position);
if (selectedIndicator != null) {//页面进入屏幕时指示器动画
selectedIndicator.setBackgroundResource(mIndicatorBackgroundResId);
mAnimatorOut.setTarget(selectedIndicator);
mAnimatorOut.start();
}
mLastPosition = position;
}
@Override public void onPageScrollStateChanged(int state) {
}
};
2.ViewPager结合design库实现tab切换
在design库中有个TabLayout可以为viewPager加上Tab标题头
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
android:orientation="vertical"
tools:context="com.example.administrator.viewpager.MainActivity">
<android.support.design.widget.TabLayout
android:id="@+id/mTabLayout"
android:layout_width="match_parent"
android:layout_height="wrap_content"></android.support.design.widget.TabLayout>
<android.support.v4.view.ViewPager
android:id="@+id/mViewPager"
android:layout_width="match_parent"
android:layout_height="match_parent"></android.support.v4.view.ViewPager>
</LinearLayout>
mTabLayout.setupWithViewPager(mViewPager);//一行代码完成绑定
更多高级的用法包括tab中添加icon等请转至这里 传送门
3.基于ViewPager实现广告轮播控件
https://github.com/daimajia/AndroidImageSlider
imageSlider.gif
源码分析，省略了部分代码：
public class SliderLayout extends RelativeLayout{
private InfiniteViewPager mViewPager;//这个ViewPager只是修改了setPageTransformer方法去掉了if (Build.VERSION.SDK_INT >= 11) 的限制，结合NineOldDroid库让动画兼容低版本
/**
* InfiniteViewPager adapter.
*/
private SliderAdapter mSliderAdapter;//这个是PagerAdapter
/**
* {@link com.daimajia.slider.library.Tricks.ViewPagerEx} indicator.
*/
private PagerIndicator mIndicator;//页面指示器
/**
* A timer and a TimerTask using to cycle the {@link com.daimajia.slider.library.Tricks.ViewPagerEx}.
*/
private Timer mCycleTimer;//用于轮播的定时器
private TimerTask mCycleTask;
/**
* For resuming the cycle, after user touch or click the {@link com.daimajia.slider.library.Tricks.ViewPagerEx}.
*/
private Timer mResumingTimer;
private TimerTask mResumingTask;
/**
* {@link com.daimajia.slider.library.Tricks.ViewPagerEx} 's transformer
*/
private BaseTransformer mViewPagerTransformer;//PageTransformer的封装用于控制页面翻页效果
public SliderLayout(Context context, AttributeSet attrs, int defStyle) {//核心代码，用于初始化ViewPager
super(context, attrs, defStyle);
mContext = context;
LayoutInflater.from(context).inflate(R.layout.slider_layout, this, true);
final TypedArray attributes = context.getTheme().obtainStyledAttributes(attrs,R.styleable.SliderLayout,
defStyle,0);
mTransformerSpan = attributes.getInteger(R.styleable.SliderLayout_pager_animation_span, 1100);
mTransformerId = attributes.getInt(R.styleable.SliderLayout_pager_animation, Transformer.Default.ordinal());
mAutoCycle = attributes.getBoolean(R.styleable.SliderLayout_auto_cycle,true);
int visibility = attributes.getInt(R.styleable.SliderLayout_indicator_visibility,0);
for(PagerIndicator.IndicatorVisibility v: PagerIndicator.IndicatorVisibility.values()){
if(v.ordinal() == visibility){
mIndicatorVisibility = v;
break;
}
}
mSliderAdapter = new SliderAdapter(mContext);
PagerAdapter wrappedAdapter = new InfinitePagerAdapter(mSliderAdapter);
mViewPager = (InfiniteViewPager)findViewById(R.id.daimajia_slider_viewpager);
mViewPager.setAdapter(wrappedAdapter);
mViewPager.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
int action = event.getAction();
switch (action) {
case MotionEvent.ACTION_UP:
recoverCycle();
break;
}
return false;
}
});
attributes.recycle();
setPresetIndicator(PresetIndicators.Center_Bottom);
setPresetTransformer(mTransformerId);
setSliderTransformDuration(mTransformerSpan,null);
setIndicatorVisibility(mIndicatorVisibility);
if(mAutoCycle){
startAutoCycle();
}
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
int action = ev.getAction();
switch (action) {
case MotionEvent.ACTION_DOWN://手指按下时候暂停轮播
pauseAutoCycle();
break;
}
return false;
}
/**
* preset transformers and their names
*/
public enum Transformer{//PageTransformer枚举
Default("Default"),
Accordion("Accordion"),
Background2Foreground("Background2Foreground"),
CubeIn("CubeIn"),
DepthPage("DepthPage"),
Fade("Fade"),
FlipHorizontal("FlipHorizontal"),
FlipPage("FlipPage"),
Foreground2Background("Foreground2Background"),
RotateDown("RotateDown"),
RotateUp("RotateUp"),
Stack("Stack"),
Tablet("Tablet"),
ZoomIn("ZoomIn"),
ZoomOutSlide("ZoomOutSlide"),
ZoomOut("ZoomOut");
private final String name;
private Transformer(String s){
name = s;
}
public String toString(){
return name;
}
public boolean equals(String other){
return (other == null)? false:name.equals(other);
}
};
}
通过分析我们可以对SliderLayout实现思路小结一下：
1.内部持有一个修改过的ViewPager控件，可以兼容低版本的页面转换动画
2.内部有一个实现了PagerAdapter的SliderAdapter适配器
3.内部持有一个PagerIndicator 页面指示器可供选择
4.维护一个定时任务用于控制轮播
5.对手势事件进行处理暂停轮播，继续轮播
6.提供了很多缺省的PageTransformer方便调用
最后在构造函数中初始化ViewPager。
