ViewPager在開發中的使用頻率非常的高，所以在此做個總結。主要包括以下幾方面：
ViewPager的簡介和作用ViewPager的適配器ViewPager的翻頁動畫簡化ViewPager的使用ViewPager結合第三方庫實現小圓點指示器效果ViewPager結合design庫實現tab切換基于ViewPager實現廣告輪播控件按照慣例，先上個效果圖
demo.gif1.ViewPager的簡介和作用ViewPager是android擴展包v4包中的類，這個類可以讓用戶左右切換當前的view1）ViewPager類直接繼承了ViewGroup類，所有它是一個容器類，可以在其中添加其他的view類。2）ViewPager類需要一個PagerAdapter適配器類給它提供數據。3）ViewPager經常和Fragment一起使用，并且提供了專門的FragmentPagerAdapter和FragmentStatePagerAdapter類供Fragment中的ViewPager使用。
2.ViewPager的適配器簡介中提到了PagerAdapter，和ListView等控件使用一樣,需要ViewPager設置PagerAdapter來完成頁面和數據的綁定，這個PagerAdapter是一個基類適配器，我們經常用它來實現app引導圖，它的子類有FragmentPagerAdapter和FragmentStatePagerAdapter,這兩個子類適配器用于和Fragment一起使用，在安卓應用中它們就像listview一樣出現的頻繁。
實現一個最基本的PagerAdapter,《必須實現四個方法》，在代碼里有注釋
public class AdapterViewpager extends PagerAdapter { PRivate List<View> mViewList; public AdapterViewpager(List<View> mViewList) { this.mViewList = mViewList; } @Override public int getCount() {//必須實現 return mViewList.size(); } @Override public boolean isViewFromObject(View view, Object object) {//必須實現 return view == object; } @Override public Object instantiateItem(ViewGroup container, int position) {//必須實現，實例化 container.addView(mViewList.get(position)); return mViewList.get(position); } @Override public void destroyItem(ViewGroup container, int position, Object object) {//必須實現，銷毀 container.removeView(mViewList.get(position)); }}實現一個最基本的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) {//必須實現 return mFragments.get(position); } @Override public int getCount() {//必須實現 return mFragments.size(); } @Override public CharSequence getPageTitle(int position) {//選擇性實現 return mFragments.get(position).getClass().getSimpleName(); }}FragmentStatePagerAdapter的實現和FragmentPagerAdapter的實現一樣就不在寫了
3個適配器的基本實現講完了是不是很簡單，那他們的區別是什么呢？PagerAdapter是基類適配器是一個通用的ViewPager適配器，相比PagerAdapter，FragmentPagerAdapter和FragmentStatePagerAdapter更專注于每一頁是Fragment的情況，而這兩個子類適配器使用情況也是有區別的。FragmentPagerAdapter適用于頁面比較少的情況，FragmentStatePagerAdapter適用于頁面比較多的情況。為什么？簡單分析下兩個適配器的源碼就可以知道了。
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被釋放后這里得到的null值 if (f != null) { return f; } } if (mCurTransaction == null) { mCurTransaction = mFragmentManager.beginTransaction(); } Fragment fragment = getItem(position);//fragment被釋放后或者是初次進入頁面拿到新的Fragment實例 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實例 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);//真正釋放了fragment實例 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);//因為fragment實例沒有被真正釋放，所以可以直接attach效率高 } else { fragment = getItem(position);//初始化頁面的時候拿到fragment的實例 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);//并沒有真正釋放fragment對象只是detach }從源碼中我們可以看出FragmentStatePagerAdapter中fragment實例在destroyItem的時候被真正釋放，所以FragmentStatePagerAdapter省內存。FragmentPagerAdapter中的fragment實例在destroyItem的時候并沒有真正釋放fragment對象只是detach，所以FragmentPagerAdapter消耗更多的內存，帶來的好處就是效率更高一些。所以得出這樣的結論：FragmentPagerAdapter適用于頁面比較少的情況，FragmentStatePagerAdapter適用于頁面比較多的情況，因此不同的場合選擇合適的適配器才是正確的做法
3.ViewPager的翻頁動畫為ViewPager設置適配器后，就可以正常使用了，接下來我們為ViewPager增加翻頁動畫，畢竟人的審美會疲勞，加上一些動畫交互會提高不少逼格~~，ViewPager提供了PageTransformer接口用于實現翻頁動畫。官方提供了PageTransformer的實現例子。
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); } }}實現翻頁動畫的關鍵就是重寫transformPage方法，方法里有兩個參數view和position,理解這兩個參數非常重要。假設有三個頁面view1，view2，view3從左至右在viewPager中顯示
往左滑動時：view1，view2，view3的position都是不斷變小的。 view1的position: 0 → -1 → 負無窮大 view2的position: 1 → 0 → -1 view3的position: 1 → 0往右滑動時：view1，view2，view3的position都是不斷變大的。 view1的position: -1 → 0 view2的position: -1 → 0 → 1 view3的position: 0 → 1→ 正無窮大當position是正負無窮大時view就離開屏幕視野了。因此最核心的控制邏輯是在[-1,0]和(0,1]這兩個區間，通過設置透明度，平移，旋轉，縮放等動畫組合可以實現各式各樣的頁面變化效果。4.簡化ViewPager的使用
不會偷懶的程序猿不是好程序員
這里只是做了最簡單的封裝，可以根據需要調整
PagerAdapter簡化
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，這樣不用每次都寫個適配器List<View> views = new ArrayList<>();...mViewPager.setAdapter(new QuickPageAdapter<View>(views));
FragmentPagerAdapter簡化
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封裝類似FragmentPagerAdapter就不寫了，基本使用講完了。
5.補充一個知識點mViewPager.setOffscreenPageLimit()//這個方法是用來控制fragment不重新走生命周期的個數的，打個比方一共4個fragment頁面，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)，那么其中有一個fragment會在切換的時候重新走一遍生命周期，FragmentStatePagerAdapter和FragmentPagerAdapter都是這樣，但是FragmentPagerAdapter設置setOffscreenPageLimit不影響fragment緩存的個數,而FragmentStatePagerAdapter緩存的fragment實例個數就是setOffscreenPageLimit設置的值+1。另外setOffscreenPageLimit的缺省值是1，設置0是無效的會被強制賦值成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;//強制賦值為1 } if (limit != mOffscreenPageLimit) { mOffscreenPageLimit = limit; populate(); } }提高篇
1.ViewPager結合第三方庫實現小圓點指示器效果https://github.com/ongakuer/CircleIndicator
screenshot.gif
使用看官方文檔很簡單。
看一下實現思路
public void setViewPager(ViewPager viewPager) { mViewpager = viewPager; if (mViewpager != null && mViewpager.getAdapter() != null) { mLastPosition = -1; createIndicators(); mViewpager.removeOnPageChangeListener(mInternalPageChangeListener); mViewpager.addOnPageChangeListener(mInternalPageChangeListener);//綁定上內部實現的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) {//這里是動畫的核心 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) {//頁面離開屏幕時指示器動畫 currentIndicator.setBackgroundResource(mIndicatorUnselectedBackgroundResId); mAnimatorIn.setTarget(currentIndicator); mAnimatorIn.start(); } View selectedIndicator = getChildAt(position); if (selectedIndicator != null) {//頁面進入屏幕時指示器動畫 selectedIndicator.setBackgroundResource(mIndicatorBackgroundResId); mAnimatorOut.setTarget(selectedIndicator); mAnimatorOut.start(); } mLastPosition = position; } @Override public void onPageScrollStateChanged(int state) { } };2.ViewPager結合design庫實現tab切換在design庫中有個TabLayout可以為viewPager加上Tab標題頭
<?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);//一行代碼完成綁定更多高級的用法包括tab中添加icon等請轉至這里 傳送門
3.基于ViewPager實現廣告輪播控件https://github.com/daimajia/AndroidImageSlider
imageSlider.gif
源碼分析，省略了部分代碼：
public class SliderLayout extends RelativeLayout{ private InfiniteViewPager mViewPager;//這個ViewPager只是修改了setPageTransformer方法去掉了if (Build.VERSION.SDK_INT >= 11) 的限制，結合NineOldDroid庫讓動畫兼容低版本 /** * InfiniteViewPager adapter. */ private SliderAdapter mSliderAdapter;//這個是PagerAdapter /** * {@link com.daimajia.slider.library.Tricks.ViewPagerEx} indicator. */ private PagerIndicator mIndicator;//頁面指示器 /** * A timer and a TimerTask using to cycle the {@link com.daimajia.slider.library.Tricks.ViewPagerEx}. */ private Timer mCycleTimer;//用于輪播的定時器 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的封裝用于控制頁面翻頁效果 public SliderLayout(Context context, AttributeSet attrs, int defStyle) {//核心代碼，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://手指按下時候暫停輪播 pauseAutoCycle(); break; } return false; } /** * preset transformers and their names */ public enum Transformer{//PageTransformer枚舉 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); } };}通過分析我們可以對SliderLayout實現思路小結一下：1.內部持有一個修改過的ViewPager控件，可以兼容低版本的頁面轉換動畫2.內部有一個實現了PagerAdapter的SliderAdapter適配器3.內部持有一個PagerIndicator 頁面指示器可供選擇4.維護一個定時任務用于控制輪播5.對手勢事件進行處理暫停輪播，繼續輪播6.提供了很多缺省的PageTransformer方便調用
最后在構造函數中初始化ViewPager。
http://www.jianshu.com/p/e5abbda4a71c
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