android系统提供的ViewPager标准方式是左右可以自由滑动，但是滑动到最左边的极限位置是第一个page，滑动到最右边的位置是最后一个page，当滑动到最左或者最右时候，就不能再滑动/滚动了，这是Android系统默认的ViewPager实现方式。

但是有些情况下开发者可能希望ViewPager能够智能的无限循环滚动回绕，比如现在总共有编号1, 2, 3, 4, 5的5个Page。

（1）当用户手指从右往左滚动到最右边/最后面的页面5时候，如果此时用户继续拖住ViewPager往左边滑动，那么ViewPager将回绕、循环到第一个Page -> 1，接着就是2,3,4,5；

（2）反过来，如果当用户手指从左往右，滑到最左边的第一个Page：1时候，如果此时继续拖住ViewPager继续从左往右滑动，那么将回绕到5，接着就是4,3,2,1.

我们把这种ViewPager称之谓“无限循环滚动回绕”的ViewPager。

这种类型的ViewPager网上有较多实现方式，现在给出一个流程较广的代码实现。

写一个测试的MainActivity.Java:

package zhangphil.demo;import java.util.Random;import android.app.Activity;import android.graphics.Color;import android.os.Bundle;import android.support.v4.view.PagerAdapter;import android.view.Gravity;import android.view.View;import android.view.ViewGroup;import android.widget.TextView;public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); LoopViewPager viewpager = (LoopViewPager) findViewById(R.id.viewpager); viewpager.setAdapter(new SamplePagerAdapter()); } public class SamplePagerAdapter extends PagerAdapter { private final Random random = new Random(); private int mSize; public SamplePagerAdapter() { mSize = 5; } public SamplePagerAdapter(int count) { mSize = count; } @Override public int getCount() { return mSize; } @Override public boolean isViewFromObject(View view, Object object) { return view == object; } @Override public void destroyItem(ViewGroup view, int position, Object object) { view.removeView((View) object); } @Override public Object instantiateItem(ViewGroup view, int position) { TextView textView = new TextView(view.getContext()); textView.setText(position + 1 + ""); textView.setBackgroundColor(0xff000000 | random.nextInt(0x00ffffff)); textView.setGravity(Gravity.CENTER); textView.setTextColor(Color.WHITE); textView.setTextSize(50); view.addView(textView, ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT); return textView; } // 增加item public void addItem() { mSize++; notifyDataSetChanged(); } // 删除item public void removeItem() { mSize--; mSize = mSize < 0 ? 0 : mSize; notifyDataSetChanged(); } }}

MainActivity.java需要的布局文件activity_main.xml：

<RelativeLayout 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" tools:context="zhangphil.demo.MainActivity" > <zhangphil.demo.LoopViewPager android:id="@+id/viewpager" android:layout_width="match_parent" android:layout_height="match_parent"/></RelativeLayout>

核心关键代码LoopViewPager.java类和LoopViewPager.java依赖的LoopPagerAdapterWrapper.java！

LoopViewPager.java：

package zhangphil.demo;import android.content.Context;import android.support.v4.view.PagerAdapter;import android.support.v4.view.ViewPager;import android.util.AttributeSet;import java.util.ArrayList;import java.util.List;public class LoopViewPager extends ViewPager { private static final boolean DEFAULT_BOUNDARY_CASHING = false; private static final boolean DEFAULT_BOUNDARY_LOOPING = true; private LoopPagerAdapterWrapper mAdapter; private boolean mBoundaryCaching = DEFAULT_BOUNDARY_CASHING; private boolean mBoundaryLooping = DEFAULT_BOUNDARY_LOOPING; private List<OnPageChangeListener> mOnPageChangeListeners; /** * helper function which may be used when implementing FragmentPagerAdapter * * @return (position-1)%count */ public static int toRealPosition(int position, int count) { position = position - 1; if (position < 0) { position += count; } else { position = position % count; } return position; } /** * If set to true, the boundary views (i.e. first and last) will never be * destroyed This may help to prevent "blinking" of some views */ public void setBoundaryCaching(boolean flag) { mBoundaryCaching = flag; if (mAdapter != null) { mAdapter.setBoundaryCaching(flag); } } public void setBoundaryLooping(boolean flag) { mBoundaryLooping = flag; if (mAdapter != null) { mAdapter.setBoundaryLooping(flag); } } @Override public void setAdapter(PagerAdapter adapter) { mAdapter = new LoopPagerAdapterWrapper(adapter); mAdapter.setBoundaryCaching(mBoundaryCaching); mAdapter.setBoundaryLooping(mBoundaryLooping); super.setAdapter(mAdapter); setCurrentItem(0, false); } @Override public PagerAdapter getAdapter() { return mAdapter != null ? mAdapter.getRealAdapter() : mAdapter; } @Override public int getCurrentItem() { return mAdapter != null ? mAdapter.toRealPosition(super.getCurrentItem()) : 0; } public void setCurrentItem(int item, boolean smoothScroll) { int realItem = mAdapter.toInnerPosition(item); super.setCurrentItem(realItem, smoothScroll); } @Override public void setCurrentItem(int item) { if (getCurrentItem() != item) { setCurrentItem(item, true); } } @Override public void setOnPageChangeListener(OnPageChangeListener listener) { addOnPageChangeListener(listener); } @Override public void addOnPageChangeListener(OnPageChangeListener listener) { if (mOnPageChangeListeners == null) { mOnPageChangeListeners = new ArrayList<>(); } mOnPageChangeListeners.add(listener); } @Override public void removeOnPageChangeListener(OnPageChangeListener listener) { if (mOnPageChangeListeners != null) { mOnPageChangeListeners.remove(listener); } } @Override public void clearOnPageChangeListeners() { if (mOnPageChangeListeners != null) { mOnPageChangeListeners.clear(); } } public LoopViewPager(Context context) { super(context); init(context); } public LoopViewPager(Context context, AttributeSet attrs) { super(context, attrs); init(context); } private void init(Context context) { if (onPageChangeListener != null) { super.removeOnPageChangeListener(onPageChangeListener); } super.addOnPageChangeListener(onPageChangeListener); } private OnPageChangeListener onPageChangeListener = new OnPageChangeListener() { private float mPreviousOffset = -1; private float mPreviousPosition = -1; @Override public void onPageSelected(int position) { int realPosition = mAdapter.toRealPosition(position); if (mPreviousPosition != realPosition) { mPreviousPosition = realPosition; if (mOnPageChangeListeners != null) { for (int i = 0; i < mOnPageChangeListeners.size(); i++) { OnPageChangeListener listener = mOnPageChangeListeners.get(i); if (listener != null) { listener.onPageSelected(realPosition); } } } } } @Override public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { int realPosition = position; if (mAdapter != null) { realPosition = mAdapter.toRealPosition(position); if (positionOffset == 0 && mPreviousOffset == 0 && (position == 0 || position == mAdapter.getCount() - 1)) { setCurrentItem(realPosition, false); } } mPreviousOffset = positionOffset; if (mOnPageChangeListeners != null) { for (int i = 0; i < mOnPageChangeListeners.size(); i++) { OnPageChangeListener listener = mOnPageChangeListeners.get(i); if (listener != null) { if (realPosition != mAdapter.getRealCount() - 1) { listener.onPageScrolled(realPosition, positionOffset, positionOffsetPixels); } else { if (positionOffset > .5) { listener.onPageScrolled(0, 0, 0); } else { listener.onPageScrolled(realPosition, 0, 0); } } } } } } @Override public void onPageScrollStateChanged(int state) { if (mAdapter != null) { int position = LoopViewPager.super.getCurrentItem(); int realPosition = mAdapter.toRealPosition(position); if (state == ViewPager.SCROLL_STATE_IDLE && (position == 0 || position == mAdapter.getCount() - 1)) { setCurrentItem(realPosition, false); } } if (mOnPageChangeListeners != null) { for (int i = 0; i < mOnPageChangeListeners.size(); i++) { OnPageChangeListener listener = mOnPageChangeListeners.get(i); if (listener != null) { listener.onPageScrollStateChanged(state); } } } } };}

LoopPagerAdapterWrapper.java：

package zhangphil.demo;import android.os.Parcelable;import android.support.v4.app.FragmentPagerAdapter;import android.support.v4.app.FragmentStatePagerAdapter;import android.support.v4.view.PagerAdapter;import android.util.SparseArray;import android.view.View;import android.view.ViewGroup;public class LoopPagerAdapterWrapper extends PagerAdapter { private PagerAdapter mAdapter; private SparseArray<ToDestroy> mToDestroy = new SparseArray<>(); private static final boolean DEFAULT_BOUNDARY_CASHING = true; private static final boolean DEFAULT_BOUNDARY_LOOPING = true; private boolean mBoundaryCaching = DEFAULT_BOUNDARY_CASHING; private boolean mBoundaryLooping = DEFAULT_BOUNDARY_LOOPING; void setBoundaryCaching(boolean flag) { mBoundaryCaching = flag; } void setBoundaryLooping(boolean flag) { mBoundaryLooping = flag; } LoopPagerAdapterWrapper(PagerAdapter adapter) { this.mAdapter = adapter; } @Override public void notifyDataSetChanged() { mToDestroy = new SparseArray<>(); super.notifyDataSetChanged(); } int toRealPosition(int position) { int realPosition = position; int realCount = getRealCount(); if (realCount == 0) return 0; if (mBoundaryLooping) { realPosition = (position - 1) % realCount; if (realPosition < 0) realPosition += realCount; } return realPosition; } public int toInnerPosition(int realPosition) { int position = (realPosition + 1); return mBoundaryLooping ? position : realPosition; } private int getRealFirstPosition() { return mBoundaryLooping ? 1 : 0; } private int getRealLastPosition() { return getRealFirstPosition() + getRealCount() - 1; } @Override public int getCount() { int count = getRealCount(); return mBoundaryLooping ? count + 2 : count; } public int getRealCount() { return mAdapter.getCount(); } public PagerAdapter getRealAdapter() { return mAdapter; } @Override public Object instantiateItem(ViewGroup container, int position) { int realPosition = (mAdapter instanceof FragmentPagerAdapter || mAdapter instanceof FragmentStatePagerAdapter) ? position : toRealPosition(position); if (mBoundaryCaching) { ToDestroy toDestroy = mToDestroy.get(position); if (toDestroy != null) { mToDestroy.remove(position); return toDestroy.object; } } return mAdapter.instantiateItem(container, realPosition); } @Override public void destroyItem(ViewGroup container, int position, Object object) { int realFirst = getRealFirstPosition(); int realLast = getRealLastPosition(); int realPosition = (mAdapter instanceof FragmentPagerAdapter || mAdapter instanceof FragmentStatePagerAdapter) ? position : toRealPosition(position); if (mBoundaryCaching && (position == realFirst || position == realLast)) { mToDestroy.put(position, new ToDestroy(container, realPosition, object)); } else { mAdapter.destroyItem(container, realPosition, object); } } /* * Delegate rest of methods directly to the inner adapter. */ @Override public void finishUpdate(ViewGroup container) { mAdapter.finishUpdate(container); } @Override public boolean isViewFromObject(View view, Object object) { return mAdapter.isViewFromObject(view, object); } @Override public void restoreState(Parcelable bundle, ClassLoader classLoader) { mAdapter.restoreState(bundle, classLoader); } @Override public Parcelable saveState() { return mAdapter.saveState(); } @Override public void startUpdate(ViewGroup container) { mAdapter.startUpdate(container); } @Override public void setPrimaryItem(ViewGroup container, int position, Object object) { mAdapter.setPrimaryItem(container, position, object); } /* * End delegation */ /** * Container class for caching the boundary views */ static class ToDestroy { ViewGroup container; int position; Object object; public ToDestroy(ViewGroup container, int position, Object object) { this.container = container; this.position = position; this.object = object; } }}

如果读者有兴趣使用，直接将上面两个核心关键代码放入到自己的项目代码包中，当作自己写的类直接使用即可，写布局时候不要再写ViewPager，而是直接像我上面写的那个布局文件一样用LoopViewPager。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。