Android ViewPager实现智能无限循环滚动回绕效果

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：

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 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 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脚本之家。