Android实现ViewPager无限循环滚动回绕
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。
