ViewPager 实现 Galler 效果, 中间大图显示，两边小图展示（优化篇）

上一张效果图：

之前的项目有一个Galley的项目，但是代码结构特别乱，别问我为什么，我也是刚接手这个项目，为了方便以后阅读和维护我对一些模块进行了重构。ViewPager实现Galler效果，但是当时时间比较急，写的比较仓促，上一篇实现了简单的效果，但是对于初始的时候左边滑动是有问题的，这是因为我们在自己的Adapter的时候对于getCount，我们想通过Integer.MAX_VALUE来实现无限循环，简单的说这个是向右（无限大）吧，所以也就只是实现了向右的循环，对于像左的还是不行的。那么怎么解决呢？首先我们对Adapter还是需要按以前的写法，其实Adapter适配器就是帮我们做界面的，其他的逻辑最好不要加，我之前的想法是用过

viewPager.addOnPageChangeListener来实现，通过判断是否滑到最左边或者最右边来实现循环滑到。

int realPosition = mAdapter.toRealPosition(position);
if (mPreviousPosition != realPosition) {
mPreviousPosition = realPosition;
if (mOuterPageChangeListener != null) {
mOuterPageChangeListener.onPageSelected(realPosition);
}
}

这里我封装了一个类，只要将你的ViewPager改为我封装好的ViewPager就行[/code]

LoopViewPager

public class LoopViewPager extends ViewPager {

private static final boolean DEFAULT_BOUNDARY_CASHING = false;

private OnPageChangeListener mOuterPageChangeListener;
private LoopPagerAdapterWrapper mAdapter;
private boolean mBoundaryCaching = DEFAULT_BOUNDARY_CASHING;

public static int toRealPosition( int position, int count ){
position = position-1;
if( position < 0 ){
position += count;
}else{
position = position%count;
}
return position;
}

public void setBoundaryCaching(boolean flag) {
mBoundaryCaching = flag;
if (mAdapter != null) {
mAdapter.setBoundaryCaching(flag);
}
}

@Override
public void setAdapter(PagerAdapter adapter) {
mAdapter = new LoopPagerAdapterWrapper(adapter);
mAdapter.setBoundaryCaching(mBoundaryCaching);
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) {
mOuterPageChangeListener = listener;
};

public LoopViewPager(Context context) {
super(context);
init();
}

public LoopViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}

private void init() {
super.setOnPageChangeListener(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 (mOuterPageChangeListener != null) {
mOuterPageChangeListener.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 (mOuterPageChangeListener != null) {
if (realPosition != mAdapter.getRealCount() - 1) {
mOuterPageChangeListener.onPageScrolled(realPosition,
positionOffset, positionOffsetPixels);
} else {
if (positionOffset > .5) {
mOuterPageChangeListener.onPageScrolled(0, 0, 0);
} else {
mOuterPageChangeListener.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 (mOuterPageChangeListener != null) {
mOuterPageChangeListener.onPageScrollStateChanged(state);
}
}
};

}

LoopPagerAdapterWrapper
[/code]
public class LoopPagerAdapterWrapper extends PagerAdapter {

private PagerAdapter mAdapter;

private SparseArray<ToDestroy> mToDestroy = new SparseArray<ToDestroy>();

private boolean mBoundaryCaching;

void setBoundaryCaching(boolean flag) {
mBoundaryCaching = flag;
}

LoopPagerAdapterWrapper(PagerAdapter adapter) {
this.mAdapter = adapter;
}

@Override
public void notifyDataSetChanged() {
mToDestroy = new SparseArray<ToDestroy>();
super.notifyDataSetChanged();
}

int toRealPosition(int position) {
int realCount = getRealCount();
if (realCount == 0)
return 0;
int realPosition = (position-1) % realCount;
if (realPosition < 0)
realPosition += realCount;

return realPosition;
}

public int toInnerPosition(int realPosition) {
int position = (realPosition + 1);
return position;
}

private int getRealFirstPosition() {
return 1;
}

private int getRealLastPosition() {
return getRealFirstPosition() + getRealCount() - 1;
}

@Override
public int getCount() {
return mAdapter.getCount() + 2;
}

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);
}
}

@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);
}

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;
}
}

}
这里需要注意对于上一篇的WelfareAdapter ，我们就按常规写法就行。[/code]
public class WelfareAdapter extends PagerAdapter {

private Context mContext;
private List<PanicBean> dataList = new ArrayList<>();

public WelfareAdapter(Context mContext) {
this.mContext = mContext;
}

public void setDatas(List<PanicBean> list) {
if (list.size() <= 0) {
dataList.clear();
notifyDataSetChanged();
return;
}
dataList.clear();
dataList.addAll(list);
notifyDataSetChanged();
}

@Override
public int getCount() {
return /*Integer.MAX_VALUE*/dataList.size();
}

@Override
public int getItemPosition(Object object) {
return POSITION_NONE;
}

@Override
public void destroyItem(View container, int position, Object object) {
}

@Override
public Object instantiateItem(ViewGroup container, int position) {
//        position %= dataList.size();
//        if (position<0){
//            position = dataList.size()+position;
//        }

PanicBean data = dataList.get(position);

ViewHolder viewHolder = null;
View view = LayoutInflater.from(mContext).inflate(
R.layout.item_finefare_layout, null);
if (viewHolder == null) {
viewHolder = new ViewHolder(view);
}
bindView(viewHolder, data);

container.addView(view, LayoutParams.MATCH_PARENT,
LayoutParams.MATCH_PARENT);

return view;
}

private void bindView(ViewHolder viewholder, final PanicBean data) {
Glide.with(mContext).load(data.pic).into(viewholder.welfareImage);

viewholder.welfareImage.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
ToastUtils.showToast("你点击了"+data.href);
}
});
}

@Override
public boolean isViewFromObject(View view, Object object) {
return view == object;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
container.removeView((View) object);
}

class ViewHolder {
@BindView(R.id.welfare_image)
RoundedImageView welfareImage;

ViewHolder(View view) {
ButterKnife.bind(this, view);
view.setTag(this);
}

public void reset() {
welfareImage.setBackground(mContext.getResources().getDrawable(R.drawable.welfare_default_icon));
}
}

}
部分源码：https://github.com/xiangzhihong/jingdongApp[/code]