一、需求
之前一篇博客使用ViewPager实现轮播图《Android ViewPager实现循环轮播图》，但是ViewPager有个天生的缺陷是View无法重用，此外ViewPager的滑动过程会频繁requestLayout，尽管可以通过addViewInLayout和removeViewInLayout配合PagerAdapter 的startUpdate和finishUpdate可以减少重绘，但在ListView和RecyclerView中仍然达不到最好的效果。因此，使用一种新的方式十分必要。
二、代码实现
RecyclerPagerView
public
class
RecyclerPagerView
extends
RecyclerView
implements
Handler
.
Callback
{
private
static
final
long
TASK_TIMEOUT =
3000
;
public
OnPageChangeListener onPageChangeListener;
private
final
Handler mRecyclerHandler;
private
final
int
MSG_PLAY_NEXT =
112233
;
private
volatile
boolean
isPlaying =
false
;
private
boolean
lastIsPlayState =
false
;
private
int
realPosition = -
1
;
public
RecyclerPagerView
(Context context)
{
this
(context,
null
);
}
public
RecyclerPagerView
(Context context, @Nullable AttributeSet attrs)
{
this
(context, attrs,
0
);
}
public
RecyclerPagerView
(Context context, @Nullable AttributeSet attrs,
int
defStyle)
{
super
(context, attrs, defStyle);
mRecyclerHandler =
new
Handler(Looper.getMainLooper(),
this
);
}
public
void
setOnPageChangeListener
(OnPageChangeListener onPageChangeListener)
{
this
.onPageChangeListener = onPageChangeListener;
if
(
this
.onPageChangeListener!=
null
){
addOnScrollListener(
this
.onPageChangeListener);
int
currentItem = getCurrentItem();
this
.onPageChangeListener.onPageSelection(currentItem);
}
}
public
int
getCurrentItem
()
{
LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();
return
linearLayoutManager.findFirstVisibleItemPosition();
}
public
void
setCurrentItem
(
int
position,
boolean
isAnimate)
{
Adapter adapter = getAdapter();
if
(adapter==
null
|| adapter.getItemCount()<=position){
return
;
}
if
(!isAnimate)
{
scrollToPosition(position);
}
else
{
smoothScrollToPosition(position);
}
}
public
void
setCurrentItem
(
int
position )
{
setCurrentItem(position,
true
);
}
@Override
public
boolean
fling
(
int
velocityX,
int
velocityY)
{
LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();
int
screenWidth = Resources.getSystem().getDisplayMetrics().widthPixels;
// views on the screen
int
lastVisibleItemPosition = linearLayoutManager.findLastVisibleItemPosition();
View lastView = linearLayoutManager.findViewByPosition(lastVisibleItemPosition);
int
firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
View firstView = linearLayoutManager.findViewByPosition(firstVisibleItemPosition);
// distance we need to scroll
int
leftMargin = (screenWidth - lastView.getWidth()) /
2
;
int
rightMargin = (screenWidth - firstView.getWidth()) /
2
+ firstView.getWidth();
int
leftEdge = lastView.getLeft();
int
rightEdge = firstView.getRight();
int
scrollDistanceLeft = leftEdge - leftMargin;
int
scrollDistanceRight = rightMargin - rightEdge;
int
targetPosition;
if
(Math.abs(velocityX) <
1500
) {
// The fling is slow -> stay at the current page if we are less than half through,
// or go to the next page if more than half through
if
(leftEdge > screenWidth /
2
) {
// go to next page
smoothScrollBy(-scrollDistanceRight,
0
);
targetPosition = firstVisibleItemPosition;
}
else
if
(rightEdge < screenWidth /
2
) {
// go to next page
smoothScrollBy(scrollDistanceLeft,
0
);
targetPosition = firstVisibleItemPosition+
1
;
}
else
{
// stay at current page
if
(velocityX >
0
) {
smoothScrollBy(-scrollDistanceRight,
0
);
}
else
{
smoothScrollBy(scrollDistanceLeft,
0
);
}
targetPosition = firstVisibleItemPosition;
}
}
else
{
// The fling is fast -> go to next page
if
(velocityX >
0
) {
smoothScrollBy(scrollDistanceLeft,
0
);
targetPosition = firstVisibleItemPosition+
1
;
}
else
{
smoothScrollBy(-scrollDistanceRight,
0
);
targetPosition = firstVisibleItemPosition;
}
}
Log.e(
"RecyclerPagerView"
,
"nextPage="
+targetPosition);
if
(
this
.onPageChangeListener!=
null
){
realPosition = targetPosition;
this
.onPageChangeListener.onPageSelection(targetPosition);
}
return
true
;
}
@Override
public
void
onScrollStateChanged
(
final
int
state)
{
super
.onScrollStateChanged(state);
if
(state == SCROLL_STATE_IDLE) {
LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();
int
screenWidth = Resources.getSystem().getDisplayMetrics().widthPixels;
int
lastVisibleItemPosition = linearLayoutManager.findLastVisibleItemPosition();
View lastView = linearLayoutManager.findViewByPosition(lastVisibleItemPosition);
int
firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
View firstView = linearLayoutManager.findViewByPosition(firstVisibleItemPosition);
// distance we need to scroll
int
leftMargin = (screenWidth - lastView.getWidth()) /
2
;
int
rightMargin = (screenWidth - firstView.getWidth()) /
2
+ firstView.getWidth();
int
leftEdge = lastView.getLeft();
int
rightEdge = firstView.getRight();
int
scrollDistanceLeft = leftEdge - leftMargin;
int
scrollDistanceRight = rightMargin - rightEdge;
int
targetPosition = -
1
;
if
(leftEdge > screenWidth /
2
) {
smoothScrollBy(-scrollDistanceRight,
0
);
targetPosition = firstVisibleItemPosition+
1
;
}
else
if
(rightEdge < screenWidth /
2
) {
smoothScrollBy(scrollDistanceLeft,
0
);
targetPosition = lastVisibleItemPosition;
}
else
{
targetPosition = firstVisibleItemPosition;
}
if
(
this
.onPageChangeListener!=
null
){
realPosition = targetPosition;
this
.onPageChangeListener.onPageSelection(targetPosition);
}
}
}
@Override
public
boolean
handleMessage
(Message msg)
{
int
what = msg.what;
switch
(what){
case
MSG_PLAY_NEXT:
showNextPage();
break
;
}
return
false
;
}
private
void
showNextPage
()
{
if
(!isPlaying){
return
;
}
if
(!canRecyclePlaying()){
isPlaying =
false
;
return
;
}
Adapter adapter = getAdapter();
int
currentItem = getCurrentItem();
if
(adapter!=
null
&& adapter.getItemCount()>
0
) {
if
(currentItem == NO_POSITION ) {
setCurrentItem(
0
);
}
else
{
setCurrentItem(currentItem+
1
);
}
}
mRecyclerHandler.sendEmptyMessageDelayed(MSG_PLAY_NEXT,TASK_TIMEOUT);
}
public
void
startPlay
()
{
if
(isPlaying){
stopPlay();
}
if
(!canRecyclePlaying()){
isPlaying =
false
;
return
;
}
isPlaying =
true
;
mRecyclerHandler.sendEmptyMessageDelayed(MSG_PLAY_NEXT,TASK_TIMEOUT);
}
@Override
public
void
setAdapter
(Adapter adapter)
{
super
.setAdapter(adapter);
if
(canRecyclePlaying()){
if
(realPosition==-
1
){
realPosition =
1000
;
}
setCurrentItem(realPosition,
false
);
}
}
private
boolean
canRecyclePlaying
()
{
Adapter adapter = getAdapter();
if
(adapter==
null
|| adapter.getItemCount()<
1
)
return
false
;
return
true
;
}
private
void
stopPlay
()
{
isPlaying =
false
;
mRecyclerHandler.removeMessages(MSG_PLAY_NEXT);
}
@Override
protected
void
onAttachedToWindow
()
{
super
.onAttachedToWindow();
if
(lastIsPlayState){
startPlay();
}
}
@Override
protected
void
onDetachedFromWindow
()
{
super
.onDetachedFromWindow();
lastIsPlayState = isPlaying;
stopPlay();
}
public
static
abstract
class
OnPageChangeListener
extends
RecyclerView
.
OnScrollListener
{
public
abstract
void
onPageSelection
(
int
position)
;
}
}
Adapter+Holder 适配器
该类的作用主要用于约束和快速接入
public class QuickViewHolder extends RecyclerView.ViewHolder{ private SparseArray mViews; private View mConvertView; private QuickViewHolder(View v){ super(v);
mConvertView = v;
mViews = new SparseArray<>();
} public static QuickViewHolder get(ViewGroup parent, int layoutId){
View convertView = LayoutInflater.from(parent.getContext()).inflate(layoutId, parent, false); return new QuickViewHolder(convertView);
} public T getView(int id){
View v = mViews.get(id); if(v == null){
v = mConvertView.findViewById(id);
mViews.put(id, v);
} return (T)v;
}
}
BannerAdapter实现，该类继承QuickAdapter
public class BannerAdapter extends QuickAdapter { public BannerAdapter(List datas) { super(datas);
} @Override
public int getLayoutId(int viewType) { return R.layout.item_banner_image;
} @Override
public void convert(QuickViewHolder holder, String url, int position) { final Resources resources = holder.itemView.getResources(); final int drawableId = resources.getIdentifier(url, "drawable", holder.itemView.getContext().getPackageName()); if(drawableId!=0) {
ImageView bannerImage = holder.getView(R.id.banner_image_item);
bannerImage.setImageResource(drawableId);
}
}
}
三、使用
final RecyclerPagerView rpv = findViewById(R.id.recycler_pager);
tipTextView = findViewById(R.id.tip_text);
LinearLayoutManager lm = new LinearLayoutManager(this,LinearLayoutManager.HORIZONTAL,false);
rpv.setLayoutManager(lm);
List imagelist = new ArrayList<>();
imagelist.add("banner_t1");
imagelist.add("banner_t2");
imagelist.add("banner_t3");
imagelist.add("banner_t4");
rpv.setAdapter(new BannerAdapter(imagelist));
rpv.setOnPageChangeListener(new PagerChangeListener(tipTextView,imagelist.size()));
rpv.startPlay();监听器
public static class PagerChangeListener extends RecyclerPagerView.OnPageChangeListener { private TextView tipTextView; private int size; public PagerChangeListener(TextView tipTextView,int size) { this.tipTextView = tipTextView; this.size = size;
} @Override
public void onPageSelection(int position) {
tipTextView.setText((position%size+1)+"/"+size);
}
}
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