实现一个自定义LayoutManager，默认只有一个抽象方法，实现的重点在于onLayoutChildren对页面的布局和滑动操作，当然还有缓存。
属性定义
mPosition是当前item的位置信息，mPositionOffset是偏移信息，mMinVy是最低的y方向的速度，这个需要根据不同屏幕尺寸来定。
private static final int MIN_VY = 300;
private int mPosition = 0;
private int mPositionOffset = 0;
private int mMinVy = 0;
private Context mContext;
布局流程
为了实现翻页效果，每次滑动都是有前景页和背景页。当翻页时，如果flip没有超过一半，当前页是primary（前景页），下一页是secondary（背景页）；当超过一半当前页已经是“下一页”（背景页）了，而刚才的当前页变成了上一页（前景页）。值得注意的是，需要把需要把item布局文件的背景设置成白色，不然会有重叠效果。翻页效果在FlipCard中实现。
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
//如果没有item，直接返回
if (getItemCount() <= 0) {
return;
}
// 跳过preLayout，preLayout主要用于支持动画
if (state.isPreLayout()) {
return;
}
fill(recycler, state);
}
private void fill(RecyclerView.Recycler recycler, RecyclerView.State state) {
checkPosition(state);
View primary;
View pre = null;
View next = null;
detachAndScrapAttachedViews(recycler);
// 当前页
primary = recycler.getViewForPosition(mPosition);
if (mPosition + 1 < state.getItemCount()) {
// 下一页
next = recycler.getViewForPosition(mPosition + 1);
}
if (mPosition - 1 >= 0) {
// 上一页
pre = recycler.getViewForPosition(mPosition - 1);
}
View secondary = null;
// 根据偏移计算是需要上一页还是下一页
if (mPositionOffset > 0) {
secondary = next;
} else if (mPositionOffset < 0) {
secondary = pre;
}
if (mPositionOffset != 0 && secondary != null) {
// 存在背景页就添加
addView(secondary);
measureChildWithMargins(secondary, 0, 0);
layoutDecorated(secondary, 0, 0, getWidth(), getHeight());
}
// 回收另一页的信息
if (pre != null && secondary !=pre) {
recycler.recycleView(pre);
}
if (next != null && secondary !=next) {
recycler.recycleView(next);
}
// 添加当前页作为前景页
addView(primary);
measureChildWithMargins(primary, 0, 0);
layoutDecorated(primary, 0, 0, getWidth(), getHeight());
if (primary instanceof FlipCard && (secondary == null || secondary instanceof FlipCard)) {
final float percent = (float) mPositionOffset / getItemHeightPositon();
// 计算滑动比例，得到flip效果
Log.d(TAG, "fill: "+percent);
if (secondary != null) {
((FlipCard) secondary).setState(false, percent);
((FlipCard) primary).setState(true, percent);
} else {
// 当在第一页或最后一页时不存在背景页（留白/刷新等）
((FlipCard) primary).setState(true, percent);
}
} else {
throw new IllegalStateException("view should be FlipCard");
}
}
private void checkPosition(RecyclerView.State state) {
// 页面最多能滑动到 -2/5*itemHeight ~ itemHeight*(state.getItemCount()-1) + itemHeight*2/5
final int itemHeight = getItemHeightPositon();
final int current = mPosition * itemHeight + mPositionOffset;
final int max = itemHeight * (state.getItemCount() - 1) + itemHeight * 2 / 5;
int pos = Math.max(-itemHeight * 2 / 5, Math.min(current, max));
mPosition = Math.round(pos / (float)itemHeight);
mPosition = mPosition >= 0 ? mPosition : 0;
mPositionOffset = pos - mPosition * itemHeight;
}
为了使翻页效果流畅，设置每一页的高度都是实际高度的2/3，这样避免完全滑满一个屏幕才完成翻页效果（不考虑惯性滑动）。
// 这样把一个item的高度设为了原来的2/3，直观的结果就是只要滑动2/3就相当于过了一个item
private int getItemHeightInPositon() {
return getHeight() * 2 / 3;
}
滑动
完成上述步骤自然还不能滑动
@Override
public boolean canScrollVertically() {
// 纵向可滑
return true;
}
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
final int before = mPosition * getItemHeightPositon() + mPositionOffset;
mPositionOffset += dy;
checkPosition(state);
final int after = mPosition * getItemHeightPositon() + mPositionOffset;
final int ans = after - before;
fill(recycler, state);
return ans;
}
@Override
public void scrollToPosition(int position) {
mPosition = position;
mPositionOffset = 0;
requestLayout();
Log.d(TAG, "scrollToPosition " + position + " position " + mPosition + " positionOffset " + mPositionOffset);
}
// 平稳滑动
@Override
public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
Log.d(TAG, "smoothScrollTo " + position + " position " + mPosition + " positionOffset " + mPositionOffset);
FlipScroller scroller = new FlipScroller(recyclerView.getContext());
scroller.setTargetPosition(position);
startSmoothScroll(scroller);
}
private class FlipScroller extends LinearSmoothScroller {
private static final String TAG = "FlipScroller";
public FlipScroller(Context context) {
super(context);
}
@Override
protected void onStop() {
super.onStop();
Log.d(TAG, "onStop: ");
}
@Override
public int calculateDyToMakeVisible(View view, int snapPreference) {
final int position = getPosition(view);
final int now = mPositionOffset + mPosition * getItemHeightPositon();
final int to = position * getItemHeightPositon();
Log.d(TAG, "calculateDyToMakeVisible: position " + position + " ans " + (to - now));
return (now - to);
}
@Override
public int calculateDxToMakeVisible(View view, int snapPreference) {
return 0;
}
@Override
protected int calculateTimeForScrolling(int dx) {
Log.d(TAG, "calculateTimeForScrolling: ");
int time = super.calculateTimeForScrolling(dx*5);
return time;
}
}
/*
以下四个方法是为了滑动时的对齐操作，不至于滑了半页然后就停在那
详细可看：https://www.jianshu.com/p/e54db232df62
*/
@Override
public PointF computeScrollVectorForPosition(int targetPosition) {
int dir = 0;
int now = mPosition * getItemHeightPositon() + mPositionOffset;
int to = targetPosition * getItemHeightPositon();
if (now > to) {
dir = -1;
} else if (now < to) {
dir = 1;
}
Log.d(TAG, "computeScrollVector " + dir + " now " + mPosition + " target " + targetPosition);
return new PointF(0, dir);
}
public int calculateDistance(View view) {
int pos = getPosition(view);
final int now = getItemHeightPositon() * mPosition + mPositionOffset;
final int to = getItemHeightPositon() * pos;
return to - now;
}
public int findTargetPosition(int vY) {
int ans = mPosition;
Log.d(TAG, "findTargetPosition: "+vY+"~"+mPositionOffset);
int absV = vY > 0 ? vY : -vY;
if (absV > mMinVy) {
if (vY * mPositionOffset > 0) {
// 速度与位置偏移同向
int d = vY > 0 ? 1 : -1;
ans += d;
} else {
ans = mPosition;
}
} else {
ans = mPosition;
}
int count = getItemCount();
if (count == 0) {
return 0;
}
ans = Math.min(count - 1, Math.max(0, ans));
return ans;
}
public View findSnapView() {
Log.d(TAG, "findSnapView: "+getChildCount()+"~"+getItemCount());
for (int i = 0;i < getChildCount(); i ++ ){
View child = getChildAt(i);
if (getPosition(child) == mPosition) {
return child;
}
}
return null;
}
// 滑动时的对齐操作
// 惯性滑动先根据findTargetSnapPosition()计算到TargetSnapView，再根据calculateDistanceToFinalSnap()计算到TargetSnapView与对齐位置的剩余距离。普通滑动就是等到滑动停止，findSnapView()找到需要对齐的View即SanpView，再calculateDistanceToFinalSnap()计算得到额外的滑动距离。
public class MySnap extends SnapHelper {
private static final String TAG = "MySnap";
@Nullable
@Override
public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager, @NonNull View targetView) {
if (layoutManager instanceof CustomLayoutManager) {
return new int[]{0, ((CustomLayoutManager) layoutManager).calculateDistance(targetView)};
} else {
throw new RuntimeException();
}
}
@Nullable
@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
CustomLayoutManager flipLayoutManager = (CustomLayoutManager) layoutManager;
return flipLayoutManager.findSnapView();
}
@Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX, int velocityY) {
if (layoutManager instanceof CustomLayoutManager) {
return ((CustomLayoutManager) layoutManager).findTargetPosition(velocityY);
} else {
throw new RuntimeException();
}
}
}
翻页View
public class FlipCard extends FrameLayout {
private static final String TAG = "FlipCard";
private Paint mScrimPaint;
private Camera mCamera;
private Matrix mMatrix;
private boolean mIsForground;
private float mPercent;
public FlipCard(Context context) {
this(context, null);
}
public FlipCard(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public FlipCard(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
mScrimPaint = new Paint();
mCamera = new Camera();
mMatrix = new Matrix();
}
public void setState(boolean isForground, float percent) {
mIsForground = isForground;
mPercent = percent;
}
@Override
public void draw(Canvas canvas) {
if (mPercent == 0) {
super.draw(canvas);
return;
}
final int height = canvas.getHeight();
final int width = canvas.getWidth();
final float percent = mPercent > 0 ? mPercent : -mPercent;
if (mIsForground) {
// clip card effect for forground view
// draw part1
int save1 = canvas.save();
if (mPercent > 0) {
canvas.clipRect(0, 0, width, height / 2);
} else {
canvas.clipRect(0, height / 2, width, height);
}
super.draw(canvas);
canvas.restoreToCount(save1);
// draw part2
if (mPercent < 0) {
canvas.clipRect(0, 0, width, height / 2);
} else {
canvas.clipRect(0, height / 2, width, height);
}
mCamera.save();
mCamera.setLocation(0f, 0f, -80);
mCamera.rotateX(mPercent * 180);
mCamera.getMatrix(mMatrix);
mCamera.restore();
mMatrix.preTranslate(-width / 2, -height / 2);
mMatrix.postTranslate(width / 2, height / 2);
canvas.concat(mMatrix);
super.draw(canvas);
mScrimPaint.setColor(0x08000000);
canvas.drawRect(0, 0, width, height, mScrimPaint);
} else {
// 作为背景，不需要有flip效果，只需要切出一半view展示出来即可
// draw shadow for underground view
final int scrimColor = (int) (0xff * (1 - percent * 2)) << 24;
mScrimPaint.setColor(scrimColor);
if (mPercent < 0) {
canvas.clipRect(0, 0, width, height / 2);
} else {
canvas.clipRect(0, height / 2, width, height);
}
super.draw(canvas);
canvas.drawRect(0, 0, width, height, mScrimPaint);
}
}
}
使用
customLayoutManager = new CustomLayoutManager(this);
recyclerView.setLayoutManager(customLayoutManager);
adapter = new CustomAdapter(myData,MainActivity.this);
recyclerView.setAdapter(adapter);
MySnap snap = new MySnap();
snap.attachToRecyclerView(recyclerView);
缓存
翻页效果带来了本身其滑动范围有限，依次只能上下翻页，因为对于缓存，首先稍微设置mCacheViews大一些，默认为2，由于每次翻页会绑定下一页，因此我设置为3来保证由本页（例：1）翻到下一页（例：2）时，不会因为加载再下一页（例：3）而导致原来的上一页（例：0）被回收。
另一项策略时预加载Prefetch，对于自定义LayoutManager，需要重写LayoutManager.collectAdjacentPrefetchPositions()方法，可以参考LinearLayoutManager实现。
包含完整源码的项目：https://github.com/starsight/Gank
参考链接
让你明明白白的使用RecyclerView——SnapHelper详解
https://medium.com/google-developers/recyclerview-prefetch-c2f269075710
Android ListView 与 RecyclerView 对比浅析–缓存机制