/*
* Copyright 2018 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.recyclerview.widget;
import android.graphics.PointF;
import android.util.DisplayMetrics;
import android.view.View;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.viewpager.widget.ViewPager;
/**
* Implementation of the {@link SnapHelper} supporting pager style snapping in either vertical or
* horizontal orientation.
*
* <p>
*
* PagerSnapHelper can help achieve a similar behavior to {@link ViewPager}.
* Set both {@link RecyclerView} and the items of the
* {@link RecyclerView.Adapter} to have
* {@link android.view.ViewGroup.LayoutParams#MATCH_PARENT} height and width and then attach
* PagerSnapHelper to the {@link RecyclerView} using {@link #attachToRecyclerView(RecyclerView)}.
*/
public class PagerSnapHelper extends SnapHelper {
private static final int MAX_SCROLL_ON_FLING_DURATION = 100; // ms
// Orientation helpers are lazily created per LayoutManager.
@Nullable
private OrientationHelper mVerticalHelper;
@Nullable
private OrientationHelper mHorizontalHelper;
@Nullable
@Override
public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,
@NonNull View targetView) {
int[] out = new int[2];
if (layoutManager.canScrollHorizontally()) {
out[0] = distanceToCenter(layoutManager, targetView,
getHorizontalHelper(layoutManager));
} else {
out[0] = 0;
}
if (layoutManager.canScrollVertically()) {
out[1] = distanceToCenter(layoutManager, targetView,
getVerticalHelper(layoutManager));
} else {
out[1] = 0;
}
return out;
}
@Nullable
@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
if (layoutManager.canScrollVertically()) {
return findCenterView(layoutManager, getVerticalHelper(layoutManager));
} else if (layoutManager.canScrollHorizontally()) {
return findCenterView(layoutManager, getHorizontalHelper(layoutManager));
}
return null;
}
@Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX,
int velocityY) {
final int itemCount = layoutManager.getItemCount();
if (itemCount == 0) {
return RecyclerView.NO_POSITION;
}
View mStartMostChildView = null;
if (layoutManager.canScrollVertically()) {
mStartMostChildView = findStartView(layoutManager, getVerticalHelper(layoutManager));
} else if (layoutManager.canScrollHorizontally()) {
mStartMostChildView = findStartView(layoutManager, getHorizontalHelper(layoutManager));
}
if (mStartMostChildView == null) {
return RecyclerView.NO_POSITION;
}
final int centerPosition = layoutManager.getPosition(mStartMostChildView);
if (centerPosition == RecyclerView.NO_POSITION) {
return RecyclerView.NO_POSITION;
}
final boolean forwardDirection;
if (layoutManager.canScrollHorizontally()) {
forwardDirection = velocityX > 0;
} else {
forwardDirection = velocityY > 0;
}
boolean reverseLayout = false;
if ((layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
RecyclerView.SmoothScroller.ScrollVectorProvider vectorProvider =
(RecyclerView.SmoothScroller.ScrollVectorProvider) layoutManager;
PointF vectorForEnd = vectorProvider.computeScrollVectorForPosition(itemCount - 1);
if (vectorForEnd != null) {
reverseLayout = vectorForEnd.x < 0 || vectorForEnd.y < 0;
}
}
return reverseLayout
? (forwardDirection ? centerPosition - 1 : centerPosition)
: (forwardDirection ? centerPosition + 1 : centerPosition);
}
@Override
protected LinearSmoothScroller createSnapScroller(RecyclerView.LayoutManager layoutManager) {
if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
return null;
}
return new LinearSmoothScroller(mRecyclerView.getContext()) {
@Override
protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
int[] snapDistances = calculateDistanceToFinalSnap(mRecyclerView.getLayoutManager(),
targetView);
final int dx = snapDistances[0];
final int dy = snapDistances[1];
final int time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)));
if (time > 0) {
action.update(dx, dy, time, mDecelerateInterpolator);
}
}
@Override
protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
}
@Override
protected int calculateTimeForScrolling(int dx) {
return Math.min(MAX_SCROLL_ON_FLING_DURATION, super.calculateTimeForScrolling(dx));
}
};
}
private int distanceToCenter(@NonNull RecyclerView.LayoutManager layoutManager,
@NonNull View targetView, OrientationHelper helper) {
final int childCenter = helper.getDecoratedStart(targetView)
+ (helper.getDecoratedMeasurement(targetView) / 2);
final int containerCenter;
if (layoutManager.getClipToPadding()) {
containerCenter = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;
} else {
containerCenter = helper.getEnd() / 2;
}
return childCenter - containerCenter;
}
/**
* Return the child view that is currently closest to the center of this parent.
*
* @param layoutManager The {@link RecyclerView.LayoutManager} associated with the attached
* {@link RecyclerView}.
* @param helper The relevant {@link OrientationHelper} for the attached {@link RecyclerView}.
*
* @return the child view that is currently closest to the center of this parent.
*/
@Nullable
private View findCenterView(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper) {
int childCount = layoutManager.getChildCount();
if (childCount == 0) {
return null;
}
View closestChild = null;
final int center;
if (layoutManager.getClipToPadding()) {
center = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;
} else {
center = helper.getEnd() / 2;
}
int absClosest = Integer.MAX_VALUE;
for (int i = 0; i < childCount; i++) {
final View child = layoutManager.getChildAt(i);
int childCenter = helper.getDecoratedStart(child)
+ (helper.getDecoratedMeasurement(child) / 2);
int absDistance = Math.abs(childCenter - center);
/** if child center is closer than previous closest, set it as closest **/
if (absDistance < absClosest) {
absClosest = absDistance;
closestChild = child;
}
}
return closestChild;
}
/**
* Return the child view that is currently closest to the start of this parent.
*
* @param layoutManager The {@link RecyclerView.LayoutManager} associated with the attached
* {@link RecyclerView}.
* @param helper The relevant {@link OrientationHelper} for the attached {@link RecyclerView}.
*
* @return the child view that is currently closest to the start of this parent.
*/
@Nullable
private View findStartView(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper) {
int childCount = layoutManager.getChildCount();
if (childCount == 0) {
return null;
}
View closestChild = null;
int startest = Integer.MAX_VALUE;
for (int i = 0; i < childCount; i++) {
final View child = layoutManager.getChildAt(i);
int childStart = helper.getDecoratedStart(child);
/** if child is more to start than previous closest, set it as closest **/
if (childStart < startest) {
startest = childStart;
closestChild = child;
}
}
return closestChild;
}
@NonNull
private OrientationHelper getVerticalHelper(@NonNull RecyclerView.LayoutManager layoutManager) {
if (mVerticalHelper == null || mVerticalHelper.mLayoutManager != layoutManager) {
mVerticalHelper = OrientationHelper.createVerticalHelper(layoutManager);
}
return mVerticalHelper;
}
@NonNull
private OrientationHelper getHorizontalHelper(
@NonNull RecyclerView.LayoutManager layoutManager) {
if (mHorizontalHelper == null || mHorizontalHelper.mLayoutManager != layoutManager) {
mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager);
}
return mHorizontalHelper;
}
}