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Forked from osama/GravitySnapHelper.java
Created November 15, 2017 09:00
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import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.LinearSnapHelper;
import android.support.v7.widget.OrientationHelper;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.RecyclerView.LayoutManager;
import android.view.Gravity;
import android.view.View;
import com.example.AppState;
/**
* A {@link LinearSnapHelper} that allows you to snap to a certain edge of the {@link RecyclerView},
* as opposed to the default center-snapping provided by {@link LinearSnapHelper}.
*
* Inspired by https://github.com/rubensousa/RecyclerViewSnap/
*/
public class GravitySnapHelper extends LinearSnapHelper {
@Nullable private OrientationHelper verticalHelper;
@Nullable private OrientationHelper horizontalHelper;
private int gravity;
//TODO Google's OrientationHelper seems to be broken in RTL carousels. Check if these flags are still needed when updating support lib.
private boolean isRTL = AppState.getAppComponent().localeManager().isCurrentLocaleRTL();
private boolean isGravityHorizontal;
private boolean isFlingingInPositiveDirection;
/**
* Constructor
*
* @param gravity The {@link Gravity} specifies the edge of the {@link RecyclerView} that items should snap to.
*/
public GravitySnapHelper(int gravity) {
switch (gravity) {
case Gravity.LEFT:
case Gravity.START:
this.gravity = Gravity.START;
this.isGravityHorizontal = true;
break;
case Gravity.RIGHT:
case Gravity.END:
this.gravity = Gravity.END;
this.isGravityHorizontal = true;
break;
case Gravity.TOP:
case Gravity.BOTTOM:
this.gravity = gravity;
break;
default:
throw new IllegalArgumentException("Illegal Gravity passed to constructor.");
}
}
@Override
@Nullable
public int[] calculateDistanceToFinalSnap(@NonNull LayoutManager layoutManager, @NonNull View targetView) {
int[] out = new int[2];
if (layoutManager.canScrollHorizontally()) {
if (gravity == Gravity.START) {
out[0] = distanceToStart(targetView, getHorizontalHelper(layoutManager));
} else { // END
out[0] = distanceToEnd(targetView, getHorizontalHelper(layoutManager));
}
} else {
out[0] = 0;
}
if (layoutManager.canScrollVertically()) {
if (gravity == Gravity.TOP) {
out[1] = distanceToStart(targetView, getVerticalHelper(layoutManager));
} else { // BOTTOM
out[1] = distanceToEnd(targetView, getVerticalHelper(layoutManager));
}
} else {
out[1] = 0;
}
return out;
}
@Override
@Nullable
public View findSnapView(LayoutManager layoutManager) {
if (layoutManager instanceof LinearLayoutManager) {
switch (gravity) {
case Gravity.START:
return findStartView(layoutManager, getHorizontalHelper(layoutManager));
case Gravity.TOP:
return findStartView(layoutManager, getVerticalHelper(layoutManager));
case Gravity.END:
return findEndView(layoutManager, getHorizontalHelper(layoutManager));
case Gravity.BOTTOM:
return findEndView(layoutManager, getVerticalHelper(layoutManager));
default:
break;
}
}
return super.findSnapView(layoutManager);
}
@Override
public int findTargetSnapPosition(LayoutManager layoutManager, int velocityX, int velocityY) {
isFlingingInPositiveDirection = isRTL ? Math.max(velocityX, velocityY) < 0 : Math.max(velocityX, velocityY) > 0;
return super.findTargetSnapPosition(layoutManager, velocityX, velocityY);
}
/**
* Calculates the passed view's distance from the start of this parent.
*/
private int distanceToStart(View targetView, OrientationHelper helper) {
if (isRTL && isGravityHorizontal) {
//TODO Check if this is still needed after updating support library. The helper currently assumes start == left and right == end.
return helper.getDecoratedEnd(targetView) - helper.getEndAfterPadding();
}
return helper.getDecoratedStart(targetView) - helper.getStartAfterPadding();
}
/**
* Calculates the passed view's distance from the end of this parent.
*/
private int distanceToEnd(View targetView, OrientationHelper helper) {
if (isRTL && isGravityHorizontal) {
//TODO Check if this is still needed after updating support library. The helper currently assumes start == left and right == end.
return helper.getDecoratedStart(targetView) - helper.getStartAfterPadding();
}
return helper.getDecoratedEnd(targetView) - helper.getEndAfterPadding();
}
/**
* Returns the child view that is currently closest to the start of this parent.
*
* @param layoutManager The {@link 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(LayoutManager layoutManager, OrientationHelper helper) {
if (layoutManager instanceof LinearLayoutManager) {
int firstChild = ((LinearLayoutManager) layoutManager).findFirstVisibleItemPosition();
if (firstChild == RecyclerView.NO_POSITION) {
return null;
}
View child = layoutManager.findViewByPosition(firstChild);
//TODO Check if this is still needed after updating support library. The helper currently assumes start == left and right == end.
boolean shouldReturnChild;
if (isRTL && isGravityHorizontal) {
shouldReturnChild = helper.getDecoratedStart(child) <= helper.getDecoratedMeasurement(child) / 2;
} else {
shouldReturnChild = helper.getDecoratedEnd(child) >= helper.getDecoratedMeasurement(child) / 2;
}
if (isFlingingInPositiveDirection
&& ((LinearLayoutManager) layoutManager).findLastCompletelyVisibleItemPosition() == layoutManager.getItemCount() - 1) {
return null;
} else if (shouldReturnChild) {
return child;
} else {
return layoutManager.findViewByPosition(firstChild + 1);
}
}
return super.findSnapView(layoutManager);
}
/**
* Returns the child view that is currently closest to the end of this parent.
*
* @param layoutManager The {@link 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 end of this parent.
*/
@Nullable
private View findEndView(LayoutManager layoutManager, OrientationHelper helper) {
if (layoutManager instanceof LinearLayoutManager) {
int lastChild = ((LinearLayoutManager) layoutManager).findLastVisibleItemPosition();
if (lastChild == RecyclerView.NO_POSITION) {
return null;
}
View child = layoutManager.findViewByPosition(lastChild);
//TODO Check if this is still needed after updating support library. The helper currently assumes start == left and right == end.
boolean shouldReturnChild;
if (isRTL && isGravityHorizontal) {
shouldReturnChild = helper.getDecoratedEnd(child) - helper.getDecoratedMeasurement(child) / 2 >= layoutManager.getPaddingLeft();
} else {
shouldReturnChild = helper.getDecoratedStart(child) + helper.getDecoratedMeasurement(child) / 2 <= helper.getTotalSpace();
}
if (!isFlingingInPositiveDirection
&& ((LinearLayoutManager) layoutManager).findFirstCompletelyVisibleItemPosition() == 0) {
return null;
} else if (shouldReturnChild) {
return child;
} else {
return layoutManager.findViewByPosition(lastChild - 1);
}
}
return super.findSnapView(layoutManager);
}
/**
* Creates and returns a vertical {@link OrientationHelper} for the passed {@link LayoutManager}.
*/
@NonNull
private OrientationHelper getVerticalHelper(LayoutManager layoutManager) {
if (verticalHelper == null) {
verticalHelper = OrientationHelper.createVerticalHelper(layoutManager);
}
return verticalHelper;
}
/**
* Creates and returns a horizontal {@link OrientationHelper} for the passed {@link LayoutManager}.
*/
@NonNull
private OrientationHelper getHorizontalHelper(LayoutManager layoutManager) {
if (horizontalHelper == null) {
horizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager);
}
return horizontalHelper;
}
}
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