SnappyRecyclerView.java

package com.luorrak.ouroboros.thread;

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;
import android.view.MotionEvent;

/**
 * Ouroboros - An 8chan browser
 * Copyright (C) 2015  Luorrak
 * <p/>
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * <p/>
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * <p/>
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
public final class SnappyRecyclerView extends RecyclerView {

    public SnappyRecyclerView(Context context) {
        super(context);
    }

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

    public SnappyRecyclerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    public boolean fling(int velocityX, int velocityY) {
        final LayoutManager lm = getLayoutManager();

        if (lm instanceof ISnappyLayoutManager) {
            super.smoothScrollToPosition(((ISnappyLayoutManager) getLayoutManager())
                    .getPositionForVelocity(velocityX, velocityY));
            return true;
        }
        return super.fling(velocityX, velocityY);
    }

    @Override
    public boolean onTouchEvent(MotionEvent e) {
        // We want the parent to handle all touch events--there's a lot going on there,
        // and there is no reason to overwrite that functionality--bad things will happen.
        final boolean ret = super.onTouchEvent(e);
        final LayoutManager lm = getLayoutManager();

        if (lm instanceof ISnappyLayoutManager
                && (e.getAction() == MotionEvent.ACTION_UP ||
                e.getAction() == MotionEvent.ACTION_CANCEL)
                && getScrollState() == SCROLL_STATE_IDLE) {
            // The layout manager is a SnappyLayoutManager, which means that the
            // children should be snapped to a grid at the end of a drag or
            // fling. The motion event is either a user lifting their finger or
            // the cancellation of a motion events, so this is the time to take
            // over the scrolling to perform our own functionality.
            // Finally, the scroll state is idle--meaning that the resultant
            // velocity after the user's gesture was below the threshold, and
            // no fling was performed, so the view may be in an unaligned state
            // and will not be flung to a proper state.
            smoothScrollToPosition(((ISnappyLayoutManager) lm).getFixScrollPos());
        }

        return ret;
    }

    /**
     * An interface that LayoutManagers that should snap to grid should implement.
     */
    public interface ISnappyLayoutManager {

        /**
         * @param velocityX
         * @param velocityY
         * @return the resultant position from a fling of the given velocity.
         */
        int getPositionForVelocity(int velocityX, int velocityY);

        /**
         * @return the position this list must scroll to to fix a state where the
         * views are not snapped to grid.
         */
        int getFixScrollPos();

    }
}