src/com/android/launcher3/BaseRecyclerView.java

/*
 * Copyright (C) 2015 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 com.android.launcher3;

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

import com.android.launcher3.util.Thunk;


/**
 * A base {@link RecyclerView}, which does the following:
 * <ul>
 *   <li> NOT intercept a touch unless the scrolling velocity is below a predefined threshold.
 *   <li> Enable fast scroller.
 * </ul>
 */
public abstract class BaseRecyclerView extends RecyclerView
        implements RecyclerView.OnItemTouchListener {

    private static final int SCROLL_DELTA_THRESHOLD_DP = 4;

    /** Keeps the last known scrolling delta/velocity along y-axis. */
    @Thunk int mDy = 0;
    private float mDeltaThreshold;

    protected final BaseRecyclerViewFastScrollBar mScrollbar;

    private int mDownX;
    private int mDownY;
    private int mLastY;

    public BaseRecyclerView(Context context) {
        this(context, null);
    }

    public BaseRecyclerView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public BaseRecyclerView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        mDeltaThreshold = getResources().getDisplayMetrics().density * SCROLL_DELTA_THRESHOLD_DP;
        mScrollbar = new BaseRecyclerViewFastScrollBar(this, getResources());

        ScrollListener listener = new ScrollListener();
        setOnScrollListener(listener);
    }

    private class ScrollListener extends OnScrollListener {
        public ScrollListener() {
            // Do nothing
        }

        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            mDy = dy;

            // TODO(winsonc): If we want to animate the section heads while scrolling, we can
            //                initiate that here if the recycler view scroll state is not
            //                RecyclerView.SCROLL_STATE_IDLE.

            onUpdateScrollbar(dy);
        }
    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        addOnItemTouchListener(this);
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        mScrollbar.setPopupView(((ViewGroup) getParent()).findViewById(R.id.fast_scroller_popup));
    }

    /**
     * We intercept the touch handling only to support fast scrolling when initiated from the
     * scroll bar.  Otherwise, we fall back to the default RecyclerView touch handling.
     */
    @Override
    public boolean onInterceptTouchEvent(RecyclerView rv, MotionEvent ev) {
        return handleTouchEvent(ev);
    }

    @Override
    public void onTouchEvent(RecyclerView rv, MotionEvent ev) {
        handleTouchEvent(ev);
    }

    /**
     * Handles the touch event and determines whether to show the fast scroller (or updates it if
     * it is already showing).
     */
    private boolean handleTouchEvent(MotionEvent ev) {
        int action = ev.getAction();
        int x = (int) ev.getX();
        int y = (int) ev.getY();
        switch (action) {
            case MotionEvent.ACTION_DOWN:
                // Keep track of the down positions
                mDownX = x;
                mDownY = mLastY = y;
                if (shouldStopScroll(ev)) {
                    stopScroll();
                }
                mScrollbar.handleTouchEvent(ev, mDownX, mDownY, mLastY);
                break;
            case MotionEvent.ACTION_MOVE:
                mLastY = y;
                mScrollbar.handleTouchEvent(ev, mDownX, mDownY, mLastY);
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                onFastScrollCompleted();
                mScrollbar.handleTouchEvent(ev, mDownX, mDownY, mLastY);
                break;
        }
        return mScrollbar.isDraggingThumb();
    }

    public void onRequestDisallowInterceptTouchEvent(boolean disallowIntercept) {
        // DO NOT REMOVE, NEEDED IMPLEMENTATION FOR M BUILDS
    }

    /**
     * Returns whether this {@link MotionEvent} should trigger the scroll to be stopped.
     */
    protected boolean shouldStopScroll(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            if ((Math.abs(mDy) < mDeltaThreshold &&
                    getScrollState() != RecyclerView.SCROLL_STATE_IDLE)) {
                // now the touch events are being passed to the {@link WidgetCell} until the
                // touch sequence goes over the touch slop.
                return true;
            }
        }
        return false;
    }

    /**
     * Returns the height of the fast scroll bar
     */
    protected int getScrollbarTrackHeight() {
        return getHeight();
    }

    /**
     * Returns the available scroll height:
     *   AvailableScrollHeight = Total height of the all items - last page height
     */
    protected abstract int getAvailableScrollHeight();

    /**
     * Returns the available scroll bar height:
     *   AvailableScrollBarHeight = Total height of the visible view - thumb height
     */
    protected int getAvailableScrollBarHeight() {
        int availableScrollBarHeight = getScrollbarTrackHeight() - mScrollbar.getThumbHeight();
        return availableScrollBarHeight;
    }

    /**
     * Returns the track color (ignoring alpha), can be overridden by each subclass.
     */
    public int getFastScrollerTrackColor(int defaultTrackColor) {
        return defaultTrackColor;
    }

    /**
     * Returns the scrollbar for this recycler view.
     */
    public BaseRecyclerViewFastScrollBar getScrollBar() {
        return mScrollbar;
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        super.dispatchDraw(canvas);
        onUpdateScrollbar(0);
        mScrollbar.draw(canvas);
    }

    /**
     * Updates the scrollbar thumb offset to match the visible scroll of the recycler view.  It does
     * this by mapping the available scroll area of the recycler view to the available space for the
     * scroll bar.
     *
     * @param scrollY the current scroll y
     */
    protected void synchronizeScrollBarThumbOffsetToViewScroll(int scrollY,
            int availableScrollHeight) {
        // Only show the scrollbar if there is height to be scrolled
        if (availableScrollHeight <= 0) {
            mScrollbar.setThumbOffsetY(-1);
            return;
        }

        // Calculate the current scroll position, the scrollY of the recycler view accounts for the
        // view padding, while the scrollBarY is drawn right up to the background padding (ignoring
        // padding)
        int scrollBarY =
                (int) (((float) scrollY / availableScrollHeight) * getAvailableScrollBarHeight());

        // Calculate the position and size of the scroll bar
        mScrollbar.setThumbOffsetY(scrollBarY);
    }

    /**
     * @return whether fast scrolling is supported in the current state.
     */
    protected boolean supportsFastScrolling() {
        return true;
    }

    /**
     * Maps the touch (from 0..1) to the adapter position that should be visible.
     * <p>Override in each subclass of this base class.
     *
     * @return the scroll top of this recycler view.
     */
    public abstract int getCurrentScrollY();

    /**
     * Maps the touch (from 0..1) to the adapter position that should be visible.
     * <p>Override in each subclass of this base class.
     */
    protected abstract String scrollToPositionAtProgress(float touchFraction);

    /**
     * Updates the bounds for the scrollbar.
     * <p>Override in each subclass of this base class.
     */
    protected abstract void onUpdateScrollbar(int dy);

    /**
     * <p>Override in each subclass of this base class.
     */
    protected void onFastScrollCompleted() {}
}