How to make a page indicator for horizontal recyclerview

Question

Any idea how to create a page indicator for recyclerview list ?

Answer 1

You can add an indicator by using RecyclerView.ItemDecoration.

Just draw some lines or circles at the bottom and use layoutManager.findFirstVisibleItemPosition() to get the current active item. Since pagers tend to fill the whole width this is an accruate way of getting the displayed item. This also allows us to calculate the scrolling distance by comparing the childs left edge to the parents.

Below you find a sample decoration that draws some lines and animates between them

public class LinePagerIndicatorDecoration extends RecyclerView.ItemDecoration {

  private int colorActive = 0xFFFFFFFF;
  private int colorInactive = 0x66FFFFFF;

  private static final float DP = Resources.getSystem().getDisplayMetrics().density;

  /**
   * Height of the space the indicator takes up at the bottom of the view.
   */
  private final int mIndicatorHeight = (int) (DP * 16);

  /**
   * Indicator stroke width.
   */
  private final float mIndicatorStrokeWidth = DP * 2;

  /**
   * Indicator width.
   */
  private final float mIndicatorItemLength = DP * 16;
  /**
   * Padding between indicators.
   */
  private final float mIndicatorItemPadding = DP * 4;

  /**
   * Some more natural animation interpolation
   */
  private final Interpolator mInterpolator = new AccelerateDecelerateInterpolator();

  private final Paint mPaint = new Paint();

  public LinePagerIndicatorDecoration() {
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeWidth(mIndicatorStrokeWidth);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setAntiAlias(true);
  }

  @Override
  public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
    super.onDrawOver(c, parent, state);

    int itemCount = parent.getAdapter().getItemCount();

    // center horizontally, calculate width and subtract half from center
    float totalLength = mIndicatorItemLength * itemCount;
    float paddingBetweenItems = Math.max(0, itemCount - 1) * mIndicatorItemPadding;
    float indicatorTotalWidth = totalLength + paddingBetweenItems;
    float indicatorStartX = (parent.getWidth() - indicatorTotalWidth) / 2F;

    // center vertically in the allotted space
    float indicatorPosY = parent.getHeight() - mIndicatorHeight / 2F;

    drawInactiveIndicators(c, indicatorStartX, indicatorPosY, itemCount);


    // find active page (which should be highlighted)
    LinearLayoutManager layoutManager = (LinearLayoutManager) parent.getLayoutManager();
    int activePosition = layoutManager.findFirstVisibleItemPosition();
    if (activePosition == RecyclerView.NO_POSITION) {
      return;
    }

    // find offset of active page (if the user is scrolling)
    final View activeChild = layoutManager.findViewByPosition(activePosition);
    int left = activeChild.getLeft();
    int width = activeChild.getWidth();

    // on swipe the active item will be positioned from [-width, 0]
    // interpolate offset for smooth animation
    float progress = mInterpolator.getInterpolation(left * -1 / (float) width);

    drawHighlights(c, indicatorStartX, indicatorPosY, activePosition, progress, itemCount);
  }

  private void drawInactiveIndicators(Canvas c, float indicatorStartX, float indicatorPosY, int itemCount) {
    mPaint.setColor(colorInactive);

    // width of item indicator including padding
    final float itemWidth = mIndicatorItemLength + mIndicatorItemPadding;

    float start = indicatorStartX;
    for (int i = 0; i < itemCount; i++) {
      // draw the line for every item
      c.drawLine(start, indicatorPosY, start + mIndicatorItemLength, indicatorPosY, mPaint);
      start += itemWidth;
    }
  }

  private void drawHighlights(Canvas c, float indicatorStartX, float indicatorPosY,
                              int highlightPosition, float progress, int itemCount) {
    mPaint.setColor(colorActive);

    // width of item indicator including padding
    final float itemWidth = mIndicatorItemLength + mIndicatorItemPadding;

    if (progress == 0F) {
      // no swipe, draw a normal indicator
      float highlightStart = indicatorStartX + itemWidth * highlightPosition;
      c.drawLine(highlightStart, indicatorPosY,
          highlightStart + mIndicatorItemLength, indicatorPosY, mPaint);
    } else {
      float highlightStart = indicatorStartX + itemWidth * highlightPosition;
      // calculate partial highlight
      float partialLength = mIndicatorItemLength * progress;

      // draw the cut off highlight
      c.drawLine(highlightStart + partialLength, indicatorPosY,
          highlightStart + mIndicatorItemLength, indicatorPosY, mPaint);

      // draw the highlight overlapping to the next item as well
      if (highlightPosition < itemCount - 1) {
        highlightStart += itemWidth;
        c.drawLine(highlightStart, indicatorPosY,
            highlightStart + partialLength, indicatorPosY, mPaint);
      }
    }
  }

  @Override
  public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    super.getItemOffsets(outRect, view, parent, state);
    outRect.bottom = mIndicatorHeight;
  }
}

Which will give you a result like the following

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There is also a blog post that goes more into detail about how the decoration works here and the full source code is available at GitHub