HeaderRenderer.java

package com.timehop.stickyheadersrecyclerview.rendering;

import android.graphics.Canvas;
import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.widget.LinearLayout;

import com.timehop.stickyheadersrecyclerview.calculation.DimensionCalculator;
import com.timehop.stickyheadersrecyclerview.util.OrientationProvider;

/**
 * Responsible for drawing headers to the canvas provided by the item decoration
 */
public class HeaderRenderer {

  private final DimensionCalculator mDimensionCalculator;
  private final OrientationProvider mOrientationProvider;

  public HeaderRenderer(OrientationProvider orientationProvider) {
    this(orientationProvider, new DimensionCalculator());
  }

  private HeaderRenderer(OrientationProvider orientationProvider,
      DimensionCalculator dimensionCalculator) {
    mOrientationProvider = orientationProvider;
    mDimensionCalculator = dimensionCalculator;
  }

  /**
   * Draws a header to a canvas, offsetting by some x and y amount
   *
   * @param recyclerView the parent recycler view for drawing the header into
   * @param canvas       the canvas on which to draw the header
   * @param header       the view to draw as the header
   * @param offset       a Rect used to define the x/y offset of the header. Specify x/y offset by setting
   *                     the {@link Rect#left} and {@link Rect#top} properties, respectively.
   */
  public void drawHeader(RecyclerView recyclerView, Canvas canvas, View header, Rect offset) {
    canvas.save();

    if (recyclerView.getLayoutManager().getClipToPadding()) {
      // Clip drawing of headers to the padding of the RecyclerView. Avoids drawing in the padding
      Rect clipRect = getClipRectForHeader(recyclerView, header);
      canvas.clipRect(clipRect);
    }

    canvas.translate(offset.left, offset.top);

    header.draw(canvas);
    canvas.restore();
  }

  /**
   * Gets a clipping rect for the header based on the margins of the header and the padding of the
   * recycler.
   * FIXME: Currently right margin in VERTICAL orientation and bottom margin in HORIZONTAL
   * orientation are clipped so they look accurate, but the headers are not being drawn at the
   * correctly smaller width and height respectively.
   *
   * @param recyclerView for which to provide a header
   * @param header       for clipping
   * @return a {@link Rect} for clipping a provided header to the padding of a recycler view
   */
  private Rect getClipRectForHeader(RecyclerView recyclerView, View header) {
    Rect headerMargins = mDimensionCalculator.getMargins(header);
    if (mOrientationProvider.getOrientation(recyclerView) == LinearLayout.VERTICAL) {
      return new Rect(
          recyclerView.getPaddingLeft(),
          recyclerView.getPaddingTop(),
          recyclerView.getWidth() - recyclerView.getPaddingRight() - headerMargins.right,
          recyclerView.getHeight() - recyclerView.getPaddingBottom());
    } else {
      return new Rect(
          recyclerView.getPaddingLeft(),
          recyclerView.getPaddingTop(),
          recyclerView.getWidth() - recyclerView.getPaddingRight(),
          recyclerView.getHeight() - recyclerView.getPaddingBottom() - headerMargins.bottom);
    }
  }

}