HeaderPositionCalculator.java

package com.timehop.stickyheadersrecyclerview;

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

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

/**
 * Calculates the position and location of header views
 */
public class HeaderPositionCalculator {

  private final StickyRecyclerHeadersAdapter mAdapter;
  private final OrientationProvider mOrientationProvider;
  private final HeaderProvider mHeaderProvider;
  private final DimensionCalculator mDimensionCalculator;

  public HeaderPositionCalculator(StickyRecyclerHeadersAdapter adapter, HeaderProvider headerProvider,
      OrientationProvider orientationProvider, DimensionCalculator dimensionCalculator) {
    mAdapter = adapter;
    mHeaderProvider = headerProvider;
    mOrientationProvider = orientationProvider;
    mDimensionCalculator = dimensionCalculator;
  }

  /**
   * Determines if an item in the list should have a header that is different than the item in the
   * list that immediately precedes it. Items with no headers will always return false.
   *
   * @param position of the list item in questions
   * @return true if this item has a different header than the previous item in the list
   * @see {@link StickyRecyclerHeadersAdapter#getHeaderId(int)}
   */
  public boolean hasNewHeader(int position) {
    if (getFirstHeaderPosition() == position) {
      return true;
    }

    if (mAdapter.getHeaderId(position) < 0 || indexOutOfBounds(position)) {
      return false;
    }

    return mAdapter.getHeaderId(position) != mAdapter.getHeaderId(position - 1);
  }

  private boolean indexOutOfBounds(int position) {
    return position < 0 || position >= mAdapter.getItemCount();
  }

  private int getFirstHeaderPosition() {
    for (int i = 0; i < mAdapter.getItemCount(); i++) {
      if (mAdapter.getHeaderId(i) >= 0) {
        return i;
      }
    }
    return -1;
  }

  public Rect getHeaderBounds(RecyclerView recyclerView, View header, View firstView, boolean firstHeader) {
    int orientation = mOrientationProvider.getOrientation(recyclerView);
    Rect bounds = getDefaultHeaderOffset(header, firstView, orientation);

    if (firstHeader && isStickyHeaderBeingPushedOffscreen(recyclerView, header)) {
      View viewAfterNextHeader = getFirstViewUnobscuredByHeader(recyclerView, header);
      int firstViewUnderHeaderPosition = recyclerView.getChildPosition(viewAfterNextHeader);
      View secondHeader = mHeaderProvider.getHeader(recyclerView, firstViewUnderHeaderPosition);
      translateHeaderWithNextHeader(recyclerView, mOrientationProvider.getOrientation(recyclerView), bounds,
          header, viewAfterNextHeader, secondHeader);
    }

    return bounds;
  }

  private Rect getDefaultHeaderOffset(View header, View firstView, int orientation) {
    int translationX, translationY;
    Rect headerMargins = mDimensionCalculator.getMargins(header);
    if (orientation == LinearLayoutManager.VERTICAL) {
      translationX = firstView.getLeft() + headerMargins.left;
      translationY = firstView.getTop() - header.getHeight() - headerMargins.bottom;
    } else {
      translationY = firstView.getTop() + headerMargins.top;
      translationX = firstView.getLeft() - header.getWidth() - headerMargins.right;
    }

    return new Rect(translationX, translationY, translationX + header.getWidth(),
        translationY + header.getHeight());
  }

  private boolean isStickyHeaderBeingPushedOffscreen(RecyclerView recyclerView, View stickyHeader) {
    View viewAfterHeader = getFirstViewUnobscuredByHeader(recyclerView, stickyHeader);
    int firstViewUnderHeaderPosition = recyclerView.getChildPosition(viewAfterHeader);

    if (firstViewUnderHeaderPosition > 0 && hasNewHeader(firstViewUnderHeaderPosition)) {
      View nextHeader = mHeaderProvider.getHeader(recyclerView, firstViewUnderHeaderPosition);
      Rect nextHeaderMargins = mDimensionCalculator.getMargins(nextHeader);
      Rect headerMargins = mDimensionCalculator.getMargins(stickyHeader);

      if (mOrientationProvider.getOrientation(recyclerView) == LinearLayoutManager.VERTICAL) {
        int topOfNextHeader = viewAfterHeader.getTop() - nextHeaderMargins.bottom - nextHeader.getHeight() - nextHeaderMargins.top;
        int bottomOfThisHeader = recyclerView.getPaddingTop() + stickyHeader.getBottom() + headerMargins.top + headerMargins.bottom;
        if (topOfNextHeader < bottomOfThisHeader) {
          return true;
        }
      } else {
        int leftOfNextHeader = viewAfterHeader.getLeft() - nextHeaderMargins.right - nextHeader.getWidth() - nextHeaderMargins.left;
        int rightOfThisHeader = recyclerView.getPaddingLeft() + stickyHeader.getRight() + headerMargins.left + headerMargins.right;
        if (leftOfNextHeader < rightOfThisHeader) {
          return true;
        }
      }
    }

    return false;
  }

  private void translateHeaderWithNextHeader(RecyclerView recyclerView, int orientation, Rect translation,
      View currentHeader, View viewAfterNextHeader, View nextHeader) {
    Rect nextHeaderMargins = mDimensionCalculator.getMargins(nextHeader);
    Rect stickyHeaderMargins = mDimensionCalculator.getMargins(currentHeader);
    if (orientation == LinearLayoutManager.VERTICAL) {
      int topOfStickyHeader = getListTop(recyclerView) + stickyHeaderMargins.top + stickyHeaderMargins.bottom;
      int shiftFromNextHeader = viewAfterNextHeader.getTop() - nextHeader.getHeight() - nextHeaderMargins.bottom - nextHeaderMargins.top - currentHeader.getHeight() - topOfStickyHeader;
      if (shiftFromNextHeader < topOfStickyHeader) {
        translation.top += shiftFromNextHeader;
      }
    } else {
      int leftOfStickyHeader = getListLeft(recyclerView) + stickyHeaderMargins.left + stickyHeaderMargins.right;
      int shiftFromNextHeader = viewAfterNextHeader.getLeft() - nextHeader.getWidth() - nextHeaderMargins.right - nextHeaderMargins.left - currentHeader.getWidth() - leftOfStickyHeader;
      if (shiftFromNextHeader < leftOfStickyHeader) {
        translation.left += shiftFromNextHeader;
      }
    }
  }

  /**
   * Returns the first item currently in the RecyclerView that is not obscured by a header.
   *
   * @param parent Recyclerview containing all the list items
   * @return first item that is fully beneath a header
   */
  private View getFirstViewUnobscuredByHeader(RecyclerView parent, View firstHeader) {
    for (int i = 0; i < parent.getChildCount(); i++) {
      View child = parent.getChildAt(i);
      if (!itemIsObscuredByHeader(parent, child, firstHeader, mOrientationProvider.getOrientation(parent))) {
        return child;
      }
    }
    return null;
  }

  /**
   * Determines if an item is obscured by a header
   *
   *
   * @param parent
   * @param item        to determine if obscured by header
   * @param header      that might be obscuring the item
   * @param orientation of the {@link RecyclerView}
   * @return true if the item view is obscured by the header view
   */
  private boolean itemIsObscuredByHeader(RecyclerView parent, View item, View header, int orientation) {
    RecyclerView.LayoutParams layoutParams = (RecyclerView.LayoutParams) item.getLayoutParams();
    Rect headerMargins = mDimensionCalculator.getMargins(header);

    if (mHeaderProvider.getHeader(parent, parent.getChildPosition(item)) != header) {
      // Resolves https://github.com/timehop/sticky-headers-recyclerview/issues/36
      // Handles an edge case where a trailing header is smaller than the current sticky header.
      return false;
    }

    if (orientation == LinearLayoutManager.VERTICAL) {
      int itemTop = item.getTop() - layoutParams.topMargin;
      int headerBottom = header.getBottom() + headerMargins.bottom + headerMargins.top;
      if (itemTop > headerBottom) {
        return false;
      }
    } else {
      int itemLeft = item.getLeft() - layoutParams.leftMargin;
      int headerRight = header.getRight() + headerMargins.right + headerMargins.left;
      if (itemLeft > headerRight) {
        return false;
      }
    }

    return true;
  }

  private int getListTop(RecyclerView view) {
    if (view.getLayoutManager().getClipToPadding()) {
      return view.getPaddingTop();
    } else {
      return 0;
    }
  }

  private int getListLeft(RecyclerView view) {
    if (view.getLayoutManager().getClipToPadding()) {
      return view.getPaddingLeft();
    } else {
      return 0;
    }
  }

}