[Android] 零碎知识汇总 - RecyclerView 顶部悬浮

package com.example.dq.app_itemdecoration;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.text.TextPaint;
import android.text.TextUtils;
import android.view.View;

/**
 * Created by DQ on 2017/8/7
 * 城市列表的分割线， 主要用于绘制字母分组
 */
public class CityItemDecoration extends RecyclerView.ItemDecoration {

  private int mGroupHeight = 60;
  private int mSideMargin = 10;
  private Paint mGroupPaint;
  private TextPaint mTextPaint;
  private GroupListener mListener;

  public CityItemDecoration(Context context, GroupListener pListener) {

    mGroupPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
    mGroupPaint.setColor(context.getResources().getColor(android.R.color.holo_blue_dark));

    mListener = pListener;

    mTextPaint = new TextPaint(Paint.ANTI_ALIAS_FLAG);
    mTextPaint.setTextSize(40);
    mTextPaint.setColor(Color.WHITE);
    mTextPaint.setTextAlign(Paint.Align.LEFT);
  }

  // 偏移距离，可以变相的理解成，分割线的高度
  @Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    super.getItemOffsets(outRect, view, parent, state);

    int position = parent.getChildAdapterPosition(view);
    // 不是省份
    if (mListener == null || TextUtils.isEmpty(mListener.getGroupName(position))) {
      return;
    }
    // 是否显示 省份标签
    if (isFirstInGroup(position)) {
      outRect.top = mGroupHeight;
    }
  }

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

    final int itemCount = state.getItemCount();
    final int childCount = parent.getChildCount();

    final int left = parent.getLeft() + parent.getPaddingLeft();
    final int right = parent.getRight() + parent.getPaddingRight();

    String preGroupName; // 标记上一个item的省份
    String currentGroupName = null; // 当前item的省份

    for (int i = 0; i < childCount; i++) {
      View child = parent.getChildAt(i);
      int position = parent.getChildAdapterPosition(child);

      preGroupName = currentGroupName;
      currentGroupName = mListener.getGroupName(position);

      // 普通城市 or 和上一次省份相同
      if (TextUtils.equals(currentGroupName, preGroupName)) {
        continue;
      }

      // 计算高度
      int bottom = Math.max(mGroupHeight, child.getTop());

      if (position < itemCount - 1) {
        String nextGroupName = mListener.getGroupName(position + 1);
        int viewBottom = child.getBottom();
        if (!currentGroupName.equals(nextGroupName) && viewBottom < bottom) {
          bottom = viewBottom;
        }
      }

      // 接下来， 就是省份的绘制了
      c.drawRect(left, bottom - mGroupHeight, right, bottom, mGroupPaint);
      // 绘制文字
      Paint.FontMetrics fm = mTextPaint.getFontMetrics();
      float baseline = bottom - (mGroupHeight - (fm.bottom - fm.top)) / 2 - fm.bottom;
      // TODO: 2017/8/7 DQ: 这里添加的是左右边距，是可以自己设置的
      float textWidth = mTextPaint.measureText(currentGroupName);
      float marginLeft = true ? 0 : right - textWidth;
      mSideMargin = Math.abs(mSideMargin);
      mSideMargin = true ? mSideMargin : -mSideMargin;
      c.drawText(currentGroupName, left + mSideMargin + marginLeft, baseline, mTextPaint);
    }
  }

  public interface GroupListener {
    String getGroupName(int position);
  }

  private boolean isFirstInGroup(int position) {
    if (position == 0) {
      return true;
    } else {
      String prevGroupId = mListener.getGroupName(position - 1);
      String groupId = mListener.getGroupName(position);
      return !TextUtils.equals(prevGroupId, groupId);
    }
  }
}