使用ItemDecoration为RecyclerView打造带悬停头部的分组列表

package jiuyuhulian.com.merchantstoreApp.view;

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.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.TypedValue;
import android.view.View;

import java.util.List;

import jiuyuhulian.com.merchantstoreApp.entity.ExchageDetailEntity;

/**
* 有分类title的 ItemDecoration
* Created by Admin on 2017/3/31.
*/

public class TitleItemDecoration extends RecyclerView.ItemDecoration {

private List<ExchageDetailEntity> mDatas;
private Paint mPaint;
private Rect mBounds;//用于存放测量文字Rect

private int mTitleHeight;//title的高
private static int COLOR_TITLE_BG = Color.parseColor("#FFDFDFDF");
private static int COLOR_TITLE_FONT = Color.parseColor("#FF000000");
private static int mTitleFontSize;//title字体大小

public TitleItemDecoration(Context context, List<ExchageDetailEntity> datas) {
super();
mDatas = datas;
mPaint = new Paint();
mBounds = new Rect();
mTitleHeight = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 30, context.getResources().getDisplayMetrics());
mTitleFontSize = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, 16, context.getResources().getDisplayMetrics());
mPaint.setTextSize(mTitleFontSize);
mPaint.setAntiAlias(true);
}

@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
super.onDraw(c, parent, state);
final int left = parent.getPaddingLeft();
final int right = parent.getWidth() - parent.getPaddingRight();
final int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = parent.getChildAt(i);
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
.getLayoutParams();
int position = params.getViewLayoutPosition();
//我记得Rv的item position在重置时可能为-1.保险点判断一下吧
if (position > -1) {
if (position == 0) {//等于0肯定要有title的
drawTitleArea(c, left, right, child, params, position);

} else {//其他的通过判断
if (null != mDatas.get(position).getMouth() && !mDatas.get(position).getMouth().equals(mDatas.get(position - 1).getMouth())) {
//不为空 且跟前一个tag不一样了，说明是新的分类，也要title
drawTitleArea(c, left, right, child, params, position);
} else {
//none
}
}
}
}
}

/**
* 绘制Title区域背景和文字的方法
*
* @param c
* @param left
* @param right
* @param child
* @param params
* @param position
*/
private void drawTitleArea(Canvas c, int left, int right, View child, RecyclerView.LayoutParams params, int position) {//最先调用，绘制在最下层
mPaint.setColor(COLOR_TITLE_BG);
c.drawRect(left, child.getTop() - params.topMargin - mTitleHeight, right, child.getTop() - params.topMargin, mPaint);
mPaint.setColor(COLOR_TITLE_FONT);
/*
Paint.FontMetricsInt fontMetrics = mPaint.getFontMetricsInt();
int baseline = (getMeasuredHeight() - fontMetrics.bottom + fontMetrics.top) / 2 - fontMetrics.top;*/

mPaint.getTextBounds(mDatas.get(position).getMouth(), 0, mDatas.get(position).getMouth().length(), mBounds);
c.drawText(mDatas.get(position).getMouth(), child.getPaddingLeft(), child.getTop() - params.topMargin - (mTitleHeight / 2 - mBounds.height() / 2), mPaint);
}

@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {//最后调用 绘制在最上层
int pos = ((LinearLayoutManager)(parent.getLayoutManager())).findFirstVisibleItemPosition();

String tag = mDatas.get(pos).getMouth();
//View child = parent.getChildAt(pos);
View child = parent.findViewHolderForLayoutPosition(pos).itemView;//出现一个奇怪的bug，有时候child为空，所以将 child = parent.getChildAt(i)。-》 parent.findViewHolderForLayoutPosition(pos).itemView
mPaint.setColor(COLOR_TITLE_BG);
c.drawRect(parent.getPaddingLeft(), parent.getPaddingTop(), parent.getRight() - parent.getPaddingRight(), parent.getPaddingTop() + mTitleHeight, mPaint);
mPaint.setColor(COLOR_TITLE_FONT);
mPaint.getTextBounds(tag, 0, tag.length(), mBounds);
c.drawText(tag, child.getPaddingLeft(),
parent.getPaddingTop() + mTitleHeight - (mTitleHeight / 2 - mBounds.height() / 2),
mPaint);
}

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
int position = ((RecyclerView.LayoutParams) view.getLayoutParams()).getViewLayoutPosition();
//我记得Rv的item position在重置时可能为-1.保险点判断一下吧
if (position > -1) {
if (position == 0) {//等于0肯定要有title的
outRect.set(0, mTitleHeight, 0, 0);
} else {//其他的通过判断
if (null != mDatas.get(position).getMouth() && !mDatas.get(position).getMouth().equals(mDatas.get(position - 1).getMouth())) {
outRect.set(0, mTitleHeight, 0, 0);//不为空 且跟前一个tag不一样了，说明是新的分类，也要title
} else {
outRect.set(0, 0, 0, 0);
}
}
}
}

}