网上的城市选择器很多，但还是亲自动手实现一下，效果如下图所示

Screenshot_2019-06-25-15-27-44.png

思路：使用RecyclerView的吸附式ItemDecoration（覆写onDrawOver方法），将分好组的城市的拼音首字母绘制到上面。触摸右侧的字母指示器IndicatorView控制RecyclerView滚动到哪个位置。
所以我们要解决的问题有：
1.RecyclerView吸附式ItemDdecoration
2.获取汉字拼音的首字母
3.根据触摸到的字母，指定Recycler View滚动到对应的位置
4.自定义View绘制“热门、A、B······Z”，重写绘制方法、测量方法，和触摸方法。

一：吸附式ItemDecoration。

在滑动的时候会依次调用onDraw和onDrawOver，其中onDraw是在ItemView的下层绘制，onDrawOver是在ItemView的上层绘制。可以在onDraw中给每一个ItemView绘制分割线，绘制区域是一个矩形，即ItemView的底边距顶部的高度，以及底边的offset。在DrawOver中绘制每一组的标题，标题的高度会在getItemOffsets中设置，为rect.top。标题位置分为三种情况，1.跟随Group第一个View移动。2.在顶部不动。3.顶部的标题被下一组的标题顶上去，即跟随该组最后一个View的底部移动。具体情况具体分析。

public class StickyItemDecoration extends RecyclerView.ItemDecoration {
    private final Paint mPaint;
    private final Paint mDividerPaint;
    private OnDrawOverListener mSticky;
    private int mGroupTitleHeight;

    public StickyItemDecoration(Context context, OnDrawOverListener sticky) {
        mSticky = sticky;
        mPaint = new Paint();
        mPaint.setStyle(Paint.Style.FILL);
        mPaint.setColor(Color.BLUE);
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setTextSize(dp2px(context, 16));
        mGroupTitleHeight = dp2px(context, 20);
        mDividerPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
    }

    @Override
    public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
        super.onDraw(c, parent, state);
        int childCount=parent.getChildCount();
        RecyclerView.LayoutManager manager = parent.getLayoutManager();
        for (int i = 0; i < childCount; i++) {
            View child=parent.getChildAt(i);
            int left=parent.getPaddingLeft() + manager.getLeftDecorationWidth(child);
            int right=parent.getWidth()-parent.getPaddingRight() - manager.getRightDecorationWidth(child);
            int top=child.getTop()-1;
            int bottom=child.getTop();
            //Item分割线
            c.drawRect(left,top,right,bottom,mDividerPaint);
        }
    }

    @Override
    public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
        super.onDrawOver(c, parent, state);
        RecyclerView.LayoutManager manager = parent.getLayoutManager();
        int childCount = parent.getChildCount();
        String preGroupName;
        String groupName = "";
        for (int i = 0; i < childCount; i++) {
            View child = parent.getChildAt(i);
            int childAdapterPosition = parent.getChildAdapterPosition(child);
            preGroupName = groupName;
            groupName = mSticky.getGroupName(childAdapterPosition);
            //和上一个标题做对比，不一样就需要绘制
            if (preGroupName == groupName) {
                continue;
            }
            int startX = parent.getPaddingLeft() + (manager != null ? manager.getLeftDecorationWidth(child) : 0);
            Paint.FontMetricsInt fontMetrics = mPaint.getFontMetricsInt();
            //标题跟随整个Group滑动
            if (child.getTop() > mGroupTitleHeight) {
                int startY = child.getTop() - (mGroupTitleHeight + fontMetrics.descent + fontMetrics.ascent) / 2;
                c.drawText(groupName, startX, startY, mPaint);
            } else {
                //位于顶部的GroupTitle，在顶部不用动，除非下边的标题顶上来了
                Log.e("TAG", "....." + childAdapterPosition);
                if (childAdapterPosition + 1 <= state.getItemCount()) {
                    //child后面的View的是下一组的第一个，并且这个child底部露出的高度已经小于标题高度了，这时候标题会被顶上去
                    if (mSticky.isGroupFirst(childAdapterPosition + 1) && child.getBottom() < mGroupTitleHeight) {
                        int startY = (child.getBottom() - (mGroupTitleHeight + fontMetrics.descent + fontMetrics.ascent) / 2);
                        c.drawText(groupName, startX, startY, mPaint);
                    } else {
                        //在顶部不用动
                        int startY = (mGroupTitleHeight - fontMetrics.descent - fontMetrics.ascent) / 2;
                        c.drawText(groupName, startX, startY, mPaint);
                    }
                }
            }
        }
    }

    @Override
    public void getItemOffsets(@NonNull Rect outRect, @NonNull View view, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        Log.e("TAG", "getItemOffsets");
        int childAdapterPosition = parent.getChildAdapterPosition(view);
        if (mSticky.isGroupFirst(childAdapterPosition)) outRect.top = mGroupTitleHeight;
        outRect.bottom = 1;
    }

    public interface OnDrawOverListener {
        String getGroupName(int position);

        boolean isGroupFirst(int position);
    }

    public static int dp2px(Context context, int dp) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, context.getResources().getDisplayMetrics());
    }
}

二：获取汉字拼音的首字

https://www.cnblogs.com/pxblog/p/10604003.html

import java.io.UnsupportedEncodingException;

/**
 * 取得给定汉字串的首字母串,即声母串
 * Title: ChineseCharToEn
 *
 * @date 注：只支持GB2312字符集中的汉字
 */
public final class ChineseCharToEn {
    private final static int[] li_SecPosValue = {1601, 1637, 1833, 2078, 2274,
            2302, 2433, 2594, 2787, 3106, 3212, 3472, 3635, 3722, 3730, 3858,
            4027, 4086, 4390, 4558, 4684, 4925, 5249, 5590};
    private final static String[] lc_FirstLetter = {"a", "b", "c", "d", "e",
            "f", "g", "h", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s",
            "t", "w", "x", "y", "z"};

    /**
     * 取得给定汉字串的首字母串,即声母串
     *
     * @param str 给定汉字串
     * @return 声母串
     */
    public static String getAllFirstLetter(String str) {
        if (str == null || str.trim().length() == 0) {
            return "";
        }

        String _str = "";
        for (int i = 0; i < str.length(); i++) {
            _str = _str + getFirstLetter(str.substring(i, i + 1));
        }

        return _str;
    }

    /**
     * 取得给定汉字的首字母,即声母
     *
     * @param chinese 给定的汉字
     * @return 给定汉字的声母
     */
    public static String getFirstLetter(String chinese) {
        if (chinese == null || chinese.trim().length() == 0) {
            return "";
        }
        chinese = conversionStr(chinese, "GB2312", "ISO8859-1");

        if (chinese.length() > 1) // 判断是不是汉字
        {
            int li_SectorCode = (int) chinese.charAt(0); // 汉字区码
            int li_PositionCode = (int) chinese.charAt(1); // 汉字位码
            li_SectorCode = li_SectorCode - 160;
            li_PositionCode = li_PositionCode - 160;
            int li_SecPosCode = li_SectorCode * 100 + li_PositionCode; // 汉字区位码
            if (li_SecPosCode > 1600 && li_SecPosCode < 5590) {
                for (int i = 0; i < 23; i++) {
                    if (li_SecPosCode >= li_SecPosValue[i]
                            && li_SecPosCode < li_SecPosValue[i + 1]) {
                        chinese = lc_FirstLetter[i];
                        break;
                    }
                }
            } else // 非汉字字符,如图形符号或ASCII码
            {
                chinese = conversionStr(chinese, "ISO8859-1", "GB2312");
                chinese = chinese.substring(0, 1);
            }
        }

        return chinese;
    }

    /**
     * 字符串编码转换
     *
     * @param str           要转换编码的字符串
     * @param charsetName   原来的编码
     * @param toCharsetName 转换后的编码
     * @return 经过编码转换后的字符串
     */
    private static String conversionStr(String str, String charsetName, String toCharsetName) {
        try {
            str = new String(str.getBytes(charsetName), toCharsetName);
        } catch (UnsupportedEncodingException ex) {
            System.out.println("字符串编码转换异常：" + ex.getMessage());
        }
        return str;
    }

    public static void main(String[] args) {
        System.out.println("获取拼音首字母：" + getAllFirstLetter("大中国南昌中大china"));
    }
}

三：RecyclerView滚动到指定位置pos

https://www.jianshu.com/p/6d5ecfdbb615

四：自定义字母指示器IndicatorView

1.绘制data中的字符串：热门、A、B、C······Z
2.测量大小onMeasure
3.重写onTouch方法，通过触摸的位置的坐标，计算出触摸的是哪个字母，并传入回调接口中

package com.app.cityselector.view;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.util.DisplayMetrics;
import android.view.MotionEvent;
import android.view.View;

import java.util.List;

public class IndicatorView extends View {

    private Context context;
    private List<String> data;
    private Paint paint;
    private float ascent;
    private float descent;
    private float textHeight;
    private float textGap;
    private float charWidth;
    private int paddingLeft;
    private int paddingRight;

    public IndicatorView(Context context) {
        this(context, null);
    }

    public IndicatorView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public IndicatorView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        this.context = context;
        initView();
    }

    private void initView() {
        paint = new Paint();
        DisplayMetrics metrics = context.getResources().getDisplayMetrics();
        paint.setTextSize(9 * metrics.density);
        ascent = paint.getFontMetrics().ascent;
        descent = paint.getFontMetrics().descent;
        textHeight = Math.abs(ascent - descent);
        charWidth = paint.measureText("A");
    }

    public void setData(List<String> data) {
        this.data = data;
        invalidate();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        float y = 0;
        y = -ascent + textGap / 2;
        float x = (getWidth() - paddingLeft - paddingRight - charWidth) / 2;
        for (int i = 0; i < data.size(); i++) {
            canvas.drawText(data.get(i), i == 0 ? 0 : x, y, paint);
            y += textHeight + textGap;
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);

        int width = widthSize;
        int height = heightSize;
        paddingLeft = getPaddingLeft();
        paddingRight = getPaddingRight();
        //wrap_content:计算得出最小的宽高，
        //match_content或具体值:撑满父容器，空出来的地方作为item间隔平均分布到item之间
        if (widthMode == MeasureSpec.EXACTLY) {
            width = widthSize;
        } else {
            width = (int) paint.measureText("热门") + paddingLeft + paddingRight;
        }
        if (heightMode == MeasureSpec.EXACTLY) {
            height = heightSize;
            textGap = (height - textHeight * data.size()) / data.size();
        } else {
            height = (int) (Math.abs(paint.getFontMetrics().ascent - paint.getFontMetrics().descent) * data.size());
        }
        setMeasuredDimension(width, height);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float y = event.getY();
        int index = (int) (y / (textGap + textHeight));
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
            case MotionEvent.ACTION_MOVE:
                if (onItemTouched != null) {
                    onItemTouched.onTouched(data.get(index), index);
                }
                break;
            case MotionEvent.ACTION_UP:
                if (onItemTouched != null) {
                    onItemTouched.onTouchedUp(data.get(index), index);
                }
                break;
        }
        return true;
    }

    OnItemTouched onItemTouched;

    public void setOnItemTouched(OnItemTouched onItemTouched) {
        this.onItemTouched = onItemTouched;
    }

    public interface OnItemTouched {
        void onTouched(String s, int pos);

        void onTouchedUp(String s, int pos);
    }

}

以上三步做好后，就完成了准备工作。
CitySelectorView中使用RecyclerView展示数据，根据右侧的字母指示器指定RecyclerView滚动到指定pos。
实体类：

public class CityBean {
    private int id;
    private String code;
    private String name;
    //getter and setter
    ......
}

CityAdapter:展示的数据有热门城市和普通城市Item，所以要区分两类itemType

package com.app.cityselector.view.adapter;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.util.TypedValue;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.TextView;

import com.app.cityselector.R;
import com.app.cityselector.bean.CityBean;

import java.util.List;

public class CityAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> {
    public static final int TYPE_HOT_CITY = 1;
    public static final int TYPE_CITY_ITEM = 2;
    private final LayoutInflater inflater;

    Context context;
    //热门城市
    List<CityBean> hotCitys;
    //全部城市
    List<CityBean> allCitys;

    public CityAdapter(Context context) {
        this.context = context;
        inflater = LayoutInflater.from(context);
    }

    public void setHotCitys(List<CityBean> hotCitys) {
        this.hotCitys = hotCitys;
    }

    public void setAllCitys(List<CityBean> allCitys) {
        this.allCitys = allCitys;
    }

    @Override
    public int getItemViewType(int position) {
        if (hotCitys != null && position == 0) {
            return TYPE_HOT_CITY;
        } else {
            return TYPE_CITY_ITEM;
        }
    }

    @NonNull
    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int type) {
        if (type == TYPE_HOT_CITY) {
            return new HotCityViewHolder(inflater.inflate(R.layout.item_hot_city, viewGroup, false));
        } else if (type == TYPE_CITY_ITEM) {
            return new CityItemViewHolder(inflater.inflate(R.layout.item_city, viewGroup, false));
        }
        return null;
    }

    @Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder viewHolder, int i) {
        if (viewHolder instanceof HotCityViewHolder) {
            HotCityViewHolder hotCityViewHolder = (HotCityViewHolder) viewHolder;
            hotCityViewHolder.bindData(hotCitys);
        } else if (viewHolder instanceof CityItemViewHolder) {
            CityItemViewHolder cityItemViewHolder = (CityItemViewHolder) viewHolder;
            CityBean cityBean = allCitys.get(hotCitys != null ? i - 1 : i);
            cityItemViewHolder.bindData(cityBean);
        }
    }

    @Override
    public int getItemCount() {
        int count = 0;
        if (hotCitys != null) {
            count++;
        }
        if (allCitys != null) {
            count += allCitys.size();
        }
        return count;
    }

    static class CityItemViewHolder extends RecyclerView.ViewHolder {

        private TextView tvCityName;

        public CityItemViewHolder(@NonNull View itemView) {
            super(itemView);
            tvCityName = itemView.findViewById(R.id.tv_city_name);
        }

        public void bindData(CityBean cityBean) {
            tvCityName.setText(cityBean.getName());
        }
    }

    static class HotCityViewHolder extends RecyclerView.ViewHolder {

        private LinearLayout llHotCityContainer;
        //热门城市的列数
        int hotColumn = 3;

        public void setHotColumn(int hotColumn) {
            this.hotColumn = hotColumn;
        }

        public HotCityViewHolder(@NonNull View itemView) {
            super(itemView);
            llHotCityContainer = itemView.findViewById(R.id.gl_hot_city_container);
        }
        //展示热门城市
        public void bindData(List<CityBean> cityBeans) {
            llHotCityContainer.removeAllViews();
            Context context = itemView.getContext();
            int halfMargin = context.getResources().getDimensionPixelSize(R.dimen.base_margin_half);
            int paddingVertical = context.getResources().getDimensionPixelSize(R.dimen.padding_vertical);
            int row = cityBeans.size() / hotColumn;
            for (int i = 0; i < row + 1; i++) {
                LinearLayout llRow = new LinearLayout(context);
                llRow.setLayoutParams(new LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT));
                llRow.setPadding(halfMargin, halfMargin, halfMargin, halfMargin);
                for (int j = 0; j < hotColumn; j++) {
                    int index = row * i + j;
                    if (index < cityBeans.size()) {
                        CityBean cityBean = cityBeans.get(index);
                        TextView textView = new TextView(context);
                        textView.setText(cityBean.getName());
                        textView.setBackgroundResource(R.drawable.bg_hot_city);
                        textView.setClickable(true);
                        textView.setPadding(0, paddingVertical, 0, paddingVertical);
                        textView.setGravity(Gravity.CENTER);
                        textView.setTextColor(context.getResources().getColor(R.color.colorText));
                        textView.setTextSize(TypedValue.COMPLEX_UNIT_SP, 14);
                        //setLayoutParams
                        LinearLayout.LayoutParams textLayoutParams = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT);
                        textLayoutParams.weight = 1;
                        textLayoutParams.leftMargin = halfMargin;
                        textLayoutParams.rightMargin = halfMargin;
                        textView.setLayoutParams(textLayoutParams);
                        llRow.addView(textView);
                    }
                }
                llHotCityContainer.addView(llRow);
            }
        }
    }
}

重点来了，当触摸右边的字母指示器时，根据字母获取RecyclerView中的城市名称的拼音的出现该字母的第一个位置，如果找不到，就定位到上一个字母出现的位置。

    /**
     * 根据ABC...Z获取第一次出现的pos
     *
     * @param s
     * @return
     */
    private int getFirstPos(String s) {
        if ("热门".equals(s)) {
            return 0;
        }
        int pos = 0;
        if (hotCity != null) {
            pos++;
        }
        int firstPos = 0;
        for (int i = 0; i < allCity.size(); i++) {
            //相等
            String firstLetter = ChinessToEn.getFirstLetter(allCity.get(i).getName()).toUpperCase();
            int compare = firstLetter.compareToIgnoreCase(s);
            if (compare == 0) {
                firstPos = i;
                break;
            } else if (compare < 0) {
                if (i > 1 && !allCity.get(i).getName().equals(allCity.get(i - 1).getName())) {
                    firstPos = i;
                }
            } else {
                break;
            }
        }
        pos += firstPos;
        return pos;
    }

获取到位置后，就可以混动RecyclerView了

 public static void moveToPosition(LinearLayoutManager manager, RecyclerView mRecyclerView, int n) {
        int firstItem = manager.findFirstVisibleItemPosition();
        int lastItem = manager.findLastVisibleItemPosition();
        if (n <= firstItem) {
            mRecyclerView.scrollToPosition(n);
        } else if (n <= lastItem) {
            int top = mRecyclerView.getChildAt(n - firstItem).getTop();
            mRecyclerView.scrollBy(0, top);
        } else {
            mRecyclerView.scrollToPosition(n);
        }
    }

CitySelectorView完成代码：

public class CitySelectorView extends FrameLayout implements View.OnClickListener {

    private Context context;
    private RecyclerView recyclerView;
    private IndicatorView indicatorView;
    private TextView tvCenter;

    private LinearLayoutManager layoutManager;
    private CityAdapter adapter;
    private List<CityBean> hotCity;
    private List<CityBean> allCity;
    private String[] indecatorData = new String[]{"热门", "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"};

    public void setHotCity(List<CityBean> hotCity) {
        this.hotCity = hotCity;
    }

    public void setAllCity(List<CityBean> allCity) {
        this.allCity = allCity;
    }

    public void notifyDataSetChanged() {
        adapter.setHotCitys(hotCity);
        adapter.setAllCitys(allCity);
        adapter.notifyDataSetChanged();
    }

    public CitySelectorView(@NonNull Context context) {
        this(context, null);
    }

    public CitySelectorView(@NonNull Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public CitySelectorView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        this.context = context;
        init();
    }

    private void init() {
        LayoutInflater.from(context).inflate(R.layout.view_city_selector, this, true);

        recyclerView = findViewById(R.id.recycler_city);
        tvCenter = findViewById(R.id.tv_center);
        indicatorView = findViewById(R.id.indicator);
        //set RecyclerView
        layoutManager = new LinearLayoutManager(context);
        recyclerView.setLayoutManager(layoutManager);
        //吸附式
        recyclerView.addItemDecoration(new StickyItemDecoration(context, new ISticky() {
            @Override
            public boolean isGroupFirst(int pos) {
                if (hotCity != null) {
                    if (pos == 0) return true;
                    if (allCity != null && allCity.size() != 0) {
                        if (pos == 1) {
                            return true;
                        } else {
                            return !ChinessToEn.getFirstLetter(allCity.get(pos - 1).getName()).equals(ChinessToEn.getFirstLetter(allCity.get(pos - 2).getName()));
                        }
                    }
                    return false;
                } else {
                    if (pos == 0) return true;
                    if (allCity != null && allCity.size() != 0) {
                        return !ChinessToEn.getFirstLetter(allCity.get(pos).getName()).equals(ChinessToEn.getFirstLetter(allCity.get(pos - 1).getName()));
                    }
                    return false;
                }
            }

            @Override
            public String getGroupTitle(int pos) {
                if (hotCity != null) {
                    if (pos == 0) return "热门";
                    if (allCity != null && allCity.size() != 0) {
                        return ChinessToEn.getFirstLetter(allCity.get(pos - 1).getName()).toUpperCase();
                    }
                } else {
                    if (allCity != null && allCity.size() != 0) {
                        return ChinessToEn.getFirstLetter(allCity.get(pos).getName()).toUpperCase();
                    }
                }
                return null;
            }
        }));
        adapter = new CityAdapter(context);
        recyclerView.setAdapter(adapter);

        indicatorView.setData(Arrays.asList(indecatorData));
        indicatorView.setOnItemTouched(new IndicatorView.OnItemTouched() {
            @Override
            public void onTouched(String s, int pos) {
                tvCenter.setVisibility(VISIBLE);
                tvCenter.setText(s);
                int firstPos = getFirstPos(s);
                L.e(firstPos);
                moveToPosition(layoutManager, recyclerView, firstPos);
            }

            @Override
            public void onTouchedUp(String s, int pos) {
                tvCenter.setVisibility(INVISIBLE);
            }
        });
    }

    @Override
    public void onClick(View v) {

    }

    /**
     * 根据ABC...Z获取第一次出现的pos
     *
     * @param s
     * @return
     */
    private int getFirstPos(String s) {
        if ("热门".equals(s)) {
            return 0;
        }
        int pos = 0;
        if (hotCity != null) {
            pos++;
        }
        int firstPos = 0;
        for (int i = 0; i < allCity.size(); i++) {
            //相等
            String firstLetter = ChinessToEn.getFirstLetter(allCity.get(i).getName()).toUpperCase();
            int compare = firstLetter.compareToIgnoreCase(s);
            if (compare == 0) {
                firstPos = i;
                break;
            } else if (compare < 0) {
                if (i > 1 && !allCity.get(i).getName().equals(allCity.get(i - 1).getName())) {
                    firstPos = i;
                }
            } else {
                break;
            }
        }
        pos += firstPos;
        return pos;
    }

    /**
     * RecyclerView 移动到当前位置，
     *
     * @param manager       设置RecyclerView对应的manager
     * @param mRecyclerView 当前的RecyclerView
     * @param n             要跳转的位置
     */
    public static void moveToPosition(LinearLayoutManager manager, RecyclerView mRecyclerView, int n) {
        int firstItem = manager.findFirstVisibleItemPosition();
        int lastItem = manager.findLastVisibleItemPosition();
        if (n <= firstItem) {
            mRecyclerView.scrollToPosition(n);
        } else if (n <= lastItem) {
            int top = mRecyclerView.getChildAt(n - firstItem).getTop();
            mRecyclerView.scrollBy(0, top);
        } else {
            mRecyclerView.scrollToPosition(n);
        }
    }
}