android用ItemDecoration实现通讯录

本来因为代码是抄的不太想自己写一篇博客，但是用的过程中出现了意外，不知道为什么粘性头部效果完全不对，无奈之下只好重新写了。特此记录：
getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state):用于设置item之间的距离，onDraw只能画在这个距离内，多出的部分不可见
onDraw(Canvas c, RecyclerView parent, RecyclerView.State state)绘制divider，只能画在getItemOffsets之内。
onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) 绘制在recyclerview上面，不影响item间的距离，也不受getItemOffsets影响。

所以粘性头部是onDrawOver部分的代码。

参考自：​​https://www.jianshu.com/p/b46a4ff7c10a​​​
SectionDecoration 内除了onDrawOver的部分，其他部分来源于这里。效果就不贴了，和参考的图差不多。
以下为完整代码：

public class SectionDecoration extends RecyclerView.ItemDecoration
    private static final String TAG = "SectionDecoration";

    private DecorationCallback callback;
    private TextPaint textPaint;
    private Paint paint;
    private int topGap;
    private Paint.FontMetrics fontMetrics;

    public SectionDecoration(Context context, DecorationCallback decorationCallback) {
        Resources res = context.getResources();
        this.callback = decorationCallback;

        paint = new Paint();
        paint.setColor(res.getColor(R.color.colorAccent));
        textPaint = new TextPaint();
        textPaint.setTypeface(Typeface.DEFAULT_BOLD);
        textPaint.setAntiAlias(true);
        textPaint.setTextSize(80);
        textPaint.setColor(Color.BLACK);
        textPaint.getFontMetrics(fontMetrics);
        textPaint.setTextAlign(Paint.Align.LEFT);
        fontMetrics = new Paint.FontMetrics();
        topGap = res.getDimensionPixelSize(R.dimen.sectioned_top);//32dp


    }


    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        int pos = parent.getChildAdapterPosition(view);
        if (isFirstInGroup(pos)) {//同组的第一个才添加padding
            outRect.top = topGap;
        } else {
            outRect.top = 0;
        }
    }

    private int charLeft = 20;

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();
        int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            View view = parent.getChildAt(i);
            int position = parent.getChildAdapterPosition(view);
            String textLine = callback.getFirstCharacter(position).toUpperCase();
            if (position == 0 || isFirstInGroup(position)) {
                float top = view.getTop() - topGap;
                float bottom = view.getTop();
                Rect bounds = new Rect();
                String str = callback.getFirstCharacter(position).toUpperCase();
                textPaint.getTextBounds(str, 0, str.length(), bounds);
                c.drawRect(left, top, right, bottom, paint);//绘制红色矩形
                c.drawText(textLine, left + charLeft, top + (topGap + bounds.height()) / 2.0f, textPaint);//绘制文本
            }
        }
    }


    private boolean isFirstInGroup(int pos) {
        if (pos == 0) {
            return true;
        } else {
            String str1 = callback.getFirstCharacter(pos).toLowerCase();
            String str2 = callback.getFirstCharacter(pos - 1).toLowerCase();
            return !str1.equals(str2);
        }
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);
        int pos = ((LinearLayoutManager)(parent.getLayoutManager())).findFirstVisibleItemPosition();
        View child = parent.findViewHolderForLayoutPosition(pos).itemView;
        int viewBottom = child.getBottom();
        if(viewBottom<topGap){//如果当前view底部距离固定头高度小于悬浮部分高度，则取下一个view的首字母
            ++pos;
            child = parent.findViewHolderForLayoutPosition(pos).itemView;
        }


        paint.setColor(parent.getResources().getColor(R.color.colorAccent));
        Rect bounds = new Rect();
        String str = callback.getFirstCharacter(pos).toUpperCase();

        int paddingTop = parent.getPaddingTop();
        c.drawRect(parent.getPaddingLeft(), paddingTop, parent.getRight() - parent.getPaddingRight(), paddingTop + topGap, paint);
        textPaint.getTextBounds(str, 0, str.length(), bounds);
        c.drawText(callback.getFirstCharacter(pos).toUpperCase(), charLeft+child.getPaddingLeft(),
                parent.getPaddingTop() + topGap - (topGap / 2 - bounds.height() / 2),
                textPaint);

    }

    public interface DecorationCallback
        String getFirstCharacter(int

AppCompatActivity.java

public class DecorationActivity extends AppCompatActivity

    private RecyclerView recyclerView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_decoration);

        recyclerView = (RecyclerView) findViewById(R.id.recycler_decoration);

        initData();
        MyRecyclerAdapter recycleAdapter = new MyRecyclerAdapter(DecorationActivity.this, mDatas);
        LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        //设置布局管理器
        recyclerView.setLayoutManager(layoutManager);
        //设置为垂直布局，这也是默认的
        layoutManager.setOrientation(OrientationHelper.VERTICAL);
        //设置Adapter
        recyclerView.setAdapter(recycleAdapter);
        //设置增加或删除条目的动画
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        //距离
//        recyclerView.addItemDecoration(new SimplePaddingDecoration(this));
        recyclerView.addItemDecoration(new SimpleDividerDecoration(this));
        recyclerView.addItemDecoration(new SectionDecoration(this, new SectionDecoration.DecorationCallback() {
            @Override
            public String getFirstCharacter(int position) {
                String contentLine = mDatas.get(position);
                if (contentLine != null && contentLine.length() > 0)
                    return contentLine.substring(0, 1);
                return "";
            }
        }));
//        recyclerView.addItemDecoration(new LeftAndRightTagDecoration(this));
//        recyclerView.addItemDecoration(new LinearLayoutColorDivider(getResources(),R.color.white,R.dimen.divider_height,LinearLayoutManager.VERTICAL));
    }

    private List<String> mDatas;

    private void initData() {
        mDatas = new ArrayList<String>();
        for (char i = 'a'; i <= 'z'; i++) {
            for (int j = 0; j < 3; j++) {
                mDatas.add(i + ""

SimpleDividerDecoration.java

package com.dyy.yonxin.library2.test.decoration;

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

import com.dyy.yonxin.library2.test.R;

public class SimpleDividerDecoration extends RecyclerView.ItemDecoration

    private int dividerHeight;
    private Paint dividerPaint;

    public SimpleDividerDecoration(Context context) {
        dividerPaint = new Paint();
        dividerPaint.setColor(context.getResources().getColor(R.color.black));
        dividerHeight = context.getResources().getDimensionPixelSize(R.dimen.divider_height);
    }


    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);
        int childCount = parent.getChildCount();
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();

        for (int i = 0; i < childCount-1; i++) {
            View view = parent.getChildAt(i);
            float top = view.getBottom();
            float