RecyclerView添加分割线

RecyclerView 并没有 divider 属性，但是我们可以通过 RecyclerView 的 addItemDecoration() 来添加分割线，该方法参数为 RecyclerView.ItemDecoration。

介绍

当 RecyclerView 添加 ItemDecoration 后，RecyclerView 在绘制每个 item 的时候，会去绘制 decorator，也就是会调用 ItemDecoration 的 onDraw() 和 onDrawOver() 方法。

RecyclerView.ItemDecoration 是抽象类，主要提供三个方法：

onDraw(Canvas c, RecyclerView parent, State state)：在绘制item（drawChild）前调用
onDrawOver(Canvas c, RecyclerView parent, State state): 在绘制item（drawChild）后调用
getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)：outRect设置 item 的偏移量，用于绘制 decorator（也就是divider）

关于 getItemOffsets 函数：
RecyclerView 添加分割线，实际上就是 RecyclerView 的 item 之间添加了用作分割线的View，自然而然后续的 item 就会有偏移量，所以用 getItemOffsets 中的 outRect 来保存 item 的偏移量，从而便于绘制 decorator。

实现

实际上在当前版本的 RecyclerView (25.3.1) 中已经有 ItemDecoration 关于分割线的默认实现类 DividerItemDecoration:

package android.support.v7.widget;

import android.annotation.SuppressLint;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.support.annotation.NonNull;
import android.support.v4.view.ViewCompat;
import android.view.View;
import android.widget.LinearLayout;

public class DividerItemDecoration extends RecyclerView.ItemDecoration {
    public static final int HORIZONTAL = LinearLayout.HORIZONTAL;
    public static final int VERTICAL = LinearLayout.VERTICAL;

    // 如果不设置，则默认的分割线为 android.R.attr.listDivider 指定的 drawable
    private static final int[] ATTRS = new int[]{ android.R.attr.listDivider };

    private Drawable mDivider;

    /**
     * Current orientation. Either {@link #HORIZONTAL} or {@link #VERTICAL}.
     */
    private int mOrientation;

    private final Rect mBounds = new Rect();

    /**
     * Creates a divider {@link RecyclerView.ItemDecoration} that can be used with a
     * {@link LinearLayoutManager}.
     *
     * @param context Current context, it will be used to access resources.
     * @param orientation Divider orientation. Should be {@link #HORIZONTAL} or {@link #VERTICAL}.
     */
    public DividerItemDecoration(Context context, int orientation) {
        final TypedArray a = context.obtainStyledAttributes(ATTRS);
        mDivider = a.getDrawable(0);
        a.recycle();
        setOrientation(orientation);
    }

    /**
     * Sets the orientation for this divider. This should be called if
     * {@link RecyclerView.LayoutManager} changes orientation.
     *
     * @param orientation {@link #HORIZONTAL} or {@link #VERTICAL}
     */
    public void setOrientation(int orientation) {
        if (orientation != HORIZONTAL && orientation != VERTICAL) {
            throw new IllegalArgumentException(
                    "Invalid orientation. It should be either HORIZONTAL or VERTICAL");
        }
        mOrientation = orientation;
    }

    /**
     * Sets the {@link Drawable} for this divider.
     *
     * @param drawable Drawable that should be used as a divider.
     */
    public void setDrawable(@NonNull Drawable drawable) {
        if (drawable == null) {
            throw new IllegalArgumentException("Drawable cannot be null.");
        }
        mDivider = drawable;
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        if (parent.getLayoutManager() == null) {
            return;
        }
        if (mOrientation == VERTICAL) {
            drawVertical(c, parent);
        } else {
            drawHorizontal(c, parent);
        }
    }

    // 绘制 RecyclerView 为垂直布局时的分割线，此时分割线为水平分割线
    @SuppressLint("NewApi")
    private void drawVertical(Canvas canvas, RecyclerView parent) {
        canvas.save();
        final int left;
        final int right;
        // 需要考虑clipToPadding的boolean值
        if (parent.getClipToPadding()) {
            left = parent.getPaddingLeft();
            right = parent.getWidth() - parent.getPaddingRight();
            canvas.clipRect(left, parent.getPaddingTop(), right,
                    parent.getHeight() - parent.getPaddingBottom());
        } else {
            left = 0;
            right = parent.getWidth();
        }

        final int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            parent.getDecoratedBoundsWithMargins(child, mBounds);
            final int bottom = mBounds.bottom + Math.round(ViewCompat.getTranslationY(child));
            final int top = bottom - mDivider.getIntrinsicHeight();
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(canvas);
        }
        canvas.restore();
    }

    // 绘制 RecyclerView 为水平布局时的分割线，此时分割线为垂直分割线
    @SuppressLint("NewApi")
    private void drawHorizontal(Canvas canvas, RecyclerView parent) {
        canvas.save();
        final int top;
        final int bottom;
        // 需要考虑clipToPadding的boolean值
        if (parent.getClipToPadding()) {
            top = parent.getPaddingTop();
            bottom = parent.getHeight() - parent.getPaddingBottom();
            canvas.clipRect(parent.getPaddingLeft(), top,
                    parent.getWidth() - parent.getPaddingRight(), bottom);
        } else {
            top = 0;
            bottom = parent.getHeight();
        }

        final int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            parent.getLayoutManager().getDecoratedBoundsWithMargins(child, mBounds);
            final int right = mBounds.right + Math.round(ViewCompat.getTranslationX(child));
            final int left = right - mDivider.getIntrinsicWidth();
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(canvas);
        }
        canvas.restore();
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
            RecyclerView.State state) {
        if (mOrientation == VERTICAL) { // 垂直方向的RecyclerView, item 的 bottom 偏移量 = 分割线高度
            outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());
        } else { // 水平方向的RecyclerView， item 的 right 偏移量 = 分割线宽度
            outRect.set(0, 0, mDivider.getIntrinsicWidth(), 0);
        }
    }
}

在代码中添加：

recyclerView.addItemDecoration(new DividerItemDecoration(recyclerView.getContext(), mLayoutManager.getOrientation()));

就有了分割线。默认的分割线效果是系统自带的 listDivider 的效果，我们也可以在主题配置文件中自定义全局的分割线，或者调用 setDivider 为每个 RecyclerView 设置单独的分割线。

网络流行代码存在的问题

目前好多博客中关于 DividerItemDecoration 中 drawVertical() 和 drawHorizontal() 方法与官方的方法其实是有出入的：

public void drawVertical(Canvas c, RecyclerView parent) {
    final int left = parent.getPaddingLeft();
    final int right = parent.getWidth() - parent.getPaddingRight();
    // 网上的方法最主要的问题是没有考虑 clipToPadding 这个参数，所以说这里缺少相应代码片段

    final int childCount = parent.getChildCount();
    // 下面这块没什么问题，和官方方案殊途同归
    // 官方的getDecoratedBoundsWithMargins实际上也是通过 LayoutParams 来获取分割线边界的
    for (int i = 0; i < childCount; i++) {
        final View child = parent.getChildAt(i);
        final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
                .getLayoutParams();
        final int top = child.getBottom() + params.bottomMargin;
        final int bottom = top + mDivider.getIntrinsicHeight();
        mDivider.setBounds(left, top, right, bottom);
        mDivider.draw(c);
    }
}

public void drawHorizontal(Canvas c, RecyclerView parent) {
    final int top = parent.getPaddingTop();
    final int bottom = parent.getHeight() - parent.getPaddingBottom();

    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();
        final int left = child.getRight() + params.rightMargin;
        final int right = left + mDivider.getIntrinsicHeight();
        mDivider.setBounds(left, top, right, bottom);
        mDivider.draw(c);
    }
}

注释中说的比较明确了，网上好多方法主要的问题在于 没有考虑 clipToPadding 属性值 。 clipToPadding 表示控件的绘制区域是否在 padding 区域外面，其默认值为 true。比如，垂直方向的 RecyclerView，当 clipToPadding=false 时，其初始绘制区域与 padding 值有关，但向上滑动时，RecylerView 的 item 会滑到 padding 区域里面。

下面用示意图来进行解释，RecyclerView 的 paddingTop = 40dp, clipToPadding = false, 下图中白色区域为 paddingTop 区域：