通过ItemDecoration实现RecyclerView分割线

RecyclerView分割线可以有多种实现，最简单的是父view底色+子view差色实现。这里主要用ItemDecoration方式来实现，这样的实现更加具有差异化的可定制性。

创建布局文件

创建布局文件：activity_recycler_view.xml

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/rv_vertical"
    android:layout_width="match_parent"
    android:layout_height="200dp" />

创建item布局文件： item_activity_recycler_vertical_view.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/tv_item"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:padding="10dp"
        tools:text="1" />
</FrameLayout>

无分割线列表

构造数据

首先构造adapter所需的数据源

private final List<String> mItemList = new ArrayList<>();

private void initData() {
    for (int i = 0; i < 100; i++) {
        mItemList.add(String.valueOf(i));
    }
}

声明Adapter类

Adapter采用static内部类+弱引用，避免内存泄漏。

private static class Adapter extends RecyclerView.Adapter<Adapter.Holder> {
    private final List<String> mItemList = new ArrayList<>();
    private final WeakReference<RecyclerViewActivity> weakReference;
    private final int mLayoutResId;
    public Adapter(RecyclerViewActivity activity, List<String> items, @LayoutRes int layoutResID) {
        this.weakReference = new WeakReference<>(activity);
        setData(items);
        mLayoutResId = layoutResID;
    }
    public void setData(List<String> items) {
        if (items != null && !items.isEmpty()) {
            this.mItemList.addAll(items);
        }
    }
    @NonNull
    @Override
    public Holder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        RecyclerViewActivity activity = weakReference.get();
        if (activity == null) {
            return null;
        } else {
            View view = LayoutInflater.from(activity).inflate(mLayoutResId, parent, false);
            return new Holder(view);
        }
    }
    @Override
    public void onBindViewHolder(@NonNull Holder holder, int position) {
        holder.mTvItem.setText(mItemList.get(position));
    }
    @Override
    public int getItemCount() {
        return mItemList.size();
    }
    private static class Holder extends RecyclerView.ViewHolder {
        private final TextView mTvItem;
        public Holder(@NonNull View itemView) {
            super(itemView);
            mTvItem = itemView.findViewById(R.id.tv_item);
        }
    }
}

初始化RecyclerView

编写一个纵向的列表，其中item_activity_recycler_vertical_view.xml为一个简单的TextView

private RecyclerView mRvVertical;

private void initView() {
    mRvVertical = findViewById(R.id.rv_vertical);
    mRvVertical.setLayoutManager(new LinearLayoutManager(this, RecyclerView.VERTICAL, false));
    mRvVertical.setAdapter(new Adapter(this, mItemList, R.layout.item_activity_recycler_vertical_view));
    ...
}

通过以上几步先看看到运行后结果吧：

往列表中绘制分割线

上面已经实现了一个简单的列表，先将在此基础上，进行增加分割线操作。

ItemDecoration

ItemDecoration: 可以给指定的item添加drawing和边距，常用于绘制分割线、高亮、边距等。其中我认为重要的方法有两个：

public void onDraw( 
    @NonNull Canvas c,
    @NonNull RecyclerView parent,
    @NonNull RecyclerView.State state 
)

public void onDrawOver( 
    @NonNull Canvas c,
    @NonNull RecyclerView parent,
    @NonNull RecyclerView.State state 
)

这两个方法都可以进行绘制，区别在于绘制的顺序，绘制先后层级关系是：onDraw->draw item views->onDrawOver。

一个简答的例子

下面写个例子看看效果：

private void initView() {
    mRvVertical = findViewById(R.id.rv_vertical);
    mRvVertical.setLayoutManager(new LinearLayoutManager(this, RecyclerView.VERTICAL, false));
    mRvVertical.setAdapter(new Adapter(this, mItemList, R.layout.item_activity_recycler_vertical_view));
    mRvVertical.addItemDecoration(new RecyclerViewItemDecoration());
    ...
}

private static class RecyclerViewItemDecoration extends RecyclerView.ItemDecoration {
        private final Paint mPaint = new Paint();
        private float mDividerSize = 0f;

        public RecyclerViewItemDecoration() {
            Resources resources = MyApp.getInstance().getResources();
            mPaint.setColor(resources.getColor(R.color.recycler_view_divider));
            mDividerSize = resources.getDimension(R.dimen.recycler_view_divider_size);
        }

        @Override
        public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
            for (int i = 0; i < parent.getChildCount(); i++) {
                View child = parent.getChildAt(i);
                c.drawRect(child.getLeft(), child.getBottom() - mDividerSize, child.getRight(), child.getBottom(), mPaint);
            }
        }

        @Override
        public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
            Paint p = new Paint();
            p.setColor(Color.BLACK);
            p.setTextSize(50);
            c.drawText("纵向列表", 100, 100, p);
        }
}

从运行结果中可以看出2点：

onDraw方法在每个item view的底部，通过drawRect成功绘制了分割线；
onDrawOver方法将文字“纵向列表”绘制在了分割线和item层级之上。

绘制较复杂的分割线

上面的例子中，我们绘制了一个简单的纵向列表分割线，下面我们来绘制横向和网格分割线：

java关键代码：

private void initView() {
        mRvVertical = findViewById(R.id.rv_vertical);
        mRvHorizontal = findViewById(R.id.rv_horizontal);
        mRvGrid = findViewById(R.id.rv_grid);

        mRvVertical.setLayoutManager(new LinearLayoutManager(this, RecyclerView.VERTICAL, false));
        mRvHorizontal.setLayoutManager(new LinearLayoutManager(this, RecyclerView.HORIZONTAL, false));
        mRvGrid.setLayoutManager(new GridLayoutManager(this, 4, RecyclerView.VERTICAL, false));

        mRvVertical.setAdapter(new Adapter(this, mItemList, R.layout.item_activity_recycler_vertical_view));
        mRvHorizontal.setAdapter(new Adapter(this, mItemList, R.layout.item_activity_recycler_horizontal_view));
        mRvGrid.setAdapter(new Adapter(this, mItemList, R.layout.item_activity_recycler_grid_view));

        mRvVertical.addItemDecoration(new RecyclerViewItemDecoration(RecyclerViewItemDecoration.VERTICAL));
        mRvHorizontal.addItemDecoration(new RecyclerViewItemDecoration(RecyclerViewItemDecoration.HORIZONTAL));
        mRvGrid.addItemDecoration(new RecyclerViewItemDecoration(RecyclerViewItemDecoration.GRID));
}

private static class RecyclerViewItemDecoration extends RecyclerView.ItemDecoration {
        public static final int HORIZONTAL = LinearLayout.HORIZONTAL;
        public static final int VERTICAL = LinearLayout.VERTICAL;
        public static final int GRID = 2;

        private final int mOrientation;
        private final Paint mPaint = new Paint();
        private float mDividerSize = 0f;

        public RecyclerViewItemDecoration(int orientation) {
            mOrientation = orientation;
            Resources resources = MyApp.getInstance().getResources();
            mPaint.setColor(resources.getColor(R.color.recycler_view_divider));
            mDividerSize = resources.getDimension(R.dimen.recycler_view_divider_size);
        }

        @Override
        public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
            for (int i = 0; i < parent.getChildCount(); i++) {
                View child = parent.getChildAt(i);
                if (mOrientation == VERTICAL) {
                    c.drawRect(child.getLeft(), child.getBottom() - mDividerSize, child.getRight(), child.getBottom(), mPaint);
                } else if (mOrientation == HORIZONTAL) {
                    c.drawRect(child.getRight() - mDividerSize, child.getTop(), child.getRight(), child.getBottom(), mPaint);
                } else if (mOrientation == GRID) {
                    //左
                    c.drawRect(child.getLeft(), child.getTop(), child.getLeft() + mDividerSize, child.getBottom(), mPaint);
                    //上
                    c.drawRect(child.getLeft(), child.getTop(), child.getRight(), child.getTop() + mDividerSize, mPaint);
                    //右
                    c.drawRect(child.getRight() - mDividerSize, child.getTop(), child.getRight(), child.getBottom(), mPaint);
                    //下
                    c.drawRect(child.getLeft(), child.getBottom() - mDividerSize, child.getRight(), child.getBottom(), mPaint);
                }
            }

            Paint p = new Paint();
            p.setColor(Color.BLACK);
            p.setTextSize(50);
            if (mOrientation == VERTICAL) {
                c.drawText("纵向列表", 100, 100, p);
            } else if (mOrientation == HORIZONTAL) {
                c.drawText("横向列表", 100, 100, p);
            } else if (mOrientation == GRID) {
                c.drawText("表格列表", 100, 100, p);
            }
            super.onDraw(c, parent, state);
        }

        @Override
        public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
            super.onDrawOver(c, parent, state);
            Paint p = new Paint();
            p.setColor(Color.RED);
            for (int i = 0; i < parent.getChildCount(); i++) {
                View child = parent.getChildAt(i);
                if (mOrientation == HORIZONTAL) {
                    c.drawRect(child.getRight() - mDividerSize, child.getTop(), child.getRight(), child.getBottom(), p);
                }
            }
        }
}

布局文件：activity_recycler_view.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.appcompat.widget.LinearLayoutCompat xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".RecyclerViewActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rv_vertical"
        android:layout_width="match_parent"
        android:layout_height="200dp" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rv_horizontal"
        android:layout_width="match_parent"
        android:layout_height="200dp" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rv_grid"
        android:layout_width="match_parent"
        android:layout_height="200dp" />
</androidx.appcompat.widget.LinearLayoutCompat>

item_activity_recycler_grid_view.xml:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/tv_item"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:padding="10dp"
        tools:text="1" />
</FrameLayout>

item_activity_recycler_horizontal_view.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/tv_item"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:gravity="center"
        android:padding="10dp"
        tools:text="1" />
</FrameLayout>

item_activity_recycler_vertical_view.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/tv_item"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:padding="10dp"
        tools:text="1" />
</FrameLayout>