前言

最近由於需求問題，需要寫一個列表。以前總是使用ListView，從來沒有用過RecyclerView。所以這次打算嘗試一下。然後就開始動手幹活了。RecyclerView佈局寫好了。然後寫adapter，一切正常。

發現沒有分割線有點難看。準備去佈局檔案裡面寫分割線，發現居然沒有divider這個屬性。後來百度了一下，原來RecyclerView設定分割線是程式碼中新增的。

recyclerView.addItemDecoration(new DividerItemDecoration(context,DividerItemDecoration.VERTICAL));

現在Google已經內建了DividerItemDecoration類，以前是沒有這個類的，需要我們自己寫，現在越來越方便了。第一個引數需要傳入Context，第二個引數是分割線的型別，我這個是表示vertical。當然，如果你有需要你完全可以設定為horizontal。你以為這樣就設定好分割線了嗎？答案顯而易見，沒有。

接下來，你需要在res目錄下drawable的資料夾裡面新建一個divider.xml檔案。名字你可以按你自己的喜好來設定。

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <solid android:color="@color/gray"/>
    <size android:height="0.1dp"/>
</shape
 
>

我這裡gray是#D0D0D0，大家也可以自己修改。

再然後，你需要在values資料夾裡面的styles檔案裡面新增這樣一行程式碼。

        <item name="android:listDivider">@drawable/divider</item>

好，寫到這裡。大功告成。先執行一下看看效果。

嗯？突然發現最後一行資料的下面也有分割線，這樣不太美觀。接下來我們繼續修改。

聰明的小夥伴可能已經知道怎麼修改了。答案就是前面提到的DividerItemDecoration這個類。我們進去看看。

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

    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);
        }
    }

    @SuppressLint("NewApi")
    private void drawVertical(Canvas canvas, RecyclerView parent) {
        canvas.save();
        final int left;
        final int right;
        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();
    }

    @SuppressLint("NewApi")
    private void drawHorizontal(Canvas canvas, RecyclerView parent) {
        canvas.save();
        final int top;
        final int bottom;
        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) {
            outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());
        } else {
            outRect.set(0, 0, mDivider.getIntrinsicWidth(), 0);
        }
    }
}

裡面有這樣一個方法drawVertical，沒錯它就是在這個方法裡面迴圈的繪製每一條分割線的。所以我們只需要將for迴圈裡面的i<childCount改為i<childCount-1就可以了。這裡我用到的是垂直方向上的，所以drawHorizontal方法裡面的資料我就沒有修改。大家自己按需求修改哈。但是DividerItemDecoration這個類我們可能不能直接修改，我們可以新建一個類DividerItemsDecoration繼承自RecyclerView.ItemDecoration，將DividerItemDecoration裡面的程式碼copy過來，將我上面說的東西修改掉，這樣就真的大功告成了。接下來執行我們的程式，就可以看到精美的
分割線了。是不是很開心。