给RecyclerView的item添加边框

BorderItemDecoration.java

public class BorderItemDecoration 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;

    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 BorderItemDecoration(Context context, int orientation) {
        final TypedArray a = context.obtainStyledAttributes(ATTRS);
        mDivider = a.getDrawable(0);
        a.recycle();
    }

    /**
     * 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;
        }

        drawVertical(c, parent);
        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);

//            if (i == 0) {
//                int tmpBottom = mBounds.top + Math.round(ViewCompat.getTranslationY(child));
//                int tmpTop = tmpBottom - mDivider.getIntrinsicHeight();
//                mDivider.setBounds(left, tmpTop, right, tmpBottom);
//                mDivider.draw(canvas);
//            }

        }
        canvas.restore();
    }

    @SuppressLint("NewApi")
    private void drawHorizontal_(Canvas canvas, RecyclerView parent) {
        canvas.save();

        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();

            int tmpBottom = mBounds.bottom + Math.round(ViewCompat.getTranslationY(child));
            int tmpTop = mBounds.top + Math.round(ViewCompat.getTranslationY(child));

            mDivider.setBounds(left, tmpTop, right, tmpBottom);
            mDivider.draw(canvas);

            // ADD
            int tmpLeft = mBounds.left + Math.round(ViewCompat.getTranslationX(child));
            int tmpRight = tmpLeft + mDivider.getIntrinsicWidth();
            mDivider.setBounds(tmpLeft, tmpTop, tmpRight, tmpBottom);
            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);

            // ADD
            int tmpLeft = mBounds.left + Math.round(ViewCompat.getTranslationX(child));
            int tmpRight = tmpLeft + mDivider.getIntrinsicWidth();
            mDivider.setBounds(tmpLeft, top, tmpRight, bottom);
            mDivider.draw(canvas);
        }
        canvas.restore();
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
                               RecyclerView.State state) {
        outRect.set(0, 0, mDivider.getIntrinsicWidth(), mDivider.getIntrinsicHeight());
    }
}