选择的答案几乎是完美的，但根据空间的不同，项目的宽度可能不相等。(对我来说，这很关键)。所以我最终用这个代码增加了一点空间，所以项目都是相同的宽度。

   class GridSpacingItemDecoration(private val columnCount: Int, @Px preferredSpace: Int, private val includeEdge: Boolean): RecyclerView.ItemDecoration() {

    /**
     * In this algorithm space should divide by 3 without remnant or width of items can have a difference
     * and we want them to be exactly the same
     */
    private val space = if (preferredSpace % 3 == 0) preferredSpace else (preferredSpace + (3 - preferredSpace % 3))

    override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State?) {
        val position = parent.getChildAdapterPosition(view)

        if (includeEdge) {

            when {
                position % columnCount == 0 -> {
                    outRect.left = space
                    outRect.right = space / 3
                }
                position % columnCount == columnCount - 1 -> {
                    outRect.right = space
                    outRect.left = space / 3
                }
                else -> {
                    outRect.left = space * 2 / 3
                    outRect.right = space * 2 / 3
                }
            }

            if (position < columnCount) {
                outRect.top = space
            }

            outRect.bottom = space

        } else {

            when {
                position % columnCount == 0 -> outRect.right = space * 2 / 3
                position % columnCount == columnCount - 1 -> outRect.left = space * 2 / 3
                else -> {
                    outRect.left = space / 3
                    outRect.right = space / 3
                }
            }

            if (position >= columnCount) {
                outRect.top = space
            }
        }
    }

}

对于这个问题，有一个非常简单而灵活的解决方案，它只使用适用于每个LayoutManager的XML。

假设你想要一个相等的X间距(例如8dp)。

在另一个布局中包装CardView项 给外层布局填充X/2 (4dp) 使外层布局背景透明

...

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="200dp"
    android:layout_height="200dp"
    android:background="@android:color/transparent"
    android:padding="4dip">

    <android.support.v7.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </android.support.v7.widget.CardView>

</LinearLayout>

给你的RecyclerView填充X/2 (4dp)

...

<android.support.v7.widget.RecyclerView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="4dp" />

就是这样。你有完美的X (8dp)间距。

上面的回答已经阐明了设置边缘处理GridLayoutManager和LinearLayoutManager的方法。

但是对于StaggeredGridLayoutManager, Pirdad Sakhizada的回答是:“它可能不太适合StaggeredGridLayoutManager”。应该是关于IndexOfSpan的问题。

您可以通过以下方式获取:

private static class MyItemDecoration extends RecyclerView.ItemDecoration {
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        int index = ((StaggeredGridLayoutManager.LayoutParams) view.getLayoutParams()).getSpanIndex();
    }
}

感谢edwardaa的回答https://stackoverflow.com/a/30701422/2227031

另一点需要注意的是:

如果total spacing和total itemWidth不等于屏幕宽度，您还需要调整itemWidth，例如，在适配器onBindViewHolder方法

Utils.init(_mActivity);
int width = 0;
if (includeEdge) {
    width = ScreenUtils.getScreenWidth() - spacing * (spanCount + 1);
} else {
    width = ScreenUtils.getScreenWidth() - spacing * (spanCount - 1);
}
int itemWidth = width / spanCount;

ConstraintLayout.LayoutParams layoutParams = (ConstraintLayout.LayoutParams) holder.imageViewAvatar.getLayoutParams();
// suppose the width and height are the same
layoutParams.width = itemWidth;
layoutParams.height = itemWidth;
holder.imageViewAvatar.setLayoutParams(layoutParams);

为了使https://stackoverflow.com/a/29905000/1649371(上面)解决方案工作，我必须修改以下方法(以及所有后续调用)

@SuppressWarnings("all")
protected int getItemSpanSize(RecyclerView parent, View view, int childIndex) {

    RecyclerView.LayoutManager mgr = parent.getLayoutManager();
    if (mgr instanceof GridLayoutManager) {
        return ((GridLayoutManager) mgr).getSpanSizeLookup().getSpanSize(childIndex);
    } else if (mgr instanceof StaggeredGridLayoutManager) {
        return ((StaggeredGridLayoutManager.LayoutParams) view.getLayoutParams()).isFullSpan() ? spanCount : 1;
    } else if (mgr instanceof LinearLayoutManager) {
        return 1;
    }

    return -1;
}

@SuppressWarnings("all")
protected int getItemSpanIndex(RecyclerView parent, View view, int childIndex) {

    RecyclerView.LayoutManager mgr = parent.getLayoutManager();
    if (mgr instanceof GridLayoutManager) {
        return ((GridLayoutManager) mgr).getSpanSizeLookup().getSpanIndex(childIndex, spanCount);
    } else if (mgr instanceof StaggeredGridLayoutManager) {
        return ((StaggeredGridLayoutManager.LayoutParams) view.getLayoutParams()).getSpanIndex();
    } else if (mgr instanceof LinearLayoutManager) {
        return 0;
    }

    return -1;
}

这就是最后对我有用的方法:

binding.rows.addItemDecoration(object: RecyclerView.ItemDecoration() {
    val px = resources.getDimensionPixelSize(R.dimen.grid_spacing)
    val spanCount = 2

    override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
        val index = parent.getChildLayoutPosition(view)
        val isLeft = (index % spanCount == 0)
        outRect.set(
            if (isLeft) px else px/2,
            0,
            if (isLeft) px/2 else px,
            px
        )
    }
})

因为我只有2列(val spanCount = 2)，我可以只做isLeft。如果有> 2列，那么我也需要一个isMiddle，两边的值都是px/2。

我希望有一种方法来获得应用程序:spanCount从直接从RecyclerView，但我不相信有。