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。

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
            }
        }
    }

}

我最后用GridLayoutManager和HeaderView为我的RecyclerView做了这样的事情。

在下面的代码中，我在每个项目之间设置了4dp空间(每个项目周围都是2dp，整个RecyclerView周围是2dp填充)。

在layout.xml:

<android.support.v7.widget.RecyclerView
    android:id="@+id/recycleview"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="2dp" />

在你的片段/活动中:

GridLayoutManager manager = new GridLayoutManager(getContext(), 3);
recyclerView.setLayoutManager(manager);
int spacingInPixels = Utils.dpToPx(2);
recyclerView.addItemDecoration(new SpacesItemDecoration(spacingInPixels));

创建SpaceItemDecoration.java:

public class SpacesItemDecoration extends RecyclerView.ItemDecoration {

    private int mSpacing;

    public SpacesItemDecoration(int spacing) {
        mSpacing = spacing;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView recyclerView, RecyclerView.State state) {
        outRect.left = mSpacing;
        outRect.top = mSpacing;
        outRect.right = mSpacing;
        outRect.bottom = mSpacing;
    }
}

在Utils.java:

public static int dpToPx(final float dp) {
    return Math.round(dp * (Resources.getSystem().getDisplayMetrics().xdpi / DisplayMetrics.DENSITY_DEFAULT));
}

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);

对于那些有问题的staggeredLayoutManager(如https://imgur.com/XVutH5u)

recyclerView的方法:

getChildAdapterPosition(view)
getChildLayoutPosition(view)

有时返回-1作为索引，所以我们可能会遇到设置itemDecor的麻烦。我的解决方案是覆盖已弃用的ItemDecoration的方法:

public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent)

而不是新手:

public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)

是这样的:

recyclerView.addItemDecoration(new RecyclerView.ItemDecoration() {
            @Override
            public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
                TheAdapter.VH vh = (TheAdapter.VH) recyclerView.findViewHolderForAdapterPosition(itemPosition);
                View itemView = vh.itemView;    //itemView is the base view of viewHolder
                //or instead of the 2 lines above maybe it's possible to use  View itemView = layoutManager.findViewByPosition(itemPosition)  ... NOT TESTED

                StaggeredGridLayoutManager.LayoutParams itemLayoutParams = (StaggeredGridLayoutManager.LayoutParams) itemView.getLayoutParams();

                int spanIndex = itemLayoutParams.getSpanIndex();

                if (spanIndex == 0)
                    ...
                else
                    ...
            }
        });

到目前为止似乎对我有用:)

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。