前言

本来来自于项目：
https://github.com/yqritc/RecyclerView-FlexibleDivider
只需要简单的几句话就能设置我们想要的效果！

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具体的用法参照原项目！
效果：

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RecyclerView 横向纵向间距设置

Looking for custom ItemDecoration to achieve equal column space for GridLayoutManager?
ItemOffsetDecoration:

public class ItemOffsetDecoration extends RecyclerView.ItemDecoration {

    private int mItemOffset;

    public ItemOffsetDecoration(int itemOffset) {
        mItemOffset = itemOffset;
    }

    public ItemOffsetDecoration(@NonNull Context context, @DimenRes int itemOffsetId) {
        this(context.getResources().getDimensionPixelSize(itemOffsetId));
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
            RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        outRect.set(mItemOffset, mItemOffset, mItemOffset, mItemOffset);
    }
}

In your source code, add ItemOffsetDecoration to your recyclerview.
Item offset value should be half size of the actual value you want to add as space between items.

mRecyclerView.setLayoutManager(new GridLayoutManager(context, NUM_COLUMNS);
ItemOffsetDecoration itemDecoration = new ItemOffsetDecoration(context, R.dimen.item_offset);
mRecyclerView.addItemDecoration(itemDecoration);

Also, set item offset value as padding for its recyclerview, and specify android:clipToPadding=false.

<android.support.v7.widget.RecyclerView
    android:id="@+id/recyclerview_grid"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:clipToPadding="false"
    android:padding="@dimen/item_offset"/>

DONE. You will get an equal spaces around items.