android - RecyclerView Item装饰间距和跨度

Question

我有一个管理间距和跨度的 GridSpacingItemDecoration 类。
这是代码:

public class GridSpacingItemDecoration extends RecyclerView.ItemDecoration
{
    private int spanCount;
    private int spacing;
    private boolean includeEdge;
    private boolean rtl;

    public GridSpacingItemDecoration(boolean rtl, int spanCount, int spacing, boolean includeEdge)
    {
        this.rtl = rtl;
        this.spanCount = spanCount;
        this.spacing = spacing;
        this.includeEdge = includeEdge;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state)
    {
        int position = parent.getChildAdapterPosition(view); // item position
        int column = position % spanCount; // item column

        if (includeEdge)
        {
            if (rtl)
            {
                outRect.right = spacing - column * spacing / spanCount; // spacing - column * ((1f / spanCount) * spacing)
                outRect.left = (column + 1) * spacing / spanCount; // (column + 1) * ((1f / spanCount) * spacing)
            }else {
                outRect.left = spacing - column * spacing / spanCount; // spacing - column * ((1f / spanCount) * spacing)
                outRect.right = (column + 1) * spacing / spanCount; // (column + 1) * ((1f / spanCount) * spacing)
            }

            if (position < spanCount)
            { // top edge
                outRect.top = spacing;
            }
            outRect.bottom = spacing; // item bottom
        } else
        {
            if (rtl){
                outRect.right = column * spacing / spanCount; // column * ((1f / spanCount) * spacing)
                outRect.left = spacing - (column + 1) * spacing / spanCount; // spacing - (column + 1) * ((1f /    spanCount) * spacing)
            }else {
                outRect.left = column * spacing / spanCount; // column * ((1f / spanCount) * spacing)
                outRect.right = spacing - (column + 1) * spacing / spanCount; // spacing - (column + 1) * ((1f /    spanCount) * spacing)
            }

            if (position >= spanCount)
            {
                outRect.top = spacing; // item top
            }
        }
    }
}

当我想要一个或多个列时，它工作得很好。 (如下图所示 - 所有间距和跨度都有效)


问题是我想使用具有不同 ViewTypes 和不同 spanCount 的 RecyclerView。这是我尝试这样做的方法:
在类中定义:

public static ArrayList<Integer> type = new ArrayList<>();

private int getTypeForPosition(int position)
{
    return type.get(position);
}

private final int HEADER = 0;
private final int CHILD = 1;
private int dpToPx(int dp)
{
    Resources r = getResources();
    return Math.round(TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, r.getDisplayMetrics()));
}

在方法中定义:

type.add(HEADER);
type.add(CHILD);
type.add(CHILD);
type.add(HEADER);
GridLayoutManager glm = new GridLayoutManager(getContext(), 2);
glm.setSpanSizeLookup(new GridLayoutManager.SpanSizeLookup() {
@Override
public int getSpanSize(int position) {
    switch(getTypeForPosition(position)) {
        case HEADER:
            return 2;
        default:
            return 1;
        }
    }
});
recyclerView.setLayoutManager(glm);
recyclerView.addItemDecoration(new GridSpacingItemDecoration(true, 1, dpToPx(8), true));
ClassAdapter classAdapter = new ClassAdapter(getContext(), classes);
recyclerView.setAdapter(classAdapter);

结果如下:

问题是:同一行中两列之间的空间(如图所示)。好像是16，是我选的两倍。
问题:如何自定义 GridSpacingItemDecoration 类以在所有项目之间具有相同的空间？

Answer 1

这样做的方法是读取 View 的布局参数。

GridLayoutManager.LayoutParams params =
        (GridLayoutManager.LayoutParams) view.getLayoutParameters()

那些layout parameters具有以下属性:

// Returns the current span index of this View.
int getSpanIndex()

// Returns the number of spans occupied by this View.
int getSpanSize()

通过这种方式，您可以检查 View 是哪一列以及它跨越多少列。

• 如果它在 0 列中，则在开始端应用完整偏移量，否则仅应用一半

• 如果 spanIndex + spanSize 等于 spanCount(它占据最后一列)，您将在末尾应用完整的偏移量，否则只应用一半。

为了更好的可重用性，您还应该考虑使用

((GridLayoutManager) parent.getLayoutManager()).getSpanCount()

获取总跨度的计数而不是在构造函数中设置它。通过这种方式，您可以动态更改/更新跨度计数，它仍然有效。

请不要忘记在转换和抛出适当的异常或其他东西之前检查 instanceof ;)

---

严格按照这些说明，我们最终得到以下装饰:

class GridSpanDecoration extends RecyclerView.ItemDecoration {
  private final int padding;

  public GridSpanDecoration(int padding) {
    this.padding = padding;
  }

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

    GridLayoutManager gridLayoutManager = (GridLayoutManager) parent.getLayoutManager();
    int spanCount = gridLayoutManager.getSpanCount();

    GridLayoutManager.LayoutParams params = (GridLayoutManager.LayoutParams) view.getLayoutParams();

    int spanIndex = params.getSpanIndex();
    int spanSize = params.getSpanSize();

    // If it is in column 0 you apply the full offset on the start side, else only half
    if (spanIndex == 0) {
      outRect.left = padding;
    } else {
      outRect.left = padding / 2;
    }

    // If spanIndex + spanSize equals spanCount (it occupies the last column) you apply the full offset on the end, else only half.
    if (spanIndex + spanSize == spanCount) {
      outRect.right = padding;
    } else {
      outRect.right = padding / 2;
    }

    // just add some vertical padding as well
    outRect.top = padding / 2;
    outRect.bottom = padding / 2;

    if(isLayoutRTL(parent)) {
      int tmp = outRect.left;
      outRect.left = outRect.right;
      outRect.right = tmp;
    }
  }

  @SuppressLint({"NewApi", "WrongConstant"})
  private static boolean isLayoutRTL(RecyclerView parent) {
    return parent.getLayoutDirection() == ViewCompat.LAYOUT_DIRECTION_RTL;
  }
}

它允许任意数量的列并将它们正确对齐。