java - Android RecyclerView DiffUtil.Callback : areContentsTheSame() and getChangePayload() giving different comparison results

Question

DiffUtil.Callback 中有 2 个需要重写的函数:

areContentsTheSame(int oldItemPosition, int newItemPosition)

getChangePayload(int oldItemPosition, int newItemPosition)

我知道前者检查内容是否已更改，我想直接使用“==”来简单地比较它们是否是同一个实例，因为我相信我的项目在更新时将是一个新实例。但同样的比较在后一个函数中给出了不同的结果。 代码如下:

public class MyDiffUtilCallback extends DiffUtil.Callback {
  List<App> mNewList;
  List<App> mOldList;

  public MyDiffUtilCallback(List<App> newList, List<App> oldList) {
    mNewList = newList;
    mOldList = oldList;
  }

  @Override
  public int getOldListSize() {
    return mOldList.size();
  }

  @Override
  public int getNewListSize() {
    return mNewList.size();
  }

  @Override
  public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
    return mNewList.get(newItemPosition).id == mOldList.get(oldItemPosition).id;
  }

  @Override
  public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
    return mNewList.get(newItemPosition) == mOldList.get(oldItemPosition);
    // This always returns false correctly whenever the item gets an update, it's working as expected
  }

  @Nullable
  @Override
  public Object getChangePayload(int oldItemPosition, int newItemPosition) {
    if (mNewList.get(newItemPosition) == mOldList.get(oldItemPosition)) {
      return null; // But this is always being executed
    }
    return mNewList.get(newItemPosition).data; // This is never executed, why?
  }
}

我不明白为什么旧项目和新项目之间的相同比较会给出不同的结果，父类(super class)是否修改了列表或其他内容？

Answer 1

来自 Android definition :

When areItemsTheSame(int, int) returns true for two items and areContentsTheSame(int, int) returns false for them, DiffUtil calls this method to get a payload about the change.

您应该尝试进行更详细的测试，我还发现了一篇关于 Medium 的好文章实现getChangePayload。我希望这对您有帮助。