java - DiffUtil.ItemCallback 有时会丢失 oldItem 并导致 areContentsTheSame() 错误地返回 true

Question

我在使用 DiffUtil.ItemCallback 库时遇到问题，其中列表项的旧状态最终会丢失并导致 oldTask 具有相同的值作为newTask。

当我选中/取消选中 ListAdapter 中的复选框并且 LiveData 观察器中的 onChanged() 调用 DiffUtil.ItemCallback 函数时，就会发生这种情况。正如您在 onBindViewHolder() 中看到的，该操作调用 setCompleted() ，从而否定 isCompleted() 的值。过期任务(红色)的预期行为是在选中复选框时变为白色。这是因为，当 isCompleted() 返回 true 时，isOverdue() 只能返回 false。

onBindViewHolder():

@Override
public void onBindViewHolder(@NonNull ViewHolder viewHolder, int i) {
    SubTask subTask = getItem(i);
    Log.d(TAG, "onBindViewHolder: CALLED");
    viewHolder.subTaskName.setText(subTask.getName());

    Calendar dueDate = subTask.getDueDate();
    viewHolder.dueDate.setText(context.getResources().getString(R.string.due_date, dueDate.getDisplayName(Calendar.DAY_OF_WEEK, Calendar.LONG, Locale.getDefault()), dueDate.getDisplayName(Calendar.MONTH, Calendar.LONG, Locale.getDefault()), dueDate.get(Calendar.DAY_OF_MONTH), dueDate.get(Calendar.YEAR)));

    viewHolder.checkBox.setOnCheckedChangeListener(null);
    //check of sub task if completed
    if (subTask.isCompleted()) {
        viewHolder.checkBox.setChecked(true);
    } else {
        viewHolder.checkBox.setChecked(false);
    }

    if (subTask.isOverdue()) {
        viewHolder.card.setCardBackgroundColor(context.getColor(R.color.red));
        viewHolder.subTaskName.setTextColor(context.getColor(R.color.colorAccent));
        viewHolder.dueDate.setTextColor(context.getColor(R.color.colorAccent));
    } else {
        viewHolder.card.setCardBackgroundColor(context.getColor(R.color.colorAccent));
        viewHolder.subTaskName.setTextAppearance(R.style.TextAppearance_AppCompat_Large);
        viewHolder.dueDate.setTextColor(context.getColor(R.color.main_task_text_color));
    }

    viewHolder.checkBox.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
        @Override
        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
            Log.d(TAG, "onCheckedChanged: called for subTask " + subTask.getName() + " isChecked = " + isChecked);
            viewModel = ViewModelProviders.of((FragmentActivity) context).get(ViewSubTasksForMainTaskViewModel.class);
            if (isChecked) {
                //if overdue, change card to white
                subTask.setCompleted(true);
                viewModel.updateSubTask(subTask);
            } else {
                //if overdue, set color back to red
                subTask.setCompleted(false);
                viewModel.updateSubTask(subTask);
            }
        }
    });

}

但是，当 areContentsTheSame() 尝试比较旧列表和新列表中的值时，有时它不应该返回 true，因为 isCompleted() 应该为旧列表项和新列表项返回不同的值。列表项的旧状态似乎以某种方式丢失了。由于未调用 onBindViewHolder()，这会导致列表项渲染不正确。 Here is a GIF这显示了应用程序的行为方式。正如您所看到的，列表项即使被选中也是红色的，列表项即使未被选中也是白色的。您还可以看到某些列表项正确呈现，但并非全部。

DiffCallback:

public class SubTaskDiffCallback {

private static final String TAG = "SubTaskDiffCallback";

static public DiffUtil.ItemCallback<SubTask> getSubTaskDiffCallback() {
    return new DiffUtil.ItemCallback<SubTask>() {
        @Override
        public boolean areItemsTheSame(@NonNull SubTask oldTask, @NonNull SubTask newTask) {
            Log.d(TAG, "areItemsTheSame: " + oldTask.getName() + " " + newTask.getName() + " " + Boolean.toString(oldTask.getId() == newTask.getId()));
            return oldTask.getId() == newTask.getId();
        }

        @Override
        public boolean areContentsTheSame(@NonNull SubTask oldTask, @NonNull SubTask newTask) {
            Log.d(TAG, "areContentsTheSame: oldTask " + oldTask.toString());
            Log.d(TAG, "areContentsTheSame: newTask " + newTask.toString());
            boolean contentsSame = oldTask.getName().equals(newTask.getName()) &&
                    oldTask.getDueDate().equals(newTask.getDueDate()) &&
                    oldTask.isCompleted() == newTask.isCompleted() &&
                    oldTask.getMainTaskId() == (newTask.getMainTaskId());
            Log.d(TAG, "areContentsTheSame = " + contentsSame);
            return contentsSame;
        }

        @Nullable
        @Override
        public Object getChangePayload(@NonNull SubTask oldTask, @NonNull SubTask newTask) {
            if (oldTask.isCompleted() != newTask.isCompleted()) {
                Log.d(TAG, "getChangePayload = false");
                return Boolean.FALSE;
            } else {
                return null;
            }
        }
    };
}
}

以下是上面链接的 GIF 中所发生事件的部分日志:

2020-02-02 15:45:21.536 8226-8226/com.johnsorhannus.divideandconquer D/ViewSTForMTAdapter: onCheckedChanged: called for subTask Wash car isChecked = true
2020-02-02 15:45:21.555 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:21.555 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:21.556 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:21.556 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:21.556 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:21.556 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:21.557 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:21.557 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:21.557 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:21.557 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = true, completed = false]
2020-02-02 15:45:21.558 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = true, completed = false]
2020-02-02 15:45:21.558 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:21.558 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = true, completed = false]
2020-02-02 15:45:21.558 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = true, completed = false]
2020-02-02 15:45:21.558 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:22.979 8226-8226/com.johnsorhannus.divideandconquer D/ViewSTForMTAdapter: onCheckedChanged: called for subTask Read OS book isChecked = true
2020-02-02 15:45:22.999 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:22.999 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:22.999 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:22.999 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:22.999 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:22.999 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:23.000 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:23.001 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:23.001 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:23.002 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = true, completed = false]
2020-02-02 15:45:23.002 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = false, completed = true]
2020-02-02 15:45:23.002 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = false
2020-02-02 15:45:23.003 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = true, completed = false]
2020-02-02 15:45:23.004 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = true, completed = false]
2020-02-02 15:45:23.004 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:23.004 8226-8226/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: getChangePayload = false
2020-02-02 15:45:23.014 8226-8226/com.johnsorhannus.divideandconquer D/ViewSTForMTAdapter: onBindViewHolder: CALLED
2020-02-02 15:45:24.275 8226-8226/com.johnsorhannus.divideandconquer D/ViewSTForMTAdapter: onCheckedChanged: called for subTask Read database book isChecked = true
2020-02-02 15:45:24.294 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:24.294 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:24.294 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:24.294 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:24.294 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:24.294 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:24.295 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:24.295 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:24.295 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:24.296 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = false, completed = true]
2020-02-02 15:45:24.296 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = false, completed = true]
2020-02-02 15:45:24.296 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:24.296 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = true, completed = false]
2020-02-02 15:45:24.297 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = false, completed = true]
2020-02-02 15:45:24.297 8226-11254/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = false
2020-02-02 15:45:24.297 8226-8226/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: getChangePayload = false
2020-02-02 15:45:24.308 8226-8226/com.johnsorhannus.divideandconquer D/ViewSTForMTAdapter: onBindViewHolder: CALLED
2020-02-02 15:45:26.590 8226-8226/com.johnsorhannus.divideandconquer D/ViewSTForMTAdapter: onCheckedChanged: called for subTask Read database book isChecked = false
2020-02-02 15:45:26.608 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:26.608 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:26.608 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:26.608 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read database book Read database book true
2020-02-02 15:45:26.608 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Read OS book Read OS book true
2020-02-02 15:45:26.608 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areItemsTheSame: Wash car Wash car true
2020-02-02 15:45:26.609 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:26.609 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Wash car, dueDate = Mar 17, 2019, overdue = false, completed = true]
2020-02-02 15:45:26.609 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:26.610 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = false, completed = true]
2020-02-02 15:45:26.610 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read OS book, dueDate = Mar 20, 2019, overdue = false, completed = true]
2020-02-02 15:45:26.610 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true
2020-02-02 15:45:26.611 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: oldTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = true, completed = false]
2020-02-02 15:45:26.611 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame: newTask SubTask[name = Read database book, dueDate = Mar 23, 2019, overdue = true, completed = false]
2020-02-02 15:45:26.611 8226-11255/com.johnsorhannus.divideandconquer D/SubTaskDiffCallback: areContentsTheSame = true

我确信 setCompleted() 更改了 isCompleted() 的值，因为当您退出并重新进入 Activity 时，it renders properly .

Answer 1

DiffUtil 通过引用存储旧项目 - 它不会复制它们。您的 SubTask 类是可变的，因此当您调用 setCompleted 时，您也将更改 DiffUtil 引用的旧类。 DiffUtil 只能与旧的、未更改的对象列表进行比较。因为您更改了旧对象，并且它仅具有对您现在更改的对象的引用，所以它不知道正确的旧值。