如何修复使用 DiffUtil 向 ListUpdateCallback 发送更新时得到的错误位置

Question

我有一个 Android 应用程序，用户可以在其中同时修改多个String项目EditText，所以我需要弄清楚发生了什么变化，以便将更改通知服务器（创建新项目、更新现有项目或删除后者），这就是为什么我使用DiffUtilwithListUpdateCallback来做到这一点。如果旧列表的大小为 2 项的问题；并且我删除了索引 0 处的项目，然后在列表末尾添加了 3 个项目，我得到了一个onInserted带有不正确位置参数的回调，导致IndexOutOfBoundsException, （这是删除旧列表中任何项目的行为，除了最后一个）请看一下这个显示问题的 GIF。

我已经尝试了以下代码，并对新列表进行了其他更改，例如在索引 1 处删除，然后在列表末尾添加 3 个项目，它工作正常！

我正在使用的数组是 Answer 类型，它是一个类：

public class Answer {
    private String id;
    private String questionId;
    private String text;
    private Integer count;
}

请注意，如果两个项目相同，则可以通过 id 标识 2 个不同的对象；然后可以通过文本识别内容。

DiffUtil.Callback

public class AnswersDiffCallback extends DiffUtil.Callback {

    List<Answer> newAnswers;
    List<Answer> oldAnswers;

    public AnswersDiffCallback(List<Answer> newAnswers, List<Answer> oldAnswers) {
        this.newAnswers = newAnswers;
        this.oldAnswers = oldAnswers;
    }

    @Override
    public int getOldListSize() {
        return oldAnswers == null ? 0 : oldAnswers.size();
    }

    @Override
    public int getNewListSize() {
        return newAnswers == null ? 0 : newAnswers.size();
    }

    @Override
    public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
        Answer oldAnswer = oldAnswers.get(oldItemPosition);
        Answer newAnswer = newAnswers.get(newItemPosition);
        return Objects.equals(oldAnswer.getId(), newAnswer.getId());
    }

    @Override
    public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
        Answer oldAnswer = oldAnswers.get(oldItemPosition);
        Answer newAnswer = newAnswers.get(newItemPosition);
        return Objects.equals(oldAnswer.getText(), newAnswer.getText());
    }
}

在 ListUpdateCallback 中，我试图记录我得到的回调，以便在我与服务器交谈之前测试它是否正常工作。

Log.d(TAG, "answers: oldAnswers = " + Utils.serializeObject(oldAnswers));
Log.d(TAG, "answers: newAnswers = " + Utils.serializeObject(newAnswers));
Log.d(TAG, "-----------------------------------------------------------------------------");
DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(new AnswersDiffCallback(newAnswers, oldAnswers), true);
diffResult.dispatchUpdatesTo(new ListUpdateCallback() {
    @Override
    public void onInserted(int position, int count) {
        try {
            Log.d(TAG, String.format("onInserted: (position, count) = (%d, %d)", position, count));
            for (int i = position; i < position + count; i++) {
                Log.d(TAG, "onInserted: newAnswer.text = " + newAnswers.get(i).getText());
            }
        } catch (Exception ex) {
            Log.e(TAG, "onInserted: Exception", ex);
        }
        Log.d(TAG, "-----------------------------------------------------------------------------");
    }

    @Override
    public void onRemoved(int position, int count) {
        try {
            Log.d(TAG, "onRemoved: (position, count) = (" + position + ", " + count + ")");
            for (int i = position; i < position + count; i++) {
                Log.d(TAG, String.format("onRemoved: (oldAnswer.id, oldAnswer.text) = (%s, %s)", oldAnswers.get(i).getId(), oldAnswers.get(i).getText()));
            }
        } catch (Exception ex) {
            Log.e(TAG, "onRemoved: Exception", ex);
        }
        Log.d(TAG, "-----------------------------------------------------------------------------");
    }

    @Override
    public void onMoved(int fromPosition, int toPosition) {
        try {
            Log.d(TAG, "onMoved: (fromPosition, toPosition) = (" + fromPosition + ", " + toPosition + ")");
            Log.d(TAG, String.format("onMoved: (oldAnswer.id, oldAnswer.text) = (%s, %s)", oldAnswers.get(fromPosition).getId(), oldAnswers.get(fromPosition).getText()));
            Log.d(TAG, String.format("onMoved: (newAnswer.id, newAnswer.text) = (%s, %s)", newAnswers.get(toPosition).getId(), newAnswers.get(toPosition).getText()));
        } catch (Exception ex) {
            Log.e(TAG, "onMoved: Exception", ex);
        }
        Log.d(TAG, "-----------------------------------------------------------------------------");
    }

    @Override
    public void onChanged(int position, int count, @Nullable Object payload) {
        try {
            Log.d(TAG, "onChanged: (position, count) = (" + position + ", " + count + ")");
            for (int i = position; i < position + count; i++) {
                Log.d(TAG, String.format("onChanged: (oldAnswer.id, oldAnswer.text) = (%s, %s)", oldAnswers.get(i).getId(), oldAnswers.get(i).getText()));
                Log.d(TAG, String.format("onChanged: (newAnswer.id, newAnswer.text) = (%s, %s)", newAnswers.get(i).getId(), newAnswers.get(i).getText()));
            }
        } catch (Exception ex) {
            Log.e(TAG, "onChanged: Exception", ex);
        }
        Log.d(TAG, "-----------------------------------------------------------------------------");
    }
});

这是我得到的例外情况的logcat：

2019-06-19 16:32:00.461 24515-24515/com.example.myApp D/AdminQuestionsFragment: answers: oldAnswers = [{"count":0,"id":"5d09a1236969e249cca42e96","questionId":"5d09a1236969e249cca42e95","text":"old answer 0"},{"count":0,"id":"5d09a1236969e249cca42e97","questionId":"5d09a1236969e249cca42e95","text":"old answer 1"}]
2019-06-19 16:32:00.501 24515-24515/com.example.myApp D/AdminQuestionsFragment: answers: newAnswers = [{"count":0,"id":"5d09a1236969e249cca42e97","questionId":"5d09a1236969e249cca42e95","text":"old answer 1"},{"text":"new answer 0"},{"text":"new answer 1"},{"text":"new answer 2"}]
2019-06-19 16:32:00.501 24515-24515/com.example.myApp D/AdminQuestionsFragment: -----------------------------------------------------------------------------
2019-06-19 16:35:19.550 24515-24515/com.example.myApp D/AdminQuestionsFragment: onInserted: (position, count) = (2, 3)
2019-06-19 16:35:19.550 24515-24515/com.example.myApp D/AdminQuestionsFragment: onInserted: newAnswer.text = new answer 1
2019-06-19 16:35:19.550 24515-24515/com.example.myApp D/AdminQuestionsFragment: onInserted: newAnswer.text = new answer 2
2019-06-19 16:35:19.599 24515-24515/com.example.myApp E/AdminQuestionsFragment: onInserted: Exception
    java.lang.IndexOutOfBoundsException: Index: 4, Size: 4
        at java.util.ArrayList.get(ArrayList.java:437)
        at com.example.myApp.views.AdminQuestionsFragment$5.onInserted(AdminQuestionsFragment.java:333)
        at androidx.recyclerview.widget.BatchingListUpdateCallback.dispatchLastEvent(BatchingListUpdateCallback.java:61)
        at androidx.recyclerview.widget.BatchingListUpdateCallback.onRemoved(BatchingListUpdateCallback.java:96)
        at androidx.recyclerview.widget.DiffUtil$DiffResult.dispatchRemovals(DiffUtil.java:921)
        at androidx.recyclerview.widget.DiffUtil$DiffResult.dispatchUpdatesTo(DiffUtil.java:836)
        at com.example.myApp.views.AdminQuestionsFragment.lambda$onActivityResult$8$AdminQuestionsFragment(AdminQuestionsFragment.java:320)
        at com.example.myApp.views.-$$Lambda$AdminQuestionsFragment$KZmQo8gdnjCYX1JsaACEVkjSd1s.onChanged(Unknown Source:8)
        at androidx.lifecycle.LiveData.considerNotify(LiveData.java:113)
        at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:126)
        at androidx.lifecycle.LiveData$ObserverWrapper.activeStateChanged(LiveData.java:424)
        at androidx.lifecycle.LiveData$LifecycleBoundObserver.onStateChanged(LiveData.java:376)
        at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:361)
        at androidx.lifecycle.LifecycleRegistry.addObserver(LifecycleRegistry.java:188)
        at androidx.lifecycle.LiveData.observe(LiveData.java:185)
        at com.example.myApp.views.AdminQuestionsFragment.onActivityResult(AdminQuestionsFragment.java:387)
        at androidx.fragment.app.FragmentActivity.onActivityResult(FragmentActivity.java:170)
        at android.app.Activity.dispatchActivityResult(Activity.java:7454)
        at android.app.ActivityThread.deliverResults(ActivityThread.java:4353)
        at android.app.ActivityThread.handleSendResult(ActivityThread.java:4402)
        at android.app.servertransaction.ActivityResultItem.execute(ActivityResultItem.java:49)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2019-06-19 16:35:19.599 24515-24515/com.example.myApp D/AdminQuestionsFragment: -----------------------------------------------------------------------------
2019-06-19 16:35:19.600 24515-24515/com.example.myApp D/AdminQuestionsFragment: onRemoved: (position, count) = (0, 1)
2019-06-19 16:35:19.603 24515-24515/com.example.myApp D/AdminQuestionsFragment: onRemoved: (oldAnswer.id, oldAnswer.text) = (5d09a1236969e249cca42e96, old answer 0)
2019-06-19 16:35:19.603 24515-24515/com.example.myApp D/AdminQuestionsFragment: -----------------------------------------------------------------------------

问题出在这一行：

2019-06-19 16:35:19.550 24515-24515/com.example.myApp D/AdminQuestionsFragment: onInserted: (position, count) = (2, 3)

为什么位置不是1？

如果我有 2 个项目的旧列表，并且我删除了索引 1 处的项目，然后在列表末尾添加了 3 个项目，则此处的更新 1是 logcat：

2019-06-19 18:25:24.368 28118-28118/com.example.myApp D/AdminQuestionsFragment: answers: oldAnswers = [{"count":0,"id":"5d09a1236969e249cca42e96","questionId":"5d09a1236969e249cca42e95","text":"old answer 0"},{"count":0,"id":"5d09a1236969e249cca42e97","questionId":"5d09a1236969e249cca42e95","text":"old answer 1"}]
2019-06-19 18:25:24.370 28118-28118/com.example.myApp D/AdminQuestionsFragment: answers: newAnswers = [{"count":0,"id":"5d09a1236969e249cca42e96","questionId":"5d09a1236969e249cca42e95","text":"old answer 0"},{"text":"new answer 0"},{"text":"new answer 1"},{"text":"new answer 2"}]
2019-06-19 18:25:24.370 28118-28118/com.example.myApp D/AdminQuestionsFragment: -----------------------------------------------------------------------------
2019-06-19 18:25:24.370 28118-28118/com.example.myApp D/AdminQuestionsFragment: onRemoved: (position, count) = (1, 1)
2019-06-19 18:25:24.371 28118-28118/com.example.myApp D/AdminQuestionsFragment: onRemoved: (oldAnswer.id, oldAnswer.text) = (5d09a1236969e249cca42e97, old answer 1)
2019-06-19 18:25:24.371 28118-28118/com.example.myApp D/AdminQuestionsFragment: -----------------------------------------------------------------------------
2019-06-19 18:25:24.372 28118-28118/com.example.myApp D/AdminQuestionsFragment: onInserted: (position, count) = (1, 3)
2019-06-19 18:25:24.372 28118-28118/com.example.myApp D/AdminQuestionsFragment: onInserted: newAnswer.text = new answer 0
2019-06-19 18:25:24.372 28118-28118/com.example.myApp D/AdminQuestionsFragment: onInserted: newAnswer.text = new answer 1
2019-06-19 18:25:24.372 28118-28118/com.example.myApp D/AdminQuestionsFragment: onInserted: newAnswer.text = new answer 2
2019-06-19 18:25:24.372 28118-28118/com.example.myApp D/AdminQuestionsFragment: -----------------------------------------------------------------------------

Answer 1

这似乎是 ListUpdateCallback 的一个已知问题。插入的 oldList 项不可能onInserted在 newList 中识别其相关位置。

https://issuetracker.google.com/issues/115701827

我的解决方案是：在 oldList 中插入一个空虚拟列表项，diffResult.dispatchUpdatesTo完成后，在相同位置用 newList 项目替换空虚拟列表项。

例如，我留下了一些调试代码和日志来识别问题：

    public static final class ListUpdate<T extends BaseIdentifier> implements ListUpdateCallback {

    private final List<T> oldList;
    private final List<T> newList;

    public ListUpdate(@NonNull List<T> oldList, @NonNull List<T> newList) {
        //logger.trace("ListUpdate" + System.lineSeparator()  + "old={}" + System.lineSeparator() + "new={}", BaseIdentifier.toString(oldList), BaseIdentifier.toString(newList));

        this.oldList = oldList;
        this.newList = newList;
    }

    public int inserts = 0;

    public boolean hasInserts() {
        return inserts > 0;
    }

    public void finishInserts() {
        if (inserts <= 0) {
            return;
        }

        //logger.trace("finishInserts inserts={}", inserts);

        ListIterator<T> oldListIterator = oldList.listIterator();
        ListIterator<T> newListIterator = newList.listIterator();

        while (inserts > 0 && oldListIterator.hasNext() && newListIterator.hasNext()) {
            T oldItem = oldListIterator.next();
            T newItem = newListIterator.next();

            if (oldItem == null) {
                //Replaces the last element returned by next()
                oldListIterator.set((T) newItem.copy());
                inserts--;
            }
        }

        if (inserts > 0 || oldList.contains(null)) {
            //There must be something wrong
            logger.error("finishInserts inserts={} remaining", inserts);
        }
    }

    /** {@inheritDoc} */
    @Override
    public void onInserted(int position, int count) {
        //logger.trace("onInserted position={} count={}", position, count);

        for (int i = 0; i < count; i++) {
            /*
            T item = newList.get(position + i);
            oldList.add(position + i, (T) item.copy());
             */
            //We don't know the related position of the newList, so we add null
            oldList.add(position + i, null);
            inserts++;
        }
    }

    /** {@inheritDoc} */
    @Override
    public void onRemoved(int position, int count) {
        //logger.trace("onRemoved position={} count={}", position, count);

        for (int i = 0; i < count; i++) {
            oldList.remove(position);
        }
    }

    /** {@inheritDoc} */
    @Override
    public void onMoved(int fromPosition, int toPosition) {
        //logger.trace("onMoved fromPosition={} toPosition={}", fromPosition, toPosition);

        T item = oldList.remove(fromPosition);
        oldList.add(toPosition, item);
    }

    /** {@inheritDoc} */
    @Override
    public void onChanged(int position, int count, Object payload) {
        logger.trace("onChanged position={} count={}", position, count);

        for (int i = 0; i < count; i++) {
            T item = newList.get(position + i);
            //noinspection unchecked
            oldList.set(position + i, (T) item.copy());
        }
    }
}

打电话就像

        DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(new BaseIdentifier.BaseIdentifierDiffUtilCallback(oldProcessEvents, processEvents));
        BaseIdentifier.ListUpdate<ProcessEvent> updater = new BaseIdentifier.ListUpdate<>(oldProcessEvents, processEvents);
        diffResult.dispatchUpdatesTo(updater);
        updater.finishInserts();