Flutter 社交媒体应用通知页面错误

Question

我现在正在开发一个社交媒体应用程序，我现在正在制作通知页面。有人关注时的警报工作正常，但当有人喜欢帖子时，您会在通知页面上收到红色颤振错误。它说“'package:cached_network_image/src/cached_network_image_provider.dart'：断言失败：第 18 行 pos 16：'url！= null'：不正确。”但为什么？ 这是页面的代码

import 'package:buddiesgram/pages/HomePage.dart';
import 'package:buddiesgram/pages/PostScreenPage.dart';
import 'package:buddiesgram/pages/ProfilePage.dart';
import 'package:buddiesgram/widgets/HeaderWidget.dart';
import 'package:buddiesgram/widgets/ProgressWidget.dart';
import 'package:cached_network_image/cached_network_image.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:timeago/timeago.dart' as tAgo;

class NotificationsPage extends StatefulWidget {
  @override
  _NotificationsPageState createState() => _NotificationsPageState();
}

class _NotificationsPageState extends State<NotificationsPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: header(context, strTitle: 'Notifications',),
      body: Container(
        child: FutureBuilder(
          future: retrieveNotifications(),
          builder: (context, dataSnapshot)
          {
            if(!dataSnapshot.hasData)
            {
              return circularProgress();
            }
            return ListView(children: dataSnapshot.data,);
          },
        ),
      ),
    );
  }

  retrieveNotifications() async
  {
    QuerySnapshot querySnapshot = await activityFeedReference.document(currentUser.id)
        .collection("feedItems").orderBy("timestamp", descending: true)
        .limit(60).getDocuments();

    List<NotificationsItem> notificationsItems = [];

    querySnapshot.documents.forEach((document){
      notificationsItems.add(NotificationsItem.fromDocument(document));
    });

    return notificationsItems;
  }

}


String notificationItemText;
Widget mediaPreview;

class NotificationsItem extends StatelessWidget
{
  final String username;
  final String type;
  final String commentData;
  final String postId;
  final String userId;
  final String userProfileImg;
  final String url;
  final Timestamp timestamp;

  NotificationsItem({this.username, this.type, this.commentData, this.postId, this.userId, this.userProfileImg, this.url, this.timestamp});

  factory NotificationsItem.fromDocument(DocumentSnapshot documentSnapshot)
  {
    return NotificationsItem(
      username: documentSnapshot["username"],
      type: documentSnapshot["type"],
      commentData: documentSnapshot["commentData"],
      postId: documentSnapshot["postId"],
      userId: documentSnapshot["userId"],
      userProfileImg: documentSnapshot["userProfileImg"],
      url: documentSnapshot["url"],
      timestamp: documentSnapshot["timestamp"],
    );
  }
  @override
  Widget build(BuildContext context)
  {
    configureMediaPreview(context);


    return Padding(
      padding: EdgeInsets.only(bottom: 2.0),
      child: Container(
        color: Colors.white54,
        child: ListTile(
          title: GestureDetector(
            onTap: ()=> displayUserProfile(context, userProfileId: userId),
            child: RichText(
              overflow: TextOverflow.ellipsis,
              text: TextSpan(
                style: TextStyle(fontSize: 14.0, color: Colors.black),
                children: [
                  TextSpan(text: username, style: TextStyle(fontWeight: FontWeight.bold)),
                  TextSpan(text: " $notificationItemText"),
                ],
              ),
            ),
          ),
          leading: CircleAvatar(
            backgroundImage: CachedNetworkImageProvider(userProfileImg),
          ),
          subtitle: Text(tAgo.format(timestamp.toDate()), overflow: TextOverflow.ellipsis,),
          trailing: mediaPreview,
        ),
      ),
    );
  }

  configureMediaPreview(context)
  {
    if(type == "comment" || type == "like")
    {
      mediaPreview = GestureDetector(
        onTap: ()=> displayFullProfile(context, userProfileId: currentUser.id),
        child: Container(
          height: 50.0,
          width: 50.0,
          child: AspectRatio(
            aspectRatio: 16/9,
            child: Container(
              decoration: BoxDecoration(
                image: DecorationImage(fit: BoxFit.cover, image: CachedNetworkImageProvider(url)),
              ),
            ),
          ),
        ),
      );
    }
    else
    {
      mediaPreview = Text("");
    }

    if(type == "like")
    {
      notificationItemText = "liked your post.";
    }
    else if(type == "comment")
    {
      notificationItemText = "replied: $commentData";
    }
    else if(type == "follow")
    {
      notificationItemText = "started following you.";
    }
    else
    {
      notificationItemText = "Error, Unknown type = $type";
    }
  }

  displayFullProfile(BuildContext context, {String userProfileId})
  {
    Navigator.push(context, MaterialPageRoute(builder: (context)=> PostScreenPage(postId: postId, userId: userId,)));
  }

  displayUserProfile(BuildContext context, {String userProfileId})
  {
    Navigator.push(context, MaterialPageRoute(builder: (context) => ProfilePage(userProfileId: userProfileId,)));
  }
}

错误是在这个页面上还是在另一个页面上？

这里是 cached_network_image_provider.dart

import 'dart:async' show Future;
import 'dart:io' show File;
import 'dart:typed_data';
import 'dart:ui' as ui show instantiateImageCodec, Codec;

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter_cache_manager/flutter_cache_manager.dart';

typedef void ErrorListener();

class CachedNetworkImageProvider
    extends ImageProvider<CachedNetworkImageProvider> {
  /// Creates an ImageProvider which loads an image from the [url], using the [scale].
  /// When the image fails to load [errorListener] is called.
  const CachedNetworkImageProvider(this.url,
      {this.scale: 1.0, this.errorListener, this.headers, this.cacheManager})
      : assert(url != null),
        assert(scale != null);

  final BaseCacheManager cacheManager;

  /// Web url of the image to load
  final String url;

  /// Scale of the image
  final double scale;

  /// Listener to be called when images fails to load.
  final ErrorListener errorListener;

  // Set headers for the image provider, for example for authentication
  final Map<String, String> headers;

  @override
  Future<CachedNetworkImageProvider> obtainKey(
      ImageConfiguration configuration) {
    return SynchronousFuture<CachedNetworkImageProvider>(this);
  }

  @override
  ImageStreamCompleter load(
      CachedNetworkImageProvider key, DecoderCallback decode) {
    return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key),
      scale: key.scale,
      informationCollector: () sync* {
        yield DiagnosticsProperty<ImageProvider>(
          'Image provider: $this \n Image key: $key',
          this,
          style: DiagnosticsTreeStyle.errorProperty,
        );
      },
    );
  }

  Future<ui.Codec> _loadAsync(CachedNetworkImageProvider key) async {
    var mngr = cacheManager ?? DefaultCacheManager();
    var file = await mngr.getSingleFile(url, headers: headers);
    if (file == null) {
      if (errorListener != null) errorListener();
      return Future<ui.Codec>.error("Couldn't download or retrieve file.");
    }
    return await _loadAsyncFromFile(key, file);
  }

  Future<ui.Codec> _loadAsyncFromFile(
      CachedNetworkImageProvider key, File file) async {
    assert(key == this);

    final Uint8List bytes = await file.readAsBytes();

    if (bytes.lengthInBytes == 0) {
      if (errorListener != null) errorListener();
      throw Exception("File was empty");
    }

    return await ui.instantiateImageCodec(bytes);
  }

  @override
  bool operator ==(dynamic other) {
    if (other.runtimeType != runtimeType) return false;
    final CachedNetworkImageProvider typedOther = other;
    return url == typedOther.url && scale == typedOther.scale;
  }

  @override
  int get hashCode => hashValues(url, scale);

  @override
  String toString() => '$runtimeType("$url", scale: $scale)';
}

有人可以帮忙吗？

Answer 1

我发现，当我使用 CircleAvatar 删除 NotificationsPage 中的前导时，错误不会出现，这意味着 CachedNetworkImageProvider 是错误制造者。但也许有人能告诉我为什么吗？