Question

我是Dart的新手，所以如果这个问题似乎微不足道或不需要，我深表歉意！

现在，我已经在全局范围内初始化了一个List<String>变量，并通过同一类中的另一种方法向其中添加了值：

// global variable
List<String> images = List(2);

getImages() async {
    // tried adding values but it is still null
    images[0] = ("https://i.pinimg.com/236x/b5/03/dd/b503dd2350f4b5c248a1f0909433fcfe.jpg");
    images[1] = ("https://i.pinimg.com/236x/04/80/ff/0480ff67b54dc03bb18da3d90e2c2a82.jpg");

    // get a Future<List<String> from another class
    List<String> items = await FirebaseStorage().getMoodBoardImages(); 
    print("items: " + items[0]); // returns a value


    print("images" + images[0]); // cause of error
    images.addAll(items);
    }

堆栈跟踪：

E/flutter (28986): [ERROR:flutter/shell/common/shell.cc(213)] Dart Error: Unhandled exception:
E/flutter (28986): NoSuchMethodError: The method '[]=' was called on null.
E/flutter (28986): Receiver: null
E/flutter (28986): Tried calling: []=(0, "https://i.pinimg.com/236x/b5/03/dd/b503dd2350f4b5c248a1f0909433fcfe.jpg")
E/flutter (28986): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
E/flutter (28986): #1 _ProfileState.getImages (package:calmante/screens/profile/profile.dart:334:11)
E/flutter (28986): #2 _ProfileState.build (package:calmante/screens/profile/profile.dart:30:5)

编辑：以下代码部分正确，因为已删除了异常，但我无法通过Flutter显示图像网址

getImages() async {
    images = List(2); // Add this
    // tried adding values but it is still null
    images[0] = ("https://i.pinimg.com/236x/b5/03/dd/b503dd2350f4b5c248a1f0909433fcfe.jpg");
    images[1] = ("https://i.pinimg.com/236x/04/80/ff/0480ff67b54dc03bb18da3d90e2c2a82.jpg");

    // get a Future<List<String> from another class
    List<String> items = await FirebaseStorage().getMoodBoardImages(); 
    print("items: " + items[0]); // returns a value


    print("images" + images[0]); // cause of error
    images.addAll(items);
}

编辑：因此，我认为上述代码仅在发生异常的位置进行了更改，因为现在它是在build方法内部触发的。

代码：

@override
  Widget build(BuildContext context) {
    getImages();
    setPalette();

    print("images build: " + images[0]);
}

StackTrace：

The following ArgumentError was thrown building Profile(dirty, state: _ProfileState#71db6):
Invalid argument(s)

The relevant error-causing widget was: 
  Profile file:///C:/Users/ugam.UG-GW-L163/AndroidStudioProjects/calmante/lib/screens/first.dart:19:5
When the exception was thrown, this was the stack: 
#0      _StringBase.+ (dart:core-patch/string_patch.dart:262:57)
#1      _ProfileState.build (package:calmante/screens/profile/profile.dart:33:28)
#2      StatefulElement.build (package:flutter/src/widgets/framework.dart:4619:28)
#3      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4502:15)
#4      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4675:11)
...

Answer 1

可能无法满足您需求的快速答案：在images上方重新初始化getImages()：

getImages() async {
    images = List(2); // Add this
    // tried adding values but it is still null
    images[0] = ("https://i.pinimg.com/236x/b5/03/dd/b503dd2350f4b5c248a1f0909433fcfe.jpg");
    images[1] = ("https://i.pinimg.com/236x/04/80/ff/0480ff67b54dc03bb18da3d90e2c2a82.jpg");

    // get a Future<List<String> from another class
    List<String> items = await FirebaseStorage().getMoodBoardImages(); 
    print("items: " + items[0]); // returns a value


    print("images" + images[0]); // cause of error
    images.addAll(items);
}

初始化后，飞镖列表为空

1 个答案: