小编Flo*_*scu的帖子

我尝试创建一个下拉列表并用几个对象填充它，这些对象代表用户可以从中选择一个的服务器，但是当我运行该应用程序时，我收到一条错误消息：

The following assertion was thrown building DropdownWidget(dirty, state: _DropdownWidgetState#1f58f): There should be exactly one item with [DropdownButton]'s value: Instance of 'ServerModel'. Either zero or 2 or more [DropdownMenuItem]s were detected with the same value

您能帮我找出我的代码中做错了什么吗？

import 'package:flutter/material.dart';\n\xe2\x80\x8b\nclass ServerSettingsPage extends StatefulWidget {\n  @override\n  _ServerSettingsPageState createState() => _ServerSettingsPageState();\n}\n\xe2\x80\x8b\nclass _ServerSettingsPageState extends State<ServerSettingsPage> {\n  @override\n  Widget build(BuildContext context) {\n    return Scaffold(\n      appBar: AppBar(\n      title: Text("Server Settings")),\n      body: _buildUI(),\n    );\n  }\n\xe2\x80\x8b\n  Widget _buildUI() {\n    return Padding(\n      padding: const EdgeInsets.fromLTRB(0, 20, 0, …

我创建了一个类，当我尝试将对象转换为 AnyObject 时收到一条警告消息。警告是：“从‘Any’到‘AnyObject’的条件转换总是成功”如何从我的文件中删除此警告？

这是我的代码：

class WebServices
{
    class func getRequest( urlString: String, successBlock :@escaping (_ response :AnyObject)->Void, errorMsg:@escaping (_ errorMessage :String)->Void )
    {
        var request = URLRequest(url: URL(string: urlString)!)
        request.httpMethod = "GET"

        let task = URLSession.shared.dataTask(with: request) { (data, urlResponse, error) in

            DispatchQueue.main.async {
                if(error == nil)
                {
                    do {
// Here is the warning 
                        let responseJSON = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? AnyObject 
                        guard let _ = responseJSON else {
                            errorMsg("Some error has been occurred!")
                            return
                        }
                        successBlock(responseJSON!) …

我使用这个库https://pub.dev/packages/pdf来生成 PDF。问题是我无法\xe2\x80\x99 设法将照片添加到我的 PDF 中。每张照片的路径都保存在List对象中。这就是我尝试实现的方式：

import \'package:pdf/widgets.dart\' as pw;\n\xe2\x80\x8b\n\xe2\x80\x8b\nFuture<void> generatePDF() async {\n\xe2\x80\x8b\npdf = pw.Document();\n\xe2\x80\x8b\n    pdf.addPage(\n      pw.MultiPage(\n        pageFormat: PdfPageFormat.a4,\n        build: (pw.Context context) => [\n          pw.Partitions(\n            children: [\n              pw.Partition(\n                child: pw.Column(\n                  crossAxisAlignment: pw.CrossAxisAlignment.start,\n                  children: <pw.Widget>[\n\xe2\x80\x8b\n                  /// Photos list\n                    pw.ListView.builder(\n                      itemCount: 1,\n                      itemBuilder: (pw.Context context, int index) {\n                        return pw.Column(\n                          children: _photosList\n                                  ?.map(\n                                    (photo) async => pw.Column(\n                                      children: <pw.Widget>[\n                                        pw.Text(\n                                            "Photo id: ${photo.photoId} - Photo type: ${photo.photoType}"), // This is displayed correctly for each photo\n                                        pw.Image(\n                                          pw.MemoryImage(\n                                            (await …