Xamarin Intent null pointer

你好，我正在尝试在Xamarin上制作一个html文件。制作文件后，我尝试有意打开它，但我不断得到一个空指针（Java.Lang.NullPointerException）。

是因为不同类的意图吗？我尝试在invoicePage.xaml中实现意图.cs但是每当我调用StartActivity（意图）时，我总是收到格式错误。

我的代码如下：

invoicePage.xaml.cs

using Android.App;
using Android.Content;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
namespace AIFieldService.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class InvoicePage : ContentPage
    {
        public InvoicePage()
        {
            InitializeComponent();
            var htmlSource = new HtmlWebViewSource();
            htmlSource.Html =
              @"<html>
                <body>
                    <h1>Xamarin.Forms</h1>
                    <p>Welcome to WebView.</p>
                </body>
            </html>";
            web.Source = htmlSource;
        }
        public async void OnCancelClicked(Object sender, EventArgs e)
        {
            await Navigation.PopAsync();
        }
        public void OnPrintClicked(Object sender, EventArgs e)
        {
           htmlMaker hm = new htmlMaker(web.Source.ToString());
           hm.write();
        }
    }
}

htmlMaker.cs

using Android.App;
using Android.Content;
using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
namespace AIFieldService
{
    [Activity(Label = "LaunchFileActivity")]
    public class htmlMaker : Activity
    {
        public string html = "";
        public htmlMaker()
        {
            html = "";
        }
        public htmlMaker(string h)
        {
            html = h;
        }

        public void write()
        {

            //This gets the full path for the "files" directory of your app, where you have permission to read/write.
            var documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
            //This creates the full file path to file
            string filePath = System.IO.Path.Combine(documentsPath, "invoice.html");
            //Check if file is there
            if (!File.Exists(filePath))
            {
                //Now create the file.
                var create = new FileStream(filePath, FileMode.Create);

                create.Dispose();
            }

            //writes to file
            File.WriteAllText(filePath, html);

            //opens file
            Android.Net.Uri uri = Android.Net.Uri.Parse(filePath);
            Intent intent = new Intent(Intent.ActionView, uri);
            //error------------------------------------
            this.StartActivity(intent);
        }
    }
}

您需要将表单代码与Android代码分开，是的，您面临的问题的一个方面可能是因为Android操作系统未正确创建htmlMaker活动。永远不应该使用 new MyActivity() 来实例化活动类，因为操作系统不会调用 OnCreate 等方法。

我的建议是使用依赖服务或消息传递中心从表单共享代码调用Android项目代码，以便您可以运行Android特定代码来编写文件并打开浏览器。我将使用消息传递中心，因为它更简单。因此，从您的OnPrintClicked处理程序开始：

public void OnPrintClicked(Object sender, EventArgs e)
{
   MessagingCenter.Send<InvoicePage, string>(this, "html", web.Source.ToString());
}

然后在 Android 项目中MainActivity.OnCreate方法中添加以下内容：

Xamarin.Forms.MessagingCenter.Subscribe<InvoicePage, string>(this, "html", (sender, html) => 
    {
        //I changed this path to be a public path so external apps can access the file. 
        //Otherwise you would have to grant Chrome access to your private app files
        var documentsPath = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDocuments).AbsolutePath;
        Directory.CreateDirectory(documentsPath);
        //This creates the full file path to file
        string filePath = System.IO.Path.Combine(documentsPath, "invoice.html");
        //writes to file (no need to create it first as the below will create if necessary)
        File.WriteAllText(filePath, html);
        //opens file
        Android.Net.Uri uri = Android.Net.Uri.FromFile(new Java.IO.File(filePath));
        Intent intent = new Intent(Intent.ActionView, uri);
        intent.AddFlags(ActivityFlags.NewTask);
        intent.SetClassName("com.android.chrome", "com.google.android.apps.chrome.Main");
        this.StartActivity(intent);
    });

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