从资产中读取文本文件的问题（xamarin）

Question

我在从资产目录中读取简单的 txt 时遇到问题。我真的无法弄清楚为什么它不起作用以及出了什么问题。

这是我写的方法代码：

        private string ReadFile(){
        var stream = Assets.Open ("sampleText.txt");
        StreamReader sr = new StreamReader (stream);
        string text = sr.ReadToEnd ();
        sr.Close ();
        return text;
    }

这是错误：

Java.Lang.NullPointerException: Exception of type 'Java.Lang.NullPointerException' was thrown.
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/data/lanes/2970/46c3f7e0/source/mono/external/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:143 
at Android.Runtime.JNIEnv.CallNonvirtualObjectMethod (IntPtr jobject, IntPtr jclass, IntPtr jmethod) [0x00084] in /Users/builder/data/lanes/2970/46c3f7e0/source/monodroid/src/Mono.Android/src/Runtime/JNIEnv.g.cs:596 
at Android.Content.ContextWrapper.get_Assets () [0x0005f] in /Users/builder/data/lanes/2970/46c3f7e0/source/monodroid/src/Mono.Android/platforms/android-19/src/generated/Android.Content.ContextWrapper.cs:154 

我在 Windows 上运行整个应用程序没有问题，只是在 android 上运行它有问题。尝试了各种方法，例如 AssetManager 的实例，但效果不佳。

文本文件被标记为 AndroidAsset。

这是我的TextChangeBench：

public class TextChangeBench : Activity
{
    public TextChangeBench (){}

    private void SaveFile(string[] tab){

        string saveLocation = "sampleTextReworked.txt";
        StreamWriter sw = new StreamWriter (saveLocation);
        foreach (string s in tab) {
            sw.Write (s);
        }
        sw.Close ();
    }

    private string ReadFile(){
        var stream = Assets.Open ("sampleText.txt");
        StreamReader sr = new StreamReader (stream);
        string text = sr.ReadToEnd ();
        sr.Close ();
        return text;
    }
    public void ChangeText(){
        try{
            File.Delete("sampleTextReworked.txt");
        }catch(FileNotFoundException e){Console.WriteLine (e);}

        try{
            string text = ReadFile ();
            char c;
            string[] newTab = new string[text.Length];
            for (int i = 0; i < text.Length; i++)
            {
                c = (char)text [i];
                if (Char.IsUpper(c))
                {
                    newTab[i] = text[i].ToString().ToLower();
                }
                else if (Char.IsLower(c))
                {
                    newTab[i] = text[i].ToString().ToUpper();
                }
                else
                {
                    newTab[i] = text[i].ToString();
                }
            }
            SaveFile(newTab);
        }
        catch(Exception e){Console.WriteLine ("{0} ", e);}
    }
}

这是我的MainActivity：

[Activity (Label = "csBench", MainLauncher = true, Icon = "@mipmap/icon")]
public class MainActivity : Activity
{
    protected override void OnCreate (Bundle savedInstanceState)
    {
        base.OnCreate (savedInstanceState);

        SetContentView (Resource.Layout.Main);
        Button startButton = FindViewById<Button> (Resource.Id.start);
        startButton.Click += delegate {

            MathBench mb = new MathBench ();
            TextChangeBench tcb = new TextChangeBench ();
            PassedTime pt = new PassedTime ();
            EditText et = (EditText)FindViewById (Resource.Id.textInfo);

            for (int i = 0; i < 5; i++) {
                pt.StartMeasuring ();
                //mb.Silnia (25);
                //mb.Fibonacci (32);
                //mb.BubbleSort ();
                tcb.ChangeText ();
                if (i == 4) {
                    pt.StopMeasuring ();

                    et.SetText(pt.ReturnResult(), TextView.BufferType.Normal);
                    //et.SetText(pt.ReturnResult().ToString());
                    //Console.WriteLine(pt.ReturnResult ());
                    pt.ResetTimers ();
                }
            }  
        };

    }
}

欢迎任何帮助，在此先感谢。

Answer 1

之所以出现此错误，是因为手动创建一个 Activity 实例（例如 TextChangeBench）会绕过 Android 框架设置过程，从而使该 Activity 处于无效状态。

Activity 用作用户交互点，将视图/屏幕绑定到应用程序中的逻辑。由于TextChangeBench 仅实现应用程序逻辑，它不需要从Activity 派生，更适合作为纯C# 类。

删除对活动的继承以简化TextChangeBench 的代码。这意味着我们不再有权访问Assets 属性，因此无法检索资产。我们可以改用全局应用程序上下文来解决这个问题：

var stream = Android.App.Application.Context.Assets.Open("sampleText.txt");

删除活动继承和使用全局上下文访问资产管理器的组合将修复Java.Lang.NullPointerException。

最终代码如下所示：

public class TextChangeBench 
{
    private void SaveFile(string[] tab)
    {

        string saveLocation = "sampleTextReworked.txt";
        StreamWriter sw = new StreamWriter(saveLocation);
        foreach (string s in tab)
        {
            sw.Write(s);
        }
        sw.Close();
    }

    private string ReadFile()
    {
        var stream = Android.App.Application.Context.Assets.Open("sampleText.txt");

        StreamReader sr = new StreamReader(stream);
        string text = sr.ReadToEnd();
        sr.Close();
        return text;
    }
    public void ChangeText()
    {
        try
        {
            File.Delete("sampleTextReworked.txt");
        }
        catch (FileNotFoundException e) { Console.WriteLine(e); }

        try
        {
            string text = ReadFile();
            char c;
            string[] newTab = new string[text.Length];
            for (int i = 0; i < text.Length; i++)
            {
                c = (char)text[i];
                if (Char.IsUpper(c))
                {
                    newTab[i] = text[i].ToString().ToLower();
                }
                else if (Char.IsLower(c))
                {
                    newTab[i] = text[i].ToString().ToUpper();
                }
                else
                {
                    newTab[i] = text[i].ToString();
                }
            }
            SaveFile(newTab);
        }
        catch (Exception e) { Console.WriteLine("{0} ", e); }
    }
}

见：

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