Question

我在项目中创建了一个包含一些数据的sqlite文件，但我不知道如何将它链接到我的应用程序。我希望数据可以在Android模拟器上加载。

我发现了一个2015年发布的教程，它不再起作用，比如新的FileAccessHelper类后无法找到GetLocalFilePath函数。教程项目似乎使用了旧版本的SQLite.net-PCL包，因为在教程项目中使用了SQLite.Net.Platform.XamarinAndroid，而这个包不再存在。有什么想法吗？

http://arteksoftware.com/deploying-a-database-file-with-a-xamarin-forms-app/

这是教程中的代码：

[Activity (Label = "People", Icon = "@drawable/icon", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsApplicationActivity  
{
    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);
        global::Xamarin.Forms.Forms.Init (this, bundle);

        string dbPath = FileAccessHelper.GetLocalFilePath ("people.db3");

        LoadApplication (new People.App (dbPath, new SQLitePlatformAndroid ()));
    }
}

FileAccessHelper.cs

public class FileAccessHelper  
{
    public static string GetLocalFilePath (string filename)
    {
        string path = Environment.GetFolderPath (Environment.SpecialFolder.Personal);
        string dbPath = Path.Combine (path, filename);

        CopyDatabaseIfNotExists (dbPath);

        return dbPath;
    }

    private static void CopyDatabaseIfNotExists (string dbPath)
    {
        if (!File.Exists (dbPath)) {
            using (var br = new BinaryReader (Application.Context.Assets.Open ("people.db3"))) {
                using (var bw = new BinaryWriter (new FileStream (dbPath, FileMode.Create))) {
                    byte[] buffer = new byte[2048];
                    int length = 0;
                    while ((length = br.Read (buffer, 0, buffer.Length)) > 0) {
                        bw.Write (buffer, 0, length);
                    }
                }
            }
        }
    }
}

Answer 1

如果您想从Xamarin.Forms调用该方法，则需要为每个平台实现一个接口，但我不会详细介绍这个示例中的所有方法。以下是有关DependencyService主题的Xamarin文档。

对于Android，您需要将DB文件放在Assets文件夹中。这是您的Android项目中需要的代码，用于复制数据库并返回其路径的接口：

[assembly: Xamarin.Forms.Dependency(typeof(FileAccessHelper))]
namespace MyNamespace.Droid
{
    class FileAccessHelper : MyXamarinFormsPage.IFileAccessHelper
    {
        public async Task<String> GetDBPathAndCreateIfNotExists()
        {
            String databaseName = "MyLite.db";
            var docFolder = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
            var dbFile = Path.Combine(docFolder, databaseName); // FILE NAME TO USE WHEN COPIED
            if (!File.Exists(dbFile))
            {
                FileStream writeStream = new FileStream(dbFile, FileMode.OpenOrCreate, FileAccess.Write);
                await Forms.Context.Assets.Open(databaseName).CopyToAsync(writeStream);
            }
            return dbFile;
        }
    }
}

对于UWP，您需要将DB文件放在根文件夹中。 UWP项目中复制文件并返回路径的界面应如下所示：

[assembly: Xamarin.Forms.Dependency(typeof(FileAccessHelper))]
namespace MyNamespace.UWP
{
    public class FileAccessHelper : MyXamarinFormsPage.IFileAccessHelper
    {
        public async Task<String> GetDBPathAndCreateIfNotExists()
        {
            String filename = "MyLite.db";
            bool isExisting = false;
            try
            {
                StorageFile storage = await ApplicationData.Current.LocalFolder.GetFileAsync(filename);
                isExisting = true;
            }
            catch (Exception)
            {
                isExisting = false;
            }
            if (!isExisting)
            {
                StorageFile databaseFile = await Package.Current.InstalledLocation.GetFileAsync(filename);
                await databaseFile.CopyAsync(ApplicationData.Current.LocalFolder, filename, NameCollisionOption.ReplaceExisting);
            }
            return Path.Combine(ApplicationData.Current.LocalFolder.Path, filename);
        }
    }
}

对于iOS，您需要将DB文件放在Resources文件夹中。然后，这是您的iOS项目中的代码：

[assembly: Xamarin.Forms.Dependency(typeof(FileAccessHelper))]
namespace MyNamespace.iOS
{
    public class FileAccessHelper : MyXamarinFormsPage.IFileAccessHelper
    {
        public async Task<String> GetDBPathAndCreateIfNotExists()
        {
            String databaseName = "MyLite.db";
            var documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
            var path = Path.Combine(documentsPath, databaseName);
            if (!File.Exists(path))
            {
                var existingDb = NSBundle.MainBundle.PathForResource("MyLite", "db");
                File.Copy(existingDb, path);
            }
            return path;
        }
    }
}

然后可以通过执行此操作从您的Xamrin.Forms项目中调用它：

public class MyXamarinFormsPage
{
    public MyXamarinFormsPage()
    {
        String DBPath = await DependencyService.Get<IFileAccessHelper>().GetDBPathAndCreateIfNotExists()
        //Store string for path
    }

    public interface IFileAccessHelper
    {
        Task<String> GetDBPathAndCreateIfNotExists();
    }
}

Answer 2

尝试使用以下内容在Android项目中分配dbpath变量（并忘记FileAccessHelper类）：

[Activity (Label = "People", Icon = "@drawable/icon", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsApplicationActivity  
{
    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);
        global::Xamarin.Forms.Forms.Init (this, bundle);

        // Retrieves the "AppHome"/files folder which is the root of your app sandbox on Android 
        var appDir = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
        // Locates your dbPath.
        string dbPath = Path.Combine(appDir , "people.db3");

        LoadApplication (new People.App (dbPath, new SQLitePlatformAndroid()));
    }
}

希望这有帮助！

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