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,但我不会详细介绍本示例中的所有工作原理。以下是有关该主题的 Xamarin 文档依赖服务 https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/dependency-service/introduction下面使用。
对于 Android,您需要将 DB 文件放置在 Assets 文件夹中。这是您在 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 文件放置在根文件夹中。 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。这是您的 iOS 项目中该界面的代码:
[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;
}
}
}
然后可以通过执行以下操作从 Xamarin.Forms 项目调用它:
public class MyXamarinFormsPage
{
public MyXamarinFormsPage()
{
String DBPath = await DependencyService.Get<IFileAccessHelper>().GetDBPathAndCreateIfNotExists()
//Store string for path
}
public interface IFileAccessHelper
{
Task<String> GetDBPathAndCreateIfNotExists();
}
}