我们在上篇和中篇对配置模型中默认提供的各种ConfigurationProvider进行了深入详尽的介绍,如果它们依然不能满足项目中的配置需求,我们可以还可以通过自定义ConfigurationProvider来支持我们希望的配置来源。配置数据的持久化方式来说,将配置存储在数据库中应该是一种非常常见的方式,接下来我们是创建一个针对数据库的ConfigurationProvider,它采用新的Entity Framework 7来完成数据库的存取操作。
  目录
  MemoryConfigurationProvider
  EnvironmentVariablesConfigurationProvider
  CommandLineConfigurationProvider
  JsonConfigurationProvider
  XmlConfiguationProvider
  IniConfigurationProvider
  自定义ConfigurationProvider
  我们将这个自定义ConfigurationProvider命名为DbConfigurationProvider。在正式对它的实现展开介绍之前,我们先来看看它在项目中的应用。我们创建一个ASP.NET Core控制台程序来演示对这个DbConfigurationProvider应用,由于我们需要使用到Entity Framework 7,并且采用SQL Server数据库,所以我们需要在project.json文件中按照如下的方式添加对“EntityFramework.MicrosoftSqlServer”这个NuGet包的依赖。
1: {
2:   ...
3:   "dependencies": {
4:     "Microsoft.Extensions.Configuration": "1.0.0-rc1-final",
5:     "EntityFramework.MicrosoftSqlServer": "7.0.0-rc1-final"
6:   },
7: }
  我们按照如下的方式读取相关配置并将绑定为一个Profile对象。我们调用自定义的扩展方法AddDatabase创建一个DbConfigurationProvider对象并将其注册到创建的ConfigurationBuilder对象上。我们在调用扩展方法AddDatabase的时候指定了连接的目标数据库,同时设置了一些初始的配置项(如果确保配置项存在于目标数据库中,这个参数是不需要指定的),它们提供了组成一个完整的Profile对象的基础数据。
1: string connectionString = "...";
2: Profile profile = new ConfigurationBuilder().AddDatabase(optionsBuilder => optionsBuilder.UseSqlServer(connectionString),
3:         new Dictionary<string, string>
4:         {
5:             ["Profile:Gender"]                    = "Male",
6:             ["Profile:Age"]                       = "18",
7:             ["Profile:ContactInfo:Email"]         = "foobar@outlook.com",
8:             ["Profile:ContactInfo:PhoneNo"]       = "123456789"
9:         })
10:     .Build().Get<Profile>("Profile");
  如上面的代码片断所示,针对自定义的DbConfigurationProvider的应用仅仅体现在我们为ConfigurationBuilder定义的扩展方法AddDatabase上,所以使用起来是非常方便的,那么这个扩展方法背后有着怎样的逻辑实现呢?DbConfigurationProvider采用Entity Framework 7以Code First的方式进行数据操作,如下所示的ApplicationSetting是表示基本配置项的POCO类型,我们将配置项的Key以小写的方式存储。另一个ApplicationSettingsContext是对应的DbContext类型。
1: [Table("ApplicationSettings")]
2: public class ApplicationSetting
3: {
4:     private string key;
5:
6:     [Key]
7:     public string Key
8:     {
9:         get { return key; }
10:         set { key = value.ToLowerInvariant(); }
11:     }
12:
13:     [Required]
14:     [MaxLength(512)]
15:     public string Value { get; set; }
16:
17:     public ApplicationSetting()
18:     {}
19:
20:     public ApplicationSetting(string key, string value)
21:     {
22:         this.Key     = key;
23:         this.Value     = value;
24:     }
25: }
26:
27: public class ApplicationSettingsContext : DbContext
28: {
29:     public ApplicationSettingsContext(DbContextOptions options) : base(options)
30:     {}
31:
32:     public DbSet<ApplicationSetting> Settings { get; set; }
33: }
  如下所示的是DbConfigurationProvider和扩展方法AddDatabase的定义。DbConfigurationProvider它的构造函数具有两个参数,一个参数类型为Action<DbContextOptionsBuilder>,用来对创建DbContext采用的DbContextOptions进行设置,另一个可选的参数用来指定一些需要自动初始化的配置项。在重写的Load方法中,我们利用创建的DbContexts从数据库中读取所有的配置项并作为自身的配置字典。
1: public class DbConfigurationProvider: ConfigurationProvider
2: {
3:     public Func<DbContextOptions> DbContextOptionsAccessor { get; private set; }
4:
5:     public DbConfigurationProvider(Action<DbContextOptionsBuilder> setup, IEnumerable<KeyValuePair<string, string>> settings = null)
6:     {
7:         DbContextOptionsBuilder<ApplicationSettingsContext> optionsBuilder = new DbContextOptionsBuilder<ApplicationSettingsContext>();
8:         setup(optionsBuilder);
9:         this.DbContextOptionsAccessor = () => optionsBuilder.Options;
10:
11:         if (settings!=null && settings.Any())
12:         {
13:             using (ApplicationSettingsContext dbContext = new ApplicationSettingsContext(this.DbContextOptionsAccessor()))
14:             {
15:                 dbContext.Database.EnsureCreated();
16:                 foreach (var item in settings)
17:                 {
18:                     ApplicationSetting setting = dbContext.Settings.FirstOrDefault(it => it.Key == item.Key.ToLowerInvariant());
19:                     if (null == setting)
20:                     {
21:                         dbContext.Settings.Add(new ApplicationSetting(item.Key, item.Value));
22:                     }
23:                     else
24:                     {
25:                         setting.Value = item.Value;
26:                     }
27:                 }
28:                 dbContext.SaveChanges();
29:             }
30:         }
31:     }
32:
33:     public override void Load()
34:     {
35:         using (ApplicationSettingsContext dbContext = new ApplicationSettingsContext(this.DbContextOptionsAccessor()))
36:         {
37:             var dictionary = dbContext.Settings.ToDictionary(it => it.Key, it => it.Value);
38:             this.Data = new Dictionary<string, string>(dictionary, StringComparer.OrdinalIgnoreCase);
39:         }
40:     }
41: }
42:
43: public static class DbConfigurationProviderExtensions
44: {
45:     public static IConfigurationBuilder AddDatabase(this IConfigurationBuilder builder, Action<DbContextOptionsBuilder> setup,
46:         IEnumerable<KeyValuePair<string, string>> settings = null)
47:     {
48:         builder.Add(new DbConfigurationProvider(setup, settings));
49:         return builder;
50:     }
51: }