（13）ASP.NET Core 中的選項模式（Options）
1.前言
選項（Options）模式是對配置（Configuration）的功能的延伸。在12章（ASP.NET Core中的配置二）Configuration中有介紹過該功能（繫結到實體類、繫結至物件圖、將陣列繫結至類）而選項模式又有個選項類（TOptions），該選項類作用是指：把選項類中的屬性與配置來源中的鍵關聯起來。舉個例，假設json檔案有個Option1鍵，選項類中也有個叫Option1的屬性名，經過選項配置，這樣就能把json中的鍵的值對映到選項類屬性值中。也可以理解在專案應用中，把一個json檔案序列化到.net類。
2.常規選項配置
選項類必須為包含公共無引數建構函式的非抽象類。在appsettings.json檔案中新增option1、option2、subsection的配置：
{
"option1": "value1_from_json",
"option2": -1,
"subsection": {
"suboption1": "subvalue1_from_json",
"suboption2": 200
},
"Logging": {
"LogLevel": {
"Default": "Warning"
}
},
"AllowedHosts": "*"
}
新建MyOptions類(Models/MyOptions.cs)，以下類MyOptions具有三種屬性：Option1和 Option2。設定預設值為可選，但以下示例中的類建構函式設定了Option1的預設值。Option2具有通過直接初始化屬性設定的預設值：
public class MyOptions
{
public MyOptions()
{
// Set default value.
Option1 = "value1_from_ctor";
}
public string Option1 { get; set; }
public int Option2 { get; set; } = 5;
}
而MyOptions類通過Configure新增到服務容器並繫結到配置：
public void ConfigureServices(IServiceCollection services)
{
// Example #1: General configuration
// Register the Configuration instance which MyOptions binds against.
services.Configure<MyOptions>(Configuration);
}也可以使用自定義ConfigurationBuilder從設定檔案載入選項配置時，確認基路徑設定正確，新增到服務容器並繫結到配置：
var configBuilder = new ConfigurationBuilder()
.SetBasePath(Directory.GetCurrentDirectory())
.AddJsonFile("appsettings.json", optional: true);
var config = configBuilder.Build();
services.Configure<MyOptions>(config);
以下頁面模型通過IOptionsMonitor<TOptions>使用建構函式依賴關係注入來訪問設定 (Pages/Index.cshtml.cs)：
public class IndexModel
{
public IndexModel(IOptionsMonitor<MyOptions> optionsAccessor)
{
_options = optionsAccessor.CurrentValue;
}
private readonly MyOptions _options;
public void OnGet()
{
// Example #1: Simple options
var option1 = _options.Option1;
var option2 = _options.Option2;
var simpleOptions = $"option1 = {option1}, option2 = {option2}";
}
}
在Home/Index控制器Action下呼叫IndexModel.OnGet方法返回包含選項值的字串：
public HomeController(IOptionsMonitor<MyOptions> optionsAccessor)
{
_optionsAccessor = optionsAccessor;
}
private readonly IOptionsMonitor<MyOptions> _optionsAccessor;
public IActionResult Index()
{
IndexModel indexModel = new IndexModel(_optionsAccessor);
indexModel.OnGet();
return View();
}
3.通過委託配置簡單選項
使用委託設定選項值。此示例應用程式使用新建MyOptionsWithDelegateConfig類 (Models/MyOptionsWithDelegateConfig.cs)：
public class MyOptionsWithDelegateConfig
{
public MyOptionsWithDelegateConfig()
{
// Set default value.
Option1 = "value1_from_ctor";
}
public string Option1 { get; set; }
public int Option2 { get; set; } = 5;
}
向服務容器新增IConfigureOptions<TOptions>服務。它通過MyOptionsWithDelegateConfig使用委託來配置繫結：
public void ConfigureServices(IServiceCollection services)
{
// Example #2: Options bound and configured by a delegate
services.Configure<MyOptionsWithDelegateConfig>(myOptions =>
{
myOptions.Option1 = "value1_configured_by_delegate";
myOptions.Option2 = 500;
});
}
以下頁面模型通過IOptionsMonitor<TOptions>使用建構函式依賴關係注入來訪問設定 (Pages/Index.cshtml.cs)：
public class IndexModel
{
private readonly MyOptionsWithDelegateConfig _optionsWithDelegateConfig;
public IndexModel(IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig)
{
_optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue;
}
public void OnGet()
{
// Example #2: Options configured by delegate
var delegate_config_option1 = _optionsWithDelegateConfig.Option1;
var delegate_config_option2 = _optionsWithDelegateConfig.Option2;
var simpleOptionsWithDelegateConfig =
$"delegate_option1 = {delegate_config_option1}, " +
$"delegate_option2 = {delegate_config_option2}";
}
}
在Home/Index控制器Action下呼叫IndexModel.OnGet方法返回包含選項值的字串：
public HomeController(IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig)
{
_optionsAccessorWithDelegateConfig = optionsAccessorWithDelegateConfig;
}
private readonly IOptionsMonitor<MyOptionsWithDelegateConfig> _optionsAccessorWithDelegateConfig;
public IActionResult Index()
{
IndexModel indexModel = new IndexModel(_optionsAccessorWithDelegateConfig);
indexModel.OnGet();
return View();
}
每次呼叫Configure都會將IConfigureOptions<TOptions>服務新增到服務容器。在前面的示例中，Option1和Option2的值同時在appsettings.json中指定，但Option1和Option2的值被配置的委託替代。當啟用多個配置服務時，指定的最後一個配置源優於其他源，由其設定配置值。執行應用程式時，頁面模型的OnGet方法返回顯示選項類值的字串。
4.子選項配置
將選項繫結到配置時，選項型別中的每個屬性都將繫結到窗體property[:sub-property:]的配置鍵。例如，MyOptions.Option1屬性將繫結到從appsettings.json中的option1屬性讀取的鍵Option1。在以下程式碼中，已向服務容器新增IConfigureOptions<TOptions>服務。它將MySubOptions繫結到appsettings.json檔案的subsection部分：
public void ConfigureServices(IServiceCollection services)
{
// Example #3: Suboptions
// Bind options using a sub-section of the appsettings.json file.
services.Configure<MySubOptions>(Configuration.GetSection("subsection"));
}
新建MySubOptions類(Models/MySubOptions.cs)將屬性SubOption1和SubOption2定義為保留選項值：
public class MySubOptions
{
public MySubOptions()
{
// Set default values.
SubOption1 = "value1_from_ctor";
SubOption2 = 5;
}
public string SubOption1 { get; set; }
public int SubOption2 { get; set; }
}
以下頁面模型通過IOptionsMonitor<TOptions>使用建構函式依賴關係注入來訪問設定(Pages/Index.cshtml.cs)：
public class IndexModel
{
private readonly MySubOptions _subOptions;
public IndexModel(IOptionsMonitor<MySubOptions> subOptionsAccessor)
{
_subOptions = subOptionsAccessor.CurrentValue;
}
public void OnGet()
{
// Example #3: Suboptions
var subOption1 = _subOptions.SubOption1;
var subOption2 = _subOptions.SubOption2;
var subOptions = $"subOption1 = {subOption1}, subOption2 = {subOption2}";
}
}
在Home/Index控制器Action下呼叫IndexModel.OnGet方法返回包含選項值的字串：
public HomeController(IOptionsMonitor<MySubOptions> subOptionsAccessor)
{
_subOptionsAccessor = subOptionsAccessor;
}
private readonly IOptionsMonitor<MySubOptions> _subOptionsAccessor;
public IActionResult Index()
{
IndexModel indexModel = new IndexModel(_subOptionsAccessor);
indexModel.OnGet();
return View();
}
5.通過IOptionsSnapshot重新載入配置資料
IOptionsSnapshot針對請求生命週期訪問和快取選項時，每個請求只能計算一次選項。以下示例演示如何在更改appsettings.json(Pages/Index.cshtml.cs)後建立新的 IOptionsSnapshot<TOptions>。在更改appsettings.json檔案和重新載入配置之前，針對伺服器的多個請求返回appsettings.json檔案提供的配置鍵值。
public class IndexModel
{
private readonly MyOptions _snapshotOptions;
public IndexModel(IOptionsSnapshot<MyOptions> snapshotOptionsAccessor)
{
_snapshotOptions = snapshotOptionsAccessor.Value;
}
public void OnGet()
{
// Example #5: Snapshot options
var snapshotOption1 = _snapshotOptions.Option1;
var snapshotOption2 = _snapshotOptions.Option2;
var snapshotOptions = $"snapshot option1 = {snapshotOption1}, " + $"snapshot option2 = {snapshotOption2}";
}
}
下面顯示從appsettings.json檔案載入的初始option1和option2值：
snapshot option1 = value1_from_json, snapshot option2 = -1
將appsettings.json檔案中的值更改為value1_from_json UPDATED和200。儲存appsettings.json 檔案。重新整理瀏覽器，檢視更新的選項值：
snapshot option1 = value1_from_json UPDATED, snapshot option2 = 200
6.包含IConfigureNamedOptions的命名選項支援
命名選項支援允許應用程式在命名選項配置之間進行區分。命名選項通過OptionsServiceCollectionExtensions.Configure進行宣告，其呼叫擴充套件方法ConfigureNamedOptions<TOptions>.Configure：
public void ConfigureServices(IServiceCollection services)
{
// Example #6: Named options (named_options_1)
// Register the ConfigurationBuilder instance which MyOptions binds against.
// Specify that the options loaded from configuration are named
// "named_options_1".
services.Configure<MyOptions>("named_options_1", Configuration);
// Example #6: Named options (named_options_2)
// Specify that the options loaded from the MyOptions class are named
// "named_options_2".
// Use a delegate to configure option values.
services.Configure<MyOptions>("named_options_2", myOptions =>
{
myOptions.Option1 = "named_options_2_value1_from_action";
});
}
通過OnGet(Pages/Index.cshtml.cs)訪問命名選項：
public class IndexModel
{
private readonly MyOptions _named_options_1;
private readonly MyOptions _named_options_2;
public IndexModel(IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
_named_options_1 = namedOptionsAccessor.Get("named_options_1");
_named_options_2 = namedOptionsAccessor.Get("named_options_2");
}
public void OnGet()
{
// Example #6: Named options
var named_options_1 =
$"named_options_1: option1 = {_named_options_1.Option1}, " +
$"option2 = {_named_options_1.Option2}";
var named_options_2 =
$"named_options_2: option1 = {_named_options_2.Option1}, " +
$"option2 = {_named_options_2.Option2}";
var namedOptions = $"{named_options_1} {named_options_2}";
}
}
在Home/Index控制器Action下呼叫IndexModel.OnGet方法返回包含選項值的字串：
public HomeController(IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
_namedOptionsAccessor = namedOptionsAccessor;
}
private readonly IOptionsSnapshot<MyOptions> _namedOptionsAccessor;
public IActionResult Index()
{
IndexModel indexModel = new IndexModel(_namedOptionsAccessor);
indexModel.OnGet();
return View();
}
5.1使用ConfigureAll方法配置所有選項
使用ConfigureAll方法可以配置所有選項例項。以下程式碼將針對包含公共值的所有配置例項配置Option1。將以下程式碼手動新增到Startup.ConfigureServices方法：
services.ConfigureAll<MyOptions>(myOptions =>
{
myOptions.Option1 = "ConfigureAll replacement value";
});
在Home/Index控制器Action下呼叫IndexModel.OnGet方法返回包含選項值的字串：
參考文獻：
ASP.NET Core 中的選項