[Web API] Web API 2 深入系列(6) Model绑定(上)

解决什么问题
Model元数据解析
- 复杂类型
ValueProvider
ValueProviderFactory

解决什么问题

Model： Action方法上的参数
Model绑定：对Action方法参数绑定

通过2个实例说明它的作用

定义控制器和特性路由

    [RoutePrefix("demo")]
    public class DemoController : ApiController
    {
        [Route("get/{x?}/{y?}/{z?}")]
        public IEnumerable<int> Get(int x, int y, int z)
        {
            yield return x;
            yield return y;
            yield return z;
        }
    }

SelfHost

    using (var server = new HttpSelfHostServer(new HttpSelfHostConfiguration("http://localhost:10000")))
    {
        server.Configuration.MapHttpAttributeRoutes();
        server.OpenAsync();
        Console.Read();
    }

请求地址:

请求结果,截图:

我们可以看到都返回了同样的结果,说明数据被绑定上了.
除了简单类型(基元类型和可空的值类型)支持绑定外,复杂类型也支持绑定.

    [Route("get2/{x}/{y}/{z}")]
    public Model Get(Model model)
    {
        return model;
    }

    [ModelBinder]
    public class Model
    {
        public string X { get; set; }
        public string Y { get; set; }
        public string Z { get; set; }
    }

请求地址:

路由地址

请求结果,截图:

同样成功绑定上!

补充:

查询参数的绑定优先级高于路由绑定

Model元数据解析

从上面的例子中,我们看到复杂类型同样能实现Model绑定.其依赖于Model元数据.

Model元数据不仅对复杂类型本身做描述,对复杂类型下的每个属性同样也有描述.

ModelMetadata则为Model元数据

public class ModelMetadata
{
    //类型
    public Type ModelType { get; }
    //是否复杂类型
    public virtual bool IsComplexType { get; }
    //是否可空类型
    public bool IsNullableValueType { get; }
    //父容器类型(root 为 null)
    public Type ContainerType { get; }
    //当前属性名
    public string PropertyName { get; }
    //当前属性值
    public object Model { get; set; }
    //所有子属性
    public virtual IEnumerable<ModelMetadata> Properties { get: }
}

复杂类型

IsComplexType判断是否为复杂类型
标准:是否允许字符串类型向该类型转换.
默认:基元类型和可空值类型

    public virtual bool IsComplexType
    {
      get { return !HasStringConverter(this.ModelType); }
    }

    internal static bool HasStringConverter(Type type)
    {
        //获取TypeConverter ,调用CanConvertFrom判断是否为复杂类型
      return TypeDescriptor.GetConverter(type).CanConvertFrom(typeof (string));
    }

定义一个TypeConverter

public class PointTypeConverter : TypeConverter
{
    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        if (value is string)
        {
            return ParsePoint(value as string);
        }
        return base.ConvertFrom(context, culture, value);
    }

    static Point ParsePoint(string value)
    {
        var point = new Point();
        var split = value.Split(',');
        point.X = double.Parse(split[0]);
        point.Y = double.Parse(split[1]);
        return point;
    }
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string);
    }
}

应用Point的TypeConverter 为 PointTypeConverter

[TypeConverter(typeof(PointTypeConverter))]
public class Point
{
    public double X { get; set; }
    public double Y { get; set; }
}

在DemoController加入一个Action

    [Route("get/{point}")]
    public Point Get(Point point)
    {
        return point;
    }

请求地址:

路由地址注意地址为1,2

Web API默认描述元数据类型为CachedDataAnnotationsModelMetadata

public class CachedDataAnnotationsModelMetadata : CachedModelMetadata<CachedDataAnnotationsMetadataAttributes>
{

}
//主要存储了Model上的特性
public class CachedDataAnnotationsMetadataAttributes
{
    public DisplayAttribute Display { get; protected set; }
    public DisplayNameAttribute DisplayName { get; protected set; }
    public DisplayFormatAttribute DisplayFormat { get; protected set; }
    public EditableAttribute Editable { get; protected set; }
    public ReadOnlyAttribute ReadOnly { get; protected set; }
    //从Model特性上赋值
    public CachedDataAnnotationsMetadataAttributes(IEnumerable<Attribute> attributes)
    {
        this.Display = attributes.OfType<DisplayAttribute>().FirstOrDefault<DisplayAttribute>();
        this.DisplayFormat = attributes.OfType<DisplayFormatAttribute>().FirstOrDefault<DisplayFormatAttribute>();
        this.DisplayName = attributes.OfType<DisplayNameAttribute>().FirstOrDefault<DisplayNameAttribute>();
        this.Editable = attributes.OfType<EditableAttribute>().FirstOrDefault<EditableAttribute>();
        this.ReadOnly = attributes.OfType<ReadOnlyAttribute>().FirstOrDefault<ReadOnlyAttribute>();
    }
}

补充:

Editable优先级高于ReadOnly

我们可以通过一个例子获取Model元数据

    //获取ModelMetadataProvider
    var provider = new HttpConfiguration().Services.GetModelMetadataProvider();
    foreach (CachedDataAnnotationsModelMetadata property in provider.GetMetadataForType(() => new Model { X = "1" }, typeof(Model)).Properties)
    {
        Console.WriteLine($"{property.GetDisplayName()}-{property.Model}-{property.ModelType}-{property.IsReadOnly}");
    }

运行截图:

ModelMetadataProvider
WebAPI利用ModelMetadataProvider 获取ModelMetadata

public abstract class ModelMetadataProvider
{
    //获取容器下所有属性元数据
    public abstract IEnumerable<ModelMetadata> GetMetadataForProperties(object container, Type containerType);
    //获取容器下指定属性元数据(modelAccessor 通过委托返回对象)
    public abstract ModelMetadata GetMetadataForProperty(Func<object> modelAccessor, Type containerType, string propertyName);
    //获取复杂数据的元数据
    public abstract ModelMetadata GetMetadataForType(Func<object> modelAccessor, Type modelType);
}

而WebAPI中默认的ModelMetadataProvider 为DataAnnotationsModelMetadataProvider,这也印证了上面的代码可行性

public class DataAnnotationsModelMetadataProvider : AssociatedMetadataProvider<CachedDataAnnotationsModelMetadata>
{
    //根据特性创建ModelMetadata
    protected override CachedDataAnnotationsModelMetadata CreateMetadataPrototype(IEnumerable<Attribute> attributes, Type containerType, Type modelType, string propertyName)
    {
      return new CachedDataAnnotationsModelMetadata(this, containerType, modelType, propertyName, attributes);
    }
    //根据原型创建ModelMetadata
    protected override CachedDataAnnotationsModelMetadata CreateMetadataFromPrototype(CachedDataAnnotationsModelMetadata prototype, Func<object> modelAccessor)
    {
      return new CachedDataAnnotationsModelMetadata(prototype, modelAccessor);
    }
}

ValueProvider

通过第1节,我们知道Model绑定的数据源有2个:路由和查询字符串.

ValueProvider可以视为数据源

IValueProvider

public interface IValueProvider
{
    //是否存在指定前缀
    bool ContainsPrefix(string prefix);
    //根据key 查找对应数据项
    ValueProviderResult GetValue(string key);
}

IEnumerableValueProvider

public interface IEnumerableValueProvider : IValueProvider
{
    //根据指定前缀 返回所有该前缀的key
    IDictionary<string, string> GetKeysFromPrefix(string prefix);
}

ValueProviderResult

public class ValueProviderResult
{
    //转换成字符串类型的值
    public string AttemptedValue { get; protected set; }
    //原始数据
    public object RawValue { get; protected set; }
    //类型转换 使用
    public CultureInfo Culture { get; protected set; }
    //类型转换
    public object ConvertTo(Type type)
    public virtual object ConvertTo(Type type, CultureInfo culture)
}

NameValuePairsValueProvider则是IEnumerableValueProvider的一个实现

public class NameValuePairsValueProvider : IEnumerableValueProvider, IValueProvider
{
    public virtual ValueProviderResult GetValue(string key)
    {
      object rawValue;
      if (this.Values.TryGetValue(key, out rawValue))
        return new ValueProviderResult(rawValue, string.Join(",", (IEnumerable<string>) rawValue), this._culture);
      return (ValueProviderResult) null;
    }
}

我们在本节开始已经说明Model绑定有2个数据来源,其对应的ValueProvider为

RouteDataValueProvider

public class RouteDataValueProvider : NameValuePairsValueProvider
{
    public RouteDataValueProvider(HttpActionContext actionContext, CultureInfo culture)
      : base(RouteDataValueProvider.GetRouteValues(actionContext.ControllerContext.RouteData), culture)
    {
    }

    internal static IEnumerable<KeyValuePair<string, string>> GetRouteValues(IHttpRouteData routeData)
    {
      foreach (KeyValuePair<string, object> keyValuePair in (IEnumerable<KeyValuePair<string, object>>) routeData.Values)
      {
        string value = keyValuePair.Value == null ? (string) null : keyValuePair.Value.ToString();
        yield return new KeyValuePair<string, string>(keyValuePair.Key, value);
      }
    }
}

QueryStringValueProvider

public class QueryStringValueProvider : NameValuePairsValueProvider
{
    public QueryStringValueProvider(HttpActionContext actionContext, CultureInfo culture)
        : base(actionContext.ControllerContext.Request.GetQueryNameValuePairs(), culture)
    {
    }
}

除了NameValuePairsValueProvider,Web API还定义了一个特殊的Provider

CompositeValueProvider 既是1个Provider 又是1个Provider集合

public class CompositeValueProvider : Collection<IValueProvider>, IEnumerableValueProvider, IValueProvider
{
    public CompositeValueProvider(IList<IValueProvider> list)
    //调用内部Provider集合
    public virtual ValueProviderResult GetValue(string key)
    //调用内部Provider集合
    public virtual IDictionary<string, string> GetKeysFromPrefix(string prefix)
}

ValueProviderFactory

ValueProvider是用来提供数据源的.Web API同时定义了ValueProviderFactory来创建ValueProvider

public abstract class ValueProviderFactory
{
    //根据HttpActionContext获取IValueProvider
    public abstract IValueProvider GetValueProvider(HttpActionContext actionContext);
}

同样,也有对应的RouteDataValueProviderFactory和QueryStringValueProviderFactory,另外,在这2个Factory中,做了同一次请求只创建一次的缓存处理.

对应的Web API也提供了CompositeValueProviderFactory

public class CompositeValueProviderFactory : ValueProviderFactory
{
    public CompositeValueProviderFactory(IEnumerable<ValueProviderFactory> factories)

    public override IValueProvider GetValueProvider(HttpActionContext actionContext)
    {
        //通过返回CompositeValueProvider来返回多个ValueProvider
        return (IValueProvider) new CompositeValueProvider(factories);
    }
}

通过ServicesContainer.GetValueProviderFactories()可以获取到HttpConfiguration注册的Factory.

而默认注册到ServicesContainer上的为DefaultServices

public DefaultServices(HttpConfiguration configuration)
{
    //由于QueryStringValueProviderFactory先注册,所以查询字符串的方式优先级高于路由数据
    this.SetMultiple<ValueProviderFactory>(new ValueProviderFactory[2]
    {
        (ValueProviderFactory) new QueryStringValueProviderFactory(),
        (ValueProviderFactory) new RouteDataValueProviderFactory()
    });
}

备注

文章中的代码并非完整WebAPI代码,一般是经过自己精简后的.
本篇内容使用MarkDown语法编辑
首发地址:http://neverc.cnblogs.com/p/6001891.html

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原文地址：https://www.cnblogs.com/neverc/p/6001891.html