asp.net mvc源码分析-Controller篇 ValueProvider

在上篇文章asp.net mvc源码分析-Action篇 IModelBinder中我们提到了ValueProvider，其实这个东西是Controller的属性，在写前面Controller的是曾打算把它写书来，后来有以下在后面用的时候在写它相关的东东吧，需求才能推动发展啊。先说明一下这个属性石很重要的，Action参数的值最总都是通过它来获取的。

public IValueProvider ValueProvider {
get {
if (_valueProvider == null) {
_valueProvider = ValueProviderFactories.Factories.GetValueProvider(ControllerContext);
}
return _valueProvider;
}
set {
_valueProvider = value;
}
}

想让我们看看ValueProviderFactories类是个什么东东

public static class ValueProviderFactories {

private static readonly ValueProviderFactoryCollection _factories = new ValueProviderFactoryCollection() {
new ChildActionValueProviderFactory(),
new FormValueProviderFactory(),
new JsonValueProviderFactory(),
new RouteDataValueProviderFactory(),
new QueryStringValueProviderFactory(),
new HttpFileCollectionValueProviderFactory(),
};

public static ValueProviderFactoryCollection Factories {
get {
return _factories;
}
}

}

看来默认就跟我们加了6个ProviderFactory啊，这个顺序很重要。

public IValueProvider GetValueProvider(ControllerContext controllerContext) {
var valueProviders = from factory in _serviceResolver.Current
let valueProvider = factory.GetValueProvider(controllerContext)
where valueProvider != null
select valueProvider;

return new ValueProviderCollection(valueProviders.ToList());
}

GetValueProvider这个返回的是一个ValueProvider集合类ValueProviderCollection，所以我认为这个方法名应该加个s改为GetValueProviders。这里的_serviceResolver.Current就是我们默认的那6个ProviderFactory

其中除JsonValueProviderFactory 有点特殊，其他的几个只要controllerContext有效都能返回valueProvider ，他们的valueProvider 类型依次对应如下：

ChildActionValueProviderFactory ->ChildActionValueProvider
FormValueProviderFactory ->FormValueProvider
RouteDataValueProviderFactory ->RouteDataValueProvider
QueryStringValueProviderFactory ->QueryStringValueProvider
HttpFileCollectionValueProviderFactory->HttpFileCollectionValueProvider

而 JsonValueProviderFactory 除了要验证controllerContext数据是否有效还需要验证 json格式是否正确，所以它直接返回了一个类DictionaryValueProvider<object>。

这里的每个ValueProvider实例类我们就不用管了，太细节了。

现在我们就可以得到一个ValueProviderCollection，它有一个很有用的方法

public virtual ValueProviderResult GetValue(string key) {
return GetValue(key, skipValidation: false);
}

public virtual ValueProviderResult GetValue(string key, bool skipValidation) {
return (from provider in this
let result = GetValueFromProvider(provider, key, skipValidation)
where result != null
select result).FirstOrDefault();
}
从先前得到的ValueProvider依次待用他们的GetValue方法，返回了一个不含ValueProviderResult为空的ValueProviderResult集合，最总返回这个集合中的第一个ValueProviderResult。

internal static ValueProviderResult GetValueFromProvider(IValueProvider provider, string key, bool skipValidation) {
// Since IUnvalidatedValueProvider is a superset of IValueProvider, it's always OK to use the
// IUnvalidatedValueProvider-supplied members if they're present. Otherwise just call the
// normal IValueProvider members.

IUnvalidatedValueProvider unvalidatedProvider = provider as IUnvalidatedValueProvider;
return (unvalidatedProvider != null) ? unvalidatedProvider.GetValue(key, skipValidation) : provider.GetValue(key);
}

为什么会出现上面provider as IUnvalidatedValueProvider这样的代码了？因为前面提到的

QueryStringValueProvider、NameValueCollectionValueProvider都继承与NameValueCollectionValueProvider

public class NameValueCollectionValueProvider : IValueProvider, IUnvalidatedValueProvider
而ChildActionValueProvider、RouteDataValueProvider、HttpFileCollectionValueProvider却继承与DictionaryValueProvider
public class DictionaryValueProvider<TValue> : IValueProvider

NameValueCollectionValueProvider的主要方法如下：

private void AddValues(NameValueCollection validatedCollection, NameValueCollection unvalidatedCollection, CultureInfo culture) {
            // Need to read keys from the unvalidated collection, as M.W.I's granular request validation is a bit touchy
            // and validated entries at the time the key or value is looked at. For example, GetKey() will throw if the
            // value fails request validation, even though the value's not being looked at (M.W.I can't tell the difference).
 
            if (unvalidatedCollection.Count > 0) {
                _prefixes.Add("");
            }
 
            foreach (string key in unvalidatedCollection) {
                if (key != null) {
                    _prefixes.UnionWith(ValueProviderUtil.GetPrefixes(key));
 
                    // need to look up values lazily, as eagerly looking at the collection might trigger validation
                    _values[key] = new ValueProviderResultPlaceholder(key, validatedCollection, unvalidatedCollection, culture);
                }
            }
        }
 public virtual ValueProviderResult GetValue(string key, bool skipValidation) {
            if (key == null) {
                throw new ArgumentNullException("key");
            }
 
            ValueProviderResultPlaceholder placeholder;
            _values.TryGetValue(key, out placeholder);
            if (placeholder == null) {
                return null;
            }
            else {
                return (skipValidation) ? placeholder.UnvalidatedResult : placeholder.ValidatedResult;
            }
        }

　而DictionaryValueProvider的主要方法：

private void AddValues(IDictionary<string, TValue> dictionary, CultureInfo culture) {
           if (dictionary.Count > 0) {
               _prefixes.Add("");
           }
 
           foreach (var entry in dictionary) {
               _prefixes.UnionWith(ValueProviderUtil.GetPrefixes(entry.Key));
 
               object rawValue = entry.Value;
               string attemptedValue = Convert.ToString(rawValue, culture);
               _values[entry.Key] = new ValueProviderResult(rawValue, attemptedValue, culture);
           }
       }
public virtual ValueProviderResult GetValue(string key) {
           if (key == null) {
               throw new ArgumentNullException("key");
           }
 
           ValueProviderResult vpResult;
           _values.TryGetValue(key, out vpResult);
           return vpResult;
       }

　　充这里可以看到ValueProviderResultPlaceholder是一个ValueProviderResult的包装类，使其数据实现延迟加载，

private sealed class ValueProviderResultPlaceholder {
           private readonly Lazy<ValueProviderResult> _validatedResultPlaceholder;
           private readonly Lazy<ValueProviderResult> _unvalidatedResultPlaceholder;
 
           public ValueProviderResultPlaceholder(string key, NameValueCollection validatedCollection, NameValueCollection unvalidatedCollection, CultureInfo culture) {
               _validatedResultPlaceholder = new Lazy<ValueProviderResult>(() => GetResultFromCollection(key, validatedCollection, culture), LazyThreadSafetyMode.None);
               _unvalidatedResultPlaceholder = new Lazy<ValueProviderResult>(() => GetResultFromCollection(key, unvalidatedCollection, culture), LazyThreadSafetyMode.None);
           }
 
           private static ValueProviderResult GetResultFromCollection(string key, NameValueCollection collection, CultureInfo culture) {
               string[] rawValue = collection.GetValues(key);
               string attemptedValue = collection[key];
               return new ValueProviderResult(rawValue, attemptedValue, culture);
           }
 
           public ValueProviderResult ValidatedResult {
               get { return _validatedResultPlaceholder.Value; }
           }
 
           public ValueProviderResult UnvalidatedResult {
               get { return _unvalidatedResultPlaceholder.Value; }
           }
       }

　　具体其他的什么细节之处我就不提了。在项目中我们往往要实现自己ValueProviderFactory，那么我们需要怎么注册它了在 Application_Start()加入ValueProviderFactories.Factories.Add(xxxx)。
我们举一个demo来说说怎么用的吧

public class CookieValueProviderFactory : ValueProviderFactory
    {
        public CookieValueProviderFactory() { }
        public override IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
            {
                throw new ArgumentNullException("controllerContext");
            }
            return new CookieValueProvider(controllerContext);
        }
    }
    public class CookieValueProvider : NameValueCollectionValueProvider
    {
        public CookieValueProvider(ControllerContext controllerContext) :
            base(GetCookies(controllerContext), CultureInfo.InvariantCulture)
        { }
 
        static NameValueCollection GetCookies(ControllerContext controllerContext)
        {
            NameValueCollection data = new NameValueCollection();
            foreach (string key in controllerContext.HttpContext.Request.Cookies.AllKeys)
            {
                data.Add(key, controllerContext.HttpContext.Request.Cookies[key].Value);
            }
            return data;
        }
    }
 public class HomeController : Controller
    {
       
        public ActionResult Index(string name)
        {
            if (string.IsNullOrEmpty(name))
            {
                Response.Cookies.Add(new HttpCookie("name", "majiang"));
                return Content("写入cookie");
            }
            else
            {
                return Content("读取cookie："+name);
            }
             
        }
      
    }
   