ASP.NET Core MVC中所提供的Model Binding功能简单但实用，其主要目的是将请求中包含的数据映射到action的方法参数中。这样就避免了开发者像在Web Forms时代那样需要从Request类中手动获取数据的繁锁操作，直接提高了开发效率。此功能继承自ASP.NET MVC，所以熟悉上一代框架开发的工程师，可以毫无障碍地继续享有它的便利。
本文想要探索下Model Binding相关的内容，这里先从源码中找到其发生的时机与场合。
在ControllerActionInvoker类的Next方法内部，可以看到对BindArgumentsAsync方法的调用，这里即会对方法的参数进行绑定数据的处理。
private Task Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted){ switch (next)
{ case State.ActionBegin:
{ var controllerContext = _controllerContext;
_cursor.Reset();
_instance = _cacheEntry.ControllerFactory(controllerContext);
_arguments = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase); var task = BindArgumentsAsync(); if (task.Status != TaskStatus.RanToCompletion)
{
next = State.ActionNext; return task;
} goto case State.ActionNext;
}
...
}
}此方法又调用了ControllerActionInvokerCacheEntry类中ControllerBinderDelegate属性，该属性是一个delegate方法，所以传入参数后可直接执行处理。
private Task BindArgumentsAsync(){
...
return _cacheEntry.ControllerBinderDelegate(_controllerContext, _instance, _arguments);
}创建ControllerActionInvokerCacheEntry的地方是前两篇文章(Controller，Action)中已经提到过的ControllerActionInvokerCache类。
public (ControllerActionInvokerCacheEntry cacheEntry, IFilterMetadata[] filters) GetCachedResult(ControllerContext controllerContext)
{
... if (!cache.Entries.TryGetValue(actionDescriptor, out var cacheEntry))
{
... var propertyBinderFactory = ControllerBinderDelegateProvider.CreateBinderDelegate(
_parameterBinder,
_modelBinderFactory,
_modelMetadataProvider,
actionDescriptor); var actionMethodExecutor = ActionMethodExecutor.GetExecutor(objectMethodExecutor);
cacheEntry = new ControllerActionInvokerCacheEntry(
filterFactoryResult.CacheableFilters,
controllerFactory,
controllerReleaser,
propertyBinderFactory,
objectMethodExecutor,
actionMethodExecutor);
cacheEntry = cache.Entries.GetOrAdd(actionDescriptor, cacheEntry);
}
... return (cacheEntry, filters);
}于是跟踪至ControllerBinderDelegateProvider类，找到CreateBinderDelegate方法。
public static ControllerBinderDelegate CreateBinderDelegate(
ParameterBinder parameterBinder,
IModelBinderFactory modelBinderFactory,
IModelMetadataProvider modelMetadataProvider,
ControllerActionDescriptor actionDescriptor){
... var parameterBindingInfo = GetParameterBindingInfo(modelBinderFactory, modelMetadataProvider, actionDescriptor);
... return Bind; async Task Bind(ControllerContext controllerContext, object controller, Dictionary<string, object> arguments) { var valueProvider = await CompositeValueProvider.CreateAsync(controllerContext); var parameters = actionDescriptor.Parameters; for (var i = 0; i < parameters.Count; i++)
{ var parameter = parameters[i]; var bindingInfo = parameterBindingInfo[i]; var modelMetadata = bindingInfo.ModelMetadata; if (!modelMetadata.IsBindingAllowed)
{ continue;
} var result = await parameterBinder.BindModelAsync(
controllerContext,
bindingInfo.ModelBinder,
valueProvider,
parameter,
modelMetadata, value: null); if (result.IsModelSet)
{
arguments[parameter.Name] = result.Model;
}
}
...
}
}这里可以看到创建绑定的delegate方法，与之对应的是之前那句_cacheEntry.ControllerBinderDelegate(_controllerContext, _instance, _arguments)代码。
public virtual async Task<ModelBindingResult> BindModelAsync(
ActionContext actionContext,
IModelBinder modelBinder,
IValueProvider valueProvider,
ParameterDescriptor parameter,
ModelMetadata metadata, object value){
... var modelBindingContext = DefaultModelBindingContext.CreateBindingContext(
actionContext,
valueProvider,
metadata,
parameter.BindingInfo,
parameter.Name);
modelBindingContext.Model = value;
... await modelBinder.BindModelAsync(modelBindingContext);
... var modelBindingResult = modelBindingContext.Result;
... return modelBindingResult;
}到了此处，就是旅程的终点。ParameterBinder类的BindModelAsync中可以找到对IModelBinder类型的BindModelAsync方法的调用。Model Binding这一操作便是在此时此地实现的。
接下来的疑问有两处，modelBinder是如何产生的，请求中的数据又是怎样与modelBinder发生联系。
ModelBinder
回到ControllerBinderDelegateProvider类的CreateBinderDelegate方法，可以看到其中调用了GetParameterBindingInfo方法。
private static BinderItem[] GetParameterBindingInfo(
IModelBinderFactory modelBinderFactory,
IModelMetadataProvider modelMetadataProvider,
ControllerActionDescriptor actionDescriptor){ var parameters = actionDescriptor.Parameters;
... var parameterBindingInfo = new BinderItem[parameters.Count]; for (var i = 0; i < parameters.Count; i++)
{ var parameter = parameters[i];
... var binder = modelBinderFactory.CreateBinder(new ModelBinderFactoryContext
{
BindingInfo = parameter.BindingInfo,
Metadata = metadata,
CacheToken = parameter,
});
parameterBindingInfo[i] = new BinderItem(binder, metadata);
} return parameterBindingInfo;
}这里的代码很明显地说明了modelBinder由ModelBinderFactory类的CreateBinder方法创建。
public IModelBinder CreateBinder(ModelBinderFactoryContext context){
...
IModelBinder binder; if (TryGetCachedBinder(context.Metadata, context.CacheToken, out binder))
{ return binder;
} var providerContext = new DefaultModelBinderProviderContext(this, context);
binder = CreateBinderCoreUncached(providerContext, context.CacheToken);
...
AddToCache(context.Metadata, context.CacheToken, binder); return binder;
}CreateBinder方法内部中如果缓存可以取到值，则从缓存内取值并直接返回，否则通过CreateBinderCoreUncached方法取值。
private IModelBinder CreateBinderCoreUncached(DefaultModelBinderProviderContext providerContext, object token){
...
IModelBinder result = null; for (var i = 0; i < _providers.Length; i++)
{ var provider = _providers[i];
result = provider.GetBinder(providerContext); if (result != null)
{ break;
}
}
... return result;
}这里的providers集合又包含哪些数据呢？可以从MvcCoreMvcOptionsSetup类中找到答案。
public void Configure(MvcOptions options){ // Set up ModelBinding
options.ModelBinderProviders.Add(new BinderTypeModelBinderProvider());
options.ModelBinderProviders.Add(new ServicesModelBinderProvider());
options.ModelBinderProviders.Add(new BodyModelBinderProvider(options.InputFormatters, _readerFactory, _loggerFactory, options));
options.ModelBinderProviders.Add(new HeaderModelBinderProvider());
options.ModelBinderProviders.Add(new FloatingPointTypeModelBinderProvider());
options.ModelBinderProviders.Add(new EnumTypeModelBinderProvider(options));
options.ModelBinderProviders.Add(new SimpleTypeModelBinderProvider());
options.ModelBinderProviders.Add(new CancellationTokenModelBinderProvider());
options.ModelBinderProviders.Add(new ByteArrayModelBinderProvider());
options.ModelBinderProviders.Add(new FormFileModelBinderProvider());
options.ModelBinderProviders.Add(new FormCollectionModelBinderProvider());
options.ModelBinderProviders.Add(new KeyValuePairModelBinderProvider());
options.ModelBinderProviders.Add(new DictionaryModelBinderProvider());
options.ModelBinderProviders.Add(new ArrayModelBinderProvider());
options.ModelBinderProviders.Add(new CollectionModelBinderProvider());
options.ModelBinderProviders.Add(new ComplexTypeModelBinderProvider());
...
}以上便是.NET Core MVC中所有被框架支持的ModelBinderProvider。
以一个最典型的FormCollectionModelBinderProvider为例。它以Metadata.ModelType的类型作为判断依据，如果是IFormCollection类型的话，则返回一个FormCollectionModelBinder对象。
public IModelBinder GetBinder(ModelBinderProviderContext context){
... var modelType = context.Metadata.ModelType;
... if (modelType == typeof(IFormCollection))
{ var loggerFactory = context.Services.GetRequiredService<ILoggerFactory>(); return new FormCollectionModelBinder(loggerFactory);
} return null;
}在CreateBinderCoreUncached方法的循环体内部会依次尝试ModelBinderProvider们是否能创建合适的ModelBinder，一旦能够生成ModelBinder，则跳出当前循环，以这个对象作为返回值。
ValueProvider
有了ModelBinder，还需要有数据才能进行绑定。而为ModelBinder提供数据的是一些ValueProvider。
MvcCoreMvcOptionsSetup类的Configure方法里，再往下找，可以看到ValueProvider们的踪影。更确切地是与之对应的工厂类们。
public void Configure(MvcOptions options){
... // Set up ValueProviders
options.ValueProviderFactories.Add(new FormValueProviderFactory());
options.ValueProviderFactories.Add(new RouteValueProviderFactory());
options.ValueProviderFactories.Add(new QueryStringValueProviderFactory());
options.ValueProviderFactories.Add(new JQueryFormValueProviderFactory());
...
}以FormValueProviderFactory为例，看一下其内部：
public Task CreateValueProviderAsync(ValueProviderFactoryContext context)
{
... var request = context.ActionContext.HttpContext.Request; if (request.HasFormContentType)
{ // Allocating a Task only when the body is form data.
return AddValueProviderAsync(context);
} return Task.CompletedTask;
}
private static async Task AddValueProviderAsync(ValueProviderFactoryContext context)
{ var request = context.ActionContext.HttpContext.Request; var valueProvider = new FormValueProvider(
BindingSource.Form,
await request.ReadFormAsync(),
CultureInfo.CurrentCulture); context.ValueProviders.Add(valueProvider);
}通过CreateValueProviderAsync方法可以得到一个FormValueProvider对象。
而这些ValueProviderFactory所创建的ValueProvider又统一被CompositeValueProvider类的CreateAsync方法聚合成CompositeValueProvider这个集合对象的内部元素。
public static async Task<CompositeValueProvider> CreateAsync(
ActionContext actionContext,
IList<IValueProviderFactory> factories){ var valueProviderFactoryContext = new ValueProviderFactoryContext(actionContext); for (var i = 0; i < factories.Count; i++)
{ var factory = factories[i]; await factory.CreateValueProviderAsync(valueProviderFactoryContext);
} return new CompositeValueProvider(valueProviderFactoryContext.ValueProviders);
}再到ControllerBinderDelegateProvider类的CreateBinderDelegate方法中，找到valueProvider创建的起始点。
async Task Bind(ControllerContext controllerContext, object controller, Dictionary<string, object> arguments){ var valueProvider = await CompositeValueProvider.CreateAsync(controllerContext); var parameters = actionDescriptor.Parameters; for (var i = 0; i < parameters.Count; i++)
{ var parameter = parameters[i]; var bindingInfo = parameterBindingInfo[i]; var modelMetadata = bindingInfo.ModelMetadata; if (!modelMetadata.IsBindingAllowed)
{ continue;
} var result = await parameterBinder.BindModelAsync(
controllerContext,
bindingInfo.ModelBinder,
valueProvider,
parameter,
modelMetadata, value: null); if (result.IsModelSet)
{
arguments[parameter.Name] = result.Model;
}
}
...
}所得到的valueProvider在ParameterBinder类的BindModelAsync方法里还要再作进一步的处理。先作为参数传入创建DefaultModelBindingContext的方法：
var modelBindingContext = DefaultModelBindingContext.CreateBindingContext(
actionContext,
valueProvider,
metadata,
parameter.BindingInfo,
parameter.Name);
再对ValueProvider作过滤处理：
return new DefaultModelBindingContext()
{
ActionContext = actionContext,
BinderModelName = binderModelName,
BindingSource = bindingSource,
PropertyFilter = propertyFilterProvider?.PropertyFilter, // Because this is the top-level context, FieldName and ModelName should be the same.
FieldName = binderModelName ?? modelName,
ModelName = binderModelName ?? modelName,
IsTopLevelObject = true,
ModelMetadata = metadata,
ModelState = actionContext.ModelState,
OriginalValueProvider = valueProvider,
ValueProvider = FilterValueProvider(valueProvider, bindingSource),
ValidationState = new ValidationStateDictionary(),
};FilterValueProvider方法最终会调用CompositeValueProvider类的Filter方法，以得到所有合适的valueProvider。
public IValueProvider Filter(BindingSource bindingSource){
... var filteredValueProviders = new List<IValueProvider>(); foreach (var valueProvider in this.OfType<IBindingSourceValueProvider>())
{ var result = valueProvider.Filter(bindingSource); if (result != null)
{
filteredValueProviders.Add(result);
}
}
... return new CompositeValueProvider(filteredValueProviders);
}那么当在ModelBinder的BindModelAsync方法里需要获取数据时，以FloatModelBinder为例：
public Task BindModelAsync(ModelBindingContext bindingContext){
... var modelName = bindingContext.ModelName; var valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);
...
}会试图从已过滤的ValueProvider中获取值。这时还是利用了CompositeValueProvider类中的方法。
public virtual ValueProviderResult GetValue(string key){ // Performance-sensitive
// Caching the count is faster for IList<T>
var itemCount = Items.Count; for (var i = 0; i < itemCount; i++)
{ var valueProvider = Items[i]; var result = valueProvider.GetValue(key); if (result != ValueProviderResult.None)
{ return result;
}
} return ValueProviderResult.None;
}这里的逻辑是从valueProvider集合中逐一尝试取值，有数据的则直接返回。
这也意味着数据绑定会以FormValueProvider到RouteValueProvider，再到QueryStringValueProvider，最后向JQueryFormValueProvider取值，这一流程执行，中间如果有任何一个能得到数据的话，则不再继续访问后面的ValueProvider。当然，前提是这些ValueProvider要不被先前的过滤处理排除在外。
若是还不明白这一顺序关系的话，可以回想下从ValueProviderFactories的添加顺序，再至ValueProvider集合生成时各个ValueProvider的顺序，就比较容易了解其中道理了。