.NET Core开发日志——Model Binding

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(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 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 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();
        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 CreateAsync(
    ActionContext actionContext,
    IList 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 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();
    foreach (var valueProvider in this.OfType())
    {
        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
    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。