接着上篇 asp.net mvc源码分析-Controller篇 ValueProvider 现在我们来看看ModelBindingContext这个对象。
ModelBindingContext bindingContext = new ModelBindingContext() {
FallbackToEmptyPrefix = (parameterDescriptor.BindingInfo.Prefix == null), // only fall back if prefix not specified
ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, parameterType),
ModelName = parameterName,
ModelState = controllerContext.Controller.ViewData.ModelState,
PropertyFilter = propertyFilter,
ValueProvider = valueProvider
};
一般情况下FallbackToEmptyPrefix 应该是true，默认parameterDescriptor.BindingInfo.Prefix为空。里面的ModelMetadata属性是ModelMetadata的一个实例。
看看它的构造函数的定义，
public ModelMetadata(ModelMetadataProvider provider, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName) 这个类涉及到的东西很多，有些我现在也不是很明白，只知道他们是干什么的，所以这个类在这里只是简单的提一下而已。
它有一个属性
public virtual bool IsComplexType {
get {
return !(TypeDescriptor.GetConverter(ModelType).CanConvertFrom(typeof(string)));
}
}
看看当前参数对象能否转化为string对象，如果可以则是简单对象，否则则是复杂对象。
这里的ModelMetadataProviders.Current是一个DataAnnotationsModelMetadataProvider实例。DataAnnotationsModelMetadataProvider继承于AssociatedMetadataProvider继承于AssociatedMetadataProvider继承于ModelMetadataProvider，这里的调用GetMetadataForType来获取ModelMetadata，而真正创建ModelMetadata的是在DataAnnotationsModelMetadataProvider的CreateMetadata，该方法定义如下：
protected override ModelMetadata CreateMetadata(IEnumerable<Attribute> attributes, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName)
该方法真正创建了一个DataAnnotationsModelMetadata实例，该类是ModelMetadata的子类。
下面 我们来看看ModelState属性=controllerContext.Controller.ViewData.ModelState
其中 ModelState非常简单
[Serializable]
public class ModelState {
private ModelErrorCollection _errors = new ModelErrorCollection();
public ValueProviderResult Value { get; set;}
public ModelErrorCollection Errors {get { return _errors; }
}
而ViewDataDictionary的ModelState是一个ModelState的字典集合类ModelStateDictionary。该属性默认就只是一个实例里面没有ModelState。
下面这句binder.BindModel(controllerContext, bindingContext)是真正绑定参数的地方，我们知道默认的binder是DefaultModelBinder，所以现在我们来看看你它的BindModel方法：
public virtual object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {
if (bindingContext == null) {
throw new ArgumentNullException("bindingContext");
}
bool performedFallback = false;
if (!String.IsNullOrEmpty(bindingContext.ModelName) && !bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)) {
// We couldn't find any entry that began with the prefix. If this is the top-level element, fall back
// to the empty prefix.
if (bindingContext.FallbackToEmptyPrefix) {
bindingContext = new ModelBindingContext() {
ModelMetadata = bindingContext.ModelMetadata,
ModelState = bindingContext.ModelState,
PropertyFilter = bindingContext.PropertyFilter,
ValueProvider = bindingContext.ValueProvider
};
performedFallback = true;
}
else {
return null;
}
}
// Simple model = int, string, etc.; determined by calling TypeConverter.CanConvertFrom(typeof(string))
// or by seeing if a value in the request exactly matches the name of the model we're binding.
// Complex type = everything else.
if (!performedFallback) {
bool performRequestValidation = ShouldPerformRequestValidation(controllerContext, bindingContext);
ValueProviderResult vpResult = bindingContext.UnvalidatedValueProvider.GetValue(bindingContext.ModelName, skipValidation: !performRequestValidation);
if (vpResult != null) {
return BindSimpleModel(controllerContext, bindingContext, vpResult);
}
}
if (!bindingContext.ModelMetadata.IsComplexType) {
return null;
}
return BindComplexModel(controllerContext, bindingContext);
}
这里面有一个判断 if (!String.IsNullOrEmpty(bindingContext.ModelName) && !bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)) 当然正常情况下
bindingContext.ModelName是不为空的，bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)则是检查所有的ValueProvider中的所有keys是否有一个包含Action中的参数名，一般我们用的最多的是ChildActionValueProviderFactory、FormValueProviderFactory、QueryStringValueProviderFactory，用ChildActionValueProviderFactory一般是因为我们经常会有这样的代码Html.RenderAction，那么正常情况下ChildActionValueProviderFactory中就应该含有这里的bindingContext.ModelName；当我们实际参数值在FormValueProviderFactory、QueryStringValueProviderFactory中，如果我们的Action参数是简单数据类型，那么ValueProviderFactory也含有该bindingContext.ModelName，例如我们的Action定义为 public ActionResult Index(string name,string age) 访问url为http://localhost:7503/home/index?name=majiang&age=27，跑的流程主要是和Html.RenderAction调用一样，bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)为true。
好让我们仔细看看
bool performRequestValidation = ShouldPerformRequestValidation(controllerContext, bindingContext);
ValueProviderResult vpResult = bindingContext.UnvalidatedValueProvider.GetValue(bindingContext.ModelName, skipValidation: !performRequestValidation);
if (vpResult != null) {
return BindSimpleModel(controllerContext, bindingContext, vpResult);
}
这几句 默认情况下performRequestValidation 为true，表示验证结果数据，而ModelBindingContext的UnvalidatedValueProvider
internal IUnvalidatedValueProvider UnvalidatedValueProvider {
get {
return (ValueProvider as IUnvalidatedValueProvider) ?? new UnvalidatedValueProviderWrapper(ValueProvider);
}
}
这里的bindingContext.UnvalidatedValueProvider.GetValue方法我想就很好明白了，不多说了。正常情况下vpResult 也不为null
internal object BindSimpleModel(ControllerContext controllerContext, ModelBindingContext bindingContext, ValueProviderResult valueProviderResult) {
bindingContext.ModelState.SetModelValue(bindingContext.ModelName, valueProviderResult);
// if the value provider returns an instance of the requested data type, we can just short-circuit
// the evaluation and return that instance
if (bindingContext.ModelType.IsInstanceOfType(valueProviderResult.RawValue)) {
return valueProviderResult.RawValue;
}
// since a string is an IEnumerable<char>, we want it to skip the two checks immediately following
if (bindingContext.ModelType != typeof(string)) {
// conversion results in 3 cases, as below
if (bindingContext.ModelType.IsArray) {
// case 1: user asked for an array
// ValueProviderResult.ConvertTo() understands array types, so pass in the array type directly
object modelArray = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, bindingContext.ModelType);
return modelArray;
}
Type enumerableType = TypeHelpers.ExtractGenericInterface(bindingContext.ModelType, typeof(IEnumerable<>));
if (enumerableType != null) {
// case 2: user asked for a collection rather than an array
// need to call ConvertTo() on the array type, then copy the array to the collection
object modelCollection = CreateModel(controllerContext, bindingContext, bindingContext.ModelType);
Type elementType = enumerableType.GetGenericArguments()[0];
Type arrayType = elementType.MakeArrayType();
object modelArray = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, arrayType);
Type collectionType = typeof(ICollection<>).MakeGenericType(elementType);
if (collectionType.IsInstanceOfType(modelCollection)) {
CollectionHelpers.ReplaceCollection(elementType, modelCollection, modelArray);
}
return modelCollection;
}
}
// case 3: user asked for an individual element
object model = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, bindingContext.ModelType);
return model;
}
BindSimpleModel方法相对简单，但是也还是比较复杂，我这里先看第一句
bindingContext.ModelState.SetModelValue(bindingContext.ModelName, valueProviderResult);
public void SetModelValue(string key, ValueProviderResult value) {
GetModelStateForKey(key).Value = value;
}
private ModelState GetModelStateForKey(string key) {
if (key == null) {
throw new ArgumentNullException("key");
}
ModelState modelState;
if (!TryGetValue(key, out modelState)) {
modelState = new ModelState();
this[key] = modelState;
}
return modelState;
}
从这里我们可以看到一个key对应一个ModelState 对应一个ValueProviderResult ；
这个 方法最后一句
object model = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, bindingContext.ModelType);方法ConvertProviderResult实际就一句
object convertedValue = valueProviderResult.ConvertTo(destinationType);意思就是把数据转换成我们需要的数据类型。
如果这里的convertedValue 为null，且bindingContext.ModelType为负责类型那么我们就要调用一次BindComplexModel，有前面的分析我们也知道bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)返回false也会调用BindComplexModel方法。
BindComplexModel方法非常复杂，这里有一句object model = bindingContext.Model; 而bindingContext.Model实际上是 return ModelMetadata.Model;具体的实现如下：
public object Model {
get {
if (_modelAccessor != null) {
_model = _modelAccessor();
_modelAccessor = null;
}
return _model;
}
set {
_model = value;
_modelAccessor = null;
_properties = null;
_realModelType = null;
}
}
默认 情况下这个_modelAccessor==null的，在ControllerActionInvoker.GetParameterValue方法中bindingContext的
ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, parameterType),参数null造成的，GetMetadataForType的具体实现：
private IEnumerable<ModelMetadata> GetMetadataForPropertiesImpl(object container, Type containerType) {
foreach (PropertyDescriptor property in GetTypeDescriptor(containerType).GetProperties()) {
Func<object> modelAccessor = container == null ? null : GetPropertyValueAccessor(container, property);
yield return GetMetadataForProperty(modelAccessor, containerType, property);
}
}
现在我们又回到DefaultModelBinder的BindComplexModel中来，这里面有一句
if (model == null) {
model = CreateModel(controllerContext, bindingContext, modelType);
}
所以一般情况下 BindComplexModel不会返回null值，大家要切记啊。
BindComplexModel会把当前数据类型依次转化为typeof(IDictionary<,>）类型如果成功就按照字典来处理，调用UpdateDictionary方法，如果转化为typeof(IDictionary<,>失败就转化为 typeof(IEnumerable<>）按照集合来处理，调用UpdateCollection方法，如果这种转化也不行的话就按照普通的强类型来处理调用BindComplexElementalModel方法，这种绑定是我们在强类型情况下用的最多的情况。BindComplexElementalModel里面的核心代码是调用
BindProperties(controllerContext, newBindingContext);方法，
private void BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) {
IEnumerable<PropertyDescriptor> properties = GetFilteredModelProperties(controllerContext, bindingContext);
foreach (PropertyDescriptor property in properties) {
BindProperty(controllerContext, bindingContext, property);
}
}
对 DefaultModelBinder的具体实现很复杂，我们在写Action时应该知道BindModel的时候里面究竟走的是BindSimpleModel还是BindComplexModel，还有参数具体是由哪个ValueProviderDictionary提供的。