接著上篇 asp.net mvc原始碼分析-Controller篇 ValueProvider 現在我們來看看ModelBindingContext這個物件。
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
};
一般情況下FallbackToEmptyPrefix 應該是true，預設parameterDescriptor.BindingInfo.Prefix為空。裡面的ModelMetadata屬性是ModelMetadata的一個例項。
看看它的建構函式的定義，
public ModelMetadata(ModelMetadataProvider provider, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName) 這個類涉及到的東西很多，有些我現在也不是很明白，只知道他們是幹什麼的，所以這個類在這裡只是簡單的提一下而已。
它有一個屬性
public virtual bool IsComplexType {
get {
return !(TypeDescriptor.GetConverter(ModelType).CanConvertFrom(typeof(string)));
}
}
看看當前引數物件能否轉化為string物件，如果可以則是簡單物件，否則則是複雜物件。
這裡的ModelMetadataProviders.Current是一個DataAnnotationsModelMetadataProvider例項。DataAnnotationsModelMetadataProvider繼承於AssociatedMetadataProvider繼承於AssociatedMetadataProvider繼承於ModelMetadataProvider，這裡的呼叫GetMetadataForType來獲取ModelMetadata，而真正建立ModelMetadata的是在DataAnnotationsModelMetadataProvider的CreateMetadata，該方法定義如下：
protected override ModelMetadata CreateMetadata(IEnumerable<Attribute> attributes, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName)
該方法真正建立了一個DataAnnotationsModelMetadata例項，該類是ModelMetadata的子類。
下面 我們來看看ModelState屬性=controllerContext.Controller.ViewData.ModelState
其中 ModelState非常簡單
[Serializable]
public class ModelState {
private ModelErrorCollection _errors = new ModelErrorCollection();
public ValueProviderResult Value { get; set;}
public ModelErrorCollection Errors {get { return _errors; }
}
而ViewDataDictionary的ModelState是一個ModelState的字典集合類ModelStateDictionary。該屬性預設就只是一個例項裡面沒有ModelState。
下面這句binder.BindModel(controllerContext, bindingContext)是真正繫結引數的地方，我們知道預設的binder是DefaultModelBinder，所以現在我們來看看你它的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);
}
這裡面有一個判斷 if (!String.IsNullOrEmpty(bindingContext.ModelName) && !bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)) 當然正常情況下
bindingContext.ModelName是不為空的，bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)則是檢查所有的ValueProvider中的所有keys是否有一個包含Action中的引數名，一般我們用的最多的是ChildActionValueProviderFactory、FormValueProviderFactory、QueryStringValueProviderFactory，用ChildActionValueProviderFactory一般是因為我們經常會有這樣的程式碼Html.RenderAction，那麼正常情況下ChildActionValueProviderFactory中就應該含有這裡的bindingContext.ModelName；當我們實際引數值在FormValueProviderFactory、QueryStringValueProviderFactory中，如果我們的Action引數是簡單資料型別，那麼ValueProviderFactory也含有該bindingContext.ModelName，例如我們的Action定義為 public ActionResult Index(string name,string age) 訪問url為http://localhost:7503/home/index?name=majiang&age=27，跑的流程主要是和Html.RenderAction呼叫一樣，bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)為true。
好讓我們仔細看看
bool performRequestValidation = ShouldPerformRequestValidation(controllerContext, bindingContext);
ValueProviderResult vpResult = bindingContext.UnvalidatedValueProvider.GetValue(bindingContext.ModelName, skipValidation: !performRequestValidation);
if (vpResult != null) {
return BindSimpleModel(controllerContext, bindingContext, vpResult);
}
這幾句 預設情況下performRequestValidation 為true，表示驗證結果資料，而ModelBindingContext的UnvalidatedValueProvider
internal IUnvalidatedValueProvider UnvalidatedValueProvider {
get {
return (ValueProvider as IUnvalidatedValueProvider) ?? new UnvalidatedValueProviderWrapper(ValueProvider);
}
}
這裡的bindingContext.UnvalidatedValueProvider.GetValue方法我想就很好明白了，不多說了。正常情況下vpResult 也不為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方法相對簡單，但是也還是比較複雜，我這裡先看第一句
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;
}
從這裡我們可以看到一個key對應一個ModelState 對應一個ValueProviderResult ；
這個 方法最後一句
object model = ConvertProviderResult(bindingContext.ModelState, bindingContext.ModelName, valueProviderResult, bindingContext.ModelType);方法ConvertProviderResult實際就一句
object convertedValue = valueProviderResult.ConvertTo(destinationType);意思就是把資料轉換成我們需要的資料型別。
如果這裡的convertedValue 為null，且bindingContext.ModelType為負責型別那麼我們就要呼叫一次BindComplexModel，有前面的分析我們也知道bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName)返回false也會呼叫BindComplexModel方法。
BindComplexModel方法非常複雜，這裡有一句object model = bindingContext.Model; 而bindingContext.Model實際上是 return ModelMetadata.Model;具體的實現如下：
public object Model {
get {
if (_modelAccessor != null) {
_model = _modelAccessor();
_modelAccessor = null;
}
return _model;
}
set {
_model = value;
_modelAccessor = null;
_properties = null;
_realModelType = null;
}
}
預設 情況下這個_modelAccessor==null的，在ControllerActionInvoker.GetParameterValue方法中bindingContext的
ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(null, parameterType),引數null造成的，GetMetadataForType的具體實現：
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);
}
}
現在我們又回到DefaultModelBinder的BindComplexModel中來，這裡面有一句
if (model == null) {
model = CreateModel(controllerContext, bindingContext, modelType);
}
所以一般情況下 BindComplexModel不會返回null值，大家要切記啊。
BindComplexModel會把當前資料型別依次轉化為typeof(IDictionary<,>）型別如果成功就按照字典來處理，呼叫UpdateDictionary方法，如果轉化為typeof(IDictionary<,>失敗就轉化為 typeof(IEnumerable<>）按照集合來處理，呼叫UpdateCollection方法，如果這種轉化也不行的話就按照普通的強型別來處理呼叫BindComplexElementalModel方法，這種繫結是我們在強型別情況下用的最多的情況。BindComplexElementalModel裡面的核心程式碼是呼叫
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);
}
}
對 DefaultModelBinder的具體實現很複雜，我們在寫Action時應該知道BindModel的時候裡面究竟走的是BindSimpleModel還是BindComplexModel，還有引數具體是由哪個ValueProviderDictionary提供的。