在我們的mvc日常開發會經常遇到什么LabelFor、EditorFor、Editor等等，這個擴展方法有很多是相似的。這里我們以EditorFor來說說吧，我覺得這個相對要復雜一點。
首先我們來看看EditorFor的定義：
public static MvcHtmlString EditorFor<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression, object additionalViewData) {
return TemplateHelpers.TemplateFor(html, expression, null /* templateName */, null /* htmlFieldName */, DataBoundControlMode.Edit, additionalViewData);
}
雖然EditorFor有很多定義，但是實際上都是調用 TemplateHelpers.TemplateFor方法。
internal static MvcHtmlString TemplateFor<TContainer, TValue>(this HtmlHelper<TContainer> html, Expression<Func<TContainer, TValue>> expression,
string templateName, string htmlFieldName, DataBoundControlMode mode,
object additionalViewData) {
return MvcHtmlString.Create(TemplateFor(html, expression, templateName, htmlFieldName, mode, additionalViewData, TemplateHelper));
}
現在大家應該知道TemplateFor方法的主要參數都有哪些了吧，但是在實際開發中我們的templateName、htmlFieldName、additionalViewData通常都是null，mode是DataBoundControlMode.Edit
我們還是舉一個例子來說說吧：
public class UserInfo
{
[StringLength(100, MinimumLength = 10)]
[Required]
public string UserName { set; get; }
}
@Html.EditorFor(model => model.UserName)
這個代碼是不是很簡單。
現在我們來看看TemplateFor的實現
return templateHelper(html,
ModelMetadata.FromLambdaExpression(expression, html.ViewData),
htmlFieldName ?? ExpressionHelper.GetExpressionText(expression),
templateName,
mode,
additionalViewData);
首先我們來看看 ModelMetadata.FromLambdaExpression(expression, html.ViewData)這句是如何獲取ModelMetadata的，具體實現如下：
public static ModelMetadata FromLambdaExpression<TParameter, TValue>(Expression<Func<TParameter, TValue>> expression,
ViewDataDictionary<TParameter> viewData) {
if (expression == null) {
throw new ArgumentNullException("expression");
}
if (viewData == null) {
throw new ArgumentNullException("viewData");
}
string propertyName = null;
Type containerType = null;
bool legalExpression = false;
// Need to verify the expression is valid; it needs to at least end in something
// that we can convert to a meaningful string for model binding purposes
switch (expression.Body.NodeType) {
// ArrayIndex always means a single-dimensional indexer; multi-dimensional indexer is a method call to Get()
case ExpressionType.ArrayIndex:
legalExpression = true;
break;
// Only legal method call is a single argument indexer/DefaultMember call
case ExpressionType.Call:
legalExpression = ExpressionHelper.IsSingleArgumentIndexer(expression.Body);
break;
// Property/field access is always legal
case ExpressionType.MemberAccess:
MemberExpression memberExpression = (MemberExpression)expression.Body;
propertyName = memberExpression.Member is PropertyInfo ? memberExpression.Member.Name : null;
containerType = memberExpression.Expression.Type;
legalExpression = true;
break;
// Parameter expression means "model => model", so we delegate to FromModel
case ExpressionType.Parameter:
return FromModel(viewData);
}
if (!legalExpression) {
throw new InvalidOperationException(MvcResources.TemplateHelpers_TemplateLimitations);
}
TParameter container = viewData.Model;
Func<object> modelAccessor = () => {
try {
return CachedExpressionCompiler.Process(expression)(container);
}
catch (NullReferenceException) {
return null;
}
};
return GetMetadataFromProvider(modelAccessor, typeof(TValue), propertyName, containerType);
}
如果我們調用的是Editor那么之力調用就是FromStringExpression方法而不是FromLambdaExpression方法，這2個方法相差不大。我們還是來看看FromLambdaExpression這個方法吧：
看了這張圖 那么 return GetMetadataFromProvider(modelAccessor, typeof(TValue), propertyName, containerType);這句代碼里面的參數相信大家都應該是到了吧。
propertyName=“UserName” containerType=MvcApp.Controllers.UserInfo，modelAccessor就是創建一個實例，實例的創建是通過 model => model.UserName這句。至於GetMetadataFromProvider這個方法就沒什么好講的了，前面的文章已經講過了，它實際是創建 了一個DataAnnotationsModelMetadata實例。
至於ExpressionHelper.GetExpressionText(expression)這句說白了默認就是返回一個屬性名稱，具體實現：
public static string GetExpressionText(LambdaExpression expression) {
// Split apart the expression string for property/field accessors to create its name
Stack<string> nameParts = new Stack<string>();
Expression part = expression.Body;
while (part != null) {
if (part.NodeType == ExpressionType.Call) {
MethodCallExpression methodExpression = (MethodCallExpression)part;
if (!IsSingleArgumentIndexer(methodExpression)) {
break;
}
nameParts.Push(
GetIndexerInvocation(
methodExpression.Arguments.Single(),
expression.Parameters.ToArray()
)
);
part = methodExpression.Object;
}
else if (part.NodeType == ExpressionType.ArrayIndex) {
BinaryExpression binaryExpression = (BinaryExpression)part;
nameParts.Push(
GetIndexerInvocation(
binaryExpression.Right,
expression.Parameters.ToArray()
)
);
part = binaryExpression.Left;
}
else if (part.NodeType == ExpressionType.MemberAccess) {
MemberExpression memberExpressionPart = (MemberExpression)part;
nameParts.Push("." + memberExpressionPart.Member.Name);
part = memberExpressionPart.Expression;
}
else if (part.NodeType == ExpressionType.Parameter) {
// Dev10 Bug #907611
// When the expression is parameter based (m => m.Something...), we'll push an empty
// string onto the stack and stop evaluating. The extra empty string makes sure that
// we don't accidentally cut off too much of m => m.Model.
nameParts.Push(String.Empty);
part = null;
}
else {
break;
}
}
// If it starts with "model", then strip that away
if (nameParts.Count > 0 && String.Equals(nameParts.Peek(), ".model", StringComparison.OrdinalIgnoreCase)) {
nameParts.Pop();
}
if (nameParts.Count > 0) {
return nameParts.Aggregate((left, right) => left + right).TrimStart('.');
}
return String.Empty;
}
這里的循環執行兩次，第一次是執行
else if (part.NodeType == ExpressionType.MemberAccess) {
MemberExpression memberExpressionPart = (MemberExpression)part;
nameParts.Push("." + memberExpressionPart.Member.Name);
part = memberExpressionPart.Expression;
}
第二次執行
else if (part.NodeType == ExpressionType.Parameter) {
// Dev10 Bug #907611
// When the expression is parameter based (m => m.Something...), we'll push an empty
// string onto the stack and stop evaluating. The extra empty string makes sure that
// we don't accidentally cut off too much of m => m.Model.
nameParts.Push(String.Empty);
part = null;
}
當然這個方法默認的返回結果這里就是UserName了,它默認就是生成html是的id和name屬性的值。
現在我們再來看看TemplateHelper方法了
internal static string TemplateHelper(HtmlHelper html, ModelMetadata metadata, string htmlFieldName, string templateName, DataBoundControlMode mode, object additionalViewData, ExecuteTemplateDelegate executeTemplate) {
// TODO: Convert Editor into Display if model.IsReadOnly is true? Need to be careful about this because
// the Model property on the ViewPage/ViewUserControl is get-only, so the type descriptor automatically
// decorates it with a [ReadOnly] attribute...
if (metadata.ConvertEmptyStringToNull && String.Empty.Equals(metadata.Model)) {
metadata.Model = null;
}
object formattedModelValue = metadata.Model;
if (metadata.Model == null && mode == DataBoundControlMode.ReadOnly) {
formattedModelValue = metadata.NullDisplayText;
}
string formatString = mode == DataBoundControlMode.ReadOnly ? metadata.DisplayFormatString : metadata.EditFormatString;
if (metadata.Model != null && !String.IsNullOrEmpty(formatString)) {
formattedModelValue = String.Format(CultureInfo.CurrentCulture, formatString, metadata.Model);
}
// Normally this shouldn't happen, unless someone writes their own custom Object templates which
// don't check to make sure that the object hasn't already been displayed
object visitedObjectsKey = metadata.Model ?? metadata.RealModelType;
if (html.ViewDataContainer.ViewData.TemplateInfo.VisitedObjects.Contains(visitedObjectsKey)) { // DDB #224750
return String.Empty;
}
ViewDataDictionary viewData = new ViewDataDictionary(html.ViewDataContainer.ViewData) {
Model = metadata.Model,
ModelMetadata = metadata,
TemplateInfo = new TemplateInfo {
FormattedModelValue = formattedModelValue,
HtmlFieldPrefix = html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(htmlFieldName),
VisitedObjects = new HashSet<object>(html.ViewContext.ViewData.TemplateInfo.VisitedObjects), // DDB #224750
}
};
if (additionalViewData != null) {
foreach (KeyValuePair<string, object> kvp in new RouteValueDictionary(additionalViewData)) {
viewData[kvp.Key] = kvp.Value;
}
}
viewData.TemplateInfo.VisitedObjects.Add(visitedObjectsKey); // DDB #224750
return executeTemplate(html, viewData, templateName, mode, GetViewNames, GetDefaultActions);
}
這個方法其實也很簡單，獲取當前model的值，以及呈現html的format格式，最后這里從新創建了一個ViewDataDictionary實例 viewData，並且把參數中的additionalViewData也合並到這個viewData中來，把當前的值 （visitedObjectsKey也就是最后呈現給textbox的value）給添加到viewData的VisitedObjects屬性中。最 后再調用
return executeTemplate(html, viewData, templateName, mode, GetViewNames, GetDefaultActions);方法。
那么現在我們應該看看ExecuteTemplate方法了：
internal static string ExecuteTemplate(HtmlHelper html, ViewDataDictionary viewData, string templateName, DataBoundControlMode mode, GetViewNamesDelegate getViewNames, GetDefaultActionsDelegate getDefaultActions) {
Dictionary<string, ActionCacheItem> actionCache = GetActionCache(html);
Dictionary<string, Func<HtmlHelper, string>> defaultActions = getDefaultActions(mode);
string modeViewPath = modeViewPaths[mode];
foreach (string viewName in getViewNames(viewData.ModelMetadata, templateName, viewData.ModelMetadata.TemplateHint, viewData.ModelMetadata.DataTypeName)) {
string fullViewName = modeViewPath + "/" + viewName;
ActionCacheItem cacheItem;
if (actionCache.TryGetValue(fullViewName, out cacheItem)) {
if (cacheItem != null) {
return cacheItem.Execute(html, viewData);
}
}
else {
ViewEngineResult viewEngineResult = ViewEngines.Engines.FindPartialView(html.ViewContext, fullViewName);
if (viewEngineResult.View != null) {
actionCache[fullViewName] = new ActionCacheViewItem { ViewName = fullViewName };
using (StringWriter writer = new StringWriter(CultureInfo.InvariantCulture)) {
viewEngineResult.View.Render(new ViewContext(html.ViewContext, viewEngineResult.View, viewData, html.ViewContext.TempData, writer), writer);
return writer.ToString();
}
}
Func<HtmlHelper, string> defaultAction;
if (defaultActions.TryGetValue(viewName, out defaultAction)) {
actionCache[fullViewName] = new ActionCacheCodeItem { Action = defaultAction };
return defaultAction(MakeHtmlHelper(html, viewData));
}
actionCache[fullViewName] = null;
}
}
throw new InvalidOperationException(
String.Format(
CultureInfo.CurrentCulture,
MvcResources.TemplateHelpers_NoTemplate,
viewData.ModelMetadata.RealModelType.FullName
)
);
}
這里的GetActionCache方法很簡單就是從當前的context.Items中獲取一個字典數據，如果沒有就實例化一個然后加入到context.Items中。GetDefaultActions方法也很簡單
internal static Dictionary<string, Func<HtmlHelper, string>> GetDefaultActions(DataBoundControlMode mode) {
return mode == DataBoundControlMode.ReadOnly ? defaultDisplayActions : defaultEditorActions;
}
其中GetViewNames就是獲取view的名稱的一個方法：
internal static IEnumerable<string> GetViewNames(ModelMetadata metadata, params string[] templateHints) {
foreach (string templateHint in templateHints.Where(s => !String.IsNullOrEmpty(s))) {
yield return templateHint;
}
// We don't want to search for Nullable<T>, we want to search for T (which should handle both T and Nullable<T>)
Type fieldType = Nullable.GetUnderlyingType(metadata.RealModelType) ?? metadata.RealModelType;
// TODO: Make better string names for generic types
yield return fieldType.Name;
if (!metadata.IsComplexType) {
yield return "String";
}
else if (fieldType.IsInterface) {
if (typeof(IEnumerable).IsAssignableFrom(fieldType)) {
yield return "Collection";
}
yield return "Object";
}
else {
bool isEnumerable = typeof(IEnumerable).IsAssignableFrom(fieldType);
while (true) {
fieldType = fieldType.BaseType;
if (fieldType == null)
break;
if (isEnumerable && fieldType == typeof(Object)) {
yield return "Collection";
}
yield return fieldType.Name;
}
}
}
默認返回參數順序是templateHints中的view，其次就是根據參數 數據類型返回相應的默認view。我這里返回的是String。
那么現在我們回到ExecuteTemplate方法中來，
string fullViewName = modeViewPath + "/" + viewName;這句就是已經找到我們的view了，如果我們先前actionCache中包含該key就直接執行該view並返回，其次通過 ViewEngines.Engines.FindPartialView來找該view，如果找到則輸出該view並返回。否則調用默認的處理方式
if (defaultActions.TryGetValue(viewName, out defaultAction)) {
actionCache[fullViewName] = new ActionCacheCodeItem { Action = defaultAction };
return defaultAction(MakeHtmlHelper(html, viewData));
}
這里的defaultAction對應則DefaultDisplayTemplates.StringTemplate，因為我的 viewName是String，這里的MakeHtmlHelper方法是根據當前的ViewContext和viewData從新實例化一個 HtmlHelper。
DefaultEditorTemplates.StringTemplate方法非常簡單：
return html.TextBox(String.Empty,
html.ViewContext.ViewData.TemplateInfo.FormattedModelValue,
CreateHtmlAttributes("text-box single-line")).ToHtmlString();
它里面主要是調用TextBox方法：
public static MvcHtmlString TextBox(this HtmlHelper htmlHelper, string name, object value, IDictionary<string, object> htmlAttributes) {
return InputHelper(htmlHelper, InputType.Text, null, name, value, (value == null) /* useViewData */, false /* isChecked */, true /* setId */, true /* isExplicitValue */, htmlAttributes);
}
private static MvcHtmlString InputHelper(HtmlHelper htmlHelper, InputType inputType, ModelMetadata metadata, string name, object value, bool useViewData, bool isChecked, bool setId, bool isExplicitValue, IDictionary<string, object> htmlAttributes) {
string fullName = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(name);
if (String.IsNullOrEmpty(fullName)) {
throw new ArgumentException(MvcResources.Common_NullOrEmpty, "name");
}
TagBuilder tagBuilder = new TagBuilder("input");
tagBuilder.MergeAttributes(htmlAttributes);
tagBuilder.MergeAttribute("type", HtmlHelper.GetInputTypeString(inputType));
tagBuilder.MergeAttribute("name", fullName, true);
string valueParameter = Convert.ToString(value, CultureInfo.CurrentCulture);
bool usedModelState = false;
switch (inputType) {
case InputType.CheckBox:
bool? modelStateWasChecked = htmlHelper.GetModelStateValue(fullName, typeof(bool)) as bool?;
if (modelStateWasChecked.HasValue) {
isChecked = modelStateWasChecked.Value;
usedModelState = true;
}
goto case InputType.Radio;
case InputType.Radio:
if (!usedModelState) {
string modelStateValue = htmlHelper.GetModelStateValue(fullName, typeof(string)) as string;
if (modelStateValue != null) {
isChecked = String.Equals(modelStateValue, valueParameter, StringComparison.Ordinal);
usedModelState = true;
}
}
if (!usedModelState && useViewData) {
isChecked = htmlHelper.EvalBoolean(fullName);
}
if (isChecked) {
tagBuilder.MergeAttribute("checked", "checked");
}
tagBuilder.MergeAttribute("value", valueParameter, isExplicitValue);
break;
case InputType.Password:
if (value != null) {
tagBuilder.MergeAttribute("value", valueParameter, isExplicitValue);
}
break;
default:
string attemptedValue = (string)htmlHelper.GetModelStateValue(fullName, typeof(string));
tagBuilder.MergeAttribute("value", attemptedValue ?? ((useViewData) ? htmlHelper.EvalString(fullName) : valueParameter), isExplicitValue);
break;
}
if (setId) {
tagBuilder.GenerateId(fullName);
}
// If there are any errors for a named field, we add the css attribute.
ModelState modelState;
if (htmlHelper.ViewData.ModelState.TryGetValue(fullName, out modelState)) {
if (modelState.Errors.Count > 0) {
tagBuilder.AddCssClass(HtmlHelper.ValidationInputCssClassName);
}
}
tagBuilder.MergeAttributes(htmlHelper.GetUnobtrusiveValidationAttributes(name, metadata));
if (inputType == InputType.CheckBox) {
// Render an additional <input type="hidden".../> for checkboxes. This
// addresses scenarios where unchecked checkboxes are not sent in the request.
// Sending a hidden input makes it possible to know that the checkbox was present
// on the page when the request was submitted.
StringBuilder inputItemBuilder = new StringBuilder();
inputItemBuilder.Append(tagBuilder.ToString(TagRenderMode.SelfClosing));
TagBuilder hiddenInput = new TagBuilder("input");
hiddenInput.MergeAttribute("type", HtmlHelper.GetInputTypeString(InputType.Hidden));
hiddenInput.MergeAttribute("name", fullName);
hiddenInput.MergeAttribute("value", "false");
inputItemBuilder.Append(hiddenInput.ToString(TagRenderMode.SelfClosing));
return MvcHtmlString.Create(inputItemBuilder.ToString());
}
return tagBuilder.ToMvcHtmlString(TagRenderMode.SelfClosing);
}
這里的InputHelper是真正生成html字符串的地方。這個方法整體比較好理解，不過要注意這個方法里面有這么一句
tagBuilder.MergeAttributes(htmlHelper.GetUnobtrusiveValidationAttributes(name, metadata));
這句就是處理對象上面的那些驗證屬性，效果如圖：
我想大家到這里應該對EditorFor方法有個了解了吧，感覺很是復雜。