WPF原始碼分析系列一:剖析WPF模板機制的內部實現(四)
(注:本文是《剖析WPF模板機制的內部實現》系列文章的第四篇,檢視上一篇文章請點這裡)
4. DataTemplate
上一篇文章我們討論了ItemsPanelTemplate類,這一篇和下一篇將討論DataTemplate類。
DataTemplate型別的變數非常多,主要有:
ComboBox.SelectionBoxItemTemplate
ContentControl.ContentTemplate
ContentPresenter.ContentTemplate
ContentPresenter.Template
DataGrid.RowHeaderTemplate
DataGridColumn.HeaderTemplate
DataGridRow.HeaderTemplate
DataGridRow.DetailsTemplate
DataGridTemplateColumn.CellTemplate
DataGridTemplateColumn.CellEditingTemplate
GridView.ColumnHeaderTemplate
GridViewColumn.HeaderTemplate
GridViewColumn.CellTemplate
GridViewHeaderRowPresenter.ColumnHeaderTemplate
GroupStyle.HeaderTemplate
HeaderedContentControl.HeaderTemplate
HeaderedItemsControl.HeaderTemplate
HierarchicalDataTemplate.ItemTemplate
ItemsControl.ItemTemplate
TabControl.SelectedContentTemplate
TabControl.ContentTemplate
我們這裡只重點分析比較重要和有代表性的三個:ContentControl.ContentTemplate,ContentPresenter.ContentTemplate和ItemsControl.ItemTemplate。本篇文章只討論前兩個,ItemsControl.ItemTemplate
4.1)ContentControl.ContentTemplate和ContentPresenter.ContentTemplate
ContentControl和ContentPresenter的父類分別是Control和FrameworkElement,這意味著ContentControl也繼承了Control.Template屬性和模板選擇機制。那麼ContentControl.ContentTemplate屬性和其Template屬性究竟有什麼關係?ContentControl和ContentPresenter的ContentTemplate屬性在模板應用的角色是什麼,二者又有什麼聯絡?
要回答這些問題,我們先看ContentPresenter.ContentTemplate的定義:
//************ContentPresenter.cs**************public static readonly DependencyProperty ContentTemplateProperty = ContentControl.ContentTemplateProperty.AddOwner( typeof(ContentPresenter), new FrameworkPropertyMetadata( (DataTemplate)null, FrameworkPropertyMetadataOptions.AffectsMeasure, new PropertyChangedCallback(OnContentTemplateChanged))); /// <summary> /// ContentTemplate is the template used to display the content of the control. /// </summary> public DataTemplate ContentTemplate { get { return (DataTemplate) GetValue(ContentControl.ContentTemplateProperty); } set { SetValue(ContentControl.ContentTemplateProperty, value); } } private static void OnContentTemplateChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) { ContentPresenter ctrl = (ContentPresenter)d; ctrl._templateIsCurrent = false; ctrl.OnContentTemplateChanged((DataTemplate) e.OldValue, (DataTemplate) e.NewValue); } protected virtual void OnContentTemplateChanged(DataTemplate oldContentTemplate, DataTemplate newContentTemplate) { Helper.CheckTemplateAndTemplateSelector("Content", ContentTemplateProperty, ContentTemplateSelectorProperty, this); // if ContentTemplate is really changing, remove the old template this.Template = null; }
首先可以注意到依賴屬性ContentTemplateProperty的註冊沒有使用DependencyProperty.Register(),而是用的ContentControl.ContentTemplateProperty.AddOwner(),此外ContentTemplate的讀寫也是用的ContentControl.ContentTemplateProperty。這意味著如果ContentPresenter處在ContentControlContent的visual tree上,那麼其ContentTemplateProperty屬性將繼承ContentControl.ContentTemplateProperty的值。這就是WPF中依賴屬性的繼承。利用同樣的方法,ContentPresenter還繼承了ContentControl.ContentProperty屬性。而我們還知道,就像ItemsControl的預設Template會包含一個ItemsPresenter控制元件(參見上一篇文章),ContentControl的預設Template模板也包含一個ContentPresenter控制元件。這意味著當ContentControl在應用模板生成visual tree時,將建立一個ContentPresenter控制元件,並把自己的ContentTemplate和Content屬性的值傳遞給它的ContentPresenter控制元件,進而觸發其呼叫自己的ApplyTemplate。ContentControl(以及ItemsPresenter)的模板應用就是這樣一個大概可以分為兩個步驟的級聯過程。
ContentControl的模板應用機制這樣基本就清楚了,不過為了搞清楚這個級聯過程的第二個步驟,我們需要進一步剖析一下ContentPresenter的模板應用機制。
首先,從回撥函式可以看出,一旦ContentPresenter.ContentTemplate屬性被改變,無論這種任何變化,ContentPresenter.Template屬性都將被清空。這個屬性的定義如下:
//***********ContentPresenter.cs************** internal static readonly DependencyProperty TemplateProperty = DependencyProperty.Register( "Template", typeof(DataTemplate), typeof(ContentPresenter), new FrameworkPropertyMetadata( (DataTemplate) null, // default value FrameworkPropertyMetadataOptions.AffectsMeasure, new PropertyChangedCallback(OnTemplateChanged))); private DataTemplate Template { get { return _templateCache; } set { SetValue(TemplateProperty, value); } } // Internal helper so FrameworkElement could see call the template changed virtual internal override void OnTemplateChangedInternal(FrameworkTemplate oldTemplate, FrameworkTemplate newTemplate) { OnTemplateChanged((DataTemplate)oldTemplate, (DataTemplate)newTemplate); } private static void OnTemplateChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) { ContentPresenter c = (ContentPresenter) d; StyleHelper.UpdateTemplateCache(c, (FrameworkTemplate) e.OldValue, (FrameworkTemplate) e.NewValue, TemplateProperty); }
這裡可以看到,ContentPresenter.Template與Control.Template、ItemsPresenter的定義如出一轍,都以_templateCache欄位作為支撐欄位,只不過Template的型別這次被換成了DataTemplate。不過與ItemsPresenter相同,ContentPresenter類也覆寫了FrameworkElement.OnPreApplyTemplate()方法,自定義了一個模板選擇機制。這個方法先是呼叫EnsureTemplate(),而後者接著又呼叫了ChooseTemplate()來根據一定的優先順序來選擇一個合適的DataTemplate,並用這個確定是非空的模板更新其Template屬性進而_templateCache欄位,從而保證Framework在呼叫ApplyTemplate()時TemplateInternal是非空的。
這裡有必要貼一下ContentPresenter.ChooseTemplate()方法的原始碼,看一下ContentPresenter選擇模板的優先順序:
//***********ContentPresenter************** /// <summary> /// Return the template to use. This may depend on the Content, or /// other properties. /// </summary> /// <remarks> /// The base class implements the following rules: /// (a) If ContentTemplate is set, use it. /// (b) If ContentTemplateSelector is set, call its /// SelectTemplate method. If the result is not null, use it. /// (c) Look for a DataTemplate whose DataType matches the /// Content among the resources known to the ContentPresenter /// (including application, theme, and system resources). /// If one is found, use it. /// (d) If the type of Content is "common", use a standard template. /// The common types are String, XmlNode, UIElement. /// (e) Otherwise, use a default template that essentially converts /// Content to a string and displays it in a TextBlock. /// Derived classes can override these rules and implement their own. /// </remarks> protected virtual DataTemplate ChooseTemplate() { DataTemplate template = null; object content = Content; // ContentTemplate has first stab template = ContentTemplate; // no ContentTemplate set, try ContentTemplateSelector if (template == null) { if (ContentTemplateSelector != null) { template = ContentTemplateSelector.SelectTemplate(content, this); } } // if that failed, try the default TemplateSelector if (template == null) { template = DefaultTemplateSelector.SelectTemplate(content, this); } return template; }
可以看出,ContentPresenter在選擇Template時,會優先選擇ContentTemplate,如果為空,則會嘗試呼叫ContentTemplateSelector.SelectTemplate()(DataTemplateSelector型別),如果再失敗,會嘗試呼叫其DefaultTemplateSelector.SelectTemplate()方法。
靜態屬性DefaultTemplateSelector是DefaultSelector型別 ,後者又繼承自DataTemplateSelector類。DefaultSelector在覆寫DataTemplateSelector.SelectTemplate()方法時引入了一套複雜的模板選擇規則,以確保最終可以返回一個有效的DataTemplate:
//*******************DefaultSelector*********************** /// <summary> /// Override this method to return an app specific <seealso cref="Template"/>. /// </summary> /// <param name="item">The data content</param> /// <param name="container">The container in which the content is to be displayed</param> /// <returns>a app specific template to apply.</returns> public override DataTemplate SelectTemplate(object item, DependencyObject container) { DataTemplate template = null; // Lookup template for typeof(Content) in resource dictionaries. if (item != null) { template = (DataTemplate)FrameworkElement.FindTemplateResourceInternal(container, item, typeof(DataTemplate)); } // default templates for well known types: if (template == null) { TypeConverter tc = null; string s; if ((s = item as string) != null) template = ((ContentPresenter)container).SelectTemplateForString(s); else if (item is UIElement) template = UIElementContentTemplate; else if (SystemXmlHelper.IsXmlNode(item)) template = ((ContentPresenter)container).SelectTemplateForXML(); else if (item is Inline) template = DefaultContentTemplate; else if (item != null && (tc = TypeDescriptor.GetConverter(ReflectionHelper.GetReflectionType(item))) != null && tc.CanConvertTo(typeof(UIElement))) template = UIElementContentTemplate; else template = DefaultContentTemplate; } return template; } }
至此,ContentPresenter在模板應用中的角色也一目瞭然了。可以看到ContentPresenter和ItemsPresenter的模板應用機制大同小異,它們都定義了一個_templateCache欄位,並都覆寫了FrameworkElement.TemplateInternal屬性,使其返回值為_templateCache欄位,同時又都覆寫FrameworkElement.OnPreApplyTemplate()方法,來嵌入一個自定義的模板選擇機制,並用選定的模板更新_templateCache欄位。於是,它們利用這種模式實現了FrameworkElement.TemplateInternal屬性的多型性,從而變相實現了FrameworkElement.ApplyTemplate()的多型性。
至此,兩個重要的DataTemplate型別ContentControl.ContentTemplate和ContentPresenter.ContentTemplate就介紹了完畢,下一篇文章將介紹DataTemplate型別的另一個重要變數ItemsControl.ItemTemplate。