ASP 2.0 数据绑定函数Eval()的机制_Asp.Net开发技巧

时间：2010-09-16 来源：缘分星空

Author: Walkdan (walkdan(at)gmail.com)

ASP.NET 2.0 的Eval()简化了掉了ASP 1.1 Container.DataItem，比如：

<%# (Container.DataItem as DataRowView)["ProductName"].ToString() %>

简化为：(去掉了类型指定 ASP 1.1, Eval通过反射实现，本文不再阐述)
<%# DataBinder.Eval(Container.DataItem, "ProductName").ToString() %>

又简化为(ASP 2.0)：
<%# Eval("ProductName") %>

Eval()是TemplateControl:Page 的方法
TemplateControl.Eval()可以自动计算出Container, 机制就是从一个dataBindingContext:Stack堆栈来获取。

1. 建立DataItem Container 栈：
    在Control.DataBind()中，建立，这样可以保证子控件的DataItem Container始终在栈顶。

    public class Control
    {
        protected virtual void DataBind(bool raiseOnDataBinding)
        {
            bool foundDataItem = false;
            if (this.IsBindingContainer)
            {
                object o = DataBinder.GetDataItem(this, out foundDataItem);
                if (foundDataItem)
                    Page.PushDataItemContext(o);        <-- 将DataItem压入堆栈
            }
            try
            {
                if (raiseOnDataBinding)
                    OnDataBinding(EventArgs.Empty);

                DataBindChildren();                     <-- 绑定子控件
            }
            finally
            {
                if (foundDataItem)
                    Page.PopDataItemContext();          <-- 将DataItem弹出堆栈
            }
        }
    }

2. 获取DataItem Container
    public class Page
    {
        public object GetDataItem()
        {
            ...
            return this._dataBindingContext.Peek();     <-- 读取堆栈顶部的DataItem Container，就是正在绑定的DataItem Container
        }
     }

   3. TemplateContro.Eval()
    public class TemplateControl
    {
        protected string Eval (string expression, string format)
        {
            return DataBinder.Eval (Page.GetDataItem(), expression, format);
        }
    }

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