MSDN Magazine - August 2008 - (Page 28)

can be enabled or disabled. When the IncrementalSearch property is updated, it enables the timer. The handler function will call Search. Once the search is complete, the timer will be disabled. One issue with a Windows Forms timer is that the interval is calculated independently of how long it takes a handler to perform an action on the Tick event. That means that if the time it takes to execute the handler is longer than the interval, there will be another Tick event waiting to go off almost immediately. In the worst case scenario, these events will stack up and make the UI Figure 5 DataGridViewIncrementalSearch Public Class DataGridViewIncrementalSearch Inherits DataGridView Private m_search As IncrementalSearch Private WithEvents m_timer As New Timer Public Property IncrementalSearch() As IncrementalSearch Get Return m_search End Get Set(ByVal value As IncrementalSearch) m_search = value m_timer.Start() End Set End Property Public Sub New() m_timer.Interval = CInt(TimeSpan.FromSeconds(0.2).TotalMilliseconds) End Sub Protected Overrides Sub Dispose(ByVal disposing As Boolean) If disposing Then m_timer.Dispose() End If MyBase.Dispose(disposing) End Sub Private Sub OnTick(ByVal sender As Object, ByVal e As EventArgs) _ Handles m_timer.Tick m_timer.Stop() If m_search Is Nothing OrElse m_search.SearchStatus = _ SearchStatus.Completed Then Return End If Dim result = m_search.Search() DataSource = Nothing DataSource = result.AllValues m_timer.Start() End Sub End Class unresponsive. In order to work around this, you can disable the timer while calling Search and re-enable it if there is more data to process (see Figure 6). No explicit column selection functionality was created for this DataGridView. For this example, I’ll let data binding choose the columns. One quirk in the code is the updating of the data source. The underlying IncrementalSearch object creates a List(Of Object) to hold the return values from the query. Every Result object contains a reference to this list instead of a copy. When updating the Data source property, the DataGridView will see that it is the same Object and will not refresh the displayed results. Setting the reference to Nothing in between will force it to reload the data. While this is a nice, easy-to-use solution, it won’t work in all scenarios. The underlying problem here is user delay. Because this is a single-threaded solution, there is no way to break the execution once you call MoveNext. Therefore, the longest you will delay the user from typing is now the DelayTime value for the IncrementalSearch class plus the longest time to find one element in the search. Sorting and Lazy Evaluation Most LINQ queries operate with lazy evaluation, which means that only a single element is processed each time MoveNext is called on the enumerator. The time it takes to find one element in the search is the same time it takes to find the next element. This allows for the result to be efficiently broken up into usable blocks of data: Dim e = From method In doc _ Where method.@Name.Contains(name) _ Where String.Compare(method.@Kind, "Method") = 0 _ Select MethodName = method.@Name ' Returns when the first matching is found Dim first = e.First() This model will break down if the underlying LINQ query processes a substantial portion or all of the values during a single call to MoveNext. This form of query is known as eager evaluation. An example of this is the Order By operator, which returns the results in a sorted fashion. Because it is sorted, the first value cannot be returned until all parts of the query have finished executing. Otherwise it couldn’t know which was the first element to return because the last element returned could be the first in terms of sorting. If you try the incremental search pattern with a query using eager evaluation, you are right back at the original problem: Dim e = From method In doc _ Where method.@Name.Contains(name) _ Where String.Compare(method.@Kind, "Method") = 0 _ Order By method.@Name _ Select MethodName = method.@Name ' The following line won't finish until the entire query is finished ' processing Dim first = e.First() Figure 6 Enabling and Disabling the Search Timer Private Sub OnTick(ByVal sender As Object, ByVal e As EventArgs) _ Handles m_timer.Tick m_timer.Stop() If m_search Is Nothing _ OrElse m_search.SearchStatus = SearchStatus.Completed Then ' Search is completed don't restart the timer Return End If ' Get the next update from the incremental search Dim result = m_search.Search() DataSource = Nothing DataSource = result.AllValues ' Restart the timer to perform another search m_timer.Start() End Sub Once again, the advantage here is in only dealing with a subset of the rows returned. Since the code is only displaying a subset of the data to the user, that is the only data that needs to be sorted at any given time. The underlying LINQ query can return the data unsorted, and then the code can sort it as more comes along. It is easy to modify the IncrementalSearch class to sort the data at the end of each Search call. There are two approaches available to searching the data. The first approach is to take advantage of the built-in List(Of T).Sort method. This has an average comBasic Instincts 28 msdn magazine

Table of Contents Feed for the Digital Edition of MSDN Magazine - August 2008

MSDN Magazine - August 2008 Toolbox CLR Inside Out Basic Instincts Cutting Edge Patterns in Practice Data 2.0 - Expose And Consume Data In A Web Services World Biztalk EDI - Build A Robust EDI Solution With BizTalk Server Silverlight - Create Animations With XAML And Expression Blend Write On! - Create Web Apps You Can Draw On With Silverlight 2 Wicked Code - Craft Custom Controls For Silverlight 2 Team System Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - August 2008

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