接下来，您要向匹配游戏中添加计时器。
添加计时器
转到 Windows 窗体设计器中的工具箱。双击**“Timer”（在“组件”**类别中）并向窗体添加计时器，其图标出现在窗体下方的灰框中，如下图所示。
Timer
.png "计时器")
单击**“timer1”图标以选择计时器。将“Interval”属性设置为 750，但将“Enabled”属性保留设置为“False”。“Interval”**属性通知计时器在计时周期之间等待的时间长度，因此，该属性通知计时器在触发第一个 Tick 事件之前等待四分之三秒（750 毫秒）。您不希望计时器在程序启动时启动。而是在玩家单击第二个标签时使用 Start() 方法启动计时器。
双击 Windows 窗体设计器中的 Timer 控件图标以添加 Tick 事件处理程序，如下面的代码所示。
''' <summary> ''' This timer is started when the player clicks ''' two icons that don't match, ''' so it counts three quarters of a second ''' and then turns itself off and hides both icons ''' </summary> ''' <remarks></remarks> Private Sub Timer1_Tick() Handles Timer1.Tick ' Stop the timer Timer1.Stop() ' Hide both icons firstClicked.ForeColor = firstClicked.BackColor secondClicked.ForeColor = secondClicked.BackColor ' Reset firstClicked and secondClicked ' so the next time a label is ' clicked, the program knows it's the first click firstClicked = Nothing secondClicked = Nothing End Sub/// <summary> /// This timer is started when the player clicks /// two icons that don't match, /// so it counts three quarters of a second /// and then turns itself off and hides both icons /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private void timer1_Tick(object sender, EventArgs e) { // Stop the timer timer1.Stop(); // Hide both icons firstClicked.ForeColor = firstClicked.BackColor; secondClicked.ForeColor = secondClicked.BackColor; // Reset firstClicked and secondClicked // so the next time a label is // clicked, the program knows it's the first click firstClicked = null; secondClicked = null; }Tick 事件处理程序执行三个操作：首先，通过调用 Stop() 方法停止计时器。然后，它使用两个引用变量 firstClicked 和 secondClicked 来捕获玩家单击的两个标签，并再次使其图标不可见。最后，它将 firstClicked 和 secondClicked 引用变量重置为 null（Visual C# 中）或 Nothing（Visual Basic 中）。这一点很重要，因为这是该程序重置本身的方式。现在，它不跟踪任何 Label 控件，并已准备好再次捕获玩家的第一次单击。
.gif "说明")
说明Timer 对象具有 Start() 方法和 Stop() 方法，分别用以启动和停止计时器。如果您在“属性”窗口中将计时器的“Enabled”属性设置为“True”，则只要程序开始运行，计时器就会开始计时。但是，如果将该属性保留设置为“False”，则在调用计时器的 Start() 方法之前，计时器不会开始计时。
说明通常，计时器会使用“Interval”属性确定要在计时周期之间等待的毫秒数，从而反复触发其 Tick 事件。您可能已经注意到在 Tick 事件中调用计时器的 Stop() 方法的方式。它会将计时器置于“单触发模式”，这样，当调用 Start() 方法时，它会等待间隔的时间长度并触发单一 Tick 事件。
若要查看正在使用的新计时器，请转至代码编辑器，将以下代码添加到 label_Click() 事件处理程序方法的顶部和底部。（您要将 if 语句添加到顶部，将三个语句添加到底部；该方法的其余部分保持相同。）
''' <summary> ''' Every label's Click event is handled by this event handler ''' </summary> ''' <param name="sender">The label that was clicked</param> ''' <param name="e"></param> ''' <remarks></remarks> Private Sub label_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label9.Click, Label8.Click, Label7.Click, Label6.Click, Label5.Click, Label4.Click, Label3.Click, Label2.Click, Label16.Click, Label15.Click, Label14.Click, Label13.Click, Label12.Click, Label11.Click, Label10.Click, Label1.Click ' The timer is only on after two non-matching ' icons have been shown to the player, ' so ignore any clicks if the timer is running If (Timer1.Enabled = True) Then Return End If Dim clickedLabel As Label = TryCast(sender, Label) If clickedLabel IsNot Nothing Then ' If the clicked label is black, the player clicked ' an icon that's already been revealed -- ' ignore the click If (clickedLabel.ForeColor = Color.Black) Then Return End If ' If firstClicked is Nothing, this is the first icon ' in the pair that the player clicked, ' so set firstClicked to the label that the player ' clicked, change its color to black, and return If (firstClicked Is Nothing) Then firstClicked = clickedLabel firstClicked.ForeColor = Color.Black Return End If ' If the player gets this far, the timer isn't ' running and firstClicked isn't Nothing, ' so this must be the second icon the player clicked ' Set its color to black secondClicked = clickedLabel secondClicked.ForeColor = Color.Black ' If the player gets this far, the player ' clicked two different icons, so start the ' timer (which will wait three quarters of ' a second, and then hide the icons) Timer1.Start() End If End Sub/// <summary> /// Every label's Click event is handled by this event handler /// </summary> /// <param name="sender">The label that was clicked</param> /// <param name="e"></param> private void label_Click(object sender, EventArgs e) { // The timer is only on after two non-matching // icons have been shown to the player, // so ignore any clicks if the timer is running if (timer1.Enabled == true) return; Label clickedLabel = sender as Label; if (clickedLabel != null) { // If the clicked label is black, the player clicked // an icon that's already been revealed -- // ignore the click if (clickedLabel.ForeColor == Color.Black) return; // If firstClicked is null, this is the first icon // in the pair that the player clicked, // so set firstClicked to the label that the player // clicked, change its color to black, and return if (firstClicked == null) { firstClicked = clickedLabel; firstClicked.ForeColor = Color.Black; return; } // If the player gets this far, the timer isn't // running and firstClicked isn't null, // so this must be the second icon the player clicked // Set its color to black secondClicked = clickedLabel; secondClicked.ForeColor = Color.Black; // If the player gets this far, the player // clicked two different icons, so start the // timer (which will wait three quarters of // a second, and then hide the icons) timer1.Start(); } }该方法顶部的代码通过检查**“Enabled”**属性来检查计时器是否已启动。这样，如果玩家单击第一个和第二个 Label 控件，且计时器启动，则单击第三个控件将不会执行任何操作。
该方法底部的代码设置 secondClicked 引用变量，以便它跟踪玩家单击的第二个 Label 控件，它将该标签的图标颜色设置为黑色以使其可见。然后，它在单触发模式下启动计时器，以便它等待 750 毫秒，然后触发 Tick 事件。然后，计时器的 Tick 事件处理程序会隐藏这两个图标，并重置 firstClicked 和 secondClicked 引用变量，以便窗体准备就绪以供玩家单击另一个图标。
保存并运行程序。单击一个图标，它将变得可见。
单击另一个图标。它会短暂显示，然后两个图标都消失。多次重复此操作。窗体现在跟踪您单击的第一个和第二个图标，并在使图标消失之前使用计时器暂停。
继续或查看
若要转到下一个教程步骤，请参见步骤 7：保持对可见。
若要返回上一个教程步骤，请参见步骤 5：添加标签引用。