Step 6: Add a Timer

  • 项目

接下来，你要向匹配游戏中添加**“Timer”**控件。计时器等待指定的毫秒数后，触发一个称为“Tick”的事件。这对于启动操作或定时重复操作很有用。在本例中，你将使用一个计时器，让玩家开始选择两个图标，而如果图标不匹配，则在短暂时间后再次隐藏这两个图标。

添加计时器

  1. 在 Windows 窗体设计器中的工具箱中，请选择**“Timer”（位于“组件”类别中），然后按 Enter 键，或双击该计时器，向窗体中添加一个计时器控件。该计时器的图标名为“Timer1”**，应显示在窗体下的空间中，如下图所示。

    Timer

    计时器

    说明说明

    如果工具箱是空的，请确保在打开工具箱前选择窗体设计器，而不是窗体的后台代码。

  2. 选择**“Timer1”图标以选中该计时器。在“属性”窗口中，从查看事件切换到查看属性。然后将计时器的“Interval”属性设置为 750，但保留“Enabled”属性的设置“False”。“Interval”**属性将通知计时器两个计时周期之间的等待时长，或何时触发 Tick 事件。值为 750 时，将通知计时器等待四分之三秒（750 毫秒）后触发 Tick 事件。只有在玩家选择第二个标签后，你才能调用 Start() 方法启动计时器。

  3. 选择 Windows 窗体设计器中的计时器控件，然后按 Enter 键或双击该计时器，以添加空的**“Tick”**事件处理程序。用下列代码替换该代码，或手动将下列代码输入到事件处理程序。

    ''' <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 事件处理程序将执行三个操作：首先，通过调用 Stop() 方法确保计时器没有运行。然后，它使用两个引用变量 firstClicked 和 secondClicked，使得玩家选择的两个标签的图标再次不可见。最后，它将 firstClicked 和 secondClicked 引用变量重置为 null（Visual C# 中）或 Nothing（Visual Basic 中）。这一步很重要，因为程序本身就是这样重置的。现在，它不跟踪任何 Label 控件，并已准备好让玩家再次选择图标。

    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。如果你在“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，如果保留该属性的设置“False”，则计时器将在 Start() 方法调用之后开始计时。通常，计时器会使用“Interval”属性确定在计时周期之间等待的毫秒数，从而反复触发其 Tick 事件。你可能已经注意到在 Tick 事件中调用计时器的 Stop() 方法的方式。这会将计时器置于“单触发模式”，意味着 Start() 方法调用时，计时器会等待指定的间隔时间，触发单个 Tick 事件，然后停止。

  4. 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。（你要将 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 Then Exit Sub 

    Dim clickedLabel = 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 Exit Sub 

        ' 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
            Exit Sub 
        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”**属性的值来检查计时器是否已启动。这样，如果玩家选择第一个和第二个 Label 控件，且计时器启动，则选择第三个控件将不会执行任何操作。

    该方法底部的代码将 secondClicked 引用变量设置为跟踪玩家选择的第二个 Label 控件，然后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，以便在等待 750 毫秒后触发单个 Tick 事件。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选择另一对图标。

  5. 保存并运行程序。选择一个图标，它将显示出来。

  6. 选择另一个图标。它会短暂显示，然后两个图标都消失。多次重复此操作。窗体现在跟踪你选择的第一个和第二个图标，并使用计时器在使图标消失之前暂停。

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