步驟 6：加入計時器

發行項
03/01/2013

接下來，您可以將計時器加入至配對遊戲。

若要加入計時器

移至 [Windows Form 設計工具] 中的 [工具箱]。按兩下 [Timer] (在 [元件] 類別中)，並將計時器加入至表單，使其圖示出現在表單下方的灰色方塊中，如下列圖片所示。

Timer
按一下 timer1 圖示以選取計時器。將 [Interval] 屬性設為 750，但是將 [Enabled] 屬性設為 [False]。 [Interval] 屬性會告訴計時器在滴答之間要等候多久，所以這會告訴計時器在它引發第一個 Tick 事件之前要等待四分之三秒 (750 毫秒)。您不想在程式啟動時啟動計時器。而是，使用 Start() 方法，以在玩家按下第二個標籤時啟動計時器。

請按兩下 [Windows Form 設計工具] 中的 Timer 控制項圖示，以加入 Tick 事件處理常式，如下列程式碼所示。

''' <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 事件處理常式會執行下列三件事情：首先，它會呼叫 Stop() 方法來停止計時器。接著，它會使用兩個參考變數 firstClicked 和 secondClicked，取得玩家所按下的兩個標籤並使其圖示再次看不見。最後，它會將 firstClicked 和 secondClicked 參考變數重設為 Visual C# 中的 null 和 Visual Basic 中的 Nothing。這很重要，因為這就是程式本身重設的方式。現在它不會追蹤任何的 Label 控制項，而且已準備好讓玩家再度按第一下。

注意事項
Timer 物件具有可以啟動和停止計時器的 Start() 方法和 Stop() 方法。當您在 [屬性] 視窗中將計時器的 [Enabled] 屬性設為 [True] 時，計時器會在程式開始時立即開始計時。但是，當您將它設為 [False] 時，則要等到呼叫其 Start() 方法時才會開始計時。

注意事項
通常，計時器會一再引發其 Tick 事件，並使用 [Interval] 屬性來決定在滴答之間要等待多少毫秒。您可能已注意到在 Tick 事件內呼叫計時器之 Stop() 方法的方式。該方式會使計時器進入「一次性模式」(One Shot Mode)，因此在呼叫 Start() 方法時，它會等候其間隔並引發單一 Tick 事件。

若要查看作用中的新計時器，請移至程式碼編輯器並將下列程式碼加入至 label_Click() 事件處理常式方法的頂端和底端 (您會將 if 陳述式加入至頂端，而將三個陳述式加入至底端；其餘的方法則維持不變)。

''' <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();
    }
}

位於方法頂端的程式碼會檢查計時器是否藉由勾選 [Enabled] 屬性來啟動。若是如此，如果玩家按下第一個和第二個 Label 控制項而且計時器啟動了，則按下第三個控制項不會執行任何動作。

位於方法底端的程式碼會設定 secondClicked 參考變數，以便追蹤玩家所按下的第二個 Label 控制項，並將該標籤的圖示色彩設成黑色，使其能被看見。然後，它會以一次性模式啟動計時器，因此會等待 750 毫秒，而後引發 Tick 事件。計時器的 Tick 事件處理常式會接著隱藏兩個圖示，並重設 firstClicked 和 secondClicked 參考變數，以便表單準備讓玩家按下另一個圖示。

儲存並執行您的程式。按一下圖示，它會成為可見狀態。
按一下另一個圖示。它會短暫出現，然後這兩個圖示會消失。重複此動作許多次。表單現在會追蹤您按下的第一個和第二個圖示，並使用計時器在圖示消失之前暫停追蹤。

若要繼續或檢視

若要移到下一個教學課程步驟，請參閱步驟 7：讓配對保持可見。
若要回到上一個教學課程步驟，請參閱步驟 5：加入標籤參考。

共用方式為

步驟 6：加入計時器

若要加入計時器

若要繼續或檢視

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