Exercise 85 in VB.NET

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Exercise 85
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The Windows focus is a very useful mechanism for making the user s interactions with a form easier Can you think of three speci c advantages controlling the input focus might have in simplifying user-interactions (Hint think of the order input is collected, what happens when an item of data obviously does not need to be entered, and what could be done if an input error was recognized)
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823 Events
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The form you manipulate at design time and all the objects you place on it provide events that act as signals sent to activate procedures on the form That can seem a
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Setting the TabIndex values for controls
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bit odd, a form sending a signal to itself, but it is this that allows for a form s core behaviour to be extended so that it can do things never thought of when the original form class was written by Microsoft Events are simply noti cations from standard signals that Windows and NET understand, such as mouse clicks and key presses, which are used to execute specially formatted subs that you write A sub that is executed in response to an event is called an event-handler, and it can take zero or more parameters that give more information about an event All of the events speci ed for the controls in VS NET pass a pair of arguments, which appear to the event-handlers as 1) a reference to the object that sent the event and 2) a reference to an EventArgs object (a member of a descendant of the EventArgs class) that contains more information about the event For example, the TextBox control sources a KeyPressed event, a handler for which appears in outline as shown in Listing 84
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Private Sub txtName_KeyPress(ByVal sender As Object, _ ByVal e As SystemWindowsFormsKeyPressEventArgs) _ Handles txtNameKeyPress Dim ch As Char ch = eKeyChar() eHandled = True End Sub Listing 84: An event-handler for the TextBoxKeyPress event
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When this event res on a form, the sender parameter will be a reference to the TextBox object that red the event The e parameter contains additional information speci c to this type of event In this case the key that was pressed to re the
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Part of the event list for a TextBox control
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event (which is being passed into the ch variable within the event-handler), and a Handled Boolean member variable, which can be used in the event-handler to indicate that the key-press has been taken care of The e parameter varies depending on the event and the control that sent it For example, a MouseMove event, which is used to indicate that the mouse is moving across a control, passes variables to give the location and button state of the mouse within the e parameter Events are coded into a class design in such a way that the Form Designer is able to provide a list of available events for an object at design time You can write code to add an event-handler to a form manually, but generally you will nd it much easier to let Visual Studio generate the outline of an event-handler for you All controls have a default event, which is the one that the Form Designer will provide a handler for if you double-click on the control at design time The default event for a Button is the Click event, for a TextBox the Change event and so on However, each control can typically source many different events To get the form designer to generate an event other than the default, simply select the control in the combo box at the top left of the form, and the desired event in the combo box at the top right (see Figure 87) Once an event-handler outline has been generated for a speci c event from a particular object, whatever code you place inside it will de ne the procedure that will be executed in response to the event An event-handler can cause other event handlers to re For example, several controls have a Changed event that is caused to re when the control s content changes in some way; a TextBox has a TextChanged event and this will change if, for any reason, the Text in the TextBox changes Therefore, an event-handler that contains a line of code like:
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