Figure 21 Owner-Owned Relationship in Visual Studio .NET

Creating QR-Code in Visual Studio .NET Figure 21 Owner-Owned Relationship
Figure 21 Owner-Owned Relationship
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When a form is activated modelessly via the Show method, by default the new form does not have an owner Setting the owner of a modeless form is a matter of setting the new form's Owner property: void button1_Click(object sender, SystemEventArgs e) { AnotherForm form = new AnotherForm(); formOwner = this; // Establish owner/owned relationship formShow(); } In the modal case, in spite of the implicit owner-owned relationship that WinForms creates, the modal form will have a null Owner property unless the Owner property is set explicitly You can do this by setting the Owner property just before the call to ShowDialog or by passing the owner form as an argument to the ShowDialog override that takes an IWin32Window[3] parameter:
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IWin32Window is an interface exposed by UI objects in WinForms that expose a Win32 HWND via the IWin32WindowHandle property
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void button1_Click(object sender, SystemEventArgs e) { AnotherForm form = new AnotherForm(); formShowDialog(this); // Passing the owner as an argument } An owner form can enumerate the list of forms it owns using the OwnedForms collection: void button1_Click(object sender, SystemEventArgs e) { AnotherForm form = new AnotherForm(); formOwner = this; formShow(); foreach( Form ownedForm in thisOwnedForms ) { MessageBoxShow(ownedFormText); } } You may have noticed that in addition to an optional owner, a form can have an optional parent, as exposed via the Parent property As it turns out, normal forms have a Parent property that is always null The one exception to this rule is MDI child forms, which I discuss later Unlike the owner-owned relationship, the parent-child relationship dictates clipping that is, a child's edge is clipped to the edge of the parent, as shown in Figure 22
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Figure 22 A Child ListBox Control Clipped to the Client Area of Its Parent Form
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The parent-child relationship is reserved for parent forms (or parent container controls) and child controls (with the exception of MDI, which is discussed later) [ Team LiB ]
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Form Lifetime
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Although the user can't see a form until either Show or ShowDialog is called, a form exists as soon as the object is created A new form object wakes up in the object's constructor , which the runtime calls when an object is first created It's during the constructor that InitializeComponent is called and therefore when all the child controls are created and initialized It's a bad idea to put custom code into the InitializeComponent function because the Designer is likely to throw it away However, if you'd like to add other controls or change anything set by the InitializeComponent method, you can do that in the constructor If the initial form implementation was generated by one of the VSNET wizards, you'll even have a helpful comment indicating where the Designer thinks that you should add your initialization code: public Form1() { // Required for Windows Form Designer support InitializeComponent(); // TODO: Add any constructor code after InitializeComponent call // Adding a control Button anotherButton = new Button(); thisControlsAdd(anotherButton); // Changing a property thisText = "Something Not Known At Design Time"; } When FormShow or FormShowDialog is called, that's the form's cue to show itself as well as all its child controls You can be notified that this has happened when the code handles the Load event: void InitializeComponent() { thisLoad += new SystemEventHandler(thisForm1_Load); } void Form1_Load(object sender, SystemEventArgs e) { MessageBoxShow("Welcome to Form1!"); } The Load event is useful for doing any final initialization right before a form is shown Also, the Load event is a good place to change the Visible property and the ShowInTaskbar property if you'd like the form to start as hidden:[4]
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Starting a form as hidden is useful for forms that need to be running but that shouldn't show themselves right away An example is a form with a notify icon in the taskbar
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void Form1_Load(object sender, EventArgs e) { // Don't show this form thisVisible = false; thisShowInTaskbar = false; }
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When a form is shown, it will become the active form It's the active form that receives keyboard input An inactive form is made active when users click on it or otherwise indicate to Windows that they would like it to be active, such as by using Alt+Tab to switch to it You can make an inactive form active programmatically by using the FormActivate method[5] When a form is made active, including when the form is first loaded, it receives the Activated event:
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Older implementations of Win32 allowed an application to set itself active on top of the currently active window, something that could be pretty annoying Modern implementations of Win32 allow an application to set a window as active only if another window in that application is currently active Some of these implementations flash a background application's button on the shell's taskbar to indicate that the application would like your attention
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void InitializeComponent() { thisActivated += new SystemEventHandler(thisForm1_Activated); } void Form1_Activated(object sender, SystemEventArgs e) { thisgameResume(); } If an application has a form that is the currently active window as far as the operating system is concerned, you can discover that using the FormActiveForm static method If FormActiveForm is null, it means that none of your application's forms is currently active To track when a form deactivates, handle the Deactivate event: void InitializeComponent() { thisDeactivate += new SystemEventHandler(thisForm1_Deactivate); } void Form1_Deactivate(object sender, SystemEventArgs e) { thisgamePause(); } If, in addition to controlling whether or not a form is active, you'd like to control its visibility, either you can use the Hide and Show methods, which set the Visible property, or you can set the Visible property directly: void hideButton_Click(object sender, SystemEventArgs e) { thisHide(); // Set Visible property indirectly thisVisible = false; // Set Visible property directly } As you might expect, there is an event that you can handle as your form flickers in and out of visual reality It's called VisibleChanged The Activated, Deactivate, and VisibleChanged events are all handy for restarting and pausing activities that require user interaction or attention, such as in a game To stop an activity altogether, you'll want to handle the Closing or the Closed event The Closing event can be canceled if users change their minds: void Form1_Closing(object sender, CancelEventArgs e) { DialogResult res = MessageBoxShow( "Abort your game ", "Game In Progress", MessageBoxButtonsYesNo); eCancel = (res == DialogResultNo); } void Form1_Closed(object sender, EventArgs e) {
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MessageBoxShow("Your game was aborted"); } Notice that during the Closing event the handler can set the CancelEventArgsCancel property to true, canceling the closing of the form This is also the best place to serialize a form's visible properties, such as size and location, before Windows closes the form On the other hand, the Closed event is merely a notification that the form has already gone away [ Team LiB ]
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