WinForms Applications

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1 Agenda WinForms Applications Writing native Windows programs Tuesday, November 2, PWindows Applications PEvents and event handlers PLayered (tiered) model of software PFocus PForm designer and controls PDialog boxes and forms PDelegates PVisual Inheritance 2 Windows vs. Console apps. Windows vs. Console apps. PA console application has a very limited userinterface < Keyboard for input < Console window (text only) for output PCompared to this a Windows program provides a much richer range of interaction methods < On-screen buttons, scroll bars, lists etc. < Multiple windows to categorize and simplify output and input 3 PThe penalty for this is a more complex application structure 4

2 Windows and Events Windows and Events Console vs. Windows PConsole programs follow an Input º Process º Output model < Easy to program PBy contrast, a Windows program can have many different forms of input < Programmer must orchestrate the various sources PEach source of input fires 'events' the program must react to Software Input Process Output 5 6 Event handlers Event handlers PEach source of events can kick off the execution of a block of code called an event handler < An event handler is simply a Sub that is called automatically by Windows in response to some external interaction Typing into a text control etc. 'Clicking' on a control Selecting an item in a list POften, the event incorporates some additional information - which key was pressed, the exact location of the mouse cursor etc. < This Information is passed in the form of parameters to the event handler 7 8

3 Layers of software PBecause of the wide range of stimuli (events) a program has to react to, the structure of a Windows application is more complex POne strategy for simplifying the structure is to try to keep application specific code and userinterface code separate < Application specific code is usually the set of classes that do the actual job required of the program, often called the 'business classes' < User-interface code accepts user input and commands and directs the business classes according to these 9 Layers of software PThese two separate parts of a system can be thought of as 'tiers' or 'layers' in a system < User-interface tier comes between the user and the business tier and is responsible for interpreting user interactions and effecting the appropriate changes in the business tier 10 A 2-Tier Application WinForms basics Handles User Interactions Implements Business Rules User- Interface Tier Business Tier PA Form is a top-level Window that is hosted and managed by the Windows operating system and which receives external interactions in the form of events PControls in Visual Basic are toolbox items that can be placed on a form to act as userinterface elements (or widgets) 11 12

4 WinForms basics The Focus PWindows considers a control to be another type of Window - a rectangular area of the screen that can receive the focus (explained later) and events PThis confuses the issue, and so the Visual Basic term 'Control' is a better description PThe system mouse can, by virtue of its pointer, be used to direct input to any visible window on a screen < The input is necessarily simple - movements, clicks, drags PThe keyboard is a richer form of input, but there is no natural way of directing its input The Focus PTo get over this, the Windows system registers which window has the current focus < In a text-oriented control, this is the cursor and acts as the insertion point for typed characters from the keyboard < In other controls, the focus changes the control's appearance slightly to indicate which control is currently receiving keyboard events < Focus can be changed using the Tab key on the keyboard < TheTabIndex of a control indicates its place in the tab order 15 Events and event handlers PEvents can originate from < Forms or controls on forms < System hardware (e.g. the real-time clock) < Other windows programs 16

5 Events and event handlers Example event handler PEvents are usually received by forms < Normal event-handlers are Subs on a form < Event-handlers can be coded into other modules, but this involves more work < A form-based event handler is usually associated with a specific event from a specific control < It has two parameters, indicating the object that sent the event (sender) and an object containing related information (e) Private Sub txtname_keypress(byval sender As Object, _ ByVal e As System.Windows.Forms.KeyPressEventArgs) _ Handles txtname.keypress Dim ch As Char ch = e.keychar() e.handled = True End Sub Example event handler Event lists PThe previous handler is set to receive KeyPress events from a text box named txtname PThe sender parameter is simply a reference to this text box PAs a KeyPress event handler, you can rightly expect that the other parameter will contain information on the key pressed -which key, whether Shift or Ctrl was pressed, etc. 19 PA typical control can source a wide range of events under different circumstances POnly events for which a handler is defined will have any effect 20

6 The Visual Studio form designer The Visual Studio form designer PThe Form-Designer performs a complex task -synchronizing the graphical design of a form with the code < The actual 'Form' is a class that inherits from the Form class. The graphical designer is simply a different view of it - a graphical class configuration panel PAs controls are added, properties changes and design wizards executed, the form code is updated to reflect the changes < Each control is a variable, and code to initialize it < Each property change is a statement that assigns a new value to a property < A double click on a control generates a skeleton event handler Windows controls PThe standard WinForms toolbox contains a wide range of controls. e.g. < Text input: TextBox, RichTextBox, ComboBox < Mouse input: CheckBox, OptionButton, ListBox, ScrollBar < Display and/or organize: Label, PictureBox, Panel < Command: Button, Menu, Timer PAll can be configured by the properties window or in code Dialog boxes PA dialog box is a form that performs complete operations < Buttons provided to confirm (usually OK) or cancel (usually Cancel) the operation < Operation typically involves a number of data values, which after editing them, the user selects OK or Cancel 23 24

7 Dialog boxes PA VB Form has AcceptButton and CancelButton properties that can refer to these buttons < This allows the DialogResult to reflect which button was pressed - True for OK, False for Cancel PA dialog box provides a Modal operation - while it is in operation, the calling code has to wait for it < This imposes a level of control over the user (one dialog at a time) 25 Forms PA dialog box is a special type of form PMore generally, any number of forms can be in operation simultaneously < This can be a more difficult scenario to control PGenerally, use a single form to input and display the properties of an object and to initiate its methods 26 Forms Delegates and event handlers W08WindowsApplication01 PA number of strategies can be taken to maintain control. e.g. < The object can 'own' (initiate, interrogate and dispose of) a form and initiate its own display, update etc. < A form can 'contain' an object, and update and display it (more suitable where one form is used to browse a collection of objects 27 PA delegate is defined as a type of Sub < A stand-in for real subs < Delegate defines name, parameters and return type, but does not contain code PThe purpose of this is to be able to include a call to a sub in code before the sub has been written < Why would you do this? - think of what an event handler is 28

8 Delegates and event handlers POnce a delegate type has been defined, a real sub can be assigned to it (just like a variable) < Then, a call to the delegate will be a call to the sub assigned to it < Basically, we want Objects to be able to pass a Method as a Parameter to other Method! < e.g. Sorting a series of values in ascending or descending order Use two separate Methods Use a Delegate that receives as a parameter a reference to the comparison method to use 29 Example of delegation The Bubble Sort Public Class CdelegateBubbleSort Public Delegate Function Comparator(ByVal element1 As Integer, _ ByVal element2 As Integer) As Boolean Public Sub SortArray(ByVal array As Integer(), ByVal Compare as Comparator) Dim i, pass As Integer For pass = 0 To array.getupperbound(0) For i = 0 To array.getupperbound(0)-1 If Compare(array(i), array(i+1)) Then Swap(array(i), array(i+1)) End If Next Next End Sub 30 Example of delegation The Bubble Sort...continuation Private Sub Swap(ByRef firstelement As Integer, _ ByRef secondelement As Integer) Dim hold As Integer hold = firstelement firstelement = secondelement secondelement = hold End Sub End Class Example of Delegates P W08WindowsApplication

9 Visual Inheritance PCode Inheritance allows a new class to be based on an existing class PA Form is a class, designed in code like any other class (form designer just displays it in a more convenient form for us) PVisual Inheritance allows us to take an initial form design (in a form builder window) and extend it by adding new controls PAs a result, the new form's code will inherit from the existing one. 33 Visual Inheritance P W08WindowsApplication03 34