Events. Event Handler Arguments 12/12/2017. EEE-425 Programming Languages (2016) 1

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1 Events Events Single Event Handlers Click Event Mouse Events Key Board Events Create and handle controls in runtime An event is something that happens. Your birthday is an event. An event in programming terminology is when something special happens. These events are so special that they are built in to the programming language. VS.NET has numerous Events that you can write code for. Event Handler Arguments Event handlers take two arguments: 1. An Object (usually sender) : Instead of sender being an integer or string variable, the type of variable set up for sender is System.Object. This stores a reference to a control (which button was clicked, for example). 2. An event arguments object (e) : An instance of type EventArgs. Class EventArgs is the base class for objects that contain event information. Events are handled by methods that live behind visual interface known as "code-behind" our job is to program these methods Form properties typically control visual appearance: Form1 form; form = new Form1(); form.windowstate = FormWindowState.Maximized; form.show(); AutoScroll BackgroundImage ControlBox FormBorderStyle (sizable?) Icon Location Size StartPosition Text (i.e. window's caption) WindowState (minimized, maximized, normal) EEE-425 Programming Languages (2016) 1

2 Actions you can perform on a form: form.hide();... form.show(); Activate: give this form the focus Close: close & release associated resources Hide: hide, but retain resources to show form later Refresh: redraw Show: make form visible on the screen, & activate ShowDialog: show modally Events you can respond to: bring up properties window double-click on event name Load: occurs just before form is shown for first time Closing: occurs as form is being closed (ability to cancel) Closed: occurs as form is definitely being closed Resize: occurs after user resizes form Click: occurs when user clicks on form's background KeyPress: occurs when form has focus & user presses key Ask user before closing form: private void Form1_Closing(object sender, System.ComponentModel.CancelEventArgs e) DialogResult r; r = MessageBox.Show("Do you really want to close?", "MyApp", MessageBoxButtons.YesNo, MessageBoxIcon.Question, MessageBoxDefaultButton.Button1); if (r == DialogResult.No) e.cancel = true; Buttons have the ability to be clicked on. When you click a button, the event that is fired is the Click Event. If you were to add a new button to a form, and then double clicked it, you would see the following code: private void button1_click(object sender, EventArgs e)... In between the round brackets, we have this: object sender, EventArgs e For example suppose we want the Button button1 to respond when clicked by showing a message box with the statement Button was clicked. n between the round brackets, we have this: object sender, EventArgs e The object keyword refers to the object which activated the event, a button in this case. This is being placed in a variable called sender. MessageBox.Show( sender.tostring() ); An event is a message sent by an object announcing that something has happened. When events occurs, information is passed to Event handlers. EEE-425 Programming Languages (2016) 2

3 EventArgs is a class. It's short for event arguments, and tells you which events was raised. The letter "e" sets up a variable to use this class. you write your line of code to this: MessageBox.Show( e.tostring() ); will then display the following: This section explains the handling of mouse events, such as clicks, presses and moves, which are generated when the mouse interacts with a control. Mouse event information is passed through class MouseEventArgs. Each mouse-event-handling method requires an Object (sender) and a MouseEventArgs object (e) as arguments. //e can use an if statement to check which mouse button was clicked. using System.Windows.Forms; Class MouseEventArgs contains information related to the mouse event, such as the x- and y-coordinates of the mouse pointer, the mouse button pressed Right, Left or Middle), the number of times the mouse was clicked Note that the x- and y-coordinates of the MouseEventArgs object are relative to the control that generated the event. Point (0,0) represents the upper-left corner of the control. The X property returns how far across, from left to right, the mouse is; the Y property returns how far down, from top to bottom, the mouse is. namespace Event private void Form1_MouseDown(object sender, MouseEventArgs e) if (e.button == MouseButtons.Left) MessageBox.Show("left button clicked"); else if (e.button == MouseButtons.Right) MessageBox.Show("Right Button clicked" ); 18 EEE-425 Programming Languages (2016) 3

4 private void panel1_mousedown(object sender, MouseEventArgs e) switch (e.button) case MouseButtons.Left: label1.text = "Left"; case MouseButtons.Middle: label1.text = "Middle"; case MouseButtons.Right: label1.text = "Right"; label2.text = Convert.ToString(e.X) + "," + Convert.ToString(e.Y); case MouseButtons.Middle: label1.text = "Middle"; OR The only thing you need is to access a property of the e variable. if (e.button == MouseButtons.Left) MessageBox.Show("left button clicked"); else if (e.button == MouseButtons.Right) MessageBox.Show("Right Button clicked" ); Key Events Generated when keys are pressed and released, there are two types: 1. KeyPress Can return a Char for any ASCII character pressed 2. KeyUp and KeyDown Test for special modifier keys Use KeyEventArgs Keyboard-Event Handling if (e.keycode == Keys.Enter) MessageBox.Show("Enter Key Pressed "); 24 EEE-425 Programming Languages (2016) 4

5 KeyPress Event using System; using System.Windows.Forms; namespace WindowsFormsApplication1 private void textbox1_keypress(object sender, KeyPressEventArgs e) if (e.keychar == (char)keys.enter) if (e.keychar == 13) using System; using System.Windows.Forms; namespace WindowsFormsApplication1 private void textbox1_keydown(object sender, KeyEventArgs e) if (e.keycode == Keys.Enter) 26 using System; using System.Windows.Forms; namespace WindowsFormsApplication1 private void textbox1_keyup(object sender, KeyEventArgs e) if (e.keycode == Keys.Enter) To see what properties the e variable has available to it, add the following to your TextBox1_KeyDown code: If (e.keycode == Keys.F1) TextBox1.Clear(); MessageBox.Show("Help!!!") 27 But these events does not fire when arrow keys are pressed. One way to get around it is to set KeyPreview as true for your Form. protected override bool ProcessCmdKey(ref Message msg, Keys keydata) switch (keydata) case Keys.Down: label1.text = "Down"; case Keys.Up: label1.text = "Up"; case Keys.Right: label1.text = "Right"; case Keys.Left: label1.text = "Left"; return base.processcmdkey(ref msg, keydata); 29 Use the Class Name and Method Name dropdown menus to create and register event handlers. The information the programmer needs to register an event is the EventArgs class (to define the event handler) and the EventHandler delegate (to register the event handler). EEE-425 Programming Languages (2016) 5

6 Mouse events (such as clicks and presses) use class MouseEventArgs (MouseEventHandler delegate) and EventArgs (EventHandler delegate). Class MouseEventArgs contains information about the x- and y-coordinates, the button used and the number of clicks. Key events are generated when keyboard s keys are pressed and released. Event KeyPress can return a Char for any ASCII character pressed. One cannot determine from a KeyPress event whether special modifier keys (such as Shift, Alt and Control) were pressed. Events KeyUp and KeyDown test for special modifier keys (using KeyEventArgs). The delegates are KeyPressEventHandler (KeyPressEventArgs) and KeyEventHandler (KeyEventArgs). Class KeyEventArgs has properties KeyCode, KeyData and KeyValue. The KeyCode property returns the key pressed, but does not give any information about modifier keys. The KeyData property includes data about modifier keys. The KeyValue property returns the key code for the key pressed as an Integer. EEE-425 Programming Languages (2016) 6