Controls Events

Introduction

An application is made of various objects or controls. During the lifetime of the application, its controls regularly send messages to the operating system to do something on their behalf. These messages are similar to human messages and must be processed appropriately. Also, most of the time, more than one application is running on the computer. The controls of such an application also send messages to the operating system. As the operating system is constantly asked to perform these assignments, because there can be so many requests presented unpredictably, the operating system leaves it up to the controls to specify what they want, when they want it, and what behavior or result they expect.

An event is the result of a message that the control needs to be dealt with. The message is what needs to be processed. The event is the action of processing the message. To process a message, it must provide at least two pieces of information: a value that carries the message and another value that indicates a method that would process the message. Both values are passed as the arguments to the event. The first must be a class derived from EventArgs. The second must be a delegate.

Event Implementation

Although there are different ways you can implement an event, there are two main ways you can initiate its coding. If the control has a default event and if you double-click it, the compiler would initiate the default event. Another technique you can use is to click the Events button in the Properties window when the form itself or the control is selected on the form. This would display a list of the events associated with a control:

The list is divided in two columns. The name of each event is displayed on the left side. You can click the name of an event to reveal a combo box. If a similar event has already been written, you can click the arrow of the combo box and select it from the list:

Similar events are those that share a behavior. Otherwise, to initiate an event double-click either the name of the event or the right field to the desired event.

When an event has been initiated, you would be transported to the Code Editor and the cursor would be positioned in the body of the event, ready to receive your instructions. To customize an event, the compiler divides its structure in three sections.

General Messages

Control Painting

While an application is opened on the screen or it needs to be shown, the operating system must display its controls. To do this, the controls colors and other visual aspects must be retrieved and restored. This is done by painting it (the control). If the form that hosts the controls was hidden somewhere such as behind another window or was minimized, when it comes up, the operating system needs to paint it.

When a control gets painted, it fires the Paint() event. The syntax of the Paint() event is:

void Paint(Object *Sender, PaintEventArgs *e);

This event is a PaintEventArgs type. To process the painting message, a PaintEventArgs argument is passed to the PaintEventHandler delegate. The PaintEventArgs parameter provides information about the area to be painted and the graphics object to paint. 

Control Resizing

When using an application, one of the actions a user can perform on a form or a control is to change its size, provided the object allows it. Also, some time to time, if possible, the user can minimize, maximize, or restore a window. Whenever any of these actions occur, the operating system must keep track of the location and size of a control. For example, if a previously minimized or maximized window is being restored, the operating system must remember where the object was previously positioned and what its dimensions were.

When the size of a control has been changed, it fires the Resize() event, which is a EventArgs type.

Keyboard Messages

Introduction

A keyboard is a hardware object attached to the computer. By default, it is used to enter recognizable symbols, letters, and other characters on a control. Each key on the keyboard displays a symbol, a letter, or a combination of those, to give an indication of what the key could be used for.

The user typically presses a key, which sends a signal to a program. The signal is analyzed to find its meaning. If the program or control that has focus is equipped to deal with the signal, it may produce the expected result. If the program or control cannot figure out what to do, it ignores the action.

Each key has a code that the operating system can recognize.

The Key Down Message

When a keyboard key is pressed, a message called KeyDown is sent. KeyDown is a KeyEventArgs type.

The Key Up Message

As opposed to the key down message that is sent when a key is down, the KeyUp message is sent when the user releases the key. Like KeyDown, KeyUp is a KeyEventArgs type.

The Key Press Message

When the user presses a key, the KeyPress message is sent. Unlike the other two keyboard messages, the key pressed for this event should (must) be a character key. KeyPress produces the KeyPressEventArgs type.

The Key argument must be a letter or a recognizable symbol. Lowercase alphabetic characters, digits, and the lower base characters such as ; , ‘ [ ] - = / are recognized as they are. For an uppercase letter or an upper base symbols, the user must press Shift + the key. The character would be identified as one entity. This means that the symbol % typed with Shift + 5 is considered as one character.

Mouse Messages

Introduction

The mouse is another object that is attached to the computer allowing the user to interact with the machine. The mouse and the keyboard can each accomplish some tasks that are not normally available on the other and both can accomplish some tasks the same way.

The mouse is equipped with two, three, or more buttons. When a mouse has two buttons, one is usually located on the left and the other is located on the right. When a mouse has three buttons, one is in the middle of the other two. The mouse is used to select a point or position on the screen. Once the user has located an item, which could also be an empty space, a letter or a word, he or she would position the mouse pointer on it.

To actually use the mouse, the user would press either the left, the middle (if any), or the right button. If the user presses the left button once, this action is called Click. If the user presses the right mouse button, the action is referred to as Right-Click. If the user presses the left button twice and very fast, the action is called Double-Click.

The Mouse Down Message

Imagine the user has located a position or an item on a document and presses one of the mouse buttons. While the button is pressed and is down, a button-down message is sent. This event is called MouseDown and is of type MouseEventArgs. To implement this event, a MouseEventArgs argument is passed to the MouseEventHandler event implementer. MouseEventArgs provides the necessary information about the event such as what button was clicked, how many times the button was clicked, and the location of the mouse.

The Mouse Up Message

After pressing a mouse button, the user usually releases it. While the button is being released, a button-up message is sent and it depends on the button, left or right, that was down. The event produced is MouseUp.

Like the MouseDown message, the MouseUp event is of type MouseEventArgs which is passed to the MouseEventHandler for processing.

The Mouse Move Message

Whenever the mouse is positioned and being moved on top of a control, a mouse event is sent. This event is called MouseMove and is of type MouseEventArgs.

Custom Message Implementation

To process Windows messages, you can use WndProc() method. Its syntax is:

virtual void WndProc(Message &Message);

In order to use this method, you must override it in your own class.