Transcript EEM336 – Microprocessors I

BIM211 – Visual Programming
Objects, Collections, and Events
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Contents
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Objects and Classes
Properties
Methods
Events
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Objects
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Objects
• C# is a true object-oriented programming
language.
• Everything you use in Visual C# is an object
– Forms and the controls you put on a form are all
objects
• All objects have attributes (called properties),
most have methods, and many have events.
• Whether creating simple applications or building
large-scale enterprise solutions, you must
understand what an object is and how it works to
be successful.
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Objects (continued)
• An object is a programming structure that
encapsulates data and functionality as a single
unit and for which the only public access is
through the programming structure’s
interfaces (properties, methods, and events).
• Whether creating simple applications or
building large-scale enterprise solutions, you
must understand what an object is and how it
works to be successful.
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“Controls”
• We used a PictureBox and two Button
objects in the last week’s class
• Both PictureBox and Button objects are
Controls
• They are different objects but they share some
properties
• The shared properties come from the class
Control.
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Properties
• The attributes of an object which specify and
return the state of the object are called
properties.
• You used some properties last week.
– What you see in Properties window are all
properties of the selected object on the form
– Text of a Button, Width and Height of a Form, and
Image of a PictureBox are all properties
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Changing Properties by Code
• By using the Properties window, you can
change the properties of an object in design
time.
• At runtime, you don’t have Properties window
• When referencing properties in code, you
specify the name of the object first, followed
by a period (.), and then the property name as
in the following syntax:
• {ObjectName}.{Property}
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Changing Properties (continued)
• If you had a button object named btnClickMe, for
example, you would reference button’s Text property
this way:
• btnClickMe.Text
• This line of code would return whatever value was
contained in the Text property of the button object
btnClickMe.
• To set a property to some value, you use an equal sign
(=). To change the Button object’s Left property, for
example, you’d use a line of code such as the following:
• btnClickMe.Left = 90;
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Storing a Property in a Temporary Variable
• The following line of code places the value of
the Text property of the button object called
btnClickMe into a temporary variable. This
statement retrieves the value of the Text
property because the Text property is
referenced on the right side of the equal sign.
• strText = btnClickMe.Text;
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Read-Only Properties
• Just as in real life, some properties can be read
but not changed.
• Those properties are called read-only properties.
• One example of a read-only property is the
Height property of the combo box control.
• Although you can view the value of the Height
property in the Properties window, you can’t
change the value—no matter how hard you try.
• If you attempt to use Visual C# code to change
the Height property, Visual C# simply changes the
value back to the default.
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Exercise
• Last week, we changed the Size of the Form in
the Properties window.
• Today, we are going to change the Width and
Height properties of the Form with Visual C#
code.
• We are going to add two buttons to the
Picture Viewer project. One button enlarges
the form when clicked, whereas the other
shrinks the form.
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Add the Buttons
Property
Value
Name
btnEnlarge
Location
338, 261
Size
21, 23
Text
^
Property
Value
Name
btnShrink
Location
365, 261
Size
21, 23
Text
v
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Enlarging Code
• Double-click the btnEnlarge button and write
the following code:
• this.Width = this.Width + 20;
• this.Height = this.Height + 20;
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Shrinking Code
• Double-click the btnShrink button and write
the following code:
• this.Width = this.Width - 20;
• this.Height = this.Height - 20;
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this
• this refers to the object to which the code
belongs (in this case, the form).
• this is a reserved word; it’s a word that you
can’t use to name objects or variables because
Visual C# has a specific meaning for it.
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Intellisense
• When you type the period, or dot, as it’s called, a
small drop-down list containing all properties and
methods that you can use appears.
• Visual Studio is smart enough to realize that this
represents the current form (more on this in a
moment), and to aid you in writing code for the
object, it gives you a drop-down list containing all
the properties and methods of the form.
• This feature is called IntelliSense.
• This prevents you from misspelling a member
name, thereby reducing compile errors.
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Intellisense
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Methods
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Understanding Methods
• In addition to properties, most objects have
methods.
• Methods are actions the object can perform,
in contrast to attributes that describe the
object.
• You can call methods like properties by using
dot:
• {ObjectName}.{MethodName}({Parameters});
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Parentheses
• Method calls in Visual C# must always have
parentheses.
• Sometimes they’ll be empty, but at other
times they’ll contain data to pass to the
method.
• Example:
• MessageBox.Show("Hello World!");
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Exercise
• We are going to modify our PictureViewer
project to include a button that, when clicked,
draws a colored border around the picture.
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Creating the Interface
• Add a new button to the form and set its properties and
location (as in pp.69).
• Write the following code into Click event:
Graphics objGraphics = this.CreateGraphics();
objGraphics.Clear(SystemColors.Control);
objGraphics.DrawRectangle(Pens.Blue,
picShowPicture.Left - 1, picShowPicture.Top - 1,
picShowPicture.Width + 1, picShowPicture.Height + 1);
objGraphics.Dispose();
• Run your program.
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Graphics Object
• A Graphics object can be used to draw some
geometrical shapes or to write some texts.
• CreateGraphics() methods of a form returns an
object which represents the client area of the
form.
• Anything drawn onto the Graphics object of the
form appears on the form.
• Notice that the values returned by a property
don’t have to be traditional values, such as
numbers or texts; they could also be objects.
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The Methods Used in the Code
• Clear() method clears the background of the
form using the color specified.
• DrawRectangle() method draws a rectangle by
using a Pen object on the specified
coordinates.
• Dispose() method cleans up the resources
used by the object (like free() in C).
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Running the Project
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Collections
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Understanding Collections
• A collection is just what its name implies: a collection
of objects.
• Collections make it easy to work with large numbers of
similar objects by enabling you to create code that
performs iterative processing on items within the
collection.
• Iterative processing is an operation that uses a loop to
perform actions on multiple objects, rather than
writing the operative code for each object.
• In addition to containing an indexed set of objects,
collections also have properties and might have
methods.
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Collections have their own properties and methods
• All collections have a
Count property.
• All collections might
have an Add() and a
Delete() method.
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Exercise
• Create a small form that tells the names of all
control it contains:
– Add some controls to the form and name them
uniquely.
– Write the code on the following slide to the Click
method of the button which is labeled as Show
Control Names.
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The Necessary Code
for (int i = 0; i < this.Controls.Count; i++)
{
MessageBox.Show("Control #" + i.ToString() +
" has the name " + this.Controls[i].Name);
}
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Another Version
int i = 1;
foreach (Control cnt in this.Controls)
{
MessageBox.Show("Control #" + i.ToString() +
" has the name " + cnt.Name);
i++;
}
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Events
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Understanding Events
• In addition to designing an interface, you have
to empower your program to perform actions
in response to both how a user interacts with
the program and how Windows interacts with
the program.
• You do this by using events.
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Event-Driven Programming
• With traditional programming languages, the first
line of code in the program executes, and the
code continues to execute in a completely
predetermined path.
• Event-driven programs have logical sections of
code placed within events. There’s no
predetermined order in which events occur; the
user often has complete control over what code
is executed in an event-driven program by
interactively triggering specific events, such as by
clicking a button.
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Event
• An event is a special kind of method used by
an object to signal state changes that might be
useful to clients (code using the object).
• Events can be called in special ways:
– Users can trigger the event (e.g. clicking a button)
– Objects can trigger their own event (e.g. timers)
– The operating system can trigger events
– You can trigger an event just by calling its method
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Exercise: Display Mouse Coordinates
• Open the PictureViewer project
• Add two labels for X and Y coordinates
• Select the picture box and create the new
MouseMove event using the Properties
window
• Enter the following code:
– lblX.Text = "X: " + e.X.ToString();
– lblY.Text = "Y: " + e.Y.ToString();
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Running the Project
• When you move the
mouse out of the
picture box, the labels
display the latest
coordinates!
• Solution: Handle the
MouseLeave event!
– lblX.Text = "";
– lblY.Text = "";
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Notes
• The event methods has two parameters:
Sender and EventArgs.
• The mouse event methods have a parameter
of type MouseEventArgs instead of EventArgs.
• Other event methods may have different type
of parameter, but still inherited from
EventArgs class.
• The EventArgs object gives useful information
related to the event.
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