Lecture 24
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Log into Windows/ACENET. Start MS VS and
open SimpleRPNCalculator project from last
class.
Questions?
Monday, March 14
CS 205 Programming for the Sciences - Lecture 24
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Outline
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Review: Events and event handlers
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Complete last class exercises
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KeyPress handler
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CS 205 Programming for the Sciences - Lecture 24
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Review: Events
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Input devices cause events that the GUI then
handles. Some examples:
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Mouse events include: Click, DoubleClick,
MouseDown, MouseMove, MouseUp, Rollover
Keyboard events include: KeyPress
In addition, there are software events
generated by the program. E.g., a timer event
can use to synchronize various independent
parts of a program.
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Review: Event Handlers
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Event handlers are methods that are called
when an event has occurred.
In the C# GUI designer, one way to attach a
handler to a GUI object is to double-click on it.
This causes the GUI designer to creates a
handler function stub for the most common
event for the element type and attaches it to the
element. E.g., Click event for Button.
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In-class Exercise, Last Class
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The rest of the buttons need individual
handlers, since they all do different things. We
will work through the rest of them.
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"Enter" button
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Operator buttons ("+", "-", "*", "/")
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"+/-" button
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Dot button (".")
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Clear and Clear Error buttons ("C" and "CE")
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Backspace button ("<-")
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"Enter" Button
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Double-click on the "Enter" button.
This button saves the first operand as a number
to be used by an operator. This operand must
be accessed by the operator Button handlers.
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Declare a double class variable operand1
initialized to 0
In btnEnter_Click( ), write code to parse
txtInput.Text as a double and assign it to operand1
This isn't quite enough, when we start entering
the second operand, the input textbox should
be cleared.
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"Enter" Button
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We could just clear the textbox after getting the
operand, but usually whatever is entered is
displayed until something new happens.
We can get this behavior by doing the following:
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Declare a bool class variable newInput initialized
to true.
In btnEnter_Click( ), set newInput to true.
In btnDigit_Click( ), check if newInput is true, and if
it is, clear the input textbox and set newInput to
false. This is done before appending the digit.
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Operator Buttons
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Double-click on the "+" button.
The result is to be stored as the first operand of
the next operation. Method btnAdd_Click( )
should do the following:
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Declare a local double variables operand2.
Parse txtInput.Text as a double and assign it to
operand2.
Sum operand1 and operand2 together, storing the
result back into operand1
Convert operand1 (the result) into a string (using
ToString( )) and assign it to txtInput.Text
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Operator Buttons
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As with the "Enter" button, when the next
operand is to be entered, the input textbox
needs to be clear, so we also need:
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Set newInput to true.
Write the code for the other operator buttons.
The only difference between these handlers is
the operation being performed.
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"+/-" Button
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Double-click on the "+/-" button
This button simply negates the current input.
However, since it is a string, we need to
compute the current value. This is done by:
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Declare a local double variable value
Parse txtInput.Text as a double and assign it to
value.
Negate value (i.e., value = ­value;)
Convert value into a string (using ToString( )) and
assign it to txtInput.Text
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Dot Button
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Double-click on the Dot button
This button appends a dot to the end of the
input textbox like the digit buttons do, except
that only one dot may appear in the input. This
can be ensured by using the Contains method
for strings like so:
if (!txtInput.Contains('.')) // no dot
{
// do like the digit button handler
}
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Clear and Clear Error Buttons
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Double-click on the Clear button
The Clear button is to reset the calculator to its
starting state: operand1 is 0, newInput is true,
input textbox is set to blank.
Double-click on the Clear Error button
The Clear Error button is to reset just the input
textbox to be blank.
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Backspace Button
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Double-click on the Backspace button
This button is to delete the last digit entered in
the input textbox. This can be done using the
Remove method on strings. This method has a
parameter that is the index of the character to
be removed. For the last character in the
textbox that would be txtInput.Text.Length - 1.
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Backspace Button
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Remove is used like so
txtInput.Text.Remove (txtInput.Text.Length­1);
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There is one problem with this. It causes an
error if the string is empty (Length is 0), so we
need to check for this before doing the
Remove.
At this point, we have a "working" calculator
that handles (only) two operands at a time.
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CS 205 Programming for the Sciences - Lecture 24
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KeyPress Handler
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Calculator applications usually let the user
enter input using the keyboard. This is the
usual use for a textbox. However, for our
calculator, we only want the digits and dot to be
allowed in the input textbox.
For other inputs like '*' or '+', we want the
application to do something different. To do
this we can write a KeyPress handler.
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KeyPress Handler
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To attach a KeyPress Handler to the input
textbox, go to the design view and click on the
input textbox.
In the Properties window, click on the lightning
bolt icon tab, if not already there. Scroll to the
KeyPress entry, then double-click on it.
This will attach a method named
txtInput_KeyPress as the handler for the
KeyPress event of the input textbox, and add a
stub for for this method in the code view.
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KeyPress Handler
click on input
textbox
lightning
bolt icon tab
double-click
on KeyPress
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KeyPress Handler
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The stub looks like:
private void txtInput_KeyPress(object sender,
KeyPressEventArgs e)
{
}
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As before, sender is the GUI element that is
responding to the event. e is an object that
contains various data. We are interested in the
KeyChar property that says what key (e.g., '+')
was pressed. This information is obtained
using e.KeyChar
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KeyPress Handler
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We can use a switch statement to test the value
of e.KeyChar and do the appropriate action.
For example, if it is the character '0' ... '9', we
want to do the same thing as the digit button.
We can combine the cases for these characters
together as shown on the next slide.
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KeyPress Handler
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Extracting a Method
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When a digit key is pressed, we want the
application to do the same thing as when a digit
button is clicked. Likewise for other valid keys.
Instead of cutting and pasting the code from
btnDigit_Click into the KeyPress handler, we
can extract the code in btnDigit_Click that
checks for new input and appends the digit into
a separate method that also can be called by
txtInput_KeyPress.
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Extracting a Method
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To do this extraction, do the following:
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Highlight the code to be made into a new method
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Go to the Refactor menu and select Extract Method
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The Extract Method dialog will appear asking for a
new method name. Enter one, e.g. ProcessDigit,
and click OK.
The new method definition is added to the code and
the selected code is replaced with a call to the new
method.
These steps are shown on the next slides.
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Extracting a Method
1. Select code to extract
2. Refactor -> Extract Method
3. Give new method a name, click OK
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Extracting a Method
call to extracted method
extracted method
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KeyPress Handler
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Now the KeyPress handler can call
ProcessDigit to do the appropriate action when
a digit key is pressed:
case '9':
ProcessDigit (e.KeyChar.ToString());
break;
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Note that the ProcessDigit parameter is a
string, so we convert e.KeyChar to a string by
calling the ToString( ) method.
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KeyPress Handler
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This doesn't quite work, yet. When we run the
program, there are two problems:
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The input digit gets added to both the beginning
and the end of the input textbox.
The input cursor is left after the first character in the
input textbox.
The first problem is due to a default handler
trying to handle another aspect of a keypress.
This is eliminated by setting e.Handled to true:
e.Handled = true; // no more processing needed
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KeyPress Handler
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The second problem is fixed by setting the
insertion point to after the last character in the
input textbox.
This is done by setting the SelectionStart
property to the TextLength property:
txtInput.SelectionStart = txtInput.TextLength;
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In-Class Exercise
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Extract methods for processing dot, the
operators, Enter, and backspace.
Add cases to txtInput_KeyPress to handle '.',
the operator characters ('+', '-', '*', '/'), Enter
('\r'), and backspace ('\b') that call the
appropriate method.
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