6.2 Scratchpad Learning Activity
This assignments covers Delegates (Funcs, Actions and Predicates) and the Observer Pattern.
Work through the stories in order. Use the acceptance criteria to check each feature, and complete the nested tasks using the specified names, values, and scenarios.
Scratchpad 6.2 LastName.zip
Scratchpad 6.2 LastName.zip6.2 Scratchpad Lab Walkthrough, Part 1 [46:18]
6.2 Scratchpad Lab Walkthrough, Part 1 [46:18]6.2 Scratchpad Lab Walkthrough, Part 2 [1:03:18]
6.2 Scratchpad Lab Walkthrough, Part 2 [1:03:18]6.2 Scratchpad Lab Walkthrough, Part 3 [28:34]
6.2 Scratchpad Lab Walkthrough, Part 3 [28:34]Call the CalculateArea method, passing in 42 and catching the result in a variable named area.
Call the CalculateArea method again, passing in 17 and assigning it to the area variable.
This is the way of calling methods that you have been used to.
We aren't really going to be making a class.
public delegate double CalculateAreaDelegate(int radius);While this looks like a method it is not. It is a TYPE of delegate. Delegates do not define a method body. Delegates only define TYPES (return and parameters). In this case a return of type double and a parameter of type int.
Define a delegate of type CalculateAreaDelegate named calculateArea.
Assign the calculateArea variable to the method CalculateArea.
CalculateAreaDelegate calculateArea = this.CalculateArea;There are no parenthesis after CalculateArea because we are not calling it; we are saying make the calculateArea variable a reference to the CalculateArea method.
Change the CalculateAreaDelegate to have an int return instead of a double return.
Return to the MainWindow. Note that now the method cannot be assigned to the delegate because the return type no longer matches CalculateArea.
Change the return of CalculateAreaDelegate back to double.
Define a local variable of type double named area and assign it to the call to calculateArea, passing in 42.
double area = calculateArea(42);Make another call to calculateArea, passing in 17 and assigning it to the area variable.
Func<int, double>Funcs are referred to as "syntactic sugar" at times because they do the work of a defined delegate such as the previous example, but on the fly.
Func<int, double> calculateArea = this.CalculateArea;A Func type declares an input and an output. In this case the input is an int and the output it a double. Multiple inputs can be declared such as <int, , string, double>double> in which int and string would be parameters and the double would be the return. The last type declared will always be return. The CalculateArea method can then be assigned to the Func because it takes an int and returns a double.
Because the parameter does not match the <int, , double>> type as defined by the Func it is not compatible.
Func<int, double>Action<string>Actions are the same as Funcs, but they do not have a return type.
Call WriteToConsole and pass in "Hello, world!" as the argument.
Call WriteToConsole and pass in "Goodbye." as the argument.
Paste the code below.
Action<string> myAction = this.WriteToConsole;The only thing declared in the < > for an Action is the parameter type(s).
Call myAction and pass in "Hello, world!"
Call myAction and pass in "Goodbye."
Action<string>Predicate<string>.Predicates are Funcs that return a boolean value.
Define a local variable named isMatch of type bool and assign it to the call to IsLongerThan5 and pass in "Hello" as the argument.
Assign isMatch to the call to IsLongerThan5 and pass in "Goodbye." as the argument.
Define a Predicate with a string parameter named isMatch and assign it to the IsLongerThan5 method.
Predicate<string> isLongString = this.IsLongerThan5;Since Predicates always return a bool type there is no reason to declare it in the < >
Call isLongString, passing in "Hello" and assigning the result to a local variable named isMatch of type bool.
Call isLongString again, passing in "Goodbye." and assigning it to the isMatch variable.
Predicate<string>.In the case of a paperboy, they would need to deliver a paper to every house regardless of if they wanted it or not. This is a "push" system. Things changed, however, when people would subscribe to the newspaper and then only subscribers would get papers. This is a "pull" system. This pull system (called the Observer Pattern) relieves the strain on the paperboy and gives control to the person.
Call the washingtonPost object's UpdateNews method and pass in the argument "Political Scandal...".
Call the bob object's ReadNews method and pass in this.washingtonPost.News
Do the same for the frank object.
Call UpdateNews and pass in a new headline.
Call ReadNews again for Bob and Frank.
There is a major issue here. Bob and Frank have no idea when the news updates so they would have to constantly be reading the news, waiting for it to update. This translates to a ton of wasted method calls on the Bob and Frank objects.
The ISubject interface represents the newspaper in this case and needs to know how to Register, Unregister, and Notify.
The IObserver represents the customer and they need to know when the paper (the ISubject) updates.
Have the Person class implement the IObserver interface,
Have the Publisher class implement the ISubject interface.
Assign a local variable of type Publisher named publisher to the subject as a Publisher.
Call ReadNews and pass in the publisher's News property.
List<IObserver> named observers.Call the washingtonPost object's RegisterObserver method and pass in this.bob.
Call RegisterObserver again and pass in this.frank.
Call the washingtonPost object's UpdateNews method and pass in a headline.
Set a breakpoint on the line of code that calls RegisterObserver.
Start the application and click the 'Paper observer pattern' button.
F10 over the calls to RegisterObserver. Bob and Frank should be added to the observers list.
F11 into the UpdateNews method.
F11 into the NotifyObservers method.
F11 through the loop. Both Bob and Frank should read the new headline.
Now instead of Bob and Frank continually refreshing their browser to check if the news has changed, whenever the news changes they will be altered and will read it then.
myAction += WriteToConsole2;What this does is plug in an additional method to the delegate so that both methods execute when the delegate is invoked/called.
Set a breakpoint on the line of code that calls myAction.
Start the application and click 'Action with method'.
Press F11. The debugger should step into the WriteToConsole method.
F11 through that method. The debugger should then step into the WriteToConsole2 method.
Step through the rest of the event handler. The output should be the same as below.
Event handlers are methods that are plugged into button delegates. A Click event, for instance, is a property on a button that takes an object type and a RoutedEventArgs type. What that means is that we can plug additional methods into that delegate type.
this.extra3Button.Click += this.extra2Button_Click;This line of code plugs the extra2Button_Click event handler into Button 3's Click event delegate.
Set breakpoints at the beginning of all ExtraButton event handlers.
Start the application and click 'Extra button 1'
F11 through the event handler.
Click 'Extra button 3'.
F11 through the event handler. The debugger should step into the extra2Button_Click event handler.
Click the 'Extra button 1' again and the 'Extra button 3' again.
F11 through the event handler. This time the debugger should go through the extra2Button event handler twice because it was plugged in a second time.
public Action<string> BroadcastNewsAction { get; set; }this.washingtonPost.BroadcastNewsAction += this.bob.ReadNews;If nothing has "subscribed to" or been "plugged into" the delegate a NullReferenceException would be thrown when trying to use it.
string.Format("{0} read: {1}", this.FirstName, news)
Return to each story and verify its acceptance criteria. Preserve the submission format and destination stated in the activity; a reading, setup guide, lab, video demonstration, and oral final may require different evidence.
Intro to Delegates Intro to Funcs Intro to Actions Intro to Predicates Paperboy Example Multicasting Use an Action to broadcast news.