Task - Creating a Custom Event

The base program is given on the Github Repository. Clone the repository to begin solving the task.

We are aiming to make a program which displays a random image received from an API whenever the user clicks the "Get Image" button.

The program retrieves a link of a random image from the API when the user clicks the "Get Image" button, and sets the "Source" property of the "mainImage" to that retrieved link.

The GetImage event handler method is binded to the Clicked EventHandler of the button. So whenever the user clicks the button, the program executes that method and makes a get request to the URL https://random.imagecdn.app/v1/image.

When the method receives a response from the API, it extracts the content from the response - which is expected to be a raw URL (link) of an image. And it passes that as a string into the OnApiResponse method which is to be defined by you.

The OnApiResponse method should execute the EventHandler of the apiEvent class which you're going to create. Moreover, it should pass the "content" forward into the event handler methods as an argument and for that you can use the apiEventArgs which has already been defined for you. You can store the content in the Link property of a new apiEventArgs instance and pass it as a second argument to the event handler execution.

So this will be the who flow of the program:

1. The user clicks Get Image;
2. Program makes a GET request;
3. The API responds with the URL of a random image;
4. The URL is passed as a string to OnApiResponse method of the apiEvent class;
5. The EventHandler of apiEvent class is invoked, in turn invoking all binded methods (event handlers);
6. The URL is sent the binded Event Handlers, hence invoking the SetImage method;
7. The image is shown on the screen;

Learning Objective:

You might be wondering, what was the need for creating this seemingly unnecessary "cycle". We could've simply written mainImage.Source = content directly inside the GetImage method where the API response is received, and by doing that we could've avoided creating the whole Event System around the API call. Moreover, In that case the program execution and code would've been much shorter:

1. The user clicks Get Image;
2. Program makes a GET request;
3. The API responds with a URL of a random image;
4. The new image is shown on the screen;

However one benefit of creating the apiEvent is that we can use this event globally in the application. Which allows us to bind methods to the Event Handler of this event from multiple different external classes.

You basically have to replicate the "Publisher" class from the following diagram:

Your Task

Following is a concise description of your task:

1. Create a new public class called apiEvent
2. Create a delegate type called apiEventHandler which can accept apiEventArgs as a second argument.
3. Create a public delegate using the recently defined delegate type. Call it OnResponse.
4. Create a delegate invoker method called OnApiResponse and it should accept one string argument. You may name the argument as "content".
5. Inside OnApiResponse, invoke the delegate if it's not null and pass the value of "content" to the event handler methods by using the apiEventArgs method.