Xamarin Apps – Input/Output

Gathering User Input

In this lesson, you’ll learn how to gather input from the user and make decisions with that information. You’ll also learn how to display information to the user.

We’re also going to explore the Model-View-Controller pattern, which is present in the design of apps made on the Xamarin platform.

Model-View-Controller (MVC)

If you’re not already familiar with the MVC pattern of application development, let’s start with an overview.

Model–view–controller (MVC) is a software architectural pattern for implementing user interfaces on computers. It divides a given software application into three interconnected parts, so as to separate internal representations of information from the ways that information is presented to or accepted from the user.

You can find a great description of this design pattern here – https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller

MVC is a standard design pattern that many developers are familiar with. Some types of Web applications will benefit from the MVC framework. Others will continue to use the traditional ASP.NET application pattern that is based on Web Forms and postbacks. Other types of Web applications will combine the two approaches; neither approach excludes the other.

The MVC framework includes the following components:

  • Models. Model objects are the parts of the application that implement the logic for the application’s data domain. Often, model objects retrieve and store model state in a database. For example, a Product object might retrieve information from a database, operate on it, and then write updated information back to a Products table in a SQL Server database.In small applications, the model is often a conceptual separation instead of a physical one. For example, if the application only reads a dataset and sends it to the view, the application does not have a physical model layer and associated classes. In that case, the dataset takes on the role of a model object.
  • Views. Views are the components that display the application’s user interface (UI). Typically, this UI is created from the model data. An example would be an edit view of a Products table that displays text boxes, drop-down lists, and check boxes based on the current state of a Productobject.
  • Controllers. Controllers are the components that handle user interaction, work with the model, and ultimately select a view to render that displays UI. In an MVC application, the view only displays information; the controller handles and responds to user input and interaction. For example, the controller handles query-string values, and passes these values to the model, which in turn might use these values to query the database.

The MVC pattern helps you create applications that separate the different aspects of the application (input logic, business logic, and UI logic), while providing a loose coupling between these elements. The pattern specifies where each kind of logic should be located in the application. The UI logic belongs in the view. Input logic belongs in the controller. Business logic belongs in the model. This separation helps you manage complexity when you build an application, because it enables you to focus on one aspect of the implementation at a time. For example, you can focus on the view without depending on the business logic.

The loose coupling between the three main components of an MVC application also promotes parallel development. For example, one developer can work on the view, a second developer can work on the controller logic, and a third developer can focus on the business logic in the model.

Using Navigation in iOS

A Storyboard is a visual representation of the appearance and flow of your application. Xamarin has introduced a Designer to allow Xamarin.iOS applications to take advantage of storyboards, so you can design your application screen visually and access the views, controllers and segues with C# for more control.

I would recommend that you read through the Xamarin documentation on Storyboards, Navigation, and Segues so you are prepared for this tutorial: https://developer.xamarin.com/guides/ios/user_interface/introduction_to_storyboards/

Video walkthrough for iOS – https://www.youtube.com/watch?v=8f2mEmbLv40

Using Navigation in Xamarin Android Apps

In this section, you’ll learn how to gather input from the user and make decisions with that information. You’ll also learn how to display information to the user. We’ll also look at how to navigate between multiple views in your Android application.

Adding an Activity

In order to add code to the app that will respond when the user clicks the button, we have to add an Activity. Your app can be made up of many activities that happen together or at various times.

An activity is a single, focused thing that the user can do. Almost all activities interact with the user, so the Activity class takes care of creating a window for you in which you can place your UI

As your apps become more complex, you will need to worry about managing the lifecycle of this activities. More information on this topic is here:http://developer.android.com/training/basics/activity-lifecycle/index.html

While activities are often presented to the user as full-screen windows, they can also be used in other ways: as floating windows (via a theme with windowIsFloating set) or embedded inside of another activity (using ActivityGroup).

There are two methods almost all subclasses of Activity will implement:

  • onCreate(Bundle) is where you initialize your activity. Most importantly, here you will usually call setContentView(int) with a layout resource defining your UI, and using findViewById(int) to retrieve the widgets in that UI that you need to interact with programmatically.
  • onPause() is where you deal with the user leaving your activity. Most importantly, any changes made by the user should at this point be committed (usually to the ContentProvider holding the data).

What do you Intend to do?

From the Android documentation:

An Intent provides a facility for performing late runtime binding between the code in different applications. Its most significant use is in the launching of activities, where it can be thought of as the glue between activities. It is basically a passive data structure holding an abstract description of an action to be performed.

That’s some engineering speak, right?

So what is an Intent? An Intent is a description of an operation to be performed. An intent allows you to start an activity in another app by describing a simple action you’d like to perform (such as “view a map” or “take a picture”) in an Intent object.

This type of intent is called an implicit intent because it does not specify the app component to start, but instead specifies an action and provides some data with which to perform the action.

An Intent in Xamarin is pretty easy. You create the Intent object, provide any additional information you want to pass to the Intent and call StartActivity to get it kicked off.

var intent = new Intent(this, typeof(YourActivityHere));                


Video walkthrough for Android – https://www.youtube.com/watch?v=R5cR34f19Fk

Congrats! You’ve made your first user input/navigation app!

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