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Introduction to Xamarin

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Injecting Platform-specific Code

7:12 with Heath Hodgert

Let's see how we can get around the limitations of our Portable Class Library by injecting platform-specific code into our shared library.

New Terms:

  • DI: Dependency Injection

Further Reading:

Android Intent
Dependency Injection
Wikipedia: Dependency Injection

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    This service is something we've all done before to avoid duplicate code, 0:00
    and it's even more useful for cross platform development. 0:03
    To make this more interesting, let's use our favorite pizza parlor's 0:07
    phone number and call them after we do the calculation. 0:12
    The problem is, making a phone call with IOS is implemented different than Android. 0:16
    To solve this problem, let's create an interface that does what we want. 0:22
    Then we can implement it for each platform and 0:26
    inject it into our pizzaCalculatorService. 0:30
    In this shared project, add an interface. 0:35
    IPhoneDialer, with a single method, call(), 0:45
    which takes a string as a phone number. 0:48
    Also, change the constructor of the PizzaCalculatorService so 1:04
    requires the IPhoneDialer. 1:08
    Now add a new method PizzaCalculatorService called 1:43
    CallPizzaParlor. 1:47
    Which we'll use the IPhoneDialer call method. 1:49
    This method is known as dependency injection which is a great pattern for 2:18
    unit testing and cross-platform development. 2:23
    Dependency injection allows us to add custom implementation as needed. 2:27
    But, because we use an interface the behavior should not change. 2:32
    There's more on dependency injection in the teacher's notes. 2:37
    In the Android project, we need to implement the IPhoneDialer interface. 2:41
    In a concrete class to create a new dialer class that implements IPhoneDialer. 2:45
    The method is simple, but Android requires a context and an intent to navigate. 3:10
    The context is the running application context which is the activity. 3:16
    So we'll pass it into the constructor of the phone dialer. 3:21
    The intent is basically details to the Android 3:25
    platform to determine which view to show. 3:29
    The intent is a very important component of Android apps, and 3:34
    there is more information in the teacher's notes. 3:37
    First, we need to create a uri using the Android Framework. 3:55
    Then we need to create our intent. 4:19
    And tell it what kind of intent it is. 4:31
    Then pass in the uri. 4:41
    Then using the context, we can start an activity with that intent we created. 4:48
    Probably want to show the results of the calculation before we call the pizza 4:58
    parlor, so add another button to the app, to call the pizza parlor. 5:02
    And we go into Resources > Layouts > Main.axml. 5:07
    And again let's go to the source. 5:14
    And add a button. 5:23
    Called callButton. 5:38
    Since we added a new string call, we'll need to go back to values, 6:11
    Strings, and add a new string for the call key. 6:20
    Using strings is good practice. 6:35
    So if you ever need to change languages, you have one place to change it. 6:37
    Now let's go back to main activity so we can find the button. 6:43
    Of type button. 6:59
    With a resource ID of callButton. 7:06

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