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ViewPager with Fragments

  • Artikkeli

ViewPager is a layout manager that lets you implement gestural navigation. Gestural navigation allows the user to swipe left and right to step through pages of data. This guide explains how to implement a swipeable UI with ViewPager, using Fragments as the data pages.

Overview

ViewPager is often used in conjunction with fragments so that it is easier to manage the lifecycle of each page in the ViewPager. In this walkthrough, ViewPager is used to create an app called FlashCardPager that presents a series of math problems on flash cards. Each flash card is implemented as a fragment. The user swipes left and right through the flash cards and taps on a math problem to reveal its answer. This app creates a Fragment instance for each flash card and implements an adapter derived from FragmentPagerAdapter. In Viewpager and Views, most of the work was done in MainActivity lifecycle methods. In FlashCardPager, most of the work will be done by a Fragment in one of its lifecycle methods.

This guide does not cover the basics of fragments – if you are not yet familiar with fragments in Xamarin.Android, see Fragments to help you get started with fragments.

Start an App Project

Create a new Android project called FlashCardPager. Next, launch the NuGet Package Manager (for more information about installing NuGet packages, see Walkthrough: Including a NuGet in your project). Find and install the Xamarin.Android.Support.v4 package as explained in Viewpager and Views.

Add an Example Data Source

In FlashCardPager, the data source is a deck of flash cards represented by the FlashCardDeck class; this data source supplies the ViewPager with item content. FlashCardDeck contains a ready-made collection of math problems and answers. The FlashCardDeck constructor requires no arguments:

FlashCardDeck flashCards = new FlashCardDeck();

The collection of flash cards in FlashCardDeck is organized such that each flash card can be accessed by an indexer. For example, the following line of code retrieves the fourth flash card problem in the deck:

string problem = flashCardDeck[3].Problem;

This line of code retrieves the corresponding answer to the previous problem:

string answer = flashCardDeck[3].Answer;

Because the implementation details of FlashCardDeck are not relevant to understanding ViewPager, the FlashCardDeck code is not listed here. The source code to FlashCardDeck is available at FlashCardDeck.cs. Download this source file (or copy and paste the code into a new FlashCardDeck.cs file) and add it to your project.

Create a ViewPager Layout

Open Resources/layout/Main.axml and replace its contents with the following XML:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.view.ViewPager
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/viewpager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    </android.support.v4.view.ViewPager>

This XML defines a ViewPager that occupies the entire screen. Note that you must use the fully-qualified name android.support.v4.view.ViewPager because ViewPager is packaged in a support library. ViewPager is available only from the Android Support Library v4; it is not available in the Android SDK.

Set up ViewPager

Edit MainActivity.cs and add the following using statements:

using Android.Support.V4.View;
using Android.Support.V4.App;

Change the MainActivity class declaration so that it is derived from FragmentActivity:

public class MainActivity : FragmentActivity

MainActivity is derived fromFragmentActivity (rather than Activity) because FragmentActivity knows how to manage the support of fragments. Replace the OnCreate method with the following code:

protected override void OnCreate(Bundle bundle)
{
    base.OnCreate(bundle);
    SetContentView(Resource.Layout.Main);
    ViewPager viewPager = FindViewById<ViewPager>(Resource.Id.viewpager);
    FlashCardDeck flashCards = new FlashCardDeck();
}

This code does the following:

  1. Sets the view from the Main.axml layout resource.

  2. Retrieves a reference to the ViewPager from the layout.

  3. Instantiates a new FlashCardDeck as the data source.

When you build and run this code, you should see a display that resembles the following screenshot:

Screenshot of FlashCardPager app with empty ViewPager

At this point, the ViewPager is empty because it is lacking the fragments that are used populate the ViewPager, and it is lacking an adapter for creating these fragments from the data in FlashCardDeck.

In the following sections, a FlashCardFragment is create to implement the functionality of each flash card, and a FragmentPagerAdapter is created to connect the ViewPager to the fragments created from data in the FlashCardDeck.

Create the Fragment

Each flash card will be managed by a UI fragment called FlashCardFragment. FlashCardFragment's view will display the information contained with a single flash card. Each instance of FlashCardFragment will be hosted by the ViewPager. FlashCardFragment's view will consist of a TextView that displays the flash card problem text. This view will implement an event handler that uses a Toast to display the answer when the user taps the flash card question.

Create the FlashCardFragment Layout

Before FlashCardFragment can be implemented, its layout must be defined. This layout is a fragment container layout for a single fragment. Add a new Android layout to Resources/layout called flashcard_layout.axml. Open Resources/layout/flashcard_layout.axml and replace its contents with the following code:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/flash_card_question"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:textAppearance="@android:style/TextAppearance.Large"
            android:textSize="100sp"
            android:layout_centerHorizontal="true"
            android:layout_centerVertical="true"
            android:text="Question goes here" />
    </RelativeLayout>

This layout defines a single flash card fragment; each fragment is comprised of a TextView that displays a math problem using a large (100sp) font. This text is centered vertically and horizontally on the flash card.

Create the Initial FlashCardFragment Class

Add a new file called FlashCardFragment.cs and replace its contents with the following code:

using System;
using Android.OS;
using Android.Views;
using Android.Widget;
using Android.Support.V4.App;

namespace FlashCardPager
{
    public class FlashCardFragment : Android.Support.V4.App.Fragment
    {
        public FlashCardFragment() { }

        public static FlashCardFragment newInstance(String question, String answer)
        {
            FlashCardFragment fragment = new FlashCardFragment();
            return fragment;
        }
        public override View OnCreateView (
            LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
        {
            View view = inflater.Inflate (Resource.Layout.flashcard_layout, container, false);
            TextView questionBox = (TextView)view.FindViewById (Resource.Id.flash_card_question);
            return view;
        }
    }
}

This code stubs out the essential Fragment definition that will be used to display a flash card. Note that FlashCardFragment is derived from the support library version of Fragment defined in Android.Support.V4.App.Fragment. The constructor is empty so that the newInstance factory method is used to create a new FlashCardFragment instead of a constructor.

The OnCreateView lifecycle method creates and configures the TextView. It inflates the layout for the fragment's TextView and returns the inflated TextView to the caller. LayoutInflater and ViewGroup are passed to OnCreateView so that it can inflate the layout. The savedInstanceState bundle contains data that OnCreateView uses to recreate the TextView from a saved state.

The fragment's view is explicitly inflated by the call to inflater.Inflate. The container argument is the view's parent, and the false flag instructs the inflater to refrain from adding the inflated view to the view's parent (it will be added when ViewPager call's the adapter's GetItem method later in this walkthrough).

Add State Code to FlashCardFragment

Like an Activity, a fragment has a Bundle that it uses to save and retrieve its state. In FlashCardPager, this Bundle is used to save the question and answer text for the associated flash card. In FlashCardFragment.cs, add the following Bundle keys to the top of the FlashCardFragment class definition:

private static string FLASH_CARD_QUESTION = "card_question";
private static string FLASH_CARD_ANSWER = "card_answer";

Modify the newInstance factory method so that it creates a Bundle object and uses the above keys to store the passed question and answer text in the fragment after it is instantiated:

public static FlashCardFragment newInstance(String question, String answer)
{
    FlashCardFragment fragment = new FlashCardFragment();

    Bundle args = new Bundle();
    args.PutString(FLASH_CARD_QUESTION, question);
    args.PutString(FLASH_CARD_ANSWER, answer);
    fragment.Arguments = args;

    return fragment;
}

Modify the fragment lifecycle method OnCreateView to retrieve this information from the passed-in Bundle and load the question text into the TextBox:

public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    string question = Arguments.GetString(FLASH_CARD_QUESTION, "");
    string answer = Arguments.GetString(FLASH_CARD_ANSWER, "");

    View view = inflater.Inflate(Resource.Layout.flashcard_layout, container, false);
    TextView questionBox = (TextView)view.FindViewById(Resource.Id.flash_card_question);
    questionBox.Text = question;

    return view;
}

The answer variable is not used here, but it will be used later when event handler code is added to this file.

Create the Adapter

ViewPager uses an adapter controller object that sits between the ViewPager and the data source (see the illustration in the ViewPager Adapter article). To access this data, ViewPager requires that you provide a custom adapter derived from PagerAdapter. Because this example uses fragments, it uses a FragmentPagerAdapter – FragmentPagerAdapter is derived from PagerAdapter. FragmentPagerAdapter represents each page as a Fragment that is persistently kept in the fragment manager for as long as the user can return to the page. As the user swipes through pages of the ViewPager, the FragmentPagerAdapter extracts information from the data source and uses it to create Fragments for the ViewPager to display.

When you implement a FragmentPagerAdapter, you must override the following:

  • Count – Read-only property that returns the number of views (pages) available.

  • GetItem – Returns the fragment to display for the specified page.

Add a new file called FlashCardDeckAdapter.cs and replace its contents with the following code:

using System;
using Android.Views;
using Android.Widget;
using Android.Support.V4.App;

namespace FlashCardPager
{
    class FlashCardDeckAdapter : FragmentPagerAdapter
    {
        public FlashCardDeckAdapter (Android.Support.V4.App.FragmentManager fm, FlashCardDeck flashCards)
            : base(fm)
        {
        }

        public override int Count
        {
            get { throw new NotImplementedException(); }
        }

        public override Android.Support.V4.App.Fragment GetItem(int position)
        {
            throw new NotImplementedException();
        }
    }
}

This code stubs out the essential FragmentPagerAdapter implementation. In the following sections, each of these methods is replaced with working code. The purpose of the constructor is to pass the fragment manager to the FlashCardDeckAdapter's base class constructor.

Implement the Adapter Constructor

When the app instantiates the FlashCardDeckAdapter, it supplies a reference to the fragment manager and an instantiated FlashCardDeck. Add the following member variable to the top of the FlashCardDeckAdapter class in FlashCardDeckAdapter.cs:

public FlashCardDeck flashCardDeck;

Add the following line of code to the FlashCardDeckAdapter constructor:

this.flashCardDeck = flashCards;

This line of code stores the FlashCardDeck instance that the FlashCardDeckAdapter will use.

Implement Count

The Count implementation is relatively simple: it returns the number of flash cards in the flash card deck. Replace Count with the following code:

public override int Count
{
    get { return flashCardDeck.NumCards; }
}

The NumCards property of FlashCardDeck returns the number of flash cards (number of fragments) in the data set.

Implement GetItem

The GetItem method returns the fragment associated with the given position. When GetItem is called for a position in the flash card deck, it returns a FlashCardFragment configured to display the flash card problem at that position. Replace the GetItem method with the following code:

public override Android.Support.V4.App.Fragment GetItem(int position)
{
    return (Android.Support.V4.App.Fragment)
        FlashCardFragment.newInstance (
            flashCardDeck[position].Problem, flashCardDeck[position].Answer);
}

This code does the following:

  1. Looks up the math problem string in the FlashCardDeck deck for the specified position.

  2. Looks up the answer string in the FlashCardDeck deck for the specified position.

  3. Calls the FlashCardFragment factory method newInstance, passing in the flash card problem and answer strings.

  4. Creates and returns a new flash card Fragment that contains the question and answer text for that position.

When the ViewPager renders the Fragment at position, it displays the TextBox containing the math problem string residing at position in the flash card deck.

Add the Adapter to the ViewPager

Now that the FlashCardDeckAdapter is implemented, it's time to add it to the ViewPager. In MainActivity.cs, add the following line of code to the end of the OnCreate method:

FlashCardDeckAdapter adapter =
    new FlashCardDeckAdapter(SupportFragmentManager, flashCards);
viewPager.Adapter = adapter;

This code instantiates the FlashCardDeckAdapter, passing in the SupportFragmentManager in the first argument. (The SupportFragmentManager property of FragmentActivity is used to get a reference to the FragmentManager – for more information about the FragmentManager, see Managing Fragments.)

The core implementation is now complete – build and run the app. You should see the first image of the flash card deck appear on the screen as shown on the left in the next screenshot. Swipe left to see more flash cards, then swipe right to move back through the flash card deck:

Example screenshots of FlashCardPager app without pager indicators

Add a Pager Indicator

This minimal ViewPager implementation displays each flash card in the deck, but it provides no indication as to where the user is within the deck. The next step is to add a PagerTabStrip. The PagerTabStrip informs the user as to which problem number is displayed and provides navigation context by displaying a hint of the previous and next flash cards.

Open Resources/layout/Main.axml and add a PagerTabStrip to the layout:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

  <android.support.v4.view.PagerTabStrip
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_gravity="top"
      android:paddingBottom="10dp"
      android:paddingTop="10dp"
      android:textColor="#fff" />

</android.support.v4.view.ViewPager>

When you build and run the app, you should see the empty PagerTabStrip displayed at the top of each flash card:

Closeup of PagerTabStrip without text

Display a Title

To add a title to each page tab, implement the GetPageTitleFormatted method in the adapter. ViewPager calls GetPageTitleFormatted (if implemented) to obtain the title string that describes the page at the specified position. Add the following method to the FlashCardDeckAdapter class in FlashCardDeckAdapter.cs:

public override Java.Lang.ICharSequence GetPageTitleFormatted(int position)
{
    return new Java.Lang.String("Problem " + (position + 1));
}

This code converts the position in the flash card deck to a problem number. The resulting string is converted into a Java String that is returned to the ViewPager. When you run the app with this new method, each page displays the problem number in the PagerTabStrip:

Screenshots of FlashCardPager with the problem number displayed above each page

You can swipe back and forth to see the problem number in the flash card deck that is displayed at the top of each flash card.

Handle User Input

FlashCardPager presents a series of fragment-based flash cards in a ViewPager, but it does not yet have a way to reveal the answer for each problem. In this section, an event handler is added to the FlashCardFragment to display the answer when the user taps on the flash card problem text.

Open FlashCardFragment.cs and add the following code to the end of the OnCreateView method just before the view is returned to the caller:

questionBox.Click += delegate
{
    Toast.MakeText(Activity.ApplicationContext,
            "Answer: " + answer, ToastLength.Short).Show();
};

This Click event handler displays the answer in a Toast that appears when the user taps the TextBox. The answer variable was initialized earlier when state information was read from the Bundle that was passed to OnCreateView. Build and run the app, then tap the problem text on each flash card to see the answer:

Screenshots of FlashCardPager app Toasts when math problem is tapped

The FlashCardPager presented in this walkthrough uses a MainActivity derived from FragmentActivity, but you can also derive MainActivity from AppCompatActivity (which also provides support for managing fragments).

Summary

This walkthrough provided a step-by-step example of how to build a basic ViewPager-based app using Fragments. It presented an example data source containing flash card questions and answers, a ViewPager layout to display the flash cards, and a FragmentPagerAdapter subclass that connects the ViewPager to the data source. To help the user navigate through the flash cards, instructions were included that explain how to add a PagerTabStrip to display the problem number at the top of each page. Finally, event handling code was added to display the answer when the user taps on a flash card problem.