37. An Introduction to Android Fragments

As you progress through the chapters of this book it will become increasingly evident that many of the design concepts behind the Android system were conceived with the goal of promoting reuse of, and interaction between, the different elements that make up an application. One such area that will be explored in this chapter involves the use of Fragments.

This chapter will provide an overview of the basics of fragments in terms of what they are and how they can be created and used within applications. The next chapter will work through a tutorial designed to show fragments in action when developing applications in Android Studio, including the implementation of communication between fragments.

37.1 What is a Fragment?

A fragment is a self-contained, modular section of an application’s user interface and corresponding behavior that can be embedded within an activity. Fragments can be assembled to create an activity during the application design phase, and added to or removed from an activity during application runtime to create a dynamically changing user interface.

Fragments may only be used as part of an activity and cannot be instantiated as standalone application elements. That being said, however, a fragment can be thought of as a functional “sub-activity” with its own lifecycle similar to that of a full activity.

Fragments are stored in the form of XML layout files and may be added to an activity either by placing appropriate <fragment> elements in the activity’s layout file, or directly through code within the activity’s class implementation.

37.2 Creating a Fragment

The two components that make up a fragment are an XML layout file and a corresponding Kotlin class. The XML layout file for a fragment takes the same format as a layout for any other activity layout and can contain any combination and complexity of layout managers and views. The following XML layout, for example, is for a fragment consisting of a ConstraintLayout with a red background containing a single TextView with a white foreground:

<?xml version="1.0" encoding="utf-8"?>

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto"

    xmlns:tools="http://schemas.android.com/tools"

    android:id="@+id/constraintLayout"

    android:layout_width="match_parent"

  &...

37.3 Adding a Fragment to an Activity using the Layout XML File

Fragments may be incorporated into an activity either by writing Kotlin code or by embedding the fragment into the activity’s XML layout file. Regardless of the approach used, a key point to be aware of is that when the support library is being used for compatibility with older Android releases, any activities using fragments must be implemented as a subclass of FragmentActivity instead of the AppCompatActivity class:

package com.example.myFragmentDemo

import androidx.fragment.app.FragmentActivity

import android.os.Bundle

 

class MainActivity : FragmentActivity() {

.

.

Fragments are embedded into activity layout files using the FragmentContainerView class. The following example layout embeds the fragment created in the previous section of this chapter into an activity layout:

<?xml version="1.0" encoding="utf-8"?>

<androidx.constraintlayout.widget.ConstraintLayout...

37.4 Adding and Managing Fragments in Code

The ease of adding a fragment to an activity via the activity’s XML layout file comes at the cost of the activity not being able to remove the fragment at runtime. In order to achieve full dynamic control of fragments during runtime, those activities must be added via code. This has the advantage that the fragments can be added, removed and even made to replace one another dynamically while the application is running.

When using code to manage fragments, the fragment itself will still consist of an XML layout file and a corresponding class. The difference comes when working with the fragment within the hosting activity. There is a standard sequence of steps when adding a fragment to an activity using code:

1. Create an instance of the fragment’s class.

2. Pass any additional intent arguments through to the class instance.

3. Obtain a reference to the fragment manager instance.

4. Call the beginTransaction() method...

37.5 Handling Fragment Events

As previously discussed, a fragment is very much like a sub-activity with its own layout, class and lifecycle. The view components (such as buttons and text views) within a fragment are able to generate events just like those in a regular activity. This raises the question as to which class receives an event from a view in a fragment; the fragment itself, or the activity in which the fragment is embedded. The answer to this question depends on how the event handler is declared.

In the chapter entitled “An Overview and Example of Android Event Handling”, two approaches to event handling were discussed. The first method involved configuring an event listener and callback method within the code of the activity. For example:

binding.button.setOnClickListener { // Code to be performed on button click }

In the case of intercepting click events, the second approach involved setting the android:onClick property within the XML layout file...

37.6 Implementing Fragment Communication

Once one or more fragments are embedded within an activity, the chances are good that some form of communication will need to take place both between the fragments and the activity, and between one fragment and another. In fact, good practice dictates that fragments do not communicate directly with one another. All communication should take place via the encapsulating activity.

In order for an activity to communicate with a fragment, the activity must identify the fragment object via the ID assigned to it. Once this reference has been obtained, the activity can simply call the public methods of the fragment object.

Communicating in the other direction (from fragment to activity) is a little more complicated. In the first instance, the fragment must define a listener interface, which is then implemented within the activity class. For example, the following code declares an interface named ToolbarListener on a fragment class named ToolbarFragment...

37.7 Summary

Fragments provide a powerful mechanism for creating re-usable modules of user interface layout and application behavior, which, once created, can be embedded in activities. A fragment consists of a user interface layout file and a class. Fragments may be utilized in an activity either by adding the fragment to the activity’s layout file, or by writing code to manage the fragments at runtime. Fragments added to an activity in code can be removed and replaced dynamically at runtime. All communication between fragments should be performed via the activity within which the fragments are embedded.

Having covered the basics of fragments in this chapter, the next chapter will work through a tutorial designed to reinforce the techniques outlined in this chapter.