For the Android Hackpack, we are going to develop HackerPad - A Simple Note taking application. The tutorial assumes a working knowledge of Kotlin. (If you need crash course in Kotlin, visit here). In this tutorial, you'll learn basic process behind making android apps such as developing an UI, storing data locally, opening activity from another activity and much more. So Let's get started!

• If you don't have Android Project installed on your computer, install it from here.
• In the Welcome to Android Studio window, click Start a new Android Studio project. Or if you have a project opened, select File > New Project.
• In the Create New Project window, enter the following values.
  • Application Name : "HackerPad"
  • Company Domain : "your_name.io" Leave the other options as they are.
• For all the other setup screens select default values and hit next.
• Check the box that says Include Kotlin support.

After some processing, Android Studio opens the IDE.

Now let's take a moment to review the most important files.

First, be sure the Project window is open (select View > Tool Windows > Project) and the Android view is selected from the drop-down list at the top of that window. You can then see the following files:

app > java > io.your_name.hackerpad > MainActivity This is the main activity (the entry point for your app). When you build and run the app, the system launches an instance of this Activity and loads its layout.

app > res > layout > activity_main.xml This XML file defines the layout for the activity's UI. It contains a TextView element with the text "Hello world!".

app > manifests > AndroidManifest.xml The manifest file describes the fundamental characteristics of the app and defines each of its components.

Gradle Scripts > build.gradle You'll see two files with this name: one for the project and one for the "app" module. Each module has its own build.gradle file, but this project currently has just one module. You'll mostly work with the module's build.gradle file to configure how the Gradle tools compile and build your app.

Press the run button (circled in following figure) and you will see a hello world written in middle of screen. You will need to add a emulator/device to run the app. Just follow the default steps to add emulator or plug your android device using usb.

Let's define our app design. It will be two activity application -

• MainActivity: This will show the list of all the notes already added and also will include a button to add a new note. Finally, it would look like this -

  

• NotesActivity: This will be the details activity where title and details of note can be added. Finally, it would look like this -

  

UI in android is build by using Design editor (Preferred way) or by using text editor. All the UI in android is declared using XML and can be found in app > res > layout.

Edit **app > res > layout > activity_main.xml ** as follows and let's see what we are doing

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ui.MainActivity">

    <ListView
        android:id="@+id/noteListView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

    </ListView>

    <android.support.v7.widget.CardView xmlns:card_view="http://schemas.android.com/apk/res-auto"
        android:id="@+id/card_view"
        android:layout_width="match_parent"
        android:layout_height="@dimen/cardview_height"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginBottom="8dp"
        card_view:cardCornerRadius="4dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.982">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="horizontal">

            <TextView
                android:id="@+id/takeNote"
                android:layout_width="269dp"
                android:layout_height="44dp"
                android:layout_margin="2dp"
                android:layout_weight="3"
                android:gravity="center_vertical|start"
                android:padding="2dp"
                android:text="@string/add_note_placeholder"
                android:textColor="@color/gray"
                android:textSize="14sp" />

            <ImageView
                android:id="@+id/takeAudioNoteView"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_margin="2dp"
                android:layout_weight="1"
                android:adjustViewBounds="true"
                android:contentDescription="@string/audio_note_placeholder"
                android:cropToPadding="false"
                android:tint="@color/colorPrimary"
                android:padding="2dp"
                android:scaleType="center"
                android:visibility="invisible"
                app:srcCompat="@drawable/ic_action_name" />
        </LinearLayout>
    </android.support.v7.widget.CardView>
</android.support.constraint.ConstraintLayout>

In this layout, we have added a <ListView> tag which is a view that can show list of elements in scrollable fashion. (Note that presently Google recommend <RecyclerView> for showing scrollable list, but for simplicity sake, we are using ListView and also it fulfills our purpose easily.) We will use ListView to show list of already added notes and we have given it id as noteListView so as to get its reference later.

Also there is card view widget added which includes a Linear Layout which layouts its children in linear fashion. This part of code builds our Take a Note action button along with Speech icon. All these codes are generated using design editor which is a simple intuitve drag and drop editor for building UI.

Similary, for notes activity, create a new activity by File > New > Activity > Empty Activity , name the activity to NotesAcitivity and selecting all the other default values. This will generate two files:-

• NotesActivity.kt in app > java > io.your_name.hackerpad
• activity_notes in app > res > layout

Edit activity_notes with this repository's activity_notes file. Its a simple layout file with two field to take user inputs as title & note along with a button to add/update the note.

Model refers to the Plain objects needed for the application which in our case is clearly a Note. So, we will define a Note class with following fields:

• Id - A string feild which will be the identifier for a particular note
• Title - A string feild which will be the title for our note
• message - A string feild which will be the message for our note
• isPinned - A boolean feild which will show if a note is pinned or not.
• isBookmarked - A boolean feild which will show if a note is bookmarked or not.

Create a new data directory in app > java > packageName. In this directoy we will create two more directory, model which will contain our plain object and repository which will handle our data storage. Copy the Note.kt file present in this repository into your project.

We will be following Reposiotry pattern to handle our data which is best explained by the following diagram:

Now let's create a interface first to define our Data handling functions. Create INotesRepository in the repository directory created in data folder and define it as follows:

public interface INotesRepository {
    fun getNotesList() : List<Note>
    fun add(note: Note)
    fun update(note: Note)
    fun getNoteById(id: String): Note?
    fun deleteNoteById(id: String): Boolean
}

As clear by the function signatures, this interface defines all the necessary operations we want to do with our data like Adding a new note, Updating it, Retreving it by Id, Deleting it and also Getting all notes.

Lets implement these in a LocalNotesRepository class.

Copy LocalNotesRepository.java file to your repository directory. Remember to change the package name. The following code is self explanatory, we have a arraylist of Note and we are using it for doing our CRUD operations.

 override fun getNotesList(): List<Note> {
        if (notes.size != 0) return notes
        val gson = Gson()
        val json = sharedPreferences.getString(prefsKey, "")
        if (json!!.trim { it <= ' ' }.isEmpty()) {
            return notes
        }
        val type = object : TypeToken<List<Note>>() {}.type
        notes = gson.fromJson<MutableList<Note>>(json, type)
        return notes
    }

    override fun add(note: Note) {
        val id = UUID.randomUUID().toString()
        note.id = id
        notes.add(note)
    }


    override fun getNoteById(id: String): Note? {
        for (note in notes) {
            if (note.id == id) {
                return note
            }
        }

        return null
    }

    override fun update(newNote: Note) {
        for (note in notes) {
            if (note.id == newNote.id) {
                note.update(newNote.title, newNote.note)
            }
        }
    }


    override fun deleteNoteById(id: String): Boolean {
        for (note in notes) {
            if (note.id == id) {
                notes.remove(note)
                return true //Success | Note with given id successfully deleted
            }
        }
        return false // Failure | Note with given id not found
    }

 

For Storage we will use Android Shared Preferences which is a small collection of Key-Value pair available to the apps and is good as storage for some simple prototyping usecases. Otherwise generally we need to access Sqlite3 database on android which comes inbuilt with every android. But for simplicity we will be using this.

We need to get access to this object from Android and we will do it in App file which will be entry point for our application where we would define all our top level objects.

Copy App class present in app>Java>packageName to your app>Java>packageName

public class App extends Application {

    internal var testingMode = false

    val prefKey = BuildConfig.APPLICATION_ID + "notesList"

    val notesRepository: INotesRepository
        get() = if (testingMode) {
            DummyNotesRepository.getInstance(this)
        } else LocalNotesRepository.getInstance(this)

    val sharedPrefs: SharedPreferences
        get() = this.getSharedPreferences(BuildConfig.APPLICATION_ID, Context.MODE_PRIVATE)

    fun saveNotes() {
        val noteList = notesRepository.getNotesList()
        val gson = Gson()
        val json = gson.toJson(noteList)
        sharedPrefs.edit()
                .putString(prefKey, json)
                .apply()
    }
    
     
    
}

As shown in above code, we have getter for our repositories and SharedPreferences as required and saveNotes() method is called when we want to store our notes on local storage. Also, since SharedPreferences can save standard objects, so for our custom Notes object we are using GSON serailiser to convert it to string and storing it as string. By, using GSON we can also easily create our Note Object back from a string.

Now its time to integrate various components and make it work. For that create a activity file using File > New > Activity > Blank Activity and call it NotesActivity. So now we have two Activity file - Main and Notes.

Now lets see what we are doing there. This is MainActivity. For brevity, I am showing only necessary codes.

 	//MainActivity.kt
    private var noteListView: ListView? = null

    private var notesRepo: INotesRepository? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        noteListView = findViewById(R.id.noteListView)
        val takeNoteView = findViewById<TextView>(R.id.takeNote)

        //You can use below AudioNoteView to add audio note functionality to the app
        //        ImageView takeAudioNoteView = findViewById(R.id.takeAudioNoteView);

        notesRepo = (application as App).notesRepository

        takeNoteView.setOnClickListener { startTakeNoteActivity() }

        populateNotes()
    }

    override fun onResume() {
        super.onResume()
        populateNotes()
    }

    

Here we have defined variables to access our previously created noteListView and created a reference for notesRepo and then in onCreate() method which is a activity lifecycle callback. we are setting up the references appropriately.

	//This is the function to start a new activity from current main activity
    private fun startTakeNoteActivity() {
        //This is the intent class which is a special object to start a new action in Android
        val intent = Intent(this, NotesActivity::class.java)
        //This is a in built function that starts a new activity as per the intent specs
        startActivity(intent)
    }

    //This is overloaded function to open note activity. This will be used for updating notes
    private fun startTakeNoteActivity(note: Note) {
        val intent = Intent(this, NotesActivity::class.java)
        intent.putExtra("noteId", note.id)
        startActivity(intent)
    }

    //This is function to show current notes in a list view
    private fun populateNotes() {

        //Get reference to the Note List from notes repository
        val noteList = notesRepo!!.getNotesList()

        /**
         * This is the adapter that creates the passed view (R.layout.note_card_view)
         * and populate a Text view present in the view with the passed data objects
         **/
        val noteArrayAdapter = ArrayAdapter(
                this, R.layout.note_card_view, R.id.note, noteList)

        noteArrayAdapter.notifyDataSetChanged()

        noteListView?.apply { 
            adapter = noteArrayAdapter
            setPadding(8, 8, 8, 8)
            dividerHeight = 8
        }

        noteListView?.onItemClickListener = AdapterView.OnItemClickListener { parent, view, position, id ->
            val note = parent.adapter.getItem(position) as Note
            startTakeNoteActivity(note)
        }
    }

    override fun onStop() {
        super.onStop()
        (application as App).saveNotes()
    }

Now lets check how we are implementing NotesActitivty.java

    private var titleView: EditText? = null
    private var noteView: EditText? = null
    private var notesRepo: INotesRepository? = null
    private var noteId: String? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_notes)

        val bundle = intent.extras

        if (bundle != null) {
            noteId = bundle.getString("noteId")
        }

        notesRepo = (application as App).notesRepository

        titleView = findViewById(R.id.noteTitle)
        noteView = findViewById(R.id.note)

        val imageButton = findViewById<Button>(R.id.imageButton)

        imageButton.setText(R.string.add_note)

        if (noteId != null) {
            imageButton.setText(R.string.update_note)
        }

        imageButton.setOnClickListener {
            addNote()
            startMainActivity()
        }

        if (noteId != null) {
            val note = notesRepo!!.getNoteById(noteId!!)
            titleView?.setText(note?.title)
            noteView?.setText(note?.note)
            return
        }

        noteView?.requestFocus()

    }

Now hit run and you can see a working prototype of your Note Taking application - HackerPad

There are lot of things that you can do on top of this application.

1. Google Search API : Use Google Search API to bring search fucntionalties to your app.
2. Google Speech API : Use Google Speech API to take audio notes or translate a given speech into English Text.
3. Google Voice API : Use Google Voice API to add custom functionalities on "Ok Google" Command.
4. Google APIs for Android : A one-stop collection for all the google apis documentation and codelabs for Android.

Android development can be overwhelming in the beginning but there are lots of learning resources available on internet. Feel free to go through them:

• Official Android Documenation: - One of the best
• Android Fundamentals Training by Google Codelab - A full list of code lab style tutorial by Google for teaching android from fundamentals to advanced. (Highly recommended!)
• Sunshine Weather App - Google CodeLab - A good end to end app following recommended architecture pattern.

HackPacks are built by the TreeHacks team and contributors to help hackers build great projects at our hackathon that happens every February at Stanford. We believe that everyone of every skill level can learn to make awesome things, and this is one way we help facilitate hacker culture. We open source our hackpacks (along with our internal tech) so everyone can learn from and use them! Feel free to use these at your own hackathons, workshops, and anything else that promotes building :)

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