Uwe's Blog: Create Android App with Android Studio and Kotlin

Uwe Sterr

Introduction

Developing a android app with MIT App Inventor was a nice experience but it seems to be limited in what can be achieved. For me it was not possible to handle several mqtt messages in one topic which could have been solved by using several topics, but that was not a solution I liked. So lets dive into Android Studio. The well done documentation helps whenever we get stuck.

Install Android studio

Installation is described at the developers webpages.

Make Your First Simple Android App with Kotlin

To create the first android app with Kotlin I followed the YouTube of Reso Coder

Use phone to run app

You can either use a

Virtual device
Real device

More on how apps can be run see https://www.homeandlearn.co.uk/android/running_android_apps.html

Using real device to run app

To use your phone as a test device you need to configure it to be used for USB debugging

On Android 4.2 and higher, you must enable this screen. To enable developer options, tap the Build Number option 7 times. You can find this option in one of the following locations, depending on your Android version:

Android 9 (API level 28) and higher: Settings > About Phone > Build Number

More detail can be found at https://developer.android.com/studio/debug/dev-options

Kotlin part

The Kotlin part of the app is shown below and can be cloned from GitHub https://github.com/ResoCoder/FirstKotlinAndroidApp


package com.example.firstapptut

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.SeekBar
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val initialTextViewTranslationY = textView_progess.translationY
        seekBar.setOnSeekBarChangeListener(object: SeekBar.OnSeekBarChangeListener{
            override fun onProgressChanged(seekBar: SeekBar?, progress: Int, fromUser: Boolean) {
                textView_progess.text = progress.toString()
                val translationDistance = (initialTextViewTranslationY + progress * resources.getDimension(R.dimen.text_anim_step) * -1)
                textView_progess.animate().translationY(translationDistance)
            }

            override fun onStartTrackingTouch(seekBar: SeekBar?) {
            }

            override fun onStopTrackingTouch(seekBar: SeekBar?) {
            }

        })
        button_rest.setOnClickListener { v ->
           seekBar.progress = 0
            textView_progess.animate().setDuration(500).rotationBy(360f)
                .translationY(initialTextViewTranslationY)
        }
            
        }
    }

Kotlin: MQTT with Eclipse Paho

The video is on youtube https://www.youtube.com/watch?v=NpURY3zE8o8. The code is on GitHub and can be read into Android Studio via

My version is on https://github.com/uwesterr/KotlinMQTTSample/tree/master

VCS
- Checkout from Version Contrlol
  - Git, input here GitHub Url https://github.com/anoop4real/KotlinMQTTSample

On the pixel 2 that looks like

To handle mqtt the Class MqttAndroidClient from eclipse is used https://www.eclipse.org/paho/files/android-javadoc/org/eclipse/paho/android/service/MqttAndroidClient.html

Kommunikation über MQTT (German)

Detailed article about mqtt communicaiton by heise Developer is given at https://www.heise.de/developer/artikel/Kommunikation-ueber-MQTT-3238975.html

The number sign (#) is a wildcard character that matches any number of levels within a topic. For example, if you subscribe to finance/stock/ibm/#, you receive messages on these topics:

finance/stock/ibm
finance/stock/ibm/closingprice
finance/stock/ibm/currentprice

for my case it is

IMU/#

Hint, all of the sudden app did not work anymore. after
- File => Sync project with gradle files
- File => Sync with file system
the APP worked es before

Android APP MAUE,

Notes on development aspects of MAUE.

MAUE: Interaction of software components within Android App

The following graph shows how the software components interact.

Activity_main.xml
- GUI definition and function are assigned to actions
MainActivity.kt
- function which are triggered by action in GUI are defined
MQTTmanager.kt
- mqttt managing class is defined, used by MainActivity.kt

MAUE: Handle pitch and roll variable

Pitch an roll are reported via mqttt from the raspi to the phone. The values are needed for

display
- after offset is subtracted
beeing send back to raspi to be stored when “RESET” button is pressed

MAUE: Handling offset values

The offset values take into consideration that the sensor might not be perfectly horizontally installed. Rotation in yaw are not taken into consideration, so roll and pitch should be roughly aligned, the cross coupling is small for small yaw angels and should be negelectable for our purpose.

offset values are stored in rapsi
are read on when connected - MainActivity.kt: fun connect(view: View){
are subtracted from measured value before displayed

Note, store measured values as offset when reset button is pressed, not measured minus offset

MAUE: Position item based on mqtt message

We want to create a visual representation of mqttt messages

IMU/pitch
IMU/roll

MAUE: Add reset button

First add the Button element in activity_main.xml

and then connect buttion click event with function To make click event work add android:onClick attribute to Declared Attributes to the Button element in your XML layout. The value for this attribute must be the name of the method you want to call in response to a click event. The Activity hosting the layout must then implement the corresponding method.

JSON mqtt

https://android.jlelse.eu/about-the-mqtt-protocol-for-iot-on-android-efb4973577b

mqtt JSON python


#!/usr/bin/python
import json
import paho.mqtt.client as mqtt


send_msg = {
        'data_to_send': variable1,
        'also_send_this': variable2
}

client.publish("topic", payload=json.dumps(send_msg), qos=2, retain=False)

https://www.programcreek.com/java-api-examples/?api=org.eclipse.paho.client.mqttv3.MqttMessage


@Override
public void messageArrived(String topic, MqttMessage message) throws Exception {
  String payload = new String(message.getPayload());
  System.out.println("Red'c command:  " + payload);
  JsonObject jsonObject = parser.parse(payload).getAsJsonObject();
  String cmd = extractCommandData(jsonObject, CMD_KEY);
  switch (cmd) {
    case "ping":
      sendResponse(pingResponse(jsonObject));
      break;
    case "randnum":
      sendResponse(randResponse(jsonObject));
      break;
    default:
      sendResponse(payload);
  }
  msgRecd = true;
}

Android Instant Apps - Sample code collection

At https://github.com/android/app-bundle-samples/tree/master/InstantApps you find a collection of sample code which can be run from within Android Studio

Select the instant feature module run configuration
Run the selected configuration

Create Android App with Android Studio and Kotlin