Using RealmRecyclerView Adapter to show list of recorded sensor data from Realm Database

In previous blog Storing Recorded Sensor Data in Realm Database we have stored the data fetched from sensors into the Realm Database by defining model classes.

In this blog, we will use the data stored in the Realm to display a list of recorded experiments in the form of well defining card view items so that it is easier for the user to understand.

For showing the list we will make use of RecyclerView  widget provided by Android which is a more advanced version of the List view and is used to display large data sets in a vertical list, horizontal list, grid, staggered grid etc.

RecyclerView  works in accordance with RecyclerView Adapter which is core engine that is responsible of inflating the layout of list items, populating the items with data, recycling of list item views when they go out of viewing screen and much more.

For this blog, we are going to use a special RecyclerView Adapter provided by Realm itself because it integrates properly with the Realm Database and handles modifying, addition, deletion or updating of Realm data automatically and efficiently.   

Step 1 Adding the dependencies

As always first we need to add the following code in our build.gradle file to add the dependency of Realm database and RealmRecyclerViewAdapter.

dependencies {
   implementation"com.android.support:recyclerview-v7:27.1.1 "
   implementation 'io.realm:android-adapters:2.1.1'
}

Step 2 Adding RecyclerView widget in our Activity layout file

First, we need to create an activity and name it as “DataLoggerActivity”, inside the layout of the Activity add the <RecyclerView> widget. This RecyclerView will act as a container of our list item.

<?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"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".activity.DataLoggerActivity">

    <android.support.v7.widget.RecyclerView
        android:layout_below="@id/top_app_bar_layout"
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
</RelativeLayout>

Step 3 Creating the layout and View holder for the list item

We have to create the layout of the list item which will be inflated by the Adapter. So for this create an XML file in res folder and name it “data_list_item.xml”. For the list of the experiments, we want to show Name of the experiment, recording time, recording date for every list item. For this we will make use of <CardView> and <TextView>. This gist shows the code of xml file.

The layout of the list item created is shown in Figure 2

Figure 1 Layout of list item showing mock information

Now we need to create a view holder for this layout which we need to pass to the Adapter, the following code shows the implementation of View Holder for above list item layout.

public class ViewHolder extends RecyclerView.ViewHolder {
   private TextView sensor, dateTime;
   ImageView deleteIcon;
   private CardView cardView;

   public ViewHolder(View itemView) {
       super(itemView);
       dateTime = itemView.findViewById(R.id.date_time);
       sensor = itemView.findViewById(R.id.sensor_name);
       deleteIcon = itemView.findViewById(R.id.delete_item);
       cardView = itemView.findViewById(R.id.data_item_card);
   }
}

Step 4 Creating the adapter for RecyclerView  

In this step, we will start by creating a class called “SensorLoggedListAdpater” and for using use the RecyclerView adapter provided by Realm we need to make this class extend the RealmRecyclerViewAdpater class.

But for that we need to pass two generic parameter:

  1. Model Class : This is class which define a Realm model, for this, we will pass a reference of “SensorLogged.class” which is defined in the previous blog as we want to show the list experiments which are stored using “SensorLogged” model class.
  2. ViewHolder : For this, we will pass the ViewHolder that we have created in Step 3.

As every RecyclerView Adapter needs a arraylist which contains the list of object containing information which we have to populate on the list item, the RealmRecyclerViewAdpater needs data in form of RealmResult to operate on, so we will create a constructor and pass in the RealmResult list in the super() method which we need to provide when we initialize this adapter in our “DataLoggerActivity” class.

public SensorLoggerListAdapter(RealmResults<SensorLogged> list, Activity context) {
   super(list, true, true);
   this.context = context;
   realm = Realm.getDefaultInstance();
}

Now we need to override two methods provided by RealmRecyclerViewAdapter class that are:

  1. public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType): In which we will inflate the layout of list item “dta_list_tem.xml” which we have created in Step 3.
  2. public void onBindViewHolder(@NonNull final ViewHolder holder, int position): In which we will populate the list item view using references stored in the ViewHolder with the data which we have provided while initializing the adapter.
@NonNull
@Override
public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
   View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.logger_data_item, parent, false);
   return new ViewHolder(itemView);
}

@Override
public void onBindViewHolder(@NonNull final ViewHolder holder, int position) {
   SensorLogged temp = getItem(position);
   holder.sensor.setText(temp.getSensor());
   Date date = new Date(temp.getDateTimeStart());
   holder.dateTime.setText(String.valueOf(sdf.format(date)));
}

Step 5 Initializing the Adapter in Data Logger Activity and connecting with RecyclerView

Now we head to our Data Logger Activity, here in OnCreate() method first we will create a object of RecyclerView, then we will initialize our adapter by passing the RealmResult<SensorLogged> list which we have queried from the Realm Database.

Then we will set the LinearLayoutManager and finally, we will connect the the Adapter with the RecyclerView.

@Override
protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_data_logger);
   ButterKnife.bind(this);

   Realm realm = Realm.getDefaultInstance();

   RealmResults<SensorLogged> results;
   String title;
  
   results = realm.where(SensorLogged.class)
           .findAll()
           .sort("dateTimeStart", Sort.DESCENDING);

   SensorLoggerListAdapter adapter = new SensorLoggerListAdapter(results, this);
   LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false);

   recyclerView.setLayoutManager(linearLayoutManager);

   recyclerView.setAdapter(adapter);
}

After following all the above steps we have finally a activity as shown in Figure 4.

Figure 2 showing a list of recorded experiments with the instrument
name and date time of an experiment

Thus we have successfully displayed a list of the experiments from the data stored in the Realm Database using RealmRecyclerViewAdapter.

Resources

  1. https://academy.realm.io/posts/android-realm-listview/ – Blog on creating a To-do list on Realm official website
  2. https://gist.github.com/Avjeet/2f350feeafff17ec855a39891d8c2d66  Gist of layout of list item used
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Export Sensor Data from the PSLab Android App

The PSLab Android App allows users to log data from the sensors connected to the PSLab hardware device. Sensor Data is stored locally but can be exported in various formats. Currently the app supports exporting data in .txt and .csv (comma-separated values) format. Exported data can be used by other users or scientists to study or analyze the data. Data can also be used by other softwares like Python, GNU octave, Matlab to further process it or visualise it in 3D. In this post, we will discuss how to export the locally stored realm data in .txt or .csv format. We will take the data of MPU6050 sensor as an example for understanding how locally logged data is exported.

Query Local Realm Data

We have attached a long click listener to sensor list view that detects which list item is selected. Clicking any sensor from sensor list for slightly longer than usual would result in a dialog popping up with the option to

  • Export Data: Results in exporting data in a format which is selected in App settings
  • Share Data: Shares sensor data with other users or on social media (yet to be implemented)
Source: PSLab Android App

As soon as the Export Data option is selected from the dialog, sensor data of the corresponding sensor is queried. The data model of the sensor and how it’s saved in the local realm database is discussed in the post Sensor Data Logging in the PSLab Android App.

RealmResults<DataMPU6050> results = realm.where(DataMPU6050.class).findAll();

Once we get the required data, we need to write it in .txt or .csv format depending on what the user has selected as a preference in App Settings.

Getting User Preference from App Settings

The format in which the sensor data should be exported is presented to the user as a preference in App Settings. Currently the app supports two formats .txt and .csv.

Source: PSLab Android App
private String format;
SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
String formatValue = preferences.getString("export_data_format_list", "0");
if ("0".equals(formatValue))
   format = "txt";
else
   format = "csv";

Export Data in .txt Format

To export the sensor data in .txt format, we need to create a .txt file in the external storage. folder variable is a path to PSLab Android folder in the external storage. If the folder doesn’t exist, it will be created.

File folder = new File(Environment.getExternalStorageDirectory() + File.separator + "PSLab Android");

After getting reference of the app folder in the external storage, we would create a text file in the PSLab Android folder. As soon as the text file is created, we initialize the FileOutputStream object to write data into the text file. The sensor data that was queried in the previous section is written into the text file just created. Finally after the complete sensor data is written, the stream is closed by stream.close() method.

FileOutputStream stream = null;
File file = new File(folder, "sensorData.txt");
try {
   stream = new FileOutputStream(file);
   for (DataMPU6050 temp : results) {
       stream.write((String.valueOf(temp.getAx()) + " " + temp.getAy() + " " + temp.getAz() + " " +
               temp.getGx() + " " + temp.getGy() + " " + temp.getGz() + " " + temp.getTemperature() + "\n").getBytes());
   }
} catch (IOException e) {
   e.printStackTrace();
} finally {
   try {
       if (stream != null) {
           stream.close();
       }
   } catch (IOException e) {
       e.printStackTrace();
   }
}

Export Data in .csv Format

Writing data in .csv format is similar to that in .txt format. As CSV stands for Comma Separated Values, which means each data value is separated by “,” (comma). It is similar to an excel sheet. The first row consists of labels that denote the type of value in that particular column. The other rows consist of the sensor data, with each row corresponding to a sample of the sensor data.

File file = new File(folder, "sensorData.csv");
PrintWriter writer;
try {
   writer = new PrintWriter(file);
   StringBuilder stringBuilder = new StringBuilder();
   stringBuilder.append("Ax,Ay,Ax,Gx,Gy,Gz,Temperature\n");
   for (DataMPU6050 temp : results) {
       stringBuilder.append(String.valueOf(temp.getAx()));
       stringBuilder.append(',');
       stringBuilder.append(String.valueOf(temp.getAy()));
       stringBuilder.append(',');
       stringBuilder.append(String.valueOf(temp.getAz()));
       stringBuilder.append(',');
       stringBuilder.append(String.valueOf(temp.getGx()));
       stringBuilder.append(',');
       stringBuilder.append(String.valueOf(temp.getGy()));
       stringBuilder.append(',');
       stringBuilder.append(String.valueOf(temp.getGz()));
       stringBuilder.append(',');
       stringBuilder.append(String.valueOf(temp.getTemperature()));
       stringBuilder.append('\n');
   }
   writer.write(stringBuilder.toString());
   writer.close();
} catch (FileNotFoundException e) {
   e.printStackTrace();
}

Resources

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Sensor Data Logging in the PSLab Android App

The PSLab Android App allows users to log data from sensors connected to the PSLab hardware device. The Connected sensors should support I2C, SPI communication protocols to communicate with the PSLab device successfully. The only prerequisite is the additional support for the particular sensor plugin in Android App. The user can log data from various sensors and measure parameters like temperature, humidity, acceleration, magnetic field, etc. These parameters are useful in predicting and monitoring the environment and in performing many experiments.

The support for the sensor plugins was added during the porting python communication library code to Java. In this post,  we will discuss how we logged real time sensor data from the PSLab Hardware Device. We used Realm database to store the sensor data locally. We have taken the MPU6050 sensor as an example to understand the complete process of logging sensor data.

Creating Realm Object for MPU6050 Sensor Data

The MPU6050 sensor gives the acceleration and gyroscope readings along the three axes X, Y and Z. So the data object storing the readings of the mpu sensor have variables to store the acceleration and gyroscope readings along all three axes.

public class DataMPU6050 extends RealmObject {

   private double ax, ay, az;
   private double gx, gy, gz;
   private double temperature;

   public DataMPU6050() {  }

   public DataMPU6050(double ax, double ay, double az, double gx, double gy, double gz, double temperature) {
       this.ax = ax;
       this.ay = ay;
       this.az = az;
       this.gx = gx;
       this.gy = gy;
       this.gz = gz;
       this.temperature = temperature;
   }

  // getter and setter for all variables
}

Creating Runnable to Start/Stop Data Logging

To sample the sensor data at 500ms interval, we created a runnable object and passed it to another thread which would prevent lagging of the UI thread. We can start/stop logging by changing the value of the boolean loggingThreadRunning on button click. TaskMPU6050 is an AsyncTask which reads each sample of sensor data from the PSLab device, it gets executed inside a while loop which is controlled by boolean loggingThreadRunning. Thread.sleep(500) pauses the thread for 500ms, this is also one of the reason to transfer the logging to another thread instead of logging the sensor data in UI thread. If such 500ms delays are incorporated in UI thread, app experience won’t be smooth for the users.

Runnable loggingRunnable = new Runnable() {
   @Override
   public void run() {
       try {
           MPU6050 sensorMPU6050 = new MPU6050(i2c);
           while (loggingThreadRunning) {
               TaskMPU6050 taskMPU6050 = new TaskMPU6050(sensorMPU6050);
               taskMPU6050.execute();
              // use lock object to synchronize threads
               Thread.sleep(500);
           }
       } catch (IOException   InterruptedException e) {
           e.printStackTrace();
       }
   }
};

Sampling of Sensor Data

We created an AsyncTask to read each sample of the sensor data from the PSLab device in the background thread. The getRaw() method read raw values from the sensor and returned an ArrayList containing the acceleration and gyro values. After the values were read successfully, they were updated in the data card in the foreground which was visible to the user. This data card acts as a real-time screen for the user. All the samples read are appended to ArrayList mpu6050DataList, when the user clicks on button Save Data, the collected samples are saved to the local realm database.

private ArrayList<DataMPU6050> mpu6050DataList = new ArrayList<>();

private class TaskMPU6050 extends AsyncTask<Void, Void, Void> {

   private MPU6050 sensorMPU6050;
   private ArrayList<Double> dataMPU6050 = new ArrayList<>();

   TaskMPU6050(MPU6050 mpu6050) {
       this.sensorMPU6050 = mpu6050;
   }

   @Override
   protected Void doInBackground(Void... params) {
       try {
           dataMPU6050 = sensorMPU6050.getRaw();
       } catch (IOException e) {
           e.printStackTrace();
       }
       return null;
   }

   @Override
   protected void onPostExecute(Void aVoid) {
       super.onPostExecute(aVoid);
       // update data card TextViews with data read.
       DataMPU6050 tempObject = new DataMPU6050(dataMPU6050.get(0), dataMPU6050.get(1), dataMPU6050.get(2),
               dataMPU6050.get(4), dataMPU6050.get(5), dataMPU6050.get(6), dataMPU6050.get(3));
       mpu6050DataList.add(tempObject);
       synchronized (lock) {
           lock.notify();
       }
   }
}
Source: PSLab Android App

There is an option for Start/Stop Logging, clicking on which will change the value of boolean loggingThreadRunning which stops starts/stops the logging thread.

When the Save Data button is clicked, all the samples of sensor data collected from the  PSLab device till that point are saved to the local realm database.

realm.beginTransaction();
for (DataMPU6050 tempObject : mpu6050DataList) {
   realm.copyToRealm(tempObject);
}
realm.commitTransaction();

Data can also be written asynchronously to the local realm database. For other methods to write to a real database refer write section of Realm docs.

Resources

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