Bangle.js Data Storage

In Bangle.js you may often want to store data over time, and then recall it later on your PC. In this tutorial we'll talk about how to do that.

There's a more general tutorial about storing data on Espruino as well.

We'll assume you've been through creating an app already so you know what's involved in creating one.

For this example, we're just going to have a menu that lets you write your current GPS location and time to a file along with a name (for example you could mark the location of every tree you find by standing by it and choosing Add Tree).

Note: We've already added a gpspoilog to the App Loader for you, so while you can upload this code yourself you'll find it is already all available for you.

Watch App

This is the code for the app - the majority of it is just creating the menu:

var menuItems = {
  "":{title:"GPS POI Log"},
  " ":{value:"No Fix"},
  "Add Tree" : ()=>addItem("Tree"),
  "Add Gate" : ()=>addItem("Gate"),
  "Add Flower" : ()=>addItem("Flower"),
  "Add Plant" : ()=>addItem("Plant")
};

var menu = E.showMenu(menuItems);
var gps = { fix : 0};
var gpsCount = 0;
// Create the file in append mode
var file = require("Storage").open("gpspoilog.csv","a");

function setStatus(msg) {
  menuItems[" "].value = msg;
  menu.draw();
}

Bangle.on('GPS',function(g) {
  gps = g;
  gpsCount++;
  var msg;
  if (g.fix) {
    msg = g.satellites + " Satellites";
  } else {
    msg = "No Fix";
  }
  setStatus(" "+"-\\|/"[gpsCount&3]);
});


function addItem(name) {
  if (!gps.fix) {
    setStatus("Ignored - no fix");
    return; // don't do anything as no fix
  }
  // The fields we want to put in out CSV file
  var csv = [
    0|getTime(), // Time to the nearest second
    gps.lat,
    gps.lon,
    gps.alt,
    name
  ];
  // Write data here
  file.write(csv.join(",")+"\n");
  setStatus("Written");
}


Bangle.loadWidgets();
Bangle.drawWidgets();
Bangle.setGPSPower(1);

First we need to open a file, which is done with this line:

var file = require("Storage").open("gpspoilog.csv","a");

This opens a file in append (a) mode. If it doesn't exist, it is created empty, but if it does then subsequent writes will go on the end.

And now, we just need to write data into the file in addItem using file.write:

file.write(csv.join(",")+"\n");

A few things are going on here:

• csv is an array, so we convert it to a string using join, which adds all elements together with a comma between them.
• write doesn't add a newline, so we need to add one with \n so we can read the data line by line later.
• write also can't currently write the character code 255 ("\xff") in files, so you need to stick to writing text, not binary data.
• write writes data as soon as it is called - there's no need to close a file when you're finished.
• A file created with require("Storage").open is different to one created with require("Storage").write since it is designed to be appended to. As such you can only access the files with require("Storage").open and not with require("Storage").write/read/erase/etc

Ok, now this is sorted, you can upload the app, run it, and get some data:

• Open the Web IDE and connect to your Bangle
• Click the down-arrow by the Upload button, choose Storage, New File, and enter gpspoilog.app.js as the name.
• Click the Upload button to uplaod the app
• Now copy/paste the following into the IDE on the left-hand side. This will create a listing for your app in the Launcher without you having to upload it via the app store:

require("Storage").write("gpspoilog.info",{
  "id":"gpspoilog",
  "name":"GPS POI Log",
  "src":"gpspoilog.app.js"
});

Now, you can go for a walk - when you have a GPS signal you can start logging some data by choosing Add Plant, etc from the menu.

Reading the data

With the IDE

The easiest way to read the data is to open the file in the Web IDE. This may be enough for many of you:

• Click the Storage icon in the middle of the screen (4 discs)
• Click the 'eye' icon next to gpspoilog.csv (StorageFile) in the window
• You should now see the file contents, and you can save this to a file if you wish.

In the app

You can access the data programmatically - either reading in chunks or line by line.

For example this code will write one line at a time:

var f = require("Storage").open("gpspoilog.csv","r");
var l = f.readLine();
while (l!==undefined) {
  console.log(l);
  l = f.readLine();
}

Reading with the App Loader

However, you can also add the loading functionality to the App Loader itself.

First, we'll need to add the app. This is basically as documented in Adding an app to the Bangle.js App Loader with one extra addition - a "interface" element in the JSON.

• Set up your own App loader as described here
• Copy the directory apps/_example_app to apps/gpspoilog
• Write the JS file above over the top of apps/gpspoilog/app.js
• Change metadata.json, as shown:

  { "id": "gpspoilog",
    "name": "GPS POI Logger",
    "shortName":"GPS POI Log",
    "icon": "app.png",
    "version":"0.01",
    "description": "A way to log points of interest with their GPS coordinates",
    "tags": "outdoors",
    "interface": "interface.html",
    "storage": [
      {"name":"gpspoilog.app.js","url":"app.js"},
      {"name":"gpspoilog.img","url":"app-icon.js","evaluate":true}
    ]
  }

Now you need to add the 'interface' file - copy the following to apps/gpspoilog/interface.html.

<html>
  <head>
    <link rel="stylesheet" href="../../css/spectre.min.css">
  </head>
  <body>
    <div id="data"></div>
    <button class="btn btn-default" id="btnSave">Save</button>
    <button class="btn btn-default" id="btnDelete">Delete</button>

    <script src="../../core/lib/interface.js"></script>
    <script>
var dataElement = document.getElementById("data");
var csvData = "";

function getData() {
  // show loading window
  Util.showModal("Loading...");
  // get the data
  dataElement.innerHTML = "";
  Util.readStorageFile(`gpspoilog.csv`,data=>{
    csvData = data.trim();
    // remove window
    Util.hideModal();
    // If no data, report it and exit
    if (data.length==0) {
      dataElement.innerHTML = "<b>No data found</b>";
      return;
    }
    // Otherwise parse the data and output it as a table
    dataElement.innerHTML = `<table>
    <tr>
      <th>Time</th>
      <th>Lat</th>
      <th>Lon</th>
      <th>Alt</th>
      <th>Type</th>
    </tr>`+data.trim().split("\n").map(l=>{
      l = l.split(",");
      return `<tr>
      <td>${(new Date(l[0]*1000)).toLocaleString()}</td>
      <td>${l[1]}</td>
      <td>${l[2]}</td>
      <td>${l[3]}</td>
      <td>${l[4]}</td>
      </tr>`
    }).join("\n")+"</table>";
  });
}

// You can call a utility function to save the data
document.getElementById("btnSave").addEventListener("click", function() {
  Util.saveCSV("gpsdata", csvData);
});
// Or you can also delete the file
document.getElementById("btnDelete").addEventListener("click", function() {
  Util.showModal("Deleting...");
  Util.eraseStorageFile("gpspoilog.csv", function() {
    Util.hideModal();
    getData();
  });
});
// Called when app starts
function onInit() {
  getData();
}

    </script>
  </body>
</html>

This does a few basic things:

• onInit is called when the page loads
• getData grabs the data from the watch
• Util.showModal displays a status screen when things are happening
• Util.readStorageFile reads the Storage file from the Bangle and returns a string containing its contents
• Util.saveCSV can pop up a save dialog
• Util.eraseStorageFile erases the file

For more information about the available functions you can check out the interface.js file.

To use your new interface HTML file:

• view your personal App Loader site
• Click Connect up the top right
• Click My Apps
• Next to GPS POI Logger there's a down arrow icon. Click it
• It'll pop up a GPS POI Logger showing you the data you recorded
• You can now save the data or delete it from the Bangle

And that's it! When you have it working as you want you can even submit it to the official App Loader site