Firefox OS-App-Anfänger-Tutorial

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Dieser Artikel bietet ein Anfängertutorial, das die wesentlichen Punkte der Entwicklung von Firefox OS-Apps bietet, einschließlich ihrer Erzeugung und wie sie sich von Standard-Web-Apps bzw. Webseiten unterscheiden. Er soll sowohl Web-Entwicklern als auch Entwicklern nativer Mobil-Apps helfen.

Firefox OS-Apps sind im Wesentlichen nichts anderes als Standardwebseiten oder Web-Apps. Man erzeugt sie mit Hilfe von Standard-Open-Web-Technologien — HTML, CSS, JavaScript, u.s.w. — und greift mit einem Web-Browser darauf zu. Die Hauptunterschiede sind, dass sie auf einem Gerät installiert werden können und auch offline funktionieren, Zugriff auf spezialisierte APIs haben, die eine Interaktion mit den Gerätefunktionen wie der Kamera, dem Gyroskop oder dem Adressbuch erlauben, und der Existenz eines soliden Entwickler-Ökosystems, einschließlich eines Marketplace für die Verteilung von freien oder auch kostenpflichtigen Apps. Ganz allgemein bieten sie dem Benutzer eine "App-Erfahrung", während sie trotzdem auf offenen, Plattform-übergreifenden Technologien beruhen.

Firefox OS-Apps haben eine niedrige Einstiegsbarriere, insbesondere für Web- und Mobil-Entwickler. Sie sind auch einfacher auf andere Plattformen portierbar als ihre nativen Äquivalente und nicht "eingezäunt".

Firefox OS

Firefox OS (also referred to by its codename Boot to Gecko — or B2G) is Mozilla's open source mobile operating system. It's based on a Linux kernel, which boots into a Gecko-based runtime that lets users install and run open web apps, Gecko being the rendering engine that the Firefox browser uses to render and display web content.

Firefox OS comes with a suite of pre-installed applications called Gaia, which handles the fundamental functions of the phone such as settings, calls, SMS, taking and storing photos, etc.

Skills Needed

As we've already mentioned Firefox OS apps are based on web technologies — HTML, CSS, and JavaScript — so if you've written a web page you already know the basics. Even if you don't have the basics you'll be able to easily follow this guide, but you may want to check out our list of Beginner's tutorials to learn more about developing with open web technologies.

Required Tools

You can build Firefox OS apps with simple free tools, or make use of your existing web editing tools. We'd suggest getting hold of:

Once you get started you will want to run your web app on a Firefox OS phone; you can get a developer preview device or install your own build on an existing device, such as various supported Google Nexus models.

Your first app

This section aims to get you up and running quickly with an installable web app, showing you how quick and easy it is to learn the basics. If you'd like to follow along with this guide, you can find our quickstart starter template repo on Github (download directly as a zip).

Quickstart app starter template

Our app does something very simple — it uses the Battery Status API to fetch the battery charge level of the device and check to see whether the battery is charging or not, and alerts the user to the status of this via a vibration (via the Vibration API) and system notification (via the Notification API).

To begin with, the starter template directory has the following structure:

  • battery-quickstart-starter-template/
    • index.html
    • images/
      • battery.svg
      • icon-128.png
      • icon-512.png
    • scripts/
      • battery.js
      • install.js
    • style/
      • style.css
    • .htaccess
    • README.md
  • index.html : The main document of our app that contains its content, and which everything else feeds into.
  • images : Contains an icon used in the app UI, plus the app icons for display on the Firefox Marketplace (if submitted there) and homescreen of the devices the app is installed on (and potentially other places too). For more information on app icons, read our Icon implementation for apps guide.
  • scripts : Will contain the JavaScript code that defines the functionality of the app; currently there are two blank files — battery.js and install.js.
  • style : contains a stylesheet, style.css, to provide the app with basic styling.
  • .htaccess: A server config file that informs web servers of the mime type of the manifest file we are adding in the next section. This makes sure that servers don't throw an error if they don't recognise the manifest file type (which some might.)
  • README.md: Markdown-based readme file that Github uses to explain what this repo is all about.

Adding a manifest

Every open web app requires a manifest.webapp file to be placed in the app's root folder: This provides important information about the app, such as version, name, description, icon location, locale strings, domains the app can be installed from, and much more.

Add the following into a new plain text file in your app's root. Name the file manifest.webapp.

{
  "name": "Battery",
  "description": "Battery provides a good template for an in-app battery/charge indicator with different visualization choices, plus ideas of what could be changed once battery level gets low.",
  "launch_path": "/index.html",
  "icons": {
    "128": "/images/icon-128.png",
    "512": "/images/icon-512.png"
  },
  "developer": {
    "name": "Chris Mills",
    "url": "http://www.conquestofsteel.co.uk"
  },
  "permissions": {
    "desktop-notification": {
      "description": "Needed for creating system notifications."
    }
  }
}

Note: For more information on exactly what is going on here, consult our App Manifests reference.

Note: paths in manifest files should be relative to the origin of the server location. So for example, if my example's root is at http://www.mysite.com/myapp/, and my icon is at http://www.mysite.com/myapp/myicon/icon.png, the icon path would be /myapp/myicon/icon.png, not /myicon/icon.png.

API permissions

There are a number of WebAPIs available that require permissions for that specific feature to be enabled. Installed apps have to register permission requests within the manifest.webapp file as seen in the "permissions" field above, which requests permission to use the system notifications controlled by the Notification API.

Different APIs require different levels of permission to access them. The three levels of permission are as follows:

  1. Normal — APIs that don't need any kind of special access permissions.
  2. Privileged — APIs available to developers to use in their applications, as long as they set access permissions in the app manifest files, and distribute them through a trusted source.
  3. Certified — APIs that control critical functions on a device, such as the call dialer and messaging services. These are generally not available for third party developers to use.

Note: For more information on what APIs require what permissions, read App permissions.

Web API functionality

JavaScript APIs are being created and enhanced as quickly as devices are. Mozilla's WebAPI effort brings dozens of standard mobile features to JavaScript APIs.

Detecting support for features

One technique employed commonly in web development is JavaScript feature detection — it involves running code to make sure a feature is supported by the browser before you actually try to use that feature. If it doesn't, you can provide some kind of fallback experience. The following snippet provides a quick example (this doesn't need to be added into your example code):

// Let's check if the browser supports notifications
if (!("Notification" in window)) {
  console.log("This browser does not support notifications.");
}

Our code for the quickstart example functionality

Inside the scripts/battery.js file, add the following code blocks one after the other, reading the code comments carefully as you go. First, we'll set up all the variables we need:

// fork the navigator.battery object depending on what prefix the viewing browser uses
var battery = navigator.battery || navigator.mozBattery || navigator.webkitBattery;
// grab the elements we need, and put them in variables
var indicator1 = document.getElementById('indicator1');
var indicator2 = document.getElementById('indicator2');
var batteryCharge = document.getElementById('battery-charge');
var batteryTop = document.getElementById('battery-top');
var chargeIcon = document.getElementById('battery-charging');

// Flag to check if battery charged/not charged has already been notified once
// 0 for first time of notification,
// 1 means "charged" has already been notified,
// 2 means "not charged" has already been notified
// This is set to the opposite after each notification, so that you don't keep
// getting repeat notifications about the same charge state.
var chargingState = 0;

Next, we'll add the main updateBatteryStatus() function, which is responsible for updating the displayed information about the battery's status whenever a battery-related event is fired:

function updateBatteryStatus() {
  // battery.level can be used to give us a percentage of bettery charge to report to
  // the app's user
  var percentage = Math.round(battery.level * 100);
  indicator1.innerHTML = "Battery charge at " + percentage + "%";
  batteryCharge.style.width = percentage + '%';

  if(percentage >= 99) {
    // report that the battery is fully charged, more or less ;-)
    batteryTop.style.backgroundColor = 'limegreen';
    batteryCharge.style.backgroundColor = 'limegreen';
    createNotification("Device battery fully charged.");
  }

  if(battery.charging) {
  // If the battery is charging
    if(chargingState == 1 || chargingState == 0) {
    // and if our chargingState flag is equal to 0 or 1
      // alter the styling to show the battery charging
      batteryTop.style.backgroundColor = 'gold';
      batteryCharge.style.backgroundColor = 'gold';
      indicator2.innerHTML = "Battery is charging";
      chargeIcon.style.visibility = 'visible';
      // notify the user with a custom notification
      createNotification("Device battery now charging.");

      // flip the chargingState flag to 2
      chargingState = 2;
    }
  } else if(!battery.charging) {
  // If the battery is NOT charging
    if(chargingState == 2 || chargingState == 0) {
    // and if our chargingState flag is equal to 0 or 2
      // alter the styling to show the battery NOT charging
      batteryTop.style.backgroundColor = 'yellow';
      batteryCharge.style.backgroundColor = 'yellow';
      indicator2.innerHTML = "Battery not charging";
      chargeIcon.style.visibility = 'hidden';
      // notify the user with a custom notification
      createNotification("Device battery is not charging.");

      // flip the chargingState flag to 1
      chargingState = 1;
    }
  }
}

Now it's time to add in the createNotification() function referenced above. When this is called, a system notification is fired containing the message passed in as its argument. This code seems a bit long-winded, but here we are both detecting support for Notifications, and handling bulletproof support for both Firefox and Chromium/Blink-based browsers.

function createNotification(message) {
  // Let's check if the browser supports notifications
  if (!("Notification" in window)) {
    console.log("This browser does not support notifications.");
  }
  // Let's check if the user is okay to get some notification
  else if (Notification.permission === "granted") {
    // If it's okay let's create a notification

    // show the notification
    var notification = new Notification('Battery status', { body: message });
    // And vibrate the device if it supports vibration API
    window.navigator.vibrate(500);
  }
  // Otherwise, we need to ask the user for permission
  // Note, Chrome does not implement the permission static property
  // So we have to check for NOT 'denied' instead of 'default'
  else if (Notification.permission !== 'denied') {
    Notification.requestPermission(function (permission) {
      // Whatever the user answers, we make sure Chrome stores the information
      if(!('permission' in Notification)) {
        Notification.permission = permission;
      }
      // If the user is okay, let's create a notification
      if (permission === "granted") {

        // show the notification
        var notification = new Notification('Battery status', { body: message });
        // And vibrate the device if it supports vibration API
        window.navigator.vibrate(500);
      }
    });
  }
}

Finally, we'll add event handlers to the battery object to let us respond to changes in the battery's charging state and charge level (by running the updateBatteryStatus() function), and then run updateBatteryStatus() once to get the show started:

// Event handler to check whether the battery has started charging or stopped charging
battery.addEventListener("chargingchange", updateBatteryStatus, false);
// Event handler to check whether the battery charge level has changed
battery.addEventListener("levelchange", updateBatteryStatus, false);

// run the central function once when the app is first loaded
updateBatteryStatus();

The comments should explain what the code does well enough, but the take home message is that it is easy to use hardware data and functionality via such APIs, with simple events and objects like chargingchange, battery, and Notification().

The JavaScript is watching for when the battery charge level changes, or when the battery stops or starts charging (the chargingchange and levelchange event listeners.) When one of these events happens, the updateBatteryStatus() function is run, which decides what notification to tell the user, updates the visual display to suit, and runs createNotification().

This final function actually fires the system notification and makes the phone vibrate to give the user some extra system-wide feedback as to what the battery status is.

Note: Check the WebAPI page frequently to keep up to date with device API statuses.

Install API functionality

In our sample app template, we've implemented an install button that you can click when viewing the app as a standard Web page, to install that site on Firefox OS as an app. The button markup is nothing special:

<button id="install">Install app on device</button>

This button's functionality will be implemented using the Install API. Add the following into your example's scripts/install.js file:

// get a reference to the install button
var button = document.getElementById('install');

// if browser has support for installable apps, run the install code; it not, hide the install button
if('mozApps' in navigator) {

    // define the manifest URL
    var manifest_url = location.href + 'manifest.webapp';

    function install(ev) {
      ev.preventDefault();
      // install the app
      var installLocFind = navigator.mozApps.install(manifest_url);
      installLocFind.onsuccess = function(data) {
        // App is installed, do something if you like
      };
      installLocFind.onerror = function() {
        // App wasn't installed, info is in
        // installapp.error.name
        alert(installLocFind.error.name);
      };
    };

    // if app is already installed, hide button. If not, add event listener to call install() on click
    var installCheck = navigator.mozApps.checkInstalled(manifest_url);
    installCheck.onsuccess = function() {

      if(installCheck.result) {
        button.style.display = "none";
      } else {
        button.addEventListener('click', install, false);
      };
    };
} else {
  button.style.display = "none";
}

Again, the comments explain what's going on quite nicely, but in brief, we first check whether the mozApps object exists in the browser (if('mozApps' in navigator)) — i.e. if the browser supports installable apps or not. If not, we just hide the install button.

Next, navigator.mozApps.checkInstalled checks whether the app defined by the manifest at manifest_url is already installed on the device. If the test returns a success, its success event is fired and the installCheck.onsuccess = function() { ... } is run.

We then test for the existence of installCheck.result; if it does exist, meaning that the app is installed, we hide the install button. If the app isn't installed, we add a click event listener to the button, so the install() function is run when the button is clicked.

When the button is clicked and the install() function is run, we install the app using navigator.mozApps.install(manifest_url), storing a reference to that installation in the installLocFind variable. You'll notice that this installation also fires success and error events, so you can run actions dependent on whether the install happened successfully or not.

Note: Installable open web apps used to have a "single app per origin" security policy, but this was lifted as of Firefox 34/Firefox OS 2.1 (read this FAQ entry for more information). If you still need to support older versions, consider hosting your apps at separate origins; one strategy is to create different subdomains for your apps.

Testing your app

At this point, your app should be finished, and you can start testing it in browsers. If the app does not seem to be working, you can find the finished source code to check against, or see the example running live. For example, it looks like this on a desktop computer:

An image of the quickstart app in a desktop browser. it shows a title saying Battery indicator, and icons to indicate that the battery is not charging.

Testing on Firefox desktop

The quickest way to test your app's basic functionality is to simply load it in Firefox desktop (open the index.html file in the browser) — this supports most of the features we are talking about here. The notifications look like so on OS X:

A system notification from Mac OS X saying that the device battery is not charging.

And in Firefox Aurora/Nightly and Firefox for Android, you can test the install functionality — these browsers include the Firefox runtime that allows installable web apps to be installed on the desktop.

Testing in the Firefox OS simulator

You can also test the app in a Firefox OS simulator via our WebIDE tool. This will give you a more realistic idea of how it will look on a real device. In short, you need to:

  1. Open WebIDE (Tools > Web Developer > WebIDE)
  2. Install a simulator using Select Runtime > Install Simulator
  3. Open the simulator using Select Runtime > [name of simulator]
  4. Select Open App > Open packaged app then navigate to your app's local directory
  5. Click the "play" button to load it inside the Firefox OS Simulator

Testing on a Firefox OS device

The vibration API won't work on these options however. To fully test this you'll need to get hold of a real Firefox OS device. If you've got one, you can connect it to your computer and install apps contained on your local drive straight onto it via WebIDE. Here's a Firefox OS screenshot showing the app running, along with a system notification.

A Firefox OS screenshot showing the app, with a notification to say that the battery is charging.

To install the app on your Firefox OS device via the App Manager:

  1. Open WebIDE (Tools > Web Developer > WebIDE)
  2. On your Firefox OS device, Select the ADB and Devtools option in the Remote Debugging developer setting
  3. Connect your phone to your desktop computer via USB
  4. Make sure you've got ADB installed on your computer, and that it is working (type the adb devices command into your terminal and see if it gives you an entry for your phone)
  5. Click the option that represents your device inside the webIDE Select Runtime menu
  6. Inside WebIDE, select Open App > Open packaged app then navigate to your app's local directory
  7. Click the "play" button to install it on the Firefox OS Device

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