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HomeBridge Plugin Development Tutorial

Click YouTube Thumbnail to watch tutorial videos in each section. Note that I am not an NodeJS expert. In fact, I just learned from other peoples code for this project. Any error correction / feedback are welcome.

Introduction

Tutorial Video #1 Introduction (Korean Language)

Before going further please,

• Make sure that you have an iPhone
• Install Homebridge on your preferred computer; i.e. Mac, PC, or Raspberry pi
• Add homebridge to your HomeKit

Bringing up plugin and adding an accessory

Tutorial Video #2 HandsOn: Adding an Accessory to Homekit (Korean Language)

Homebridge Plugin is in a form of NodeJS Module so the first step is to creat a module. Homebridge only searches module with name starting with "homebridge-" and checks the keyword section of package.json file; the keyword should be homebridge-plugin.

I made a directory "homebridge-tutorial" to store my codes. Under the directory two files are required to be a module: i.e. index.js and package.json.

index.js

// index.js
"use strict";
module.exports = function (homebridge) {
};

package.json

{
  "name": "homebridge-tutorial",
  "version": "0.0.1",
  "description": "Description for Homebridge Tutorial",
  "license": "the most strict license",
  "keywords": [
    "homebridge-plugin"
  ],
  "engines": {
    "node": ">=10.17.0",
    "homebridge": ">=0.4.8"
  },
  "dependencies": {}
}

Now time to install my module so that homebridge can load. In the directory where package.json lives, run the following command. It will create a symbolic link in nodejs module path that points our module directory.

sudo npm link

Refresh your homebridge config web interface. Plugin should be found there.

Adding an accessory

When homebridge code calls our module it passes itself. We added an argument of our function called 'homebridge' to recieve it. Note that name can be anything.

Now with homebridge object, we can call homebridge related function so we call homebridge object an API (application programming interface).

To add an accessory, call registerAccessory function of homebridge object as shown in the example below.

module.exports = function (homebridge) {
    /*
        API.registerAccessory(PluginIdentifier,
            AccessoryName, AccessoryPluginConstructor)
    */
   homebridge.registerAccessory("homebridge-tutorial",
            "SensMan Volume", volume); 
};

When we call this API function, homebridge wants to know which plugin is calling it. That is why it requires the first argument to be the plugin name. Second argument is accessory name and it can be anything. In many homekit products, it is usually the product name. The last one is called a constructor. Constructor is what we will define and it is the core of our accessory.

Now save index.js and restart homebridge then print out the log.

$ sudo hb-service restart <- restart homebridge
$ hb-service logs <- see logs. Ctrl+C to stop

This will print out some error because we have not defined the constructor. And it is proof that our plugin is loaded by homebridge. So we made a progress

[6/18/2020, 2:49:59 AM] ERROR INITIALIZING PLUGIN homebridge-tutorial:
[6/18/2020, 2:49:59 AM] ReferenceError: volume is not defined ...

Constructor Definition

function volume(log, config, api) {
    this.log = log;
    this.config = config;
    this.homebridge = api;

    if (this.config.defaultVolume)
        this.defaultVolume = this.config.defaultVolume;
    else
        this.defaultVolume = 10;

    this.log('Volume accessory is Created!');
    this.log('defaultVolume is ' + this.defaultVolume);
};

After adding this code, restarting homebrdige won't generate errors. Instead it says

[6/18/2020, 3:42:07 AM] Loaded plugin: [email protected]
[6/18/2020, 3:42:07 AM] Registering accessory 'homebridge-tutorial.SensMan Volume'

Create Accessory Instances

Add accessories in config.json file, which is homebridge's configuration file. Please properly use the commas not to get any errors.

"accessories": [
        {
            "name": "TV Volume",
            "accessory": "SensMan Volume"
        },
        {
            "name": "Radio Volume",
            "accessory": "SensMan Volume",
            "defaultVolume": 90
        }
    ]

Homebridge will call our constructor for each accessory. It also calls getServices to know what devices are included in my accessory. For example, despite of the accessory name, SensMan Volume may contain a lightbulb and a fan.

We have to define getServices function like below. Currently, we do not return any service, which means that there will be no actual device showing in homekit.

volume.prototype = {
    getServices: function() {}
}

When homebridge calls our constructor. It passes three variables. The first one is log. Using this object, my plugin can write into homebridge's log file. Second one is configuration file. In case of above example, "defaultVolume: 90" is passed by this variable. Last one is again the homebridge object itself.

In our volume constructor function, we first store the three passed variabled in 'this' for later use. Then, check if defaultVolume is defined for the accessory. If defined store the value in 'this' again. If not defined store a default of defaultVolume, which is 10.

Lastly, prints out some strings in log file. What do you expect to see from the log?? Here it is.

[6/18/2020, 3:56:24 AM] [TV Volume] Initializing SensMan Volume accessory...
[6/18/2020, 3:56:24 AM] [TV Volume] Volume accessory is Created!
[6/18/2020, 3:56:24 AM] [TV Volume] defaultVolume is 10
[6/18/2020, 3:56:24 AM] [TV Volume] Accessory SensMan Volume returned empty set of services. Won't adding it to the bridge!
[6/18/2020, 3:56:24 AM] [Radio Volume] Initializing SensMan Volume accessory...
[6/18/2020, 3:56:24 AM] [Radio Volume] Volume accessory is Created!
[6/18/2020, 3:56:24 AM] [Radio Volume] defaultVolume is 90
[6/18/2020, 3:56:24 AM] [Radio Volume] Accessory SensMan Volume returned empty set of services. Won't adding it to the bridge!

So far, our code had nothing to do with Homekit. The real homekit thing is here called service. Let's add a lightbulb service in our plugin.

Adding a lightbulb Service

Now we will add lightbulb Service in Constructor function. Before moving on let's define some useful global variables for convenience. Service and Characteristic objects are two key objects that we will be frequently using so I am giving them short names. Put the following code right after "use strict";

// declare variables for easy access to often-used long-named variables
let Service, Characteristic;

And add the following code right before "registerAccessory" function.

    Service = homebridge.hap.Service;
    Characteristic = homebridge.hap.Characteristic;

Now let's add Service in volume function.

    this.bulb = new Service.Lightbulb(this.config.name);
    // Set up Event Handler for bulb on/off
    this.bulb.getCharacteristic(Characteristic.On)
        .on("get", this.getPower.bind(this))
        .on("set", this.setPower.bind(this));
    
    this.log('all event handler was setup.')

"new" will create a lightbulb service with the name defined in config; i.e. TV Volume or Radio Volume from homebridge's config.json file. You will see the names dispalyed in you home app tiles.

Homekit's lightbulb requires "On Characterstic" to be defined. "On Characteristic" is about the state of the lightbulb, i.e. on or off. You have to defines two event handlers; (1) event when homekit asks current lightbulb state and (2) event when user changed the lightbulb state in home app. They are called get and set events. To define the event handlers, we first get the Characteristic object using getCharacteristic function. Use .on function to add event handlers to get and set events.

Finally we added a service. So let's update getService function again. It should return information about this accessory and the created service.

        if (!this.bulb) return [];
        this.log('Homekit asked to report service');
        const infoService =  
            new Service.AccessoryInformation();
        infoService
            .setCharacteristic(Characteristic.Manufacturer,
                'SensMan')
        return [infoService, this.bulb];
}

First if the service is not created yet, return nothing. Then, we create information object and add some information. I added Manufacturer information but there are more you can add. Finally, we return information and the service that we created.

Now the last step! We need to define the event handlers. For get event, we return the current state. For now let's always return true. For set event, we do nothing. Note that all event handler is given 'callback' function we must be called only 'once'. Event driven programming is asynchronous; i.e. after homebridge calls one of this handler it does not wait the handler finishes its job. It is our event handler to call 'callback' function which is part of homebridge that takes care of things after handler finishes all job. The first argument of homebrige's callback is always err. For now we use null to indicate no error.

volume.prototype = {
    getServices: function() {
        // ... already defined above
    },    
    getPower: function(callback) {
        this.log('Homekit Asked Power State');
        callback(null,true)
    },
    setPower: function(on, callback) {
        this.log('Homekit Gave New Power State' + ' ' + on);
        callback(null)
    }
}

Now restart homebridge and open your iphone. Voila! You see two lightbulb accessories in your home app.

Tutorial Video #3 Summary (Korean Language)

Event Handling

Tutorial Video #4 Event Handler (Korean Language)

User Scenario Specification

Before we write up any code in event handler, we have to define what to implement. Sometimes people are not patient and rush into coding but this will make things complicated unless you are lucky. Knowing what you will code is very important.

Here is what I defined as use cases.

(1) User tap lightbulb tile to mute or unmute the speaker

• lightbulb off -> volume becomes zero
• lightbulb on -> volume becomes defaultVolume

(2) User slide lightbulb slidebar to between 0~100

• brightness set to x % -> volume becomes x

For the first use case, we make use of ON Characteristics. For the next one, we do of Brightness Characteristics.

Homekit Callback Scenarios

(1) Homekit UI update (eg. open home app, etc)

• On Characteristic -> Get
• Brightness Characteristic -> Get

(2) Tile toggle

• Brightness Characteristic -> Set 100%
• On Characteristic -> Set True

(3) Slider change

• Brightness Characteristic -> Set x %
• On Characteristic -> Set x (True if x>0, False otherwise)

Volume Control Code

How to adjust volume of a speaker device depends on the hardware. Some speaker does not have this capability at all; some can be impelmented very easily whereas some can't. For this reason, I'll not go in details about this.

In my case, I have Sonos's system in my living room. There are plenty of libraries and sources codes that works for chaning volume with Sonos. Since my preferred language is Python and I have no idea of NodeJS, I am forced to code Python to control my Speaker. So my plugin (NodeJS) has to call my Python code and I think the most elegant way is to utilize REST.

Long story short, I need to code the following in NodeJS for given tasks.

• HTTP POST to http://127.0.0.1:5000/volume with volume data "volume":xx will change the volume to xx.
• HTTP GET from http://127.0.0.1:5000/volume will return current volume information

Coding to Complete

Add Brightness Slider

Just like we added On Characteristic, Adding a Brightness Characteristic of bulb service will add a slider to the UI.

this.bulb.getCharacteristic(Characteristic.On)
    .on("get", this.getPower.bind(this))
    .on("set", this.setPower.bind(this));
this.bulb.getCharacteristic(Characteristic.Brightness)
    .on("get", this.getVolume.bind(this))
    .on("set", this.setVolume.bind(this));

Getting Power State and Volume

According to my experiment, get brightness is always called after get power state. We impelment speaker volume reading in get power state using http module. The HW volume data is stored as this.vol and getPower reports this.vol>0 to homebridge.

Soon after getPower is called, getVolume is called. Since this.vol is already updated, the function simply reports this.vol value.

getPower: function(callback) {
        this.log('getPower');

        // read speaker volume info
        let req = http.get('http://localhost:5000/volume', res => {
            let recv_data = '';
            res.on('data', chunk => { recv_data += chunk});
            res.on('end', () => {
                // recv_data contains volume info.
                let vol = JSON.parse(recv_data).volume; // vol = [0,100]
                this.log('Read from Sonos; volume: ' + vol);
                this.vol = vol;

                callback(null, this.vol > 0);
            });
        });
        req.on('error', err => {
            this.log("Error in getPower: "+ err.message);
            callback(err);
        })
    },
getVolume: function (callback) {
        this.log('getVolume')

        // callback with volume read in getPower
        callback(null,this.vol)
    },

Setting Power State and Volume

User's toggle and slide actions should be distinguished because both actions will cause setPower event. From experiment, assuming that setPower is always called after setVolume is fair. So in setVolume, we define a variable (triggeredby) to indicate that setVolume is just called. In setPower, the variable is checked so if setVolume is called right before, it will use the volume from UI to adjust HW. If not, it is from tile UI, which is toggling, the volume is either set to 0 or defaultValue.

The HW adjustment is again via http module.

After sending a command to speaker HW in setPower, UI update is called. UI update function asks homekit UI to be updated after 100ms delay. This is necessary because upon toggling on action, homekit UI assumes 100% brightness; the actual HW volume is not 100% but UI thinks it is 100%. The 100ms is enough to wait until homekit change the UI to 100%, and our update code will ask UI change to the right value.

setPower: function(on, callback) {
        let new_vol;
        if(this.triggeredby=='slider') {
            this.log('setPower triggered by slider')
            new_vol = this.vol;
            delete this.triggeredby;
        } else {
            this.log('setPower ' + on)
            new_vol = on ? this.defaultVolume : 0;
        }

        let toSend = '{"volume": ' + new_vol + '}';
        let options = {
            host: 'localhost',
            port: 5000,
            path: '/volume',
            method: 'POST',
            headers: {
                'Content-Type': 'application/json',
                'Content-Length': toSend.length
            }
        }

        let req = http.request(options, res => {
            let recv_data = '';
            res.on('data', chunk => {recv_data += chunk})
        });

        req.on('error', err=>{
            this.log('Error in setPower:' + err.message);
            callback(err);
        });

        req.end(toSend)
        this.log('Request sent to set volume to ' + new_vol)
        this.vol = new_vol;

        this.updateUI();
                
        callback(null);
    },
    setVolume: function (vol, callback) {
        if(vol==100) {callback(null); return;}
        this.log('setVolume ' + vol);

        this.vol = vol;
        this.triggeredby = 'slider';

        callback(null);
    },
    updateUI: function () {
        setTimeout( () => {
            this.bulb.getCharacteristic(Characteristic.Brightness).updateValue(this.vol);
            this.bulb.getCharacteristic(Characteristic.On).updateValue(this.vol>0);
        }, 100);
    }

Polling

If an external non-homkit device changes the volume of speaker HW, homekit never knows. In this case, homekit UI and our plugin shall have wrong information. For this reason, I implemented a polling mechanism, which checks the volume of speaker HW per every 1 sec and update the UI.

function volume(log, config, api) {
    // ... existing codes
    if (this.config.refreshInterval)
        this.refreshInterval = this.config.refreshInterval;
    else
        this.refreshInterval = 1000;
    this.timer = setTimeout(this.poll.bind(this), this.refreshInterval);
}

poll: function() {
        if(this.timer) clearTimeout(this.timer);
        this.timer = null;

        // volume update from Sonos
        this.getPower( (err, poweron) => {  //this.vol updated.
            // update UI
            this.updateUI();
        });

        this.timer = setTimeout(this.poll.bind(this), this.refreshInterval)
    }