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Build Apps with Sencha Ext JS

The goal of this doc is to get you started on building Chrome Apps with the Sencha Ext JS framework. To achieve this goal, we will dive into a media player app built by Sencha. The source code and API Documentation are available on GitHub.

This app discovers a user's available media servers, including media devices connected to the pc and software that manages media over the network. Users can browse media, play over the network, or save offline.

Here are the key things you must do to build a media player app using Sencha Ext JS:

  • Create manifest, manifest.json.
  • Create event page, background.js.
  • Sandbox app's logic.
  • Communicate between Chrome App and sandboxed files.
  • Discover media servers.
  • Explore and play media.
  • Save media offline.

Create manifest

All Chrome Apps require a manifest file which contains the information Chrome needs to launch apps. As indicated in the manifest, the media player app is "offline_enabled"; media assets can be saved locally, accessed and played regardless of connectivity.

The "sandbox" field is used to sandbox the app's main logic in a unique origin. All sandboxed content is exempt from the Chrome App Content Security Policy, but cannot directly access the Chrome App APIs. The manifest also includes the "socket" permission; the media player app uses the socket API to connect to a media server over the network. 

{
    "name": "Video Player",
    "description": "Features network media discovery and playlist management",
    "version": "1.0.0",
    "manifest_version": 2,
    "offline_enabled": true,
    "app": {
        "background": {
            "scripts": [
                "background.js"
            ]
        }
    },
    ...

    "sandbox": {
        "pages": ["sandbox.html"]
    },
    "permissions": [
        "experimental",
        "http://*/*",
        "unlimitedStorage",
        {
            "socket": [
                "tcp-connect",
                "udp-send-to",
                "udp-bind"
            ]
        }
    ]
}

Create event page

All Chrome Apps require background.js to launch the application. The media player's main page, index.html, opens in a window with the specified dimensions: 

chrome.app.runtime.onLaunched.addListener(function(launchData) {
    var opt = {
        width: 1000,
        height: 700
    };

    chrome.app.window.create('index.html', opt, function (win) {
        win.launchData = launchData;
    });

});

Sandbox app's logic

Chrome Apps run in a controlled environment that enforces a strict Content Security Policy (CSP). The media player app needs some higher privileges to render the Ext JS components. To comply with CSP and execute the app logic, the app's main page, index.html, creates an iframe that acts as a sandbox environment: 

<iframe id="sandbox-frame" sandbox="allow-scripts" src="sandbox.html"></iframe>

The iframe points to sandbox.html which includes the files required for the Ext JS application: 

<html>
<head>
    <link rel="stylesheet" type="text/css" href="resources/css/app.css" />'
    <script src="sdk/ext-all-dev.js"></script>'
    <script src="lib/ext/data/PostMessage.js"></script>'
    <script src="lib/ChromeProxy.js"></script>'
    <script src="app.js"></script>
</head>
<body></body>
</html>

The app.js script executes all the Ext JS code and renders the media player views. Since this script is sandboxed, it cannot directly access the Chrome App APIs. Communication between app.js and non-sandboxed files is done using the HTML5 Post Message API.

Communicate between files

In order for the media player app to access Chrome App APIs, like query the network for media servers, app.js posts messages to index.js. Unlike the sandboxed app.js, index.js can directly access the Chrome App APIs.

index.js creates the iframe: 

var iframe = document.getElementById('sandbox-frame');

iframeWindow = iframe.contentWindow;

And listens for messages from the sandboxed files: 

window.addEventListener('message', function(e) {
    var data= e.data,
        key = data.key;

    console.log('[index.js] Post Message received with key ' + key);

    switch (key) {
        case 'extension-baseurl':
            extensionBaseUrl(data);
            break;

        case 'upnp-discover':
            upnpDiscover(data);
            break;

        case 'upnp-browse':
            upnpBrowse(data);
            break;

        case 'play-media':
            playMedia(data);
            break;

        case 'download-media':
            downloadMedia(data);
            break;

        case 'cancel-download':
            cancelDownload(data);
            break;

        default:
            console.log('[index.js] unidentified key for Post Message: "' + key + '"');
    }
}, false);

In the following example, app.js sends a message to index.js requesting the key 'extension-baseurl': 

Ext.data.PostMessage.request({
    key: 'extension-baseurl',
    success: function(data) {
        //...
    }
});

index.js receives the request, assigns the result, and replies by sending the Base URL back: 

function extensionBaseUrl(data) {
    data.result = chrome.extension.getURL('/');
    iframeWindow.postMessage(data, '*');
}

Discover media servers

There's a lot that goes into discovering media servers. At a high level, the discovery workflow is initiated by a user action to search for available media servers. The MediaServer controller posts a message to index.js; index.js listens for this message and when received, calls Upnp.js.

The Upnp library uses the Chrome App socket API to connect the media player app with any discovered media servers and receive media data from the media server. Upnp.js also uses soapclient.js to parse the media server data. The remainder of this section describes this workflow in more detail.

Post message

When a user clicks the Media Servers button in the center of the media player app, MediaServers.js calls discoverServers(). This function first checks for any outstanding discovery requests, and if true, aborts them so the new request can be initiated. Next, the controller posts a message to index.js with a key upnp-discovery, and two callback listeners: 

me.activeDiscoverRequest = Ext.data.PostMessage.request({
    key: 'upnp-discover',
    success: function(data) {
        var items = [];
        delete me.activeDiscoverRequest;

        if (serversGraph.isDestroyed) {
            return;
        }

        mainBtn.isLoading = false;
        mainBtn.removeCls('pop-in');
        mainBtn.setIconCls('ico-server');
        mainBtn.setText('Media Servers');

        //add servers
        Ext.each(data, function(server) {
            var icon,
                urlBase = server.urlBase;

            if (urlBase) {
                if (urlBase.substr(urlBase.length-1, 1) === '/'){
                        urlBase = urlBase.substr(0, urlBase.length-1);
                }
            }

            if (server.icons && server.icons.length) {
                if (server.icons[1]) {
                    icon = server.icons[1].url;
                }
                else {
                    icon = server.icons[0].url;
                }

                icon = urlBase + icon;
            }

            items.push({
                itemId: server.id,
                text: server.friendlyName,
                icon: icon,
                data: server
            });
        });

        ...
    },
    failure: function() {
        delete me.activeDiscoverRequest;
                
        if (serversGraph.isDestroyed) {
            return;
        }
                
        mainBtn.isLoading = false;
        mainBtn.removeCls('pop-in');
        mainBtn.setIconCls('ico-error');
        mainBtn.setText('Error...click to retry');
    }
});

Call upnpDiscover()

index.js listens for the 'upnp-discover' message from app.js and responds by calling upnpDiscover(). When a media server is discovered, index.js extracts the media server domain from the parameters, saves the server locally, formats the media server data, and pushes the data to the MediaServer controller.

Parse media server data

When Upnp.js discovers a new media server, it then retrieves a description of the device and sends a Soaprequest to browse and parse the media server data; soapclient.js parses the media elements by tag name into a document.

Connect to media server

Upnp.js connects to discovered media servers and receives media data using the Chrome App socket API: 

socket.create("udp", {}, function(info) {
    var socketId = info.socketId;
        
    //bind locally
    socket.bind(socketId, "0.0.0.0", 0, function(info) {
            
        //pack upnp message
        var message = String.toBuffer(UPNP_MESSAGE);
            
        //broadcast to upnp
        socket.sendTo(socketId, message, UPNP_ADDRESS, UPNP_PORT, function(info) {
                
            // Wait 1 second
            setTimeout(function() {
                    
                //receive
                socket.recvFrom(socketId, function(info) {

                    //unpack message
                    var data        = String.fromBuffer(info.data),
                        servers     = [],
                        locationReg = /^location:/i;
                            
                    //extract location info
                    if (data) {
                        data = data.split("\r\n");
                            
                        data.forEach(function(value) {
                            if (locationReg.test(value)){
                                servers.push(value.replace(locationReg, "").trim());
                            }
                        });
                    }
                            
                    //success
                    callback(servers);
                });
                    
            }, 1000);
        });
    });
});

Explore and play media

The MediaExplorer controller lists all the media files inside a media server folder and is responsible for updating the breadcrumb navigation in the media player app window. When a user selects a media file, the controller posts a message to index.js with the 'play-media' key: 

onFileDblClick: function(explorer, record) {
    var serverPanel, node,
        type    = record.get('type'),
        url     = record.get('url'),
        name    = record.get('name'),
        serverId= record.get('serverId');
            
    if (type === 'audio' || type === 'video') {
        Ext.data.PostMessage.request({
            key     : 'play-media',
            params  : {
                url: url,
                name: name,
                type: type
            }
        });
    }
},

index.js listens for this post message and responds by calling playMedia(): 

function playMedia(data) {
    var type        = data.params.type,
        url         = data.params.url,
        playerCt    = document.getElementById('player-ct'),
        audioBody   = document.getElementById('audio-body'),
        videoBody   = document.getElementById('video-body'),
        mediaEl     = playerCt.getElementsByTagName(type)[0],
        mediaBody   = type === 'video' ? videoBody : audioBody,
        isLocal     = false;

    //save data
    filePlaying = {
        url : url,
        type: type,
        name: data.params.name
    };

    //hide body els
    audioBody.style.display = 'none';
    videoBody.style.display = 'none';

    var animEnd = function(e) {

        //show body el
        mediaBody.style.display = '';

        //play media
        mediaEl.play();

        //clear listeners
        playerCt.removeEventListener( 'transitionend', animEnd, false );
        animEnd = null;
    };

    //load media
    mediaEl.src = url;
    mediaEl.load();

    //animate in player
    playerCt.addEventListener( 'transitionend', animEnd, false );
    playerCt.style.transform = "translateY(0)";

    //reply postmessage
    data.result = true;
    sendMessage(data);
}

Save media offline

Most of the hard work to save media offline is done by the filer.js library. You can read more this library in Introducing filer.js.

The process kicks off when a user selects one or more files and initiates the 'Take offline' action. The MediaExplorer controller posts a message to index.js with a key 'download-media'; index.js listens for this message and calls the downloadMedia() function to initiate the download process: 

function downloadMedia(data) {
        DownloadProcess.run(data.params.files, function() {
            data.result = true;
            sendMessage(data);
        });
    }

The DownloadProcess utility method creates an xhr request to get data from the media server and waits for completion status. This initiates the onload callback which checks the received content and saves the data locally using the filer.js function: 

filer.write(
    saveUrl,
    {
        data: Util.arrayBufferToBlob(fileArrayBuf),
        type: contentType
    },
    function(fileEntry, fileWriter) {

        console.log('file saved!');

        //increment downloaded
        me.completedFiles++;

        //if reached the end, finalize the process
        if (me.completedFiles === me.totalFiles) {

            sendMessage({
                key             : 'download-progresss',
                totalFiles      : me.totalFiles,
                completedFiles  : me.completedFiles
            });

            me.completedFiles = me.totalFiles = me.percentage = me.downloadedFiles = 0;
            delete me.percentages;

            //reload local
            loadLocalFiles(callback);
        }
    },
    function(e) {
        console.log(e);
    }
);

When the download process is finished, MediaExplorer updates the media file list and the media player tree panel.

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