hello.js, world.js - A minimal application that connects to another.
wget.js - Uses the network service to load a URL.
--- Running Mojo Applications ---
A Mojo application written in JavaScript is launched with mojo_shell like this:
mojo_shell
Where js-application-url is either a file or an http URL that names a JS source file. The JS file itself must begin with a Mojo “shebang” that specifies the Mojo URL of the JS content handler. In other words, the first line of the JS source file must be:
#!mojo:js_content_handler
Following the shebang should be a single AMD module called “main” whose value is an Application class. The JS content handler will create an instance of the Application and make it the client of the Mojo shell. The JS content handler is itself a Mojo application and it's responsible for creating an instance of V8 and loading the “main” JS module and all of the modules the main module depends on.
The overall structure of a JS Mojo application is this:
#!mojo:js_content_handler
define(“main”, [], function() { function Application(appShell, url) { }
Application.prototype.initialize = function(args) {
}
Application.prototype.acceptConnection = function(url, spHandle) {
}
return Application;
});
The hello.js example is little more than this basic skeleton.
The JS content handler loads the “main” module and makes an instance of its value, which must be the application‘s class. The application’s constructor is passed two arguments:
appShell - a pointer to the Mojo shell. Typically this will be wrapped by a Shell object, see below.
url - the URL this application was loaded from as a String.
The initialize() and acceptConnection() methods are defined by application.mojom and they‘re needed because the JS content handler makes the JS application the Mojo shell’s client.
--- Mojo Application Structure ---
Mojo applications can connect to services provided by other applications and they can provide services of their own. A service is an implementation of a Mojo interface that was defined as part of a Mojo module in a “.mojom” file.
To implement a service you‘ll need the JS “bindings” for the Mojo interface. The bindings are generated by the build system and end up in files whose name is the same as the ‘.mojom’ file with a ‘.js’ suffix. It’s often helpful to look at the generated ‘mojom.js’ files.
The JS Shell class simplifies connecting to applications and services. It‘s a wrapper for the Application’s appShell argument.
The Shell's connectToService() method returns a “proxy” to a service provided by another application.
The JS bindings for a Mojo interface‘s API are delivered as a JS module whose name is based on the ‘.mojom’ file’s path. For example, to use the Mojo network service you need the JS module based on network_service.mojom:
define(“main”, [ “mojo/services/public/interfaces/network/network_service.mojom”, “mojo/services/public/js/shell”, ] function(netModule, shellModule) { function Application(appShell, url) { this.shell = new shellModule.Shell(appShell); }
Application.prototype.initialize = function(args) {
var netService = this.shell.connectToService(
"mojo:network_service", netModule.NetworkService);
}
...
return Application;
});
The first connectToService() parameter is the Mojo URL for the network service application and the second is the JS “interface” object for NetworkService. The JS interface object's properties identify the (generated) JS bindings classes used to provide or connect to a service. For example (from network_service.mojom.js):
var NetworkService = { name: ‘mojo::NetworkService’, // Fully qualified Mojo interface name. proxyClass: NetworkServiceProxy, stubClass: NetworkServiceStub, // ... };
The ‘proxyClass’ is used to access another application's NetworkService and the ‘stubClass’ is used to create an implementation of NetworkService.
In the netService case above the Shell connects to the Mojo application at “mojo:network_service”, then connects its service called ‘NetworkService.name’ with an instance of ‘NetworkService.proxyClass’. The proxy instance is returned.
--- Interface Parameters ---
Mojo functions with interface valued parameters allow one to request a service from a service or to provide a service to a service. The indirect_service example demonstrates this.
The NetworkService CreateURLLoader() method has an interface request parameter:
interface NetworkService { CreateURLLoader(URLLoader& loader); // Return a URLLoader to the caller. ... }
Interface request parameters can be specified as an instance of the interface‘s proxy class. Ordinary interface parameters can be specified as an instance of the interface’s stub class. The stub class constructor has an optional delegate parameter that defines the stub's implementation.
Here's an example of an interface request parameter taken from wget.js:
var urlLoader = new loader.URLLoader.proxyClass; netService.createURLLoader(urlLoader); // interface& parameter
var urlRequest = new loader.URLRequest({ url: “http://www.cnn.com”, method: “GET”, auto_follow_redirects: true });
urlLoader.start(urlRequest).then(function(result) { // ..Do something with result.response });
--- Mojo Responses are Promises ---
Mojo functions can return zero or more values called a “response”. For example the EchoString function below returns a string or null.
interface EchoService { EchoString(string? value) => (string? value); };
The response is delivered to the function caller asynchronously. In C++ the caller provides a Callback object whose Run() method has one argument for each response parameter. In JS, Mojo functions that specify a response return a Promise object. The Promise resolves to an object with one property per response parameter. In the EchoString case that would be something like {value: “foo”}.
Similarly, the implementation of a Mojo interface functions that specify a response, must return a Promise. The implementation of EchoString() could be written like this:
MyEchoStringImpl.prototype.EchoString = function(s) { return Promise.resolve({value: s}); };