signalkit

Signals & delegates

npm install signalkit
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signalkit

Tiny signals library. Signals are an alternative to EventEmitter wherein each possible event is represented by a unique object to which listeners can be attached. There are a couple of benefits to this approach:

Anonymous signal objects can be passed around independently of their owner object

In the example below, MyObserver could be passed any signal at all:

function MyObserver(signal) {
    signal.connect(function() {
        // do something
    })
}

With EventEmitter, we'd need to attach to a fixed event of a given object, or pass in the event name explicitly. I find this less elegant.

function MyObserver(obj, eventName) {
    obj.on(eventName || 'data', function() {
        // do something
    })
}

No subtle bugs caused by misspelt event names

obj.on('foobar', function() { ... });
obj.emit('foobaz'); // nothing happens

Contrast with:

var obj = {};
obj.foobar = new Signal();
obj.foobar.connect(function() { ... });
obj.foobaz.emit(); // runtime error

Example

var Signal = require('signalkit').Signal;
var signal = new Signal('foo');

var unsub1 = signal.connect(function(name) {
    console.log("handler 1: " + name);
});

var foo = {
    bar: function(name) {
        console.log("handler 2: " + name);
    }
};

var unsub2 = signal.connect(foo, 'bar');

console.log("emit fred...");
signal.emit("fred");

console.log("emit bob...");
unsub1();
signal.emit("bob");

console.log("emit alice...");
unsub2();
signal.emit("alice");

API

Signals

var signal = new Signal(name)

Create a new signal with a given name.

signal.connect(fn)

Connect a supplied function so it will be called when this Signal is emitted.

Returns a function that can be called to cancel the connection.

signal.connect(object, methodName)

Connect a supplied object/method to this Signal, i.e. call object[methodName]() when signal is emitted. The method lookup is lazy; that is, object[methodName] is resolved each time the signal is fired, rather than being bound at the time of connection.

Returns a function that can be called to cancel the connection.

signal.once(fn)

As per signal.connect(fn), except that the function is called once and once only.

Returns a function that can be called to cancel the connection.

signal.once(object, methodName)

As per signal.connect(object, methodName), except that the method is called once and once only.

Returns a function that can be called to cancel the connection.

signal.emit(args...)

Emit this signal, invoking each connected function with the given arguments. All handlers are guaranteed to fire; any errors thrown by handlers will be caught and re-raised asynchronously.

signal.clear()

Remove all connections.

Error handling

You can catch - and handle - any errors thrown during event handling by overriding signal.onError, which receives the thrown error as an argument. Your error handler may return false to prevent any successive handlers from being fired. The default behaviour is to continue firing the remaining handlers and rethrow the error asynchronously.

The default error handler for all signals can be changed by setting Signal.prototype.onError.

TODO

  • Async emit. Not sure if this should be specified at signal creation time or emission time.
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