README

diagnostic-channel

This package provides a channel to connect diagnostic event publishers and subscribers. It includes a way to preserve context for the published event too. What that context is and contains is deliberately left unspecified.

Usage

Subscribe to an event:

const channel = require('diagnostic-channel').channel;

channel.subscribe('someEvent', function (event) {
    // do something about the event
});

The properties of the event object passed to the subscriber handler function are determined by the publisher.

Publish an event

var someData = { myField: "myData" };
const channel = require('diagnostic-channel').channel;

channel.publish('someEvent', someData);

Publish an event with context

Preserving context on publication may require some additional effort. A correlation handler can be added with the addContextPreservation method, and then the bindToContext method can be used to set up context using the provided handler before calling its received function.

channel.addContextPreservation((callback) => {
    return Zone.current.wrap(callback);
});

function doWork(args, callback) {
    // In some context...
    doBatchedAsyncWork(args, channel.bindToContext((result) => {
        channel.publish('someEvent', {result: result});
        callback(result);
    }))
}

API

channel.subscribe(name: string, callback: (event: any) => void): void

Register the callback to be called when publish is invoked with a matching name. The callback will be given the object that is passed to the publish call.

If the callback throws an error, it will be silently ignored. If the callback modifies the event object, any subsequent subscribers will see the modified object, and it may also impact the original code's execution.

publish

channel.publish(name: string, event: any): void

Trigger each subscribed callback for the same named event, passing the event object to each.

Subscribers may modify the event object.

unsubscribe

channel.unsubscribe(name: string, callback: (event: any) => void): void

Remove a previously registered callback from the named event. This uses function equality so it must be a reference to the same function, not an equivalent function.

addContextPreservation

channel.addContextPreservation(preserver: (callback: Function) => Function)

Pushes the provided context preservation function onto a 'stack' of functions to preserve context.

The context preservation function preserver is expected to capture the current context and return a function that when invoked restores this preserved context and only then calls the provided callback with the originally provided arguments. Before returning, this second function should also restore the previous context.

A simple example preserving the Zone.js context:

channel.addContextPreservation((callback) => {
    return Zone.current.wrap(callback);
});

A more general, but somewhat contrived, example, where the 'context' is a global object called context:

var context = { value: 1 };

channel.addContextPreservation((callback) => {
    var preservedContext = context;
    return function () {
        var priorContext = context;
        context = preservedContext;
        var result = callback.apply(this, arguments);
        context = priorContext;
        return result;
    }
});

bindToContext

channel.bindToContext(callback: Function)

Returns a function which will call the callback after applying each of the registered context preservation functions, and return the result of the callback after unwinding each of the context preservation functions.

For example, when using Zone.js:

channel.addContextPreservation((callback) => Zone.current.wrap(callback));

var z1 = Zone.current.fork({name: 'zone 1'});
var z2 = Zone.current.fork({name: 'zone 2'});

var z1BoundFunc = z1.run(() => channel.bindToContext(() => Zone.current.name));

var result = z2.run(() => z1BoundFunc());

Because the function was bound in zone 1, result will be zone 1.

registerMonkeyPatch

channel.registerMonkeyPatch(packageName: string, patcher: {versionSpecifier: string, patch: (any, path: string) => any});

In order to inject publishing and context preservation behavior into third party libraries, we support monkey patching libraries as they are require'd.

Calling this function will register a candidate monkey patcher to be applied when a future require(packageName) is called. If the package's version is a semver match for the versionSpecifier range, then the original object for that package is passed to the patch function, along with the path to the module, and the patch function should return a patched version which will end up as the result of the require.

For a simple example where we patch a doSomethingAsync method of the foo module to preserve the current context when invoking a callback:

function patchFunction(originalPackage, packagePath) {
    var originalFooAsync = foo.doSomethingAsync;
    foo.doSomethingAsync = function () {
        var callback = arguments[arguments.length-1];
        if (callback && typeof callback == 'function') {
            arguments[arguments.length-1] = channel.bindToContext(callback);
        }
        return originalFooAsync.apply(this, arguments);
    }
    return originalPackage;
}

var patcher = {
    versionSpecifier: ">= 1.0.0 < 2.0.0",
    patch: patchFunction
};

channel.registerMonkeyPatch('foo', patcher);

var foo = require('foo');
// Now foo.doSomethingAsync will be the patched version, assuming that the version of the foo package found by require() falls within the 1.0.0 - 2.0.0 range.

Usage no npm install needed!