feat(@embark/core): Add events oracle

Add the ability to request an event before it’s command handler has been set. Also, add the ability to emit an event before a listener has been set.

The most useful use case for this is to allow modules/plugins to load asynchronously and without need to know their load order beforehand. This let’s us request events in advance of having the event command handler set by the module/plugin.

BEFORE:
```
this.events.request(“event:name”, () => {
  // never fired
});
this.events.setCommandHandler(“event:name”, cb);
```

AFTER:
```
this.events.request(“event:name”, () => {
  // YAY it fires!
});
this.events.setCommandHandler(“event:name”, cb);
```

packages/embark/src/lib/core/events.js

@@ -1,4 +1,5 @@
 var EventEmitter = require('events');
+const cloneDeep = require('lodash.clonedeep');
 
 function warnIfLegacy(eventName) {
   const legacyEvents = [];
@@ -20,6 +21,14 @@ EventEmitter.prototype._maxListeners = 350;
 const _on         = EventEmitter.prototype.on;
 const _once       = EventEmitter.prototype.once;
 const _setHandler = EventEmitter.prototype.setHandler;
+const _removeAllListeners = EventEmitter.prototype.removeAllListeners;
+
+const toFire = [];
+
+EventEmitter.prototype.removeAllListeners = function(requestName) {
+  delete toFire[requestName];
+  return _removeAllListeners.call(this, requestName);
+};
 
 EventEmitter.prototype.on = function(requestName, cb) {
   log("listening to event: ", requestName);
@@ -45,7 +54,19 @@ EventEmitter.prototype.request = function() {
 
   log("requesting: ", requestName);
   warnIfLegacy(requestName);
-  return this.emit('request:' + requestName, ...other_args);
+  const listenerName = 'request:' + requestName;
+
+  // if we don't have a command handler set for this event yet,
+  // store it and fire it once a command handler is set
+  if (!this.listeners(listenerName).length) {
+    if(!toFire[listenerName]) {
+      toFire[listenerName] = [];
+    }
+    toFire[listenerName].push(other_args);
+    return;
+  }
+
+  return this.emit(listenerName, ...other_args);
 };
 
 EventEmitter.prototype.setCommandHandler = function(requestName, cb) {
@@ -53,14 +74,51 @@ EventEmitter.prototype.setCommandHandler = function(requestName, cb) {
   let listener = function(_cb) {
     cb.call(this, ...arguments);
   };
+  const listenerName = 'request:' + requestName;
+
   // unlike events, commands can only have 1 handler
-  this.removeAllListeners('request:' + requestName);
-  return this.on('request:' + requestName, listener);
+  _removeAllListeners.call(this, listenerName);
+
+  // if this event was requested prior to the command handler
+  // being set up,
+  // 1. delete the premature request(s) from the toFire array so they are not fired again
+  // 2. Add an event listener for future requests
+  // 3. call the premature request(s) bound
+  const prematureListenerArgs = cloneDeep(toFire[listenerName]);
+  if (prematureListenerArgs) {
+    delete toFire[listenerName];
+    // Assign listener here so that any requests bound inside the
+    // initial listener callback will be bound (see unit tests for an example)
+    this.on(listenerName, listener);
+    prematureListenerArgs.forEach((prematureArgs) => {
+      cb.call(this, ...prematureArgs);
+    });
+    return;
+  }
+  return this.on(listenerName, listener);
 };
 
 EventEmitter.prototype.setCommandHandlerOnce = function(requestName, cb) {
   log("setting command handler for: ", requestName);
-  return this.once('request:' + requestName, function(_cb) {
+
+  const listenerName = 'request:' + requestName;
+
+  // if this event was requested prior to the command handler
+  // being set up,
+  // 1. delete the premature request(s) from the toFire array so they are not fired again
+  // 2. call the premature request(s) bound
+  // Do not bind an event listener for future requests as this is meant to be fired
+  // only once.
+  const prematureListenerArgs = cloneDeep(toFire[listenerName]);
+  if (prematureListenerArgs) {
+    delete toFire[listenerName];
+    prematureListenerArgs.forEach((prematureArgs) => {
+      cb.call(this, ...prematureArgs);
+    });
+    return;
+  }
+
+  return this.once(listenerName, function(_cb) {
     cb.call(this, ...arguments);
   });
 };

packages/embark/src/test/events.js

@@ -0,0 +1,120 @@
+/*globals describe, it, before, beforeEach*/
+const {File, Types} = require("../lib/core/file");
+const Assert = require("assert");
+const {expect} = require("chai");
+const fs = require("../lib/core/fs");
+const Events = require("../lib/core/events");
+
+let events;
+const testEventName = "testevent";
+
+describe('embark.Events', function () {
+  this.timeout(10000);
+  before(() => {
+    events = new Events();
+  });
+
+  beforeEach(() => {
+    events.removeAllListeners(testEventName);
+    events.removeAllListeners(`request:${testEventName}`);
+  });
+
+  describe('Set event listeners', function () {
+    it('should be able to listen to an event emission', (done) => {
+      events.on(testEventName, ({isTest}) => {
+        expect(isTest).to.be.true;
+        done();
+      });
+      events.emit(testEventName, { isTest: true });
+    });
+
+    it('should be able to listen to an event emission once', (done) => {
+      events.once(testEventName, ({isTest}) => {
+        expect(isTest).to.be.true;
+        done();
+      });
+      events.emit(testEventName, { isTest: true });
+    });
+  });
+  describe('Set command handlers', function() {
+    it('should be able to set a command handler and request the event', (done) => {
+      events.setCommandHandler(testEventName, () => {
+        Assert.ok(true);
+        done();
+      });
+      events.request(testEventName);
+    });
+
+    it('should be able to set a command handler with data and request the event', (done) => {
+      events.setCommandHandler(testEventName, (options, cb) => {
+        expect(options.isTest).to.be.true;
+        cb(options);
+      });
+      events.request(testEventName, { isTest: true }, (data) => {
+        expect(data.isTest).to.be.true;
+        done();
+      });
+    });
+
+    it('should be able to set a command handler with data and request the event once', (done) => {
+      events.setCommandHandlerOnce(testEventName, (options, cb) => {
+        expect(options.isTest).to.be.true;
+        cb(options);
+      });
+      events.request(testEventName, { isTest: true }, (data) => {
+        expect(data.isTest).to.be.true;
+        events.request(testEventName, { isTest: true }, (data) => {
+          Assert.fail("Should not call the requested event again, as it was set with once only");
+        });
+        done();
+      });
+    });
+
+    it('should be able to request an event before a command handler has been set', (done) => {
+      let testData = { isTest: true, manipulatedCount: 0 };
+      events.request(testEventName, testData, (dataFirst) => {
+        expect(dataFirst.isTest).to.be.true;
+        expect(dataFirst.manipulatedCount).to.equal(1);
+        expect(dataFirst.isAnotherTest).to.be.undefined;
+      });
+      events.request(testEventName, testData, (dataSecond) => {
+        expect(dataSecond.isTest).to.be.true;
+        expect(dataSecond.manipulatedCount).to.equal(2);
+        expect(dataSecond.isAnotherTest).to.be.undefined;
+
+        dataSecond.isAnotherTest = true;
+        events.request(testEventName, dataSecond, (dataThird) => {
+          expect(dataThird.isTest).to.be.true;
+          expect(dataThird.manipulatedCount).to.equal(3);
+          expect(dataThird.isAnotherTest).to.be.true;
+          done();
+        });
+      });
+      events.setCommandHandler(testEventName, (options, cb) => {
+        expect(options.isTest).to.be.true;
+        options.manipulatedCount++;
+        cb(options);
+      });
+    });
+
+    it('should be able to request an event before a command handler has been set once', (done) => {
+      let testData = { isTest: true, manipulatedCount: 0 };
+      events.request(testEventName, testData, (data) => {
+        expect(data.isTest).to.be.true;
+        expect(data.manipulatedCount).to.equal(1);
+        expect(data.isAnotherTest).to.be.undefined;
+        events.request(testEventName, data, (_dataSecondRun) => {
+          Assert.fail("Should not call the requested event again, as it was set with once only");
+        });
+      });
+      events.request(testEventName, testData, (_dataThirdRun) => {
+        done();
+      });
+      events.setCommandHandlerOnce(testEventName, (options, cb) => {
+        expect(options.isTest).to.be.true;
+        options.manipulatedCount = options.manipulatedCount + 1;
+        cb(options);
+      });
+    });
+  });
+});