Ask/answer opcode implementation with runtime events.

src/blocks/scratch3_sensing.js

@@ -7,6 +7,21 @@ class Scratch3SensingBlocks {
          * @type {Runtime}
          */
         this.runtime = runtime;
+
+        /**
+         * The "answer" block value.
+         * @type {string}
+         */
+        this._answer = '';
+
+        /**
+         * The list of queued questions and respective `resolve` callbacks.
+         * @type {!Array}
+         */
+        this._questionList = [];
+
+        this.runtime.on('ANSWER', this._onAnswer.bind(this));
+        this.runtime.on('PROJECT_STOP_ALL', this._clearAllQuestions.bind(this));
     }
 
     /**
@@ -28,10 +43,51 @@ class Scratch3SensingBlocks {
             sensing_keypressed: this.getKeyPressed,
             sensing_current: this.current,
             sensing_dayssince2000: this.daysSince2000,
-            sensing_loudness: this.getLoudness
+            sensing_loudness: this.getLoudness,
+            sensing_askandwait: this.askAndWait,
+            sensing_answer: this.getAnswer
         };
     }
 
+    _onAnswer (answer) {
+        this._answer = answer;
+        const questionObj = this._questionList.shift();
+        if (questionObj) {
+            const resolve = questionObj[1];
+            resolve();
+            this._askNextQuestion();
+        }
+    }
+
+    _enqueueAsk (question, resolve) {
+        this._questionList.push([question, resolve]);
+    }
+
+    _askNextQuestion () {
+        if (this._questionList.length > 0) {
+            this.runtime.emit('QUESTION', this._questionList[0][0]);
+        }
+    }
+
+    _clearAllQuestions () {
+        this._questionList = [];
+        this.runtime.emit('QUESTION', null);
+    }
+
+    askAndWait (args) {
+        return new Promise(resolve => {
+            const isQuestionAsked = this._questionList.length > 0;
+            this._enqueueAsk(args.QUESTION, resolve);
+            if (!isQuestionAsked) {
+                this._askNextQuestion();
+            }
+        });
+    }
+
+    getAnswer () {
+        return this._answer;
+    }
+
     touchingObject (args, util) {
         const requestedObject = args.TOUCHINGOBJECTMENU;
         if (requestedObject === '_mouse_') {

test/unit/blocks_sensing.js

@@ -0,0 +1,34 @@
+const test = require('tap').test;
+const Sensing = require('../../src/blocks/scratch3_sensing');
+const Runtime = require('../../src/engine/runtime');
+
+test('getPrimitives', t => {
+    const rt = new Runtime();
+    const s = new Sensing(rt);
+    t.type(s.getPrimitives(), 'object');
+    t.end();
+});
+
+test('ask and answer', t => {
+    const rt = new Runtime();
+    const s = new Sensing(rt);
+
+    const expectedQuestion = 'a question';
+    const expectedAnswer = 'the answer';
+
+    // Test is written out of order because of promises, follow the (#) comments.
+    rt.addListener('QUESTION', question => {
+        // (2) Assert the question is correct, then emit the answer
+        t.strictEqual(question, expectedQuestion);
+        rt.emit('ANSWER', expectedAnswer);
+    });
+
+    // (1) Emit the question.
+    const promise = s.askAndWait({QUESTION: expectedQuestion});
+
+    // (3) Ask block resolves after the answer is emitted.
+    promise.then(() => {
+        t.strictEqual(s.getAnswer(), expectedAnswer);
+        t.end();
+    });
+});