Allow customizing the task scheduler.

Closes #17

asynciterator.ts

@@ -6,6 +6,23 @@
 import { EventEmitter } from 'events';
 import queueMicrotask from 'queue-microtask';
 
+let taskScheduler: TaskScheduler = queueMicrotask;
+
+/** Schedules the given ask for asynchronous execution. */
+export function scheduleTask(task: () => void): void {
+  taskScheduler(task);
+}
+
+/** Returns the asynchronous task scheduler. */
+export function getTaskScheduler(): TaskScheduler {
+  return taskScheduler;
+}
+
+/** Sets the asynchronous task scheduler. */
+export function setTaskScheduler(scheduler: TaskScheduler): void {
+  taskScheduler = scheduler;
+}
+
 /**
   ID of the INIT state.
   An iterator is initializing if it is preparing main item generation.
@@ -90,7 +107,7 @@ export class AsyncIterator<T> extends EventEmitter {
         if (!eventAsync)
           this.emit('end');
         else
-          queueMicrotask(() => this.emit('end'));
+          taskScheduler(() => this.emit('end'));
       }
     }
     return valid;
@@ -210,7 +227,7 @@ export class AsyncIterator<T> extends EventEmitter {
     @protected
   */
   protected _endAsync() {
-    queueMicrotask(() => this._end());
+    taskScheduler(() => this._end());
   }
 
   /**
@@ -236,7 +253,7 @@ export class AsyncIterator<T> extends EventEmitter {
       this._readable = readable;
       // If the iterator became readable, emit the `readable` event
       if (readable)
-        queueMicrotask(() => this.emit('readable'));
+        taskScheduler(() => this.emit('readable'));
     }
   }
 
@@ -308,7 +325,7 @@ export class AsyncIterator<T> extends EventEmitter {
       return properties && properties[propertyName];
     // If the value has been set, send it through the callback
     if (properties && (propertyName in properties)) {
-      queueMicrotask(() => callback(properties[propertyName]));
+      taskScheduler(() => callback(properties[propertyName]));
     }
     // If the value was not set, store the callback for when the value will be set
     else {
@@ -336,7 +353,7 @@ export class AsyncIterator<T> extends EventEmitter {
     const callbacks = propertyCallbacks[propertyName];
     if (callbacks) {
       delete propertyCallbacks[propertyName];
-      queueMicrotask(() => {
+      taskScheduler(() => {
         for (const callback of callbacks)
           callback(value);
       });
@@ -502,7 +519,7 @@ function waitForDataListener(this: AsyncIterator<any>, eventName: string) {
     this.removeListener('newListener', waitForDataListener);
     addSingleListener(this, 'readable', emitData);
     if (this.readable)
-      queueMicrotask(() => emitData.call(this));
+      taskScheduler(() => emitData.call(this));
   }
 }
 // Emits new items though `data` events as long as there are `data` listeners
@@ -710,7 +727,7 @@ export class BufferedIterator<T> extends AsyncIterator<T> {
   constructor({ maxBufferSize = 4, autoStart = true } = {}) {
     super(INIT);
     this.maxBufferSize = maxBufferSize;
-    queueMicrotask(() => this._init(autoStart));
+    taskScheduler(() => this._init(autoStart));
   }
 
   /**
@@ -886,7 +903,7 @@ export class BufferedIterator<T> extends AsyncIterator<T> {
     // Acquire reading lock to avoid recursive reads
     if (!this._reading) {
       this._reading = true;
-      queueMicrotask(() => {
+      taskScheduler(() => {
         // Release reading lock so _fillBuffer` can take it
         this._reading = false;
         this._fillBuffer();
@@ -1072,7 +1089,7 @@ export class TransformIterator<S, D = S> extends BufferedIterator<D> {
     const next = () => {
       // Continue transforming until at least `count` items have been pushed
       if (this._pushedCount < count && !this.closed)
-        queueMicrotask(() => this._readAndTransform(next, done));
+        taskScheduler(() => this._readAndTransform(next, done));
       else
         done();
     };
@@ -1220,10 +1237,10 @@ export class SimpleTransformIterator<S, D = S> extends TransformIterator<S, D> {
   /* Tries to read and transform items */
   protected _read(count: number, done: () => void) {
     const next = () => this._readAndTransformSimple(count, nextAsync, done);
+    this._readAndTransformSimple(count, nextAsync, done);
     function nextAsync() {
-      queueMicrotask(next);
+      taskScheduler(next);
     }
-    this._readAndTransformSimple(count, nextAsync, done);
   }
 
   /* Reads and transform items */
@@ -1858,3 +1875,5 @@ type SourceExpression<T> =
 
 type InternalSource<T> =
   AsyncIterator<T> & { _destination: AsyncIterator<any> };
+
+type TaskScheduler = (task: () => void) => void;

test/AsyncIterator-test.js

@@ -4,10 +4,10 @@ import {
   CLOSED,
   ENDED,
   DESTROYED,
+  scheduleTask,
 } from '../asynciterator.mjs';
 
 import { EventEmitter } from 'events';
-import queueMicrotask from 'queue-microtask';
 
 describe('AsyncIterator', () => {
   describe('The AsyncIterator module', () => {
@@ -408,7 +408,7 @@ describe('AsyncIterator', () => {
     before(() => {
       iterator = new AsyncIterator();
       captureEvents(iterator, 'data', 'readable', 'end');
-      iterator._destroy = (error, callback) => queueMicrotask(callback);
+      iterator._destroy = (error, callback) => scheduleTask(callback);
       iterator.destroy();
     });
 

test/BufferedIterator-test.js

@@ -2,10 +2,10 @@ import {
   AsyncIterator,
   BufferedIterator,
   CLOSED,
+  scheduleTask,
 } from '../asynciterator.mjs';
 
 import { EventEmitter } from 'events';
-import queueMicrotask from 'queue-microtask';
 
 describe('BufferedIterator', () => {
   describe('The BufferedIterator function', () => {
@@ -301,7 +301,7 @@ describe('BufferedIterator', () => {
     function createIterator(options) {
       const iterator = new BufferedIterator(options);
       iterator._read = function (count, done) {
-        queueMicrotask(() => {
+        scheduleTask(() => {
           this.close();
           done();
         });
@@ -660,8 +660,8 @@ describe('BufferedIterator', () => {
       describe('after `read` is called and the iterator has been closed', () => {
         before(() => {
           iterator.read();
-          queueMicrotask(() => { _readDone(); });
-          queueMicrotask(() => { iterator.close(); });
+          scheduleTask(() => { _readDone(); });
+          scheduleTask(() => { iterator.close(); });
         });
 
         it('should have emitted the `end` event', () => {
@@ -1292,7 +1292,7 @@ describe('BufferedIterator', () => {
       const iterator = new BufferedIterator(options);
       iterator._read = function (count, done) {
         this._push('a');
-        queueMicrotask(() => {
+        scheduleTask(() => {
           this._push('b');
           this._push('c');
           done();
@@ -1408,9 +1408,7 @@ describe('BufferedIterator', () => {
     before(done => {
       iterator = new BufferedIterator({ autoStart: false });
       iterator._read = (count, callback) => { readDone = callback; };
-      // `queueMicrotask` because reading directly after construction does not call `_read`;
-      // this is necessary to enable attaching a `_begin` hook after construction
-      queueMicrotask(() => { iterator.read(); done(); });
+      scheduleTask(() => { iterator.read(); done(); });
     });
 
     it('should cause an exception', () => {
@@ -1637,7 +1635,7 @@ describe('BufferedIterator', () => {
     before(() => {
       iterator = new BufferedIterator();
       iterator._begin = function (done) {
-        queueMicrotask(() => {
+        scheduleTask(() => {
           this._push('x');
           this._push('y');
           done();
@@ -1947,7 +1945,7 @@ describe('BufferedIterator', () => {
         done();
       };
       iterator._flush = function (done) {
-        queueMicrotask(() => {
+        scheduleTask(() => {
           this._push('x');
           this._push('y');
           done();
@@ -2206,7 +2204,7 @@ describe('BufferedIterator', () => {
         done();
       };
       iterator._flush = function (done) {
-        queueMicrotask(() => {
+        scheduleTask(() => {
           this._push('x');
           this._push('y');
           done();

test/MultiTransformIterator-test.js

@@ -6,10 +6,10 @@ import {
   EmptyIterator,
   SingletonIterator,
   ArrayIterator,
+  scheduleTask,
 } from '../asynciterator.mjs';
 
 import { EventEmitter } from 'events';
-import queueMicrotask from 'queue-microtask';
 
 describe('MultiTransformIterator', () => {
   describe('The MultiTransformIterator function', () => {
@@ -146,7 +146,7 @@ describe('MultiTransformIterator', () => {
       iterator = new MultiTransformIterator(source);
       iterator._createTransformer = sinon.spy(() => {
         const transformer = new BufferedIterator();
-        setImmediate(() => transformer.close());
+        setTimeout(() => transformer.close(), 0);
         return transformer;
       });
     });
@@ -181,7 +181,7 @@ describe('MultiTransformIterator', () => {
       iterator = new MultiTransformIterator(source);
       iterator._createTransformer = sinon.spy(item => {
         const transformer = new BufferedIterator();
-        queueMicrotask(() => {
+        scheduleTask(() => {
           transformer._push(`${item}1`);
           transformer.close();
         });
@@ -209,7 +209,7 @@ describe('MultiTransformIterator', () => {
       iterator = new MultiTransformIterator(source);
       iterator._createTransformer = sinon.spy(item => {
         const transformer = new BufferedIterator();
-        queueMicrotask(() => {
+        scheduleTask(() => {
           transformer._push(`${item}1`);
           transformer._push(`${item}2`);
           transformer._push(`${item}3`);
@@ -300,7 +300,7 @@ describe('MultiTransformIterator', () => {
       iterator = new MultiTransformIterator(source);
       iterator._createTransformer = sinon.spy(item => {
         const transformer = new BufferedIterator();
-        queueMicrotask(() => {
+        scheduleTask(() => {
           transformer.emit('error', new Error(`Error ${item}`));
         });
         return transformer;
@@ -319,7 +319,7 @@ describe('MultiTransformIterator', () => {
       source = new ArrayIterator(['a', 'b', 'c', 'd', 'e', 'f']);
       const multiTransform = sinon.spy(item => {
         const transformer = new BufferedIterator();
-        queueMicrotask(() => {
+        scheduleTask(() => {
           transformer._push(`${item}1`);
           transformer._push(`${item}2`);
           transformer._push(`${item}3`);
@@ -356,7 +356,7 @@ describe('MultiTransformIterator', () => {
       source = new ArrayIterator(['a', 'b', 'c', 'd', 'e', 'f']);
       const multiTransform = sinon.spy(item => {
         const transformer = new BufferedIterator();
-        queueMicrotask(() => {
+        scheduleTask(() => {
           transformer._push(`${item}1`);
           transformer._push(`${item}2`);
           transformer._push(`${item}3`);

test/SimpleTransformIterator-test.js

@@ -6,10 +6,10 @@ import {
   EmptyIterator,
   ArrayIterator,
   IntegerIterator,
+  scheduleTask,
 } from '../asynciterator.mjs';
 
 import { EventEmitter } from 'events';
-import queueMicrotask from 'queue-microtask';
 
 describe('SimpleTransformIterator', () => {
   describe('The SimpleTransformIterator function', () => {
@@ -163,7 +163,7 @@ describe('SimpleTransformIterator', () => {
       source = new ArrayIterator(['a', 'b', 'c']);
       transform = sinon.spy(function (item, done) {
         this._push(item + (++i));
-        queueMicrotask(done);
+        scheduleTask(done);
       });
       iterator = new SimpleTransformIterator(source, transform);
     });

test/TaskScheduler-test.js

@@ -0,0 +1,51 @@
+import {
+  scheduleTask,
+  getTaskScheduler,
+  setTaskScheduler,
+} from '../asynciterator.mjs';
+
+describe('TaskScheduler', () => {
+  describe('scheduleTask', () => {
+    it('is a function', () => {
+      expect(scheduleTask).to.be.an.instanceof(Function);
+    });
+
+    it('schedules a task', done => {
+      scheduleTask(done);
+    });
+  });
+
+  describe('getTaskScheduler', () => {
+    it('is a function', () => {
+      expect(getTaskScheduler).to.be.an.instanceof(Function);
+    });
+
+    it('returns a task scheduler', done => {
+      const scheduler = getTaskScheduler();
+      scheduler(done);
+    });
+  });
+
+  describe('setTaskScheduler', () => {
+    it('is a function', () => {
+      expect(setTaskScheduler).to.be.an.instanceof(Function);
+    });
+
+    it('allows setting the task scheduler', () => {
+      const scheduler = getTaskScheduler();
+      expect(getTaskScheduler()).to.equal(scheduler);
+
+      const newScheduler = sinon.spy();
+      setTaskScheduler(newScheduler);
+      expect(getTaskScheduler()).to.equal(newScheduler);
+      expect(newScheduler).to.not.have.been.called;
+
+      const task = sinon.spy();
+      scheduleTask(task);
+      expect(newScheduler).to.have.been.calledOnce;
+      expect(newScheduler).to.have.been.calledWith(task);
+
+      setTaskScheduler(scheduler);
+    });
+  });
+});

test/TransformIterator-test.js

@@ -5,10 +5,10 @@ import {
   ArrayIterator,
   TransformIterator,
   wrap,
+  scheduleTask,
 } from '../asynciterator.mjs';
 
 import { EventEmitter } from 'events';
-import queueMicrotask from 'queue-microtask';
 
 describe('TransformIterator', () => {
   describe('The TransformIterator function', () => {
@@ -476,7 +476,7 @@ describe('TransformIterator', () => {
     before(() => {
       iterator = new TransformIterator(source = new ArrayIterator(['a', 'b', 'c']));
       iterator._transform = function (item, done) {
-        queueMicrotask(() => {
+        scheduleTask(() => {
           iterator._push(`${item}1`);
           iterator._push(`${item}2`);
           done();
@@ -588,7 +588,7 @@ describe('TransformIterator', () => {
       iterator = new TransformIterator(source);
       iterator._transform = sinon.spy(function (item, done) {
         this._push(item + (++i));
-        queueMicrotask(done);
+        scheduleTask(done);
       });
     });