Timer class, promise based, so clear/setTimeout doesn't grow unwieldly

src/utils/Timer.js

@@ -0,0 +1,124 @@
+/*
+Copyright 2018 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+A countdown timer, exposing a promise api.
+A timer starts in a non-started state,
+and needs to be started by calling `start()`` on it first.
+
+Timers can be `abort()`-ed which makes the promise reject prematurely.
+
+Once a timer is finished or aborted, it can't be started again
+(because the promise should not be replaced). Instead, create
+a new one through `clone()` or `cloneIfRun()`.
+*/
+export default class Timer {
+
+    constructor(timeout) {
+        this._timeout = timeout;
+        this._onTimeout = this._onTimeout.bind(this);
+        this._setNotStarted();
+    }
+
+    _setNotStarted() {
+        this._timerHandle = null;
+        this._startTs = null;
+        this._promise = new Promise((resolve, reject) => {
+            this._resolve = resolve;
+            this._reject = reject;
+        }).finally(() => {
+            this._timerHandle = null;
+        });
+    }
+
+    _onTimeout() {
+        const now = Date.now();
+        const elapsed = now - this._startTs;
+        if (elapsed >= this._timeout) {
+            this._resolve();
+            this._setNotStarted();
+        } else {
+            const delta = this._timeout - elapsed;
+            this._timerHandle = setTimeout(this._onTimeout, delta);
+        }
+    }
+
+    changeTimeout(timeout) {
+        if (timeout === this._timeout) {
+            return;
+        }
+        console.log(`changing timer timeout from ${this._timeout} to ${timeout}`);
+        const isSmallerTimeout = timeout < this._timeout;
+        this._timeout = timeout;
+        if (this.isRunning() && isSmallerTimeout) {
+            clearTimeout(this._timerHandle);
+            this._onTimeout();
+        }
+    }
+
+    /**
+     * if not started before, starts the timer.
+     */
+    start() {
+        if (!this.isRunning()) {
+            this._startTs = Date.now();
+            this._timerHandle = setTimeout(this._onTimeout, this._timeout);
+        }
+        return this;
+    }
+
+    /**
+     * (re)start the timer. If it's running, reset the timeout. If not, start it.
+     */
+    restart() {
+        if (this.isRunning()) {
+            // don't clearTimeout here as this method
+            // can be called in fast succession,
+            // instead just take note and compare
+            // when the already running timeout expires
+            this._startTs = Date.now();
+            return this;
+        } else {
+            return this.start();
+        }
+    }
+
+    /**
+     * if the timer is running, abort it,
+     * and reject the promise for this timer.
+     */
+    abort() {
+        if (this.isRunning()) {
+            clearTimeout(this._timerHandle);
+            this._reject(new Error("Timer was aborted."));
+            this._setNotStarted();
+        }
+        return this;
+    }
+
+    /**
+     *promise that will resolve when the timer elapses,
+     *or is rejected when abort is called
+     *@return {Promise}
+     */
+    finished() {
+        return this._promise;
+    }
+
+    isRunning() {
+        return this._timerHandle !== null;
+    }
+}