Add methods to mapboxgl namespace to allow preloading of workers.

src/index.js

@@ -23,6 +23,7 @@ import {Debug} from './util/debug';
 import {isSafari} from './util/util';
 import {setRTLTextPlugin, getRTLTextPluginStatus} from './source/rtl_text_plugin';
 import WorkerPool from './util/worker_pool';
+import {prewarm, clearPrewarmedResources} from './util/global_worker_pool';
 import {clearTileCache} from './util/tile_request_cache';
 import {PerformanceUtils} from './util/performance';
 
@@ -46,6 +47,39 @@ const exported = {
     MercatorCoordinate,
     Evented,
     config,
+    /**
+     * Initializes resources like WebWorkers that can be shared across maps to lower load
+     * times in some situations. `mapboxgl.workerUrl` and `mapboxgl.workerCount`, if being
+     * used, must be set before `prewarm()` is called to have an effect.
+     *
+     * By default, the lifecycle of these resources is managed automatically, and they are
+     * lazily initialized when a Map is first created. By invoking `prewarm()`, these
+     * resources will be created ahead of time, and will not be cleared when the last Map
+     * is removed from the page. This allows them to be re-used by new Map instances that
+     * are created later. They can be manually cleared by calling
+     * `mapboxgl.clearPrewarmedResources()`. This is only necessary if your web page remains
+     * active but stops using maps altogether.
+     *
+     * This is primarily useful when using GL-JS maps in a single page app, wherein a user
+     * would navigate between various views that can cause Map instances to constantly be
+     * created and destroyed.
+     *
+     * @function prewarm
+     * @example
+     * mapboxgl.prewarm()
+     */
+    prewarm,
+    /**
+     * Clears up resources that have previously been created by `mapboxgl.prewarm()`.
+     * Note that this is typically not necessary. You should only call this function
+     * if you expect the user of your app to not return to a Map view at any point
+     * in your application.
+     *
+     * @function clearPrewarmedResources
+     * @example
+     * mapboxgl.clearPrewarmedResources()
+     */
+    clearPrewarmedResources,
 
     /**
      * Gets and sets the map's [access token](https://www.mapbox.com/help/define-access-token/).

src/util/global_worker_pool.js

@@ -1,6 +1,6 @@
 // @flow
 
-import WorkerPool from './worker_pool';
+import WorkerPool, {PRELOAD_POOL_ID} from './worker_pool';
 
 let globalWorkerPool;
 
@@ -15,3 +15,21 @@ export default function getGlobalWorkerPool () {
     }
     return globalWorkerPool;
 }
+
+export function prewarm() {
+    const workerPool = getGlobalWorkerPool();
+    workerPool.acquire(PRELOAD_POOL_ID);
+}
+
+export function clearPrewarmedResources() {
+    const pool = globalWorkerPool;
+    if (pool) {
+        // Remove the pool only if all maps that referenced the preloaded global worker pool have been removed.
+        if (pool.isPreloaded() && pool.numActive() === 1) {
+            pool.release(PRELOAD_POOL_ID);
+            globalWorkerPool = null;
+        } else {
+            console.warn('Could not clear WebWorkers since there are active Map instances that still reference it. The pre-warmed WebWorker pool can only be cleared when all map instances have been removed with map.remove()');
+        }
+    }
+}

src/util/worker_pool.js

@@ -4,21 +4,23 @@ import WebWorker from './web_worker';
 import type {WorkerInterface} from './web_worker';
 import browser from './browser';
 
+export const PRELOAD_POOL_ID = 'mapboxgl_preloaded_worker_pool';
+
 /**
  * Constructs a worker pool.
  * @private
  */
 export default class WorkerPool {
     static workerCount: number;
 
-    active: {[_: number]: boolean};
+    active: {[_: number | string]: boolean};
     workers: Array<WorkerInterface>;
 
     constructor() {
         this.active = {};
     }
 
-    acquire(mapId: number): Array<WorkerInterface> {
+    acquire(mapId: number | string): Array<WorkerInterface> {
         if (!this.workers) {
             // Lazily look up the value of mapboxgl.workerCount so that
             // client code has had a chance to set it.
@@ -32,15 +34,23 @@ export default class WorkerPool {
         return this.workers.slice();
     }
 
-    release(mapId: number) {
+    release(mapId: number | string) {
         delete this.active[mapId];
-        if (Object.keys(this.active).length === 0) {
+        if (this.numActive() === 0) {
             this.workers.forEach((w) => {
                 w.terminate();
             });
             this.workers = (null: any);
         }
     }
+
+    isPreloaded(): boolean {
+        return !!this.active[PRELOAD_POOL_ID];
+    }
+
+    numActive(): number {
+        return Object.keys(this.active).length;
+    }
 }
 
 const availableLogicalProcessors = Math.floor(browser.hardwareConcurrency / 2);