add Map#isSourceLoaded #3691 (#4033)

* add Map#isSourceLoaded #3691

* fix the case then Map is constructed without a style and throw an error instead of firing event #3691

* fire an error on checking if non-existant source is loaded #3691

js/ui/map.js

@@ -733,6 +733,23 @@ class Map extends Camera {
         return this;
     }
 
+    /**
+     * Returns a Boolean indicating whether the source is loaded.
+     *
+     * @param {string} id The ID of the source to be checked.
+     * @returns {boolean} A Boolean indicating whether the source is loaded.
+     */
+    isSourceLoaded(id) {
+        const source = this.style && this.style.sourceCaches[id];
+        if (source === undefined) {
+            this.fire('error', {
+                error: new Error(`There is no source with ID '${id}'`)
+            });
+            return;
+        }
+        return source.loaded();
+    }
+
     /**
      * Adds a [custom source type](#Custom Sources), making it available for use with
      * {@link Map#addSource}.

test/js/ui/map.test.js

@@ -311,6 +311,33 @@ test('Map', (t) => {
             });
         });
 
+        t.test('fires an error on checking if non-existant source is loaded', (t) => {
+            const style = createStyle();
+            const map = createMap({style: style});
+
+            map.on('load', () => {
+                map.on('error', ({ error }) => {
+                    t.match(error.message, /There is no source with ID/);
+                    t.end();
+                });
+                map.isSourceLoaded('geojson');
+            });
+        });
+
+        t.test('#isSourceLoaded', (t) => {
+            const style = createStyle();
+            const map = createMap({style: style});
+
+            map.on('load', () => {
+                map.on('source.load', () => {
+                    t.equal(map.isSourceLoaded('geojson'), true, 'true when loaded');
+                    t.end();
+                });
+                map.addSource('geojson', createStyleSource());
+                t.equal(map.isSourceLoaded('geojson'), false, 'false before loaded');
+            });
+        });
+
         t.test('returns the style with added layers', (t) => {
             const style = createStyle();
             const map = createMap({style: style});