fix: do not start multiple watchers

In some cases (for example `tns preview` and scan QR code simultaneously with several devices) we start multiple watchers per platform. We should always have single watcher per platform (one from webpack and one native in fact).

Author: rosen-vladimirov

lib/controllers/prepare-controller.ts

@@ -80,24 +80,24 @@ export class PrepareController extends EventEmitter {
 				nativeFilesWatcher: null,
 				webpackCompilerProcess: null
 			};
-		}
+			await this.startJSWatcherWithPrepare(platformData, projectData, prepareData); // -> start watcher + initial compilation
+			const hasNativeChanges = await this.startNativeWatcherWithPrepare(platformData, projectData, prepareData); // -> start watcher + initial prepare
+			const result = { platform: platformData.platformNameLowerCase, hasNativeChanges };
 
-		await this.startJSWatcherWithPrepare(platformData, projectData, prepareData); // -> start watcher + initial compilation
-		const hasNativeChanges = await this.startNativeWatcherWithPrepare(platformData, projectData, prepareData); // -> start watcher + initial prepare
+			const hasPersistedDataWithNativeChanges = this.persistedData.find(data => data.platform === result.platform && data.hasNativeChanges);
+			if (hasPersistedDataWithNativeChanges) {
+				result.hasNativeChanges = true;
+			}
 
-		const result = { platform: platformData.platformNameLowerCase, hasNativeChanges };
-		const hasPersistedDataWithNativeChanges = this.persistedData.find(data => data.platform === result.platform && data.hasNativeChanges);
-		if (hasPersistedDataWithNativeChanges) {
-			result.hasNativeChanges = true;
-		}
+			// TODO: Do not persist this in `this` context. Also it should be per platform.
+			this.isInitialPrepareReady = true;
 
-		this.isInitialPrepareReady = true;
+			if (this.persistedData && this.persistedData.length) {
+				this.emitPrepareEvent({ files: [], hasOnlyHotUpdateFiles: false, hasNativeChanges: result.hasNativeChanges, hmrData: null, platform: platformData.platformNameLowerCase });
+			}
 
-		if (this.persistedData && this.persistedData.length) {
-			this.emitPrepareEvent({ files: [], hasOnlyHotUpdateFiles: false, hasNativeChanges: result.hasNativeChanges, hmrData: null, platform: platformData.platformNameLowerCase });
+			return result;
 		}
-
-		return result;
 	}
 
 	private async startJSWatcherWithPrepare(platformData: IPlatformData, projectData: IProjectData, prepareData: IPrepareData): Promise<void> {