[flutter_tools] eagerly set asset directory path, cache flutter views,...

[flutter_tools] eagerly set asset directory path, cache flutter views, simplify error handling (#68978)

Performs some small cleanup on the hot reload code path.

- Combines nested try/catch into single try catch, update on clause now that package:vm_service is used and Map does not need to be caught.
- Cache FlutterViews for the lifetime of the hot reload method handler
- Set asset directory path once during startup and remove conditional set during hot reload

packages/flutter_tools/lib/src/run_hot.dart

@@ -110,7 +110,6 @@ class HotRunner extends ResidentRunner {
   final Map<String, List<int>> benchmarkData = <String, List<int>>{};
 
   DateTime firstBuildTime;
-  bool _shouldResetAssetDirectory = true;
 
   void _addBenchmarkData(String name, int value) {
     benchmarkData[name] ??= <int>[];
@@ -205,7 +204,7 @@ class HotRunner extends ResidentRunner {
           ),
         );
       }
-    } on Exception catch (error) {
+    } on DevFSException catch (error) {
       globals.printError('Error initializing DevFS: $error');
       return 3;
     }
@@ -226,6 +225,14 @@ class HotRunner extends ResidentRunner {
         device.generator.accept();
       }
       final List<FlutterView> views = await device.vmService.getFlutterViews();
+      final Uri deviceAssetsDirectoryUri = device.devFS.baseUri.resolveUri(globals.fs.path.toUri(getAssetBuildDirectory()));
+      await Future.wait<void>(views.map<Future<void>>(
+        (FlutterView view) => device.vmService.setAssetDirectory(
+          assetsDirectory: deviceAssetsDirectoryUri,
+          uiIsolateId: view.uiIsolate.id,
+          viewId: view.id,
+        )
+      ));
       for (final FlutterView view in views) {
         globals.printTrace('Connected to $view.');
       }
@@ -729,7 +736,25 @@ class HotRunner extends ResidentRunner {
           );
         },
       );
-    } on vm_service.RPCError {
+    } on vm_service.RPCError catch (error) {
+      String errorMessage = 'hot reload failed to complete';
+      int errorCode = 1;
+      if (error.code == kIsolateReloadBarred) {
+        errorCode = error.code;
+        errorMessage = 'Unable to hot reload application due to an unrecoverable error in '
+                      'the source code. Please address the error and then use "R" to '
+                      'restart the app.\n'
+                      '${error.message} (error code: ${error.code})';
+        HotEvent('reload-barred',
+          targetPlatform: targetPlatform,
+          sdkName: sdkName,
+          emulator: emulator,
+          fullRestart: false,
+          reason: reason,
+          nullSafety: usageNullSafety,
+          fastReassemble: null,
+        ).send();
+      } else {
         HotEvent('exception',
           targetPlatform: targetPlatform,
           sdkName: sdkName,
@@ -739,7 +764,8 @@ class HotRunner extends ResidentRunner {
           reason: reason,
           fastReassemble: null,
         ).send();
-      return OperationResult(1, 'hot reload failed to complete', fatal: true);
+      }
+      return OperationResult(errorCode, errorMessage, fatal: true);
     } finally {
       status.cancel();
     }
@@ -764,20 +790,6 @@ class HotRunner extends ResidentRunner {
     ];
   }
 
-  Future<void> _resetAssetDirectory(FlutterDevice device) async {
-    final Uri deviceAssetsDirectoryUri = device.devFS.baseUri.resolveUri(
-      globals.fs.path.toUri(getAssetBuildDirectory()));
-    assert(deviceAssetsDirectoryUri != null);
-    final List<FlutterView> views = await device.vmService.getFlutterViews();
-    await Future.wait<void>(views.map<Future<void>>(
-      (FlutterView view) => device.vmService.setAssetDirectory(
-        assetsDirectory: deviceAssetsDirectoryUri,
-        uiIsolateId: view.uiIsolate.id,
-        viewId: view.id,
-      )
-    ));
-  }
-
   Future<OperationResult> _reloadSources({
     String targetPlatform,
     String sdkName,
@@ -786,8 +798,10 @@ class HotRunner extends ResidentRunner {
     String reason,
     void Function(String message) onSlow,
   }) async {
+    final Map<FlutterDevice, List<FlutterView>> viewCache = <FlutterDevice, List<FlutterView>>{};
     for (final FlutterDevice device in flutterDevices) {
       final List<FlutterView> views = await device.vmService.getFlutterViews();
+      viewCache[device] = views;
       for (final FlutterView view in views) {
         if (view.uiIsolate == null) {
           return OperationResult(2, 'Application isolate not found', fatal: true);
@@ -806,20 +820,15 @@ class HotRunner extends ResidentRunner {
     }
     String reloadMessage;
     final Stopwatch vmReloadTimer = Stopwatch()..start();
-    Map<String, dynamic> firstReloadDetails;
-    try {
+    Map<String, Object> firstReloadDetails;
     const String entryPath = 'main.dart.incremental.dill';
     final List<Future<DeviceReloadReport>> allReportsFutures = <Future<DeviceReloadReport>>[];
+
     for (final FlutterDevice device in flutterDevices) {
-        if (_shouldResetAssetDirectory) {
-          // Asset directory has to be set only once when the engine switches from
-          // running from bundle to uploaded files.
-          await _resetAssetDirectory(device);
-          _shouldResetAssetDirectory = false;
-        }
       final List<Future<vm_service.ReloadReport>> reportFutures = await _reloadDeviceSources(
         device,
-          entryPath, pause: pause,
+        entryPath,
+        pause: pause,
       );
       allReportsFutures.add(Future.wait(reportFutures).then(
         (List<vm_service.ReloadReport> reports) async {
@@ -864,45 +873,20 @@ class HotRunner extends ResidentRunner {
       globals.printTrace('reloaded $loadedLibraryCount of $finalLibraryCount libraries');
       reloadMessage = 'Reloaded $loadedLibraryCount of $finalLibraryCount libraries';
     }
-    } on Map<String, dynamic> catch (error, stackTrace) {
-      globals.printTrace('Hot reload failed: $error\n$stackTrace');
-      final int errorCode = error['code'] as int;
-      String errorMessage = error['message'] as String;
-      if (errorCode == kIsolateReloadBarred) {
-        errorMessage = 'Unable to hot reload application due to an unrecoverable error in '
-                       'the source code. Please address the error and then use "R" to '
-                       'restart the app.\n'
-                       '$errorMessage (error code: $errorCode)';
-        HotEvent('reload-barred',
-          targetPlatform: targetPlatform,
-          sdkName: sdkName,
-          emulator: emulator,
-          fullRestart: false,
-          reason: reason,
-          nullSafety: usageNullSafety,
-          fastReassemble: null,
-        ).send();
-        return OperationResult(errorCode, errorMessage);
-      }
-      return OperationResult(errorCode, '$errorMessage (error code: $errorCode)');
-    } on Exception catch (error, stackTrace) {
-      globals.printTrace('Hot reload failed: $error\n$stackTrace');
-      return OperationResult(1, '$error');
-    }
+
     // Record time it took for the VM to reload the sources.
     _addBenchmarkData('hotReloadVMReloadMilliseconds', vmReloadTimer.elapsed.inMilliseconds);
     final Stopwatch reassembleTimer = Stopwatch()..start();
     await _evictDirtyAssets();
 
-    // Check if any isolates are paused and reassemble those
-    // that aren't.
+    // Check if any isolates are paused and reassemble those that aren't.
     final Map<FlutterView, vm_service.VmService> reassembleViews = <FlutterView, vm_service.VmService>{};
     final List<Future<void>> reassembleFutures = <Future<void>>[];
     String serviceEventKind;
     int pausedIsolatesFound = 0;
     bool failedReassemble = false;
     for (final FlutterDevice device in flutterDevices) {
-      final List<FlutterView> views = await device.vmService.getFlutterViews();
+      final List<FlutterView> views = viewCache[device];
       for (final FlutterView view in views) {
         // Check if the isolate is paused, and if so, don't reassemble. Ignore the
         // PostPauseEvent event - the client requesting the pause will resume the app.