Support multi-line log messages on iOS devices (#17327)

packages/flutter_tools/lib/src/ios/devices.dart

@@ -415,7 +415,10 @@ String decodeSyslog(String line) {
 }
 
 class _IOSDeviceLogReader extends DeviceLogReader {
-  RegExp _lineRegex;
+  // Matches a syslog line from the runner.
+  RegExp _runnerLineRegex;
+  // Matches a syslog line from any app.
+  RegExp _anyLineRegex;
 
   _IOSDeviceLogReader(this.device, ApplicationPackage app) {
     _linesController = new StreamController<String>.broadcast(
@@ -428,7 +431,11 @@ class _IOSDeviceLogReader extends DeviceLogReader {
     // iOS 9 format:  Runner[297] <Notice>:
     // iOS 10 format: Runner(Flutter)[297] <Notice>:
     final String appName = app == null ? '' : app.name.replaceAll('.app', '');
-    _lineRegex = new RegExp(appName + r'(\(Flutter\))?\[[\d]+\] <[A-Za-z]+>: ');
+    _runnerLineRegex = new RegExp(appName + r'(\(Flutter\))?\[[\d]+\] <[A-Za-z]+>: ');
+    // Similar to above, but allows ~arbitrary components instead of "Runner"
+    // and "Flutter". The regex tries to strike a balance between not producing
+    // false positives and not producing false negatives.
+    _anyLineRegex = new RegExp(r'\w+(\([^)]*\))?\[\d+\] <[A-Za-z]+>: ');
   }
 
   final IOSDevice device;
@@ -445,8 +452,8 @@ class _IOSDeviceLogReader extends DeviceLogReader {
   void _start() {
     iMobileDevice.startLogger().then<Null>((Process process) {
       _process = process;
-      _process.stdout.transform(utf8.decoder).transform(const LineSplitter()).listen(_onLine);
-      _process.stderr.transform(utf8.decoder).transform(const LineSplitter()).listen(_onLine);
+      _process.stdout.transform(utf8.decoder).transform(const LineSplitter()).listen(_newLineHandler());
+      _process.stderr.transform(utf8.decoder).transform(const LineSplitter()).listen(_newLineHandler());
       _process.exitCode.whenComplete(() {
         if (_linesController.hasListener)
           _linesController.close();
@@ -454,14 +461,35 @@ class _IOSDeviceLogReader extends DeviceLogReader {
     });
   }
 
-  void _onLine(String line) {
-    final Match match = _lineRegex.firstMatch(line);
+  // Returns a stateful line handler to properly capture multi-line output.
+  //
+  // For multi-line log messages, any line after the first is logged without
+  // any specific prefix. To properly capture those, we enter "printing" mode
+  // after matching a log line from the runner. When in printing mode, we print
+  // all lines until we find the start of another log message (from any app).
+  Function _newLineHandler() {
+    bool printing = false;
+
+    return (String line) {
+      if (printing) {
+        if (!_anyLineRegex.hasMatch(line)) {
+          _linesController.add(decodeSyslog(line));
+          return;
+        }
 
-    if (match != null) {
-      final String logLine = line.substring(match.end);
-      // Only display the log line after the initial device and executable information.
-      _linesController.add(decodeSyslog(logLine));
-    }
+        printing = false;
+      }
+
+      final Match match = _runnerLineRegex.firstMatch(line);
+
+      if (match != null) {
+        final String logLine = line.substring(match.end);
+        // Only display the log line after the initial device and executable information.
+        _linesController.add(decodeSyslog(logLine));
+
+        printing = true;
+      }
+    };
   }
 
   void _stop() {

packages/flutter_tools/test/ios/devices_test.dart

@@ -125,5 +125,40 @@ f577a7903cc54959be2e34bc4f7f80b7009efcf4
     }, overrides: <Type, Generator>{
       IMobileDevice: () => mockIMobileDevice,
     });
+
+    testUsingContext('includes multi-line Flutter logs in the output', () async {
+      when(mockIMobileDevice.startLogger()).thenAnswer((Invocation invocation) {
+        final Process mockProcess = new MockProcess();
+        when(mockProcess.stdout).thenAnswer((Invocation invocation) =>
+            new Stream<List<int>>.fromIterable(<List<int>>['''
+  Runner(Flutter)[297] <Notice>: This is a multi-line message,
+  with another Flutter message following it.
+  Runner(Flutter)[297] <Notice>: This is a multi-line message,
+  with a non-Flutter log message following it.
+  Runner(libsystem_asl.dylib)[297] <Notice>: libMobileGestalt
+  '''.codeUnits]));
+        when(mockProcess.stderr)
+            .thenAnswer((Invocation invocation) => const Stream<List<int>>.empty());
+        // Delay return of exitCode until after stdout stream data, since it terminates the logger.
+        when(mockProcess.exitCode)
+            .thenAnswer((Invocation invocation) => new Future<int>.delayed(Duration.zero, () => 0));
+        return new Future<Process>.value(mockProcess);
+      });
+
+      final IOSDevice device = new IOSDevice('123456');
+      final DeviceLogReader logReader = device.getLogReader(
+        app: new BuildableIOSApp(projectBundleId: 'bundleId'),
+      );
+
+      final List<String> lines = await logReader.logLines.toList();
+      expect(lines, <String>[
+        'This is a multi-line message,',
+        '  with another Flutter message following it.',
+        'This is a multi-line message,',
+        '  with a non-Flutter log message following it.',
+      ]);
+    }, overrides: <Type, Generator>{
+      IMobileDevice: () => mockIMobileDevice,
+    });
   });
 }