Replace println calls with enforcable signature

Having the `log` call return a `List<String>` instead of `void` has several
advantages. It makes the printers testable. The `buffer` field was not
accessable in the tests. It also decouples printers from the main logger. Yet an
other advantage is that, instead of imploring implementers of custom printer
classes to use `println` we now enforce it via the type interface. This leads to
less confusion.

Author: haarts

README.md

@@ -116,14 +116,12 @@ A very basic printer could look like this:
 ```dart
 class MyPrinter extends LogPrinter {
   @override
-  void log(LogEvent event) {
-    println(event.message);
+  List<String> log(LogEvent event) {
+    return [event.message];
   }
 }
 ```
 
-**Important:** Every implementation has to send its output using the `println()` method.
-
 If you created a cool `LogPrinter` which might be helpful to others, feel free to open a pull request. :)
 
 

lib/src/log_printer.dart

@@ -8,18 +8,11 @@ import 'package:logger/src/logger.dart';
 ///
 /// You can implement a `LogPrinter` from scratch or extend [PrettyPrinter].
 abstract class LogPrinter {
-  List<String> buffer;
-
   void init() {}
 
   /// Is called every time a new [LogEvent] is sent and handles printing or
   /// storing the message.
-  void log(LogEvent event);
+  List<String> log(LogEvent event);
 
   void destroy() {}
-
-  /// Sends a line to the [LogOutput].
-  void println(String line) {
-    buffer.add(line);
-  }
 }

lib/src/logger.dart

@@ -49,7 +49,6 @@ class Logger {
   final LogFilter _filter;
   final LogPrinter _printer;
   final LogOutput _output;
-  List<String> _outputBuffer = [];
   bool _active = true;
 
   /// Create a new instance of Logger.
@@ -66,7 +65,6 @@ class Logger {
         _printer = printer ?? PrettyPrinter(),
         _output = output ?? ConsoleOutput() {
     _filter.init();
-    _printer.buffer = _outputBuffer;
     _filter.level = level ?? Logger.level;
     _printer.init();
     _output.init();
@@ -117,27 +115,23 @@ class Logger {
       for (var callback in _logCallbacks) {
         callback(logEvent);
       }
-      _printer.log(logEvent);
+      var output = _printer.log(logEvent);
 
-      if (_outputBuffer.isNotEmpty) {
-        var outputEvent = OutputEvent(level, _outputBuffer);
+      if (output.isNotEmpty) {
+        var outputEvent = OutputEvent(level, output);
         for (var callback in _outputCallbacks) {
           callback(outputEvent);
         }
         _output.output(outputEvent);
-        _outputBuffer = [];
-        _printer.buffer = _outputBuffer;
       }
     }
   }
 
   /// Closes the logger and releases all resources.
   void close() {
     _active = false;
-    _outputBuffer = null;
     _filter.destroy();
     _printer.destroy();
-    _printer.buffer = null;
     _output.destroy();
   }
 

lib/src/printers/pretty_printer.dart

@@ -82,7 +82,7 @@ class PrettyPrinter extends LogPrinter {
   }
 
   @override
-  void log(LogEvent event) {
+  List<String> log(LogEvent event) {
     var messageStr = stringifyMessage(event.message);
 
     String stackTraceStr;
@@ -101,7 +101,13 @@ class PrettyPrinter extends LogPrinter {
       timeStr = getTime();
     }
 
-    formatAndPrint(event.level, messageStr, timeStr, errorStr, stackTraceStr);
+    return _formatAndPrint(
+      event.level,
+      messageStr,
+      timeStr,
+      errorStr,
+      stackTraceStr,
+    );
   }
 
   String formatStackTrace(StackTrace stackTrace, int methodCount) {
@@ -190,40 +196,48 @@ class PrettyPrinter extends LogPrinter {
     }
   }
 
-  formatAndPrint(Level level, String message, String time, String error,
-      String stacktrace) {
+  List<String> _formatAndPrint(
+    Level level,
+    String message,
+    String time,
+    String error,
+    String stacktrace,
+  ) {
+    List<String> buffer = [];
     var color = _getLevelColor(level);
-    println(color(_topBorder));
+    buffer.add(color(_topBorder));
 
     if (error != null) {
       var errorColor = _getErrorColor(level);
       for (var line in error.split('\n')) {
-        println(
+        buffer.add(
           color('$verticalLine ') +
               errorColor.resetForeground +
               errorColor(line) +
               errorColor.resetBackground,
         );
       }
-      println(color(_middleBorder));
+      buffer.add(color(_middleBorder));
     }
 
     if (stacktrace != null) {
       for (var line in stacktrace.split('\n')) {
-        println('${color}$verticalLine $line');
+        buffer.add('${color}$verticalLine $line');
       }
-      println(color(_middleBorder));
+      buffer.add(color(_middleBorder));
     }
 
     if (time != null) {
-      println(color('$verticalLine $time'));
-      println(color(_middleBorder));
+      buffer.add(color('$verticalLine $time'));
+      buffer.add(color(_middleBorder));
     }
 
     var emoji = _getEmoji(level);
     for (var line in message.split('\n')) {
-      println(color('$verticalLine $emoji$line'));
+      buffer.add(color('$verticalLine $emoji$line'));
     }
-    println(color(_bottomBorder));
+    buffer.add(color(_bottomBorder));
+
+    return buffer;
   }
 }

lib/src/printers/simple_printer.dart

@@ -22,11 +22,11 @@ class SimplePrinter extends LogPrinter {
   SimplePrinter({this.printTime = false});
 
   @override
-  void log(LogEvent event) {
+  List<String> log(LogEvent event) {
     var messageStr = stringifyMessage(event.message);
     var errorStr = event.error != null ? "  ERROR: ${event.error}" : "";
     var timeStr = printTime ? "TIME: ${DateTime.now().toIso8601String()}" : "";
-    println("${levelPrefixes[event.level]} $timeStr $messageStr$errorStr");
+    return ["${levelPrefixes[event.level]} $timeStr $messageStr$errorStr"];
   }
 
   String stringifyMessage(dynamic message) {

test/logger_test.dart

@@ -3,7 +3,7 @@ import 'dart:math';
 import 'package:test/test.dart';
 import 'package:logger/logger.dart';
 
-typedef PrinterCallback = void Function(
+typedef PrinterCallback = List<String> Function(
     Level level, dynamic message, dynamic error, StackTrace stackTrace);
 
 class _AlwaysFilter extends LogFilter {
@@ -22,8 +22,13 @@ class _CallbackPrinter extends LogPrinter {
   _CallbackPrinter(this.callback);
 
   @override
-  void log(LogEvent event) {
-    callback(event.level, event.message, event.error, event.stackTrace);
+  List<String> log(LogEvent event) {
+    return callback(
+      event.level,
+      event.message,
+      event.error,
+      event.stackTrace,
+    );
   }
 }
 
@@ -37,6 +42,7 @@ void main() {
     printedMessage = m;
     printedError = e;
     printedStackTrace = s;
+    return [];
   });
 
   setUp(() {