Allow setting a canvas for an engine at any point

lib/src/Engine.dart

@@ -22,6 +22,11 @@ class Engine {
   num _updateLag = 0.0;
   num _drawLag = 0.0;
 
+  /// The Canvas to Paint on
+  ///
+  /// Manually define or change the canvas the engine should render to. Should
+  /// be used if no canvas was defined at engine creation and it should be
+  /// rendered later.
   html.CanvasElement canvas;
   Grid _grid = new Grid(GRID_X, GRID_Y);
   bool running = false;
@@ -68,15 +73,30 @@ class Engine {
     }
   }
 
+  /// Update Engine Logic
+  ///
+  /// Updates the logic of the engine by one tick. Should usually not be called
+  /// directly, since it is automatically taken care of by the processing function.
+  /// If simulation should be advanced manually one time, prefer using step().
   void update() {
     if (!_grid.update()) running = false;
   }
 
+  /// Advances Logic One Update
+  ///
+  /// Moves logic of the engine one step forward and pauses the
+  /// simulation. Does not automatically re-render the new state
+  /// (though this should usually not pose a problem).
   void step() {
     running = false;
     _grid.update();
   }
 
+  /// Renders the Current Simulation State
+  ///
+  /// Renders the simulation once. Will usually automatically be called by
+  /// the internal engine processing. Does not do anything if no canvas is
+  /// defined.
   void render([num interp]) {
     if(canvas != null) _grid.render(canvas, interp);
   }

test/src/engine_test.dart

@@ -1,22 +1,32 @@
+import 'dart:html' as html;
+
 @TestOn('browser')
 
 import 'package:rules_of_living/src/Engine.dart';
 import 'package:test/test.dart';
 
 void main() {
+  Engine sut;
+  setUp(() {
+    sut = Engine();
+  });
+
   test("Engine can be instantiated without canvas", () {
-    expect(Engine(), isNot(throwsNoSuchMethodError));
+    expect(sut, isNot(throwsNoSuchMethodError));
   });
 
   test("Engine does not throw errors when calling render directly", () {
-    Engine sut = new Engine();
     // anonymous function necessary since throws can not use functions with args
     expect(() => sut.render(), isNot(throwsNoSuchMethodError));
   });
 
   test("Engine does not throw errors when processing without attached canvas", () {
-    Engine sut = new Engine();
     // anonymous function necessary since throws can not use functions with args
     expect(() => sut.process(1000), isNot(throwsNoSuchMethodError));
   });
+
+  test("setCanvas allows setting a canvas for an engine at any point", () {
+    sut.canvas = new html.CanvasElement();
+    expect(sut.canvas, isNotNull);
+  });
 }