diff --git a/examples/layers/raw/canvas.dart b/examples/layers/raw/canvas.dart
index 4e046e68cc..3f955c8156 100644
--- a/examples/layers/raw/canvas.dart
+++ b/examples/layers/raw/canvas.dart
@@ -2,19 +2,32 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This example shows how to use the ui.Canvas interface to draw various shapes
+// with gradients and transforms.
+
 import 'dart:ui' as ui;
 import 'dart:math' as math;
 import 'dart:typed_data';
 
 ui.Picture paint(ui.Rect paintBounds) {
+  // First we create a PictureRecorder to record the commands we're going to
+  // feed in the canvas. The PictureRecorder will eventually produce a Picture,
+  // which is an immutable record of those commands.
   ui.PictureRecorder recorder = new ui.PictureRecorder();
+
+  // Next, we create a canvas from the recorder. The canvas is an interface
+  // which can receive drawing commands. The canvas interface is modeled after
+  // the SkCanvas interface from Skia. The paintBounds establishes a "cull rect"
+  // for the canvas, which lets the implementation discard any commands that
+  // are entirely outside this rectangle.
   ui.Canvas canvas = new ui.Canvas(recorder, paintBounds);
-  ui.Size size = paintBounds.size;
 
   ui.Paint paint = new ui.Paint();
+  canvas.drawPaint(new ui.Paint()..color = const ui.Color(0xFFFFFFFF));
+
+  ui.Size size = paintBounds.size;
   ui.Point mid = size.center(ui.Point.origin);
   double radius = size.shortestSide / 2.0;
-  canvas.drawPaint(new ui.Paint()..color = const ui.Color(0xFFFFFFFF));
 
   canvas.save();
   canvas.translate(-mid.x/2.0, ui.window.size.height*2.0);
@@ -46,6 +59,11 @@ ui.Picture paint(ui.Rect paintBounds) {
   canvas.restore();
 
   canvas.translate(0.0, 50.0);
+
+  // A DrawLooper is a powerful painting primitive that lets you apply several
+  // blending and filtering passes over a sequence of commands "in a loop". For
+  // example, the looper below draws the circle tree times, once with a blur,
+  // then with a gradient blend, and finally with just an offset.
   ui.LayerDrawLooperBuilder builder = new ui.LayerDrawLooperBuilder()
     ..addLayerOnTop(
         new ui.DrawLooperLayerInfo()
@@ -90,6 +108,10 @@ ui.Picture paint(ui.Rect paintBounds) {
   paint.drawLooper = builder.build();
   canvas.drawCircle(ui.Point.origin, radius, paint);
 
+  // When we're done issuing painting commands, we end the recording an receive
+  // a Picture, which is an immutable record of the commands we've issued. You
+  // can draw a Picture into another canvas or include it as part of a
+  // composited scene.
   return recorder.endRecording();
 }
 
diff --git a/examples/layers/raw/hello_world.dart b/examples/layers/raw/hello_world.dart
index 1c7f83e80e..0583943413 100644
--- a/examples/layers/raw/hello_world.dart
+++ b/examples/layers/raw/hello_world.dart
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This example shows how to put some pixels on the screen using the raw
+// interface to the engine.
+
 import 'dart:ui' as ui;
 import 'dart:typed_data';
 
@@ -12,18 +15,38 @@ import 'package:sky_services/pointer/pointer.mojom.dart';
 ui.Color color;
 
 ui.Picture paint(ui.Rect paintBounds) {
+  // First we create a PictureRecorder to record the commands we're going to
+  // feed in the canvas. The PictureRecorder will eventually produce a Picture,
+  // which is an immutable record of those commands.
   ui.PictureRecorder recorder = new ui.PictureRecorder();
+
+  // Next, we create a canvas from the recorder. The canvas is an interface
+  // which can receive drawing commands. The canvas interface is modeled after
+  // the SkCanvas interface from Skia. The paintBounds establishes a "cull rect"
+  // for the canvas, which lets the implementation discard any commands that
+  // are entirely outside this rectangle.
   ui.Canvas canvas = new ui.Canvas(recorder, paintBounds);
+
+  // The commands draw a circle in the center of the screen.
   ui.Size size = paintBounds.size;
+  canvas.drawCircle(
+    size.center(ui.Point.origin),
+    size.shortestSide * 0.45,
+    new ui.Paint()..color = color
+  );
 
-  double radius = size.shortestSide * 0.45;
-  ui.Paint paint = new ui.Paint()..color = color;
-  canvas.drawCircle(size.center(ui.Point.origin), radius, paint);
-
+  // When we're done issuing painting commands, we end the recording an receive
+  // a Picture, which is an immutable record of the commands we've issued. You
+  // can draw a Picture into another canvas or include it as part of a
+  // composited scene.
   return recorder.endRecording();
 }
 
 ui.Scene composite(ui.Picture picture, ui.Rect paintBounds) {
+  // The device pixel ratio gives an approximate ratio of the size of pixels on
+  // the device's screen to "normal" sized pixels. We commonly work in logical
+  // pixels, which are then scalled by the device pixel ratio before being drawn
+  // on the screen.
   final double devicePixelRatio = ui.window.devicePixelRatio;
   ui.Rect sceneBounds = new ui.Rect.fromLTWH(
       0.0,
@@ -31,49 +54,83 @@ ui.Scene composite(ui.Picture picture, ui.Rect paintBounds) {
       ui.window.size.width * devicePixelRatio,
       ui.window.size.height * devicePixelRatio
   );
+
+  // This transform scales the x and y coordinates by the devicePixelRatio.
   Float64List deviceTransform = new Float64List(16)
     ..[0] = devicePixelRatio
     ..[5] = devicePixelRatio
     ..[10] = 1.0
     ..[15] = 1.0;
+
+  // We build a very simple scene graph with two nodes. The root node is a
+  // transform that scale its children by the device pixel ratio. This transform
+  // lets us paint in "logical" pixels which are converted to device pixels by
+  // this scaling operation.
   ui.SceneBuilder sceneBuilder = new ui.SceneBuilder(sceneBounds)
     ..pushTransform(deviceTransform)
     ..addPicture(ui.Offset.zero, picture)
     ..pop();
+
+  // When we're done recording the scene, we call build() to obtain an immutable
+  // record of the scene we've recorded.
   return sceneBuilder.build();
 }
 
 void beginFrame(Duration timeStamp) {
   ui.Rect paintBounds = ui.Point.origin & ui.window.size;
+  // First, record a picture with our painting commands.
   ui.Picture picture = paint(paintBounds);
+  // Second, include that picture in a scene graph.
   ui.Scene scene = composite(picture, paintBounds);
+  // Third, instruct the engine to render that scene graph.
   ui.window.render(scene);
 }
 
-void handlePopRoute() {
-  print('Pressed back button.');
-}
-
+// Pointer input arrives as an array of bytes. The format for the data is
+// defined by pointer.mojom, which generates serializes and parsers for a
+// number of languages, including Dart, C++, Java, and Go.
 void handlePointerPacket(ByteData serializedPacket) {
+  // We wrap the byte data up into a Mojo Message object, which we then
+  // deserialize according to the mojom definition.
   bindings.Message message = new bindings.Message(serializedPacket, <core.MojoHandle>[], serializedPacket.lengthInBytes, 0);
   PointerPacket packet = PointerPacket.deserialize(message);
 
+  // The deserialized pointer packet contains a number of pointer movements,
+  // which we iterate through and process.
   for (Pointer pointer in packet.pointers) {
     if (pointer.type == PointerType.down) {
-      color = new ui.Color.fromARGB(255, 0, 0, 255);
+      // If the pointer went down, we change the color of the circle to blue.
+      color = const ui.Color(0xFF0000FF);
+      // Rather than calling paint() synchronously, we ask the engine to
+      // schedule a frame. The engine will call onBeginFrame when it is actually
+      // time to produce the frame.
       ui.window.scheduleFrame();
     } else if (pointer.type == PointerType.up) {
-      color = new ui.Color.fromARGB(255, 0, 255, 0);
+      // Similarly, if the pointer went up, we change the color of the circle to
+      // green and schedule a frame. It's harmless to call scheduleFrame many
+      // times because the engine will ignore redundant requests up until the
+      // point where the engine calls onBeginFrame, which signals the boundary
+      // between one frame and another.
+      color = const ui.Color(0xFF00FF00);
       ui.window.scheduleFrame();
     }
   }
 }
 
+// This function is the primary entry point to your application. The engine
+// calls main() as soon as it has loaded your code.
 void main() {
+  // Print statements go either go to stdout or to the system log, as
+  // appropriate for the operating system.
   print('Hello, world');
-  color = new ui.Color.fromARGB(255, 0, 255, 0);
+  color = const ui.Color(0xFF00FF00);
+  // The engine calls onBeginFrame whenever it wants us to produce a frame.
   ui.window.onBeginFrame = beginFrame;
-  ui.window.onPopRoute = handlePopRoute;
+  // The engine calls onPointerPacket whenever it had updated information about
+  // the pointers directed at our app.
   ui.window.onPointerPacket = handlePointerPacket;
+  // Here we kick off the whole process by asking the engine to schedule a new
+  // frame. The engine will eventually call onBeginFrame when it is time for us
+  // to actually produce the frame.
   ui.window.scheduleFrame();
 }
diff --git a/examples/layers/raw/spinning_square.dart b/examples/layers/raw/spinning_square.dart
index 7ebbac22b3..c4f464a880 100644
--- a/examples/layers/raw/spinning_square.dart
+++ b/examples/layers/raw/spinning_square.dart
@@ -2,49 +2,60 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:developer';
+// This example shows how to perform a simple animation using the raw interface
+// to the engine.
+
 import 'dart:math' as math;
 import 'dart:typed_data';
 import 'dart:ui' as ui;
 
-Duration timeBase = null;
-
 void beginFrame(Duration timeStamp) {
-  Timeline.timeSync('beginFrame', () {
-    if (timeBase == null)
-      timeBase = timeStamp;
-    double delta = (timeStamp - timeBase).inMicroseconds / Duration.MICROSECONDS_PER_MILLISECOND;
+  // The timeStamp argument to beginFrame indicates the timing information we
+  // should use to clock our animations. It's important to use timeStamp rather
+  // than reading the system time because we want all the parts of the system to
+  // coordinate the timings of their animations. If each component read the
+  // system clock independently, the animations that we processed later would be
+  // slightly ahead of the animations we processed earlier.
 
-    // paint
-    ui.Rect paintBounds = ui.Point.origin & ui.window.size;
-    ui.PictureRecorder recorder = new ui.PictureRecorder();
-    ui.Canvas canvas = new ui.Canvas(recorder, paintBounds);
-    canvas.translate(paintBounds.width / 2.0, paintBounds.height / 2.0);
-    canvas.rotate(math.PI * delta / 1800);
-    canvas.drawRect(new ui.Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
-                    new ui.Paint()..color = const ui.Color.fromARGB(255, 0, 255, 0));
-    ui.Picture picture = recorder.endRecording();
+  // PAINT
 
-    // composite
-    final double devicePixelRatio = ui.window.devicePixelRatio;
-    ui.Rect sceneBounds = new ui.Rect.fromLTWH(
-      0.0,
-      0.0,
-      ui.window.size.width * devicePixelRatio,
-      ui.window.size.height * devicePixelRatio
-    );
-    Float64List deviceTransform = new Float64List(16)
-      ..[0] = devicePixelRatio
-      ..[5] = devicePixelRatio
-      ..[10] = 1.0
-      ..[15] = 1.0;
-    ui.SceneBuilder sceneBuilder = new ui.SceneBuilder(sceneBounds)
-      ..pushTransform(deviceTransform)
-      ..addPicture(ui.Offset.zero, picture)
-      ..pop();
-    ui.window.render(sceneBuilder.build());
-  });
+  ui.Rect paintBounds = ui.Point.origin & ui.window.size;
+  ui.PictureRecorder recorder = new ui.PictureRecorder();
+  ui.Canvas canvas = new ui.Canvas(recorder, paintBounds);
+  canvas.translate(paintBounds.width / 2.0, paintBounds.height / 2.0);
 
+  // Here we determine the rotation according to the timeStamp given to us by
+  // the engine.
+  double t = timeStamp.inMicroseconds / Duration.MICROSECONDS_PER_MILLISECOND / 1800.0;
+  canvas.rotate(math.PI * (t % 1.0));
+
+  canvas.drawRect(new ui.Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
+                  new ui.Paint()..color = const ui.Color.fromARGB(255, 0, 255, 0));
+  ui.Picture picture = recorder.endRecording();
+
+  // COMPOSITE
+
+  final double devicePixelRatio = ui.window.devicePixelRatio;
+  ui.Rect sceneBounds = new ui.Rect.fromLTWH(
+    0.0,
+    0.0,
+    ui.window.size.width * devicePixelRatio,
+    ui.window.size.height * devicePixelRatio
+  );
+  Float64List deviceTransform = new Float64List(16)
+    ..[0] = devicePixelRatio
+    ..[5] = devicePixelRatio
+    ..[10] = 1.0
+    ..[15] = 1.0;
+  ui.SceneBuilder sceneBuilder = new ui.SceneBuilder(sceneBounds)
+    ..pushTransform(deviceTransform)
+    ..addPicture(ui.Offset.zero, picture)
+    ..pop();
+  ui.window.render(sceneBuilder.build());
+
+  // After rendering the current frame of the animation, we ask the engine to
+  // schedule another frame. The engine will call beginFrame again when its time
+  // to produce the next frame.
   ui.window.scheduleFrame();
 }
 
diff --git a/examples/layers/raw/text.dart b/examples/layers/raw/text.dart
index ce9ddc8d7e..24e90fbc9d 100644
--- a/examples/layers/raw/text.dart
+++ b/examples/layers/raw/text.dart
@@ -2,9 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This example shows how to draw some bi-directional text using the raw
+// interface to the engine.
+
 import 'dart:ui' as ui;
 import 'dart:typed_data';
 
+// A paragraph represents a rectangular region that contains some text.
 ui.Paragraph paragraph;
 
 ui.Picture paint(ui.Rect paintBounds) {
@@ -15,8 +19,9 @@ ui.Picture paint(ui.Rect paintBounds) {
   canvas.drawRect(new ui.Rect.fromLTRB(-100.0, -100.0, 100.0, 100.0),
                   new ui.Paint()..color = const ui.Color.fromARGB(255, 0, 255, 0));
 
-  canvas.translate(paragraph.maxWidth / -2.0, (paragraph.maxWidth / 2.0) - 125);
-  paragraph.paint(canvas, ui.Offset.zero);
+  // The paint method of Pargraph draws the contents of the paragraph unto the
+  // given canvas.
+  paragraph.paint(canvas, new ui.Offset(paragraph.maxWidth / -2.0, (paragraph.maxWidth / 2.0) - 125));
 
   return recorder.endRecording();
 }
@@ -49,19 +54,37 @@ void beginFrame(Duration timeStamp) {
 }
 
 void main() {
+  // To create a paragraph of text, we use ParagraphBuilder.
   ui.ParagraphBuilder builder = new ui.ParagraphBuilder()
+    // We first push a style that turns the text blue.
     ..pushStyle(new ui.TextStyle(color: const ui.Color(0xFF0000FF)))
-    ..addText("Hello, ")
+    ..addText('Hello, ')
+    // The next run of text will be bold.
     ..pushStyle(new ui.TextStyle(fontWeight: ui.FontWeight.bold))
-    ..addText("world. ")
+    ..addText('world. ')
+    // The pop() command signals the end of the bold styling.
     ..pop()
-    ..addText("هذا هو قليلا طويلة من النص الذي يجب التفاف .")
+    // We add text to the paragraph in logical order. The paragraph object
+    // understands bi-directional text and will compute the visual ordering
+    // during layout.
+    ..addText('هذا هو قليلا طويلة من النص الذي يجب التفاف .')
+    // The second pop() removes the blue color.
     ..pop()
-    ..addText(" و أكثر قليلا لجعله أطول. ");
+    // We can add more text with the default styling.
+    ..addText(' و أكثر قليلا لجعله أطول. ')
+    ..addText('สวัสดี');
+  // When we're done adding styles and text, we build the Paragraph object, at
+  // which time we can apply styling that affects the entire paragraph, such as
+  // left, right, or center alignment. Once built, the contents of the paragraph
+  // cannot be altered, but sizing and positioning information can be updated.
   paragraph = builder.build(new ui.ParagraphStyle())
+    // Next, we supply a maximum width that the text is permitted to occupy.
     ..maxWidth = 180.0
+    // ... and we ask the paragraph to the visual position of each its glyphs as
+    // well as its overall size, subject to its sizing constraints.
     ..layout();
 
+  // Finally, we register our beginFrame callback and kick off the first frame.
   ui.window.onBeginFrame = beginFrame;
   ui.window.scheduleFrame();
 }
diff --git a/examples/layers/rendering/autolayout.dart b/examples/layers/rendering/autolayout.dart
index 17a90c35b5..c2c30c540b 100644
--- a/examples/layers/rendering/autolayout.dart
+++ b/examples/layers/rendering/autolayout.dart
@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:ui';
+// This example shows how to use the Cassowary autolayout system directly in the
+// underlying render tree.
 
 import 'package:cassowary/cassowary.dart' as al;
 import 'package:flutter/rendering.dart';
diff --git a/examples/layers/rendering/custom_coordinate_systems.dart b/examples/layers/rendering/custom_coordinate_systems.dart
index e28dfbc164..8b5d295da3 100644
--- a/examples/layers/rendering/custom_coordinate_systems.dart
+++ b/examples/layers/rendering/custom_coordinate_systems.dart
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This example shows how to build a render tree with a non-cartesian coordinate
+// system. Most of the guts of this examples are in src/sector_layout.dart.
+
 import 'package:flutter/rendering.dart';
 import 'src/sector_layout.dart';
 
diff --git a/examples/layers/rendering/flex_layout.dart b/examples/layers/rendering/flex_layout.dart
index 9066762127..945c83ea34 100644
--- a/examples/layers/rendering/flex_layout.dart
+++ b/examples/layers/rendering/flex_layout.dart
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This example shows how to use flex layout directly in the underlying render
+// tree.
+
 import 'package:flutter/rendering.dart';
 
 import 'src/solid_color_box.dart';
diff --git a/examples/layers/rendering/hello_world.dart b/examples/layers/rendering/hello_world.dart
index 904414a73b..84f39a6424 100644
--- a/examples/layers/rendering/hello_world.dart
+++ b/examples/layers/rendering/hello_world.dart
@@ -2,12 +2,20 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This example shows how to show the text 'Hello, world.' using the underlying
+// render tree.
+
 import 'package:flutter/rendering.dart';
 
 void main() {
+  // We use RenderingFlutterBinding to attach the render tree to the window.
   new RenderingFlutterBinding(
+    // The root of our render tree is a RenderPositionedBox, which centers its
+    // child both vertically and horizontally.
     root: new RenderPositionedBox(
       alignment: const FractionalOffset(0.5, 0.5),
+      // We use a RenderParagraph to display the text "Hello, world." without
+      // any explicit styling.
       child: new RenderParagraph(new PlainTextSpan('Hello, world.'))
     )
   );
diff --git a/examples/layers/rendering/spinning_square.dart b/examples/layers/rendering/spinning_square.dart
index 04df327c99..5ed6ffd44c 100644
--- a/examples/layers/rendering/spinning_square.dart
+++ b/examples/layers/rendering/spinning_square.dart
@@ -2,43 +2,59 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// This example shows how to perform a simple animation using the underlying
+// render tree.
+
 import 'dart:math' as math;
 
 import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 
 void main() {
-  // A green box...
+  // We first create a render object that represents a green box.
   RenderBox green = new RenderDecoratedBox(
     decoration: const BoxDecoration(backgroundColor: const Color(0xFF00FF00))
   );
-  // of a certain size...
+  // Second, we wrap that green box in a render object that forces the green box
+  // to have a specific size.
   RenderBox square = new RenderConstrainedBox(
     additionalConstraints: const BoxConstraints.tightFor(width: 200.0, height: 200.0),
     child: green
   );
-  // With a given rotation (starts off as the identity transform)...
+  // Third, we wrap the sized green square in a render object that applies rotation
+  // transform before painting its child. Each frame of the animation, we'll
+  // update the transform of this render object to cause the green square to
+  // spin.
   RenderTransform spin = new RenderTransform(
     transform: new Matrix4.identity(),
     alignment: const FractionalOffset(0.5, 0.5),
     child: square
   );
-  // centered...
+  // Finally, we center the spinning green square...
   RenderBox root = new RenderPositionedBox(
     alignment: const FractionalOffset(0.5, 0.5),
     child: spin
   );
-  // on the screen.
+  // and attach it to the window.
   new RenderingFlutterBinding(root: root);
 
-  // A repeating animation every 1800 milliseconds...
+  // To make the square spin, we use an animation that repeats every 1800
+  // milliseconds.
   AnimationController animation = new AnimationController(
     duration: const Duration(milliseconds: 1800)
   )..repeat();
-  // From 0.0 to math.PI.
+  // The animation will produce a value between 0.0 and 1.0 each frame, but we
+  // want to rotate the square using a value between 0.0 and math.PI. To change
+  // the range of the animation, we use a Tween.
   Tween<double> tween = new Tween<double>(begin: 0.0, end: math.PI);
+  // We add a listener to the animation, which will be called every time the
+  // animation ticks.
   animation.addListener(() {
-    // Each frame of the animation, set the rotation of the square.
+    // This code runs every tick of the animation and sets a new transform on
+    // the "spin" render object by evaluating the tween on the current value
+    // of the animation. Setting this value will mark a number of dirty bits
+    // inside the render tree, which cause the render tree to repaint with the
+    // new transform value this frame.
     spin.transform = new Matrix4.rotationZ(tween.evaluate(animation));
   });
 }
diff --git a/examples/layers/rendering/touch_input.dart b/examples/layers/rendering/touch_input.dart
index 54304b1bad..68f442122e 100644
--- a/examples/layers/rendering/touch_input.dart
+++ b/examples/layers/rendering/touch_input.dart
@@ -2,7 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/material.dart';
+// This example shows how to use process input events in the underlying render
+// tree.
+
+import 'package:flutter/material.dart'; // Imported just for its color palette.
 import 'package:flutter/rendering.dart';
 
 // Material design colors. :p
@@ -34,12 +37,23 @@ class Dot {
 }
 
 /// A render object that draws dots under each pointer.
-class RenderDots extends RenderConstrainedBox {
-  RenderDots() : super(additionalConstraints: const BoxConstraints.expand());
+class RenderDots extends RenderBox {
+  RenderDots();
 
   /// State to remember which dots to paint.
   final Map<int, Dot> _dots = <int, Dot>{};
 
+  /// Indicates that the size of this render object depends only on the
+  /// layout constraints provided by the parent.
+  bool get sizedByParent => true;
+
+  /// By selecting the biggest value permitted by the incomming constraints
+  /// during layout, this function makes this render object as large as
+  /// possible (i.e., fills the entire screen).
+  void performResize() {
+    size = constraints.biggest;
+  }
+
   /// Makes this render object hittable so that it receives pointer events.
   bool hitTestSelf(Point position) => true;
 
@@ -49,6 +63,10 @@ class RenderDots extends RenderConstrainedBox {
     if (event is PointerDownEvent) {
       Color color = _kColors[event.pointer.remainder(_kColors.length)];
       _dots[event.pointer] = new Dot(color: color)..update(event);
+      // We call markNeedsPaint to indicate that our painting commands have
+      // changed and that paint needs to be called before displaying a new frame
+      // to the user. It's harmless to call markNeedsPaint multiple times
+      // because the render tree will ignore redundant calls.
       markNeedsPaint();
     } else if (event is PointerUpEvent || event is PointerCancelEvent) {
       _dots.remove(event.pointer);
@@ -62,31 +80,50 @@ class RenderDots extends RenderConstrainedBox {
   /// Issues new painting commands.
   void paint(PaintingContext context, Offset offset) {
     final Canvas canvas = context.canvas;
+    // The "size" property indicates the size of that this render box was
+    // alotted during layout. Here we paint our bounds white. Notice that we're
+    // located at "offset" from the origin of the canvas' coordinate system.
+    // Passing offset during the render tree's paint walk is an optimization to
+    // avoid having to change the origin of the canvas's coordinate system too
+    // often.
     canvas.drawRect(offset & size, new Paint()..color = const Color(0xFFFFFFFF));
+
+    // We iterate through our model and paint each dot.
     for (Dot dot in _dots.values)
       dot.paint(canvas, offset);
-    super.paint(context, offset);
   }
 }
 
 void main() {
+  // Create some styled text to tell the user to interact with the app.
   RenderParagraph paragraph = new RenderParagraph(
     new StyledTextSpan(
       new TextStyle(color: Colors.black87),
-      [ new PlainTextSpan("Touch me!") ]
+      <TextSpan>[ new PlainTextSpan("Touch me!") ]
     )
   );
+  // A stack is a render object that layers its children on top of each other.
+  // The bottom later is our RenderDots object, and on top of that we show the
+  // text.
   RenderStack stack = new RenderStack(
     children: <RenderBox>[
       new RenderDots(),
       paragraph,
     ]
   );
-  // Prevent the RenderParagraph from filling the whole screen so
-  // that it doesn't eat events.
+  // The "parentData" field of a render object is controlled by the render
+  // object's parent render object. Now that we've added the paragraph as a
+  // child of the RenderStack, the paragraph's parentData field has been
+  // populated with a StackParentData, which we can use to provide input to the
+  // stack's layout algorithm.
+  //
+  // We use the StackParentData of the paragraph to position the text in the top
+  // left corner of the screen.
   final StackParentData paragraphParentData = paragraph.parentData;
   paragraphParentData
     ..top = 40.0
     ..left = 20.0;
+
+  // Finally, we attach the render tree we've built to the screen.
   new RenderingFlutterBinding(root: stack);
 }