Overview

The Sky render tree is a low-level layout and painting system based on a
retained tree of objects that inherit from RenderObject. Most
developers using Sky will not need to interact directly with the rendering tree.
Instead, most developers should use Sky widgets, which
are built using the render tree.

Flutter

Flutter is a new way to build high-performance, cross-platform mobile apps.
Flutter is optimized for today’s, and tomorrow’s, mobile devices. We are
focused on low-latency input and high frame rates on Android and iOS.

• To start building apps with Flutter, please see the
  getting started guide.
• To learn more about the Flutter framework, please see the tutorial.

Base Model

The base class for every node in the render tree is
RenderObject, which defines the base layout model. The base
layout mode is extremely general and can accomodate a large number of more
concrete layout models that can co-exist in the same tree. For example, the base
model does not commit to a fixed number of dimensions or even a cartesian
coordinate system. In this way, a single render tree can contain render objects
operating in three-dimensional space together with other render objects
operating in two-dimensional space, e.g., on the face of a cube in the three-
dimensional space. Moreover, the two-dimensional layout might be partially
computed in cartesian coordinates and partially computed in polar coordinates.
These distinct models can interact during layout, for example determining the
size of the cube by the height of a block of text on the cube’s face.

Not entirely free-wheeling, the base model does impose some structure on the
render tree:

• Subclasses of RenderObject must implement a performLayout function that
  takes as input a constraints object provided by its parent. RenderObject
  has no opinion about the structure of this object and different layout models
  use different types of constraints. However, whatever type they choose must
  implement operator== in such a way that performLayout produces the same
  output for two constraints objects that are operator==.

• Implementations of performLayout are expected to call layout on their
  children. When calling layout, a RenderObject must use the
  parentUsesSize parameter to declare whether its performLayout function
  depends on information read from the child. If the parent doesn’t declare
  that it uses the child’s size, the edge from the parent to the child becomes
  a relayout boundary, which means the child (and its subtree) might undergo
  layout without the parent undergoing layout.

• Subclasses of RenderObject must implement a paint function that draws a
  visual representation of the object onto a PaintingCanvas. If
  the RenderObject has children, the RenderObject is responsible for
  painting its children using the paintChild function on the
  PaintingCanvas.

• Subclasses of RenderObject must call adoptChild whenever they add a
  child. Similarly, they must call dropChild whenever they remove a child.

• Most subclasses of RenderObject will implement a hitTest function that
  lets clients query the render tree for objects that intersect with a given
  user input location. RenderObject itself does not impose a particular
  type signature on hitTest, but most implementations will take an argument
  of type HitTestResult (or, more likely, a model-specific subclass of
  HitTestResult) as well as an object that describes the location at which
  the user provided input (e.g., a Point for a two-dimensional cartesian
  model).

• Finally, subclasses of RenderObject can override the default, do-nothing
  implemenations of handleEvent and rotate to respond to user input and
  screen rotation, respectively.

The base model also provides two mixins for common child models:

• RenderObjectWithChildMixin is useful for subclasses of RenderObject that
  have a unique child.

• ContainerRenderObjectMixin is useful for subclasses of RenderObject that
  have a child list.

Subclasses of RenderObject are not required to use either of these child
models and are free to invent novel child models for their specific use cases.

Parent Data

TODO(ianh): Describe the parent data concept.

The setupParentData() method is automatically called for each child
when the child’s parent is changed. However, if you need to
preinitialise the parentData member to set its values before you add
a node to its parent, you can preemptively call that future parent’s
setupParentData() method with the future child as the argument.

TODO(ianh): Discuss putting per-child configuration information for
the parent on the child’s parentData.

If you change a child’s parentData dynamically, you must also call
markNeedsLayout() on the parent, otherwise the new information will
not take effect until something else triggers a layout.

Box Model

Dimensions

All dimensions are expressed as logical pixel units. Font sizes are
also in logical pixel units. Logical pixel units are approximately
96dpi, but the precise value varies based on the hardware, in such a
way as to optimise for performance and rendering quality while keeping
interfaces roughly the same size across devices regardless of the
hardware pixel density.

Logical pixel units are automatically converted to device (hardware)
pixels when painting by applying an appropriate scale factor.

TODO(ianh): Define how you actually get the device pixel ratio if you
need it, and document best practices around that.

EdgeDims

BoxConstraints

Bespoke Models

Using the provided subclasses

render_box.dart

RenderConstrainedBox

RenderShrinkWrapWidth

RenderOpacity

RenderColorFilter

RenderClipRect

RenderClipOval

RenderPadding

RenderPositionedBox

RenderImage

RenderDecoratedBox

RenderTransform

RenderSizeObserver

RenderCustomPaint

RenderBlock (render_block.dart)

RenderFlex (render_flex.dart)

RenderParagraph (render_paragraph.dart)

RenderStack (render_stack.dart)

Writing new subclasses

The RenderObject contract

If you want to define a RenderObject that uses a new coordinate
system, then you should inherit straight from RenderObject. Examples
of doing this can be found in RenderBox, which deals in
rectangles in cartesian space, and in the sector_layout.dart
example, which
implements a toy model based on polar coordinates. The RenderView
class, which is used internally to adapt from the host system to this
rendering framework, is another example.

A subclass of RenderObject must fulfill the following contract:

• It must fulfill the AbstractNode contract when
  dealing with children. Using RenderObjectWithChildMixin or
  ContainerRenderObjectMixin can make this easier.

• Information about the child managed by the parent, e.g. typically
  position information and configuration for the parent’s layout,
  should be stored on the parentData member; to this effect, a
  ParentData subclass should be defined and the setupParentData()
  method should be overriden to initialise the child’s parent data
  appropriately.

• Layout constraints must be expressed in a Constraints subclass. This
  subclass must implement operator== (and hashCode).

• Whenever the layout needs updating, the markNeedsLayout() method
  should be called.

• Whenever the rendering needs updating without changing the layout,
  the markNeedsPaint() method should be called. (Calling
  markNeedsLayout() implies a call to markNeedsPaint(), so you
  don’t need to call both.)

• The subclass must override performLayout() to perform layout based
  on the constraints given in the constraints member. Each object is
  responsible for sizing itself; positioning must be done by the
  object calling performLayout(). Whether positioning is done before
  or after the child’s layout is a decision to be made by the class.
  TODO(ianh): Document sizedByParent, performResize(), rotate

• TODO(ianh): Document painting, hit testing, debug*

The ParentData contract

Using RenderObjectWithChildMixin

Using ContainerRenderObjectMixin (and ContainerParentDataMixin)

This mixin can be used for classes that have a child list, to manage
the list. It implements the list using linked list pointers in the
parentData structure.

TODO(ianh): Document this mixin.

Subclasses must follow the following contract, in addition to the
contracts of any other classes they subclass:

• If the constructor takes a list of children, it must call addAll()
  with that list.

TODO(ianh): Document how to walk the children.

The RenderBox contract

A RenderBox subclass is required to implement the following contract:

• It must fulfill the AbstractNode contract when
  dealing with children. Note that using RenderObjectWithChildMixin
  or ContainerRenderObjectMixin takes care of this for you, assuming
  you fulfill their contract instead.

• If it has any data to store on its children, it must define a
  BoxParentData subclass and override setupParentData() to initialise
  the child’s parent data appropriately, as in the following example.
  (If the subclass has an opinion about what type its children must
  be, e.g. the way that RenderBlock wants its children to be
  RenderBox nodes, then change the setupParentData() signature
  accordingly, to catch misuse of the method.)

1
2
3
4
5
6
7

  class FooParentData extends BoxParentData { ... }

  // In RenderFoo
  void setupParentData(RenderObject child) {
    if (child.parentData is! FooParentData)
      child.parentData = new FooParentData();
  }

• The class must encapsulate a layout algorithm that has the following
  features:

** It uses as input a set of constraints, described by a
BoxConstraints object, and a set of zero or more children, as
determined by the class itself, and has as output a Size (which is
set on the object’s own size field), and positions for each child
(which are set on the children’s parentData.position field).

** The algorithm can decide the Size in one of two ways: either
exclusively based on the given constraints (i.e. it is effectively
sized entirely by its parent), or based on those constraints and
the dimensions of the children.

In the former case, the class must have a sizedByParent getter that
returns true, and it must have a performResize() method that uses
the object’s constraints member to size itself by setting the
size member. The size must be consistent, a given set of
constraints must always result in the same size.

In the latter case, it will inherit the default sizedByParent
getter that returns false, and it will size itself in the
performLayout() function described below.

The sizedByParent distinction is purely a performance
optimisation. It allows nodes that only set their size based on the
incoming constraints to skip that logic when they need to be
re-laid-out, and, more importantly, it allows the layout system to
treat the node as a layout boundary, which reduces the amount of
work that needs to happen when the node is marked as needing
layout.

• The following methods must report numbers consistent with the output
  of the layout algorithm used:

  • double getMinIntrinsicWidth(BoxConstraints constraints) must
    return the width that fits within the given constraints below which
    making the width constraint smaller would not increase the
    resulting height, or, to put it another way, the narrowest width at
    which the box can be rendered without failing to lay the children
    out within itself.

    For example, the minimum intrinsic width of a piece of text like “a
    b cd e”, where the text is allowed to wrap at spaces, would be the
    width of “cd”.

  • double getMaxIntrinsicWidth(BoxConstraints constraints) must
    return the width that fits within the given constraints above which
    making the width constraint larger would not decrease the resulting
    height.

    For example, the maximum intrinsic width of a piece of text like “a
    b cd e”, where the text is allowed to wrap at spaces, would be the
    width of the whole “a b cd e” string, with no wrapping.

  • double getMinIntrinsicHeight(BoxConstraints constraints) must
    return the height that fits within the given constraints below
    which making the height constraint smaller would not increase the
    resulting width, or, to put it another way, the shortest height at
    which the box can be rendered without failing to lay the children
    out within itself.

    The minimum intrinsic height of a width-in-height-out algorithm,
    like English text layout, would be the height of the text at the
    width that would be used given the constraints. So for instance,
    given the text “hello world”, if the constraints were such that it
    had to wrap at the space, then the minimum intrinsic height would
    be the height of two lines (and the appropriate line spacing). If
    the constraints were such that it all fit on one line, then it
    would be the height of one line.

    • double getMaxIntrinsicHeight(BoxConstraints constraints) must
      return the height that fits within the given constraints above
      which making the height constraint larger would not decrease the
      resulting width. If the height depends exclusively on the width,
      and the width does not depend on the height, then
      getMinIntrinsicHeight() and getMaxIntrinsicHeight() will return the
      same number given the same constraints.

      In the case of English text, the maximum intrinsic height is the
      same as the minimum instrinsic height.

• The box must have a performLayout() method that encapsulates the
  layout algorithm that this class represents. It is responsible for
  telling the children to lay out, positioning the children, and, if
  sizedByParent is false, sizing the object.

  Specifically, the method must walk over the object’s children, if
  any, and for each one call child.layout() with a BoxConstraints
  object as the first argument, and a second argument named
  parentUsesSize which is set to true if the child’s resulting size
  will in any way influence the layout, and omitted (or set to false)
  if the child’s resulting size is ignored. The children’s positions
  (child.parentData.position) must then be set.

  (Calling layout() can result in the child’s own performLayout()
  method being called recursively, if the child also needs to be laid
  out. If the child’s constraints haven’t changed and the child is not
  marked as needing layout, however, this will be skipped.)

  The parent must not set a child’s size directly. If the parent
  wants to influence the child’s size, it must do so via the
  constraints that it passes to the child’s layout() method.

  If an object’s sizedByParent is false, then its performLayout()
  must also size the object (by setting size), otherwise, the size
  must be left untouched.

• The size member must never be set to an infinite value.

• The box must also implement hitTestChildren().
  TODO(ianh): Define this better

• The box must also implement paint().
  TODO(ianh): Define this better

Using RenderProxyBox

The Hit Testing contract

Performance rules of thumb

• Avoid using transforms where mere maths would be sufficient (e.g.
  draw your rectangle at x,y rather than translating by x,y and
  drawing it at 0,0).

• Avoid using save/restore on canvases.

Useful debugging tools

This is a quick way to dump the entire render tree to the console every frame.
This can be quite useful in figuring out exactly what is going on when
working with the render tree.

1
2
3
4
5
6

import 'package:sky/animation.dart';
import 'package:sky/rendering.dart';

scheduler.addPersistentFrameCallback((_) {
  SkyBinding.instance.debugDumpRenderTree();
});

Next steps:

• Getting started guide
• Tutorial introduction
• Widget gallery
• API documentation