HelpSizing class

Sizing

Sizes, Constraints and Positions

Rules

  • Constraints go down
  • Sizes go up
  • Parent sets position

Process

For an arbitrary widget X, its parent Y, and its children Z

  1. Y passes its constraints down to X
    • min/max height/width
  2. X passes its constraints down to Z
  3. X asks Z what size they are
    • width/height
  4. X sets positions of Z
  5. X tells Y its final size

Limitations

  • Size defined in widget only within constraints of parent
  • Widget does not know/decide its position
  • Defining alignment must be specific or child size may be ignored

Mechanisms

  • RenderBox
    • Underlying object used to render widgets

Types of Constraints

As Big As Possible

  • e.g.,
    • Center
    • ListView
    • Container (null width and height)

Same Size if Possible

  • e.g.,
    • Transform
    • Opacity
    • Container (non null width or height)

Fixed Size if Possible

  • e.g.,
    • Image
    • Text

BoxConstraints

  • Passed to Container.constraints
  • Can specify max/min width/height

LayoutBuilder

  • Provides parent constraints to child
  • Builds at layout time

FractionallySizedBox

  • Provides percentage of parent size to child
ParentWidget(
  child: FractionallySizedBox(
        widthFactor: 0.5,
        heightFactor: 0.5,
        child: Container(
          /// this container won't be larger than
          /// half of its parent size
        ),
  )
)

Constructors

HelpSizing()

Properties

hashCode → int
The hash code for this object.
no setterinherited
runtimeType → Type
A representation of the runtime type of the object.
no setterinherited

Methods

noSuchMethod(Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
inherited
toString() → String
A string representation of this object.
inherited

Operators

operator ==(Object other) → bool
The equality operator.
inherited