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the tight constraints it received from its parent
(the screen) to loose constraints for its child
(the container).
if you revisit example 3,
the center allows the red container to be smaller,
but not bigger than the screen.
<topic_end>
<topic_start>
unbounded constraints
info note
you might be directed here if the framework
detects a problem involving box constraints.
the flex section below might also apply.
in certain situations,
a box’s constraint is unbounded, or infinite.
this means that either the maximum width or
the maximum height is set to double.infinity.
a box that tries to be as big as possible won’t
function usefully when given an unbounded constraint and,
in debug mode, throws an exception.
the most common case where a render box ends up
with an unbounded constraint is within a flex box
(row or column),
and within a scrollable region
(such as ListView and other ScrollView subclasses).
ListView, for example,
tries to expand to fit the space available
in its cross-direction
(perhaps it’s a vertically-scrolling block and
tries to be as wide as its parent).
if you nest a vertically scrolling ListView
inside a horizontally scrolling ListView,
the inner list tries to be as wide as possible,
which is infinitely wide,
since the outer one is scrollable in that direction.
the next section describes the error you might
encounter with unbounded constraints in a flex widget.
<topic_end>
<topic_start>
flex
a flex box (row and column) behaves
differently depending on whether its
constraint is bounded or unbounded in
its primary direction.
a flex box with a bounded constraint in its
primary direction tries to be as big as possible.
a flex box with an unbounded constraint
in its primary direction tries to fit its children
in that space. each child’s flex value must be
set to zero, meaning that you can’t use
expanded when the flex box is inside
another flex box or a scrollable;
otherwise it throws an exception.
the cross direction
(width for column or height for row),
must never be unbounded,
or it can’t reasonably align its children.
<topic_end>
<topic_start>
learning the layout rules for specific widgets
knowing the general layout rule is necessary, but it’s not enough.
each widget has a lot of freedom when applying the general rule,
so there is no way of knowing how it behaves by just reading
the widget’s name.
if you try to guess, you’ll probably guess wrong.
you can’t know exactly how a widget behaves unless
you’ve read its documentation, or studied its source-code.
the layout source-code is usually complex,
so it’s probably better to just read the documentation.
however, if you decide to study the layout source-code,
you can easily find it by using the navigating capabilities
of your IDE.
here’s an example:
find a column in your code and navigate to its
source code. to do this, use command+B (macos)
or control+B (windows/linux) in android studio or IntelliJ.
you’ll be taken to the basic.dart file.
since column extends flex, navigate to the flex
source code (also in basic.dart).
scroll down until you find a method called
createRenderObject(). as you can see,
this method returns a RenderFlex.
this is the render-object for the column.
now navigate to the source-code of RenderFlex,
which takes you to the flex.dart file.
scroll down until you find a method called
performLayout(). this is the method that does
the layout for the column.
original article by marcelo glasberg
marcelo originally published this content as
flutter: the advanced layout rule even beginners must know
on medium. we loved it and asked that he allow us to publish
in on docs.flutter.dev, to which he graciously agreed. thanks, marcelo!
you can find marcelo on GitHub and pub.dev.
also, thanks to simon lightfoot for creating the
header image at the top of the article.
info note
to better understand how flutter implements layout
constraints, check out the following 5-minute video:
decoding flutter: unbounded height and width