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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 GlasbergMarcelo 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 |
<topic_end> |
<topic_start>Hot reload |
Flutter’s hot reload feature helps you quickly and |
easily experiment, build UIs, add features, and fix bugs. |
Hot reload works by injecting updated source code files |
into the running Dart Virtual Machine (VM). |
After the VM updates classes with the new versions of fields and functions, |
the Flutter framework automatically rebuilds the widget tree, |
allowing you to quickly view the effects of your changes.<topic_end> |
<topic_start> |
How to perform a hot reload |
To hot reload a Flutter app:If you’re working in an IDE/editor that supports Flutter’s IDE tools, |
select Save All (cmd-s/ctrl-s), |
or click the hot reload button on the toolbar.If you’re running the app at the command line using flutter run, |
enter r in the terminal window.After a successful hot reload operation, |
you’ll see a message in the console similar to:The app updates to reflect your change, |
and the current state of the app is preserved. |
Your app continues to execute from where it was prior |
to run the hot reload command. |
The code updates and execution continues.What is the difference between hot reload, hot restart, |
and full restart?Flutter web currently supports hot restart but not |
hot reload. |
Controls for run, run debug, hot reload, and hot restart in Android StudioA code change has a visible effect only if the modified |
Dart code is run again after the change. Specifically, |
a hot reload causes all the existing widgets to rebuild. |
Only code involved in the rebuilding of the widgets |
is automatically re-executed. The main() and initState() |
functions, for example, are not run again.<topic_end> |
<topic_start> |
Special cases |
The next sections describe specific scenarios that involve |
hot reload. In some cases, small changes to the Dart code |
enable you to continue using hot reload for your app. |
In other cases, a hot restart, or a full restart is needed.<topic_end> |
<topic_start> |
An app is killed |
Hot reload can break when the app is killed. |
For example, if the app was in the background for too long.<topic_end> |
<topic_start> |
Compilation errors |
When a code change introduces a compilation error, |
hot reload generates an error message similar to:In this situation, simply correct the errors on the |
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