Lesser known classes and functions from the Dart core libraries

Help Dart
21

Yeah @mraleph went into this in details in this talk

Basically instead of JsonDecoder, it gives you a decoder that goes Utf8 → Json directly without converting the Utf8 bytes into a string first

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22

Thanks for the link, @Hari_07!

Another cool functionality from the Dart SDK that I didn’t know about until recently is the FutureRecord2 extension and its kin.

It lets you do parallel awaits more easily and readably than Future.wait() or [...].wait. Because it works on records. So:

var (text, image) = await (fetchText(), fetchImage()).wait;

Using record instead of iterable means that the types of text and image are correctly inferred, whatever they are.

23 Likes
23

Oh this is very cool. Thanks!

24

Flutter team can make a valuable video of the week with all those pearls.
How many cool stuffs to us are casual to them is beyond me!

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26

I use unawaited function - dart:async library - Dart API when I want to silence the unawaited_futures linter rule or when I want to communicate that a Future is deliberately not awaited.

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27

Records destructuring.

I was a bit confused with the syntax how to destructure/split the record fields into separate variables (similar to here), and I think it’s quite useful to refresh this one. In the example below you can use entire record boundaries but if you only care about each field, you can assign it to local variables dayStart and dayEnd. I never know what is the order and suspect there might be others like me :slight_smile:

void main() {
  final boundaries = getDayBoundaries(DateTime.now());
  print('Day start: ${boundaries.start}, day end: ${boundaries.end}');

  final (start: dayStart, end: dayEnd) = getDayBoundaries(DateTime.now());
  print('Day start: ${dayStart}, day end: ${dayEnd}');
}

({DateTime start, DateTime end}) getDayBoundaries(DateTime day) {
  return (
    start: DateTime(day.year, day.month, day.day, 0),
    end: DateTime(day.year, day.month, day.day, 23, 59, 59, 999, 999)
  );
}
1 Like
28

You can even use the direct assignation, it will respect the record naming:

  final (:start, :end) = getDayBoundaries(DateTime.now());
  print('Day start: ${start}, day end: ${end}');
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29

Maybe we could do a separate thread on this. What I don’t like is that the syntax for the destructuring looks almost as if you define a new record.

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30

Good idea! this forum need a place where we would demonstrate the (new) languages features and what tricks we came with … also discuss our personal tastes :smiley:

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31

These are all great. I didn’t know maxBy/minBy existed, the ephemeral cache technique, or adding spacing to the column so one doesn’t have to add padding widgets. One I use which doesn’t jump out from linters but can be found if doing searches for it is when you need to compare collections in == and hashcode. is listEquals/setEquals/mapEquals and Object.hashAll for the corresponding hashing function. so something like:

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      other is Timeline &&
          runtimeType == other.runtimeType &&
          listEquals(words, other.words);

  @override
  int get hashCode => Object.hashAll(words);
6 Likes
32

Not directly a function, but I just found out how to test if a dynamic object supports a certain function without calling it

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{21F6A5F7-6F2D-4595-81DF-27F57E49095D}845×243 23.7 KB

from

5 Likes
33

Dart support reproductible shuffle or stable shuffle.
The moment you generate a seed you have an idem-potent series of pseudo-random actions. So it’s not adapted to security, few tools are but if perfect for creating a perception of randomness and testability.

Check this exemple the same numbers and even operations like shuffle are executed:


  final seed = 22;
  final r1 = Random(seed);
  final r2 = Random(seed);
  
  print(r1.nextInt(100).toString());
  print(r2.nextInt(100).toString());
  
  final l1 = List.generate(10, (idx) => idx)..shuffle(r1);
  final l2 = List.generate(10, (idx) => idx)..shuffle(r2);
  
  print(l1.first);
  print(l2.first);
2 Likes
34

Yes I think firstWhere and first are footguns that should be accompanied by some more obvious warnings. Something like firstOrThrow would be better I think.

1 Like
35

This is going pretty deep, but I just found out there’s a Dart VM pragma that hints the attached debugger that it should break at an exception even though that exception is caught. This seems really helpful for things like Sentry, where you want to catch exceptions and send them to the cloud but you also want to see the error immediately when you’re debugging.

@pragma('vm:notify-debugger-on-exception')
void doSomething() {
  try {
    methodThatMayThrow();
  } catch (exception, stack) {
    // ... catch and report the error ...
  }
}

More info here.

8 Likes