What is the use for a late local variable?-CodePudding

In Dart, we can declare a local variable without initializing it (and yes, it works with null safety too):

void conditionalInit(bool something) {
  int x;

  if (something) {
    x = 1;
    print(x); // OK
  }
}

If so, what is the added benefit of late modifier? The only one I can think of is that it silences the error of messy conditions like this:

void conditionalInit(bool something) {
  int x;

  if (something) {
    x = 1;
    print(x); // OK
  }

  if (something) {
    print(x); // This only compiles with late declaration
  }
}

But that should be avoided anyway.

So is there a legitimate use for local late variables?

CodePudding user response：

Here is one possible use for a local late variable:

import 'dart:async';

Stream<int> countToFive() {
  late StreamController<int> controller;

  controller = StreamController(
    onListen: () {
      for (int i = 0; i < 5; i  ) {
        controller.add(i   1);
      }
      controller.close();
    },
  );
  
  return controller.stream;
}

Future<void> main() async {
  await for (final number in countToFive()) {
    print(number);
  }
}

Because controller is late it is possible to reference controller within the onListen callback passed into the constructor of the StreamController.

CodePudding user response：

I think the main use case is lazy initialization. A late variable will only be evaluated when it is accessed, which can be useful for costly computations which may not be required at runtime.

CodePudding user response：

late is also useful to assign immutable final variables after initialization.

void conditionalInit(bool something) {
    late final int x;

    if (something) {
        x = 1;
        print(x); // OK
    }

} }