Provide function context in dart-CodePudding

So i need to run some code that needs to get some data later.

I imagine it like:

voin runWithContext(void Function () fn, dynamic context) {
  // ... magic
  fn();
}

And later in the call stack of fn()

void otherFn() {
   dynamic context = getContext();
}

If functions are not async we could just store the context as global variable, but the main requirement to support dart:async

I was looking into dart:mirrors and stacktrace but i can't find no way to bind some data.

CodePudding user response：

You can do this with Zones. Zones, and their "zone variables" are the canonical way to create a context that is preserved throughout an asynchronous computation).

Since you want single global function to access the context, it will have to be untyped (or typed at the one type that you need, but not generally reusable). That would me me want to reconsider the design, but if it works for you, I'd do it as:

import "dart:async";

/// Container for a mutable value.
class _Context {
  dynamic value;
}
final _Context _rootContext = _Context();

R runWithNewContext<R>(R Function() body) =>
  runZoned(body, zoneValues: {
      #_context: _Context()..value = context,
  });

dynamic get context => (Zone.current[#_context] ?? _rootContext).value;
set context(dynamic value) {
  (Zone.current[#_context] ?? _rootContext).value = value;
}

If you don't need the mutability, you can simplify things a little, but not much.

The typed and unmodifable alternative is something like

class _Box { // Distinguishable from `null`, even if value is `null`
  final Object? value;
  _Box(this.value);
}
class ZoneStorage<T> {
  final _Box _initialValue;
  ZoneStorage(T initialValue) : _initialValue = _Box(initialValue);
  
  R update<R>(T newValue, R Function() body) =>
    runZoned(body, zoneValues: {this: _Box(newValue)});

  T get value => 
     (Zone.current[this] as _Box? ?? _initialValue).value as T;
}

That allows you to create multiple independent zone stores, like:

  var zs1 = ZoneStorage<int>(1);
  var zs2 = ZoneStorage<String>("yup");
  zs1.update(42, () {
    print(zs1.value);
    print(zs2.value);
  });
  zs2.update("yup yup yup", () {
    print(zs1.value);
    print(zs2.value);
  });

CodePudding user response：

So the thing that is need is https://api.dart.dev/stable/2.17.3/dart-async/Zone-class.html

Hope that answer will help someone else