Reorganize compute docs and add sample code (#100253)

packages/flutter/lib/src/foundation/isolates.dart

@@ -11,44 +11,83 @@ import '_isolates_io.dart'
 ///
 /// {@macro flutter.foundation.compute.types}
 ///
-/// Instances of [ComputeCallback] must be sendable between isolates i.e.
-/// top-level functions, static methods or a closures that only capture objects
-/// sendable between isolates or an instance methods which instance properties
-/// are also sendable.
+/// Instances of [ComputeCallback] must be functions that can be sent to an
+/// isolate.
+/// {@macro flutter.foundation.compute.callback}
 ///
-/// {@macro flutter.foundation.compute.closure.note}
-///
-/// {@macro flutter.foundation.compute.limitations}
+/// {@macro flutter.foundation.compute.types}
 typedef ComputeCallback<Q, R> = FutureOr<R> Function(Q message);
 
 /// The signature of [compute], which spawns an isolate, runs `callback` on
 /// that isolate, passes it `message`, and (eventually) returns the value
 /// returned by `callback`.
 ///
+/// {@macro flutter.foundation.compute.usecase}
+///
+/// The function used as `callback` must be one that can be sent to an isolate.
+/// {@macro flutter.foundation.compute.callback}
+///
+/// {@macro flutter.foundation.compute.types}
+///
+/// The `debugLabel` argument can be specified to provide a name to add to the
+/// [Timeline]. This is useful when profiling an application.
+typedef ComputeImpl = Future<R> Function<Q, R>(ComputeCallback<Q, R> callback, Q message, { String? debugLabel });
+
+/// A function that spawns an isolate and runs the provided `callback` on that
+/// isolate, passes it the provided `message`, and (eventually) returns the
+/// value returned by `callback`.
+///
+/// {@template flutter.foundation.compute.usecase}
 /// This is useful for operations that take longer than a few milliseconds, and
 /// which would therefore risk skipping frames. For tasks that will only take a
 /// few milliseconds, consider [SchedulerBinding.scheduleTask] instead.
+/// {@endtemplate}
+///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=5AxWC49ZMzs}
+///
+/// {@tool snippet}
+/// The following code uses the [compute] function to check whether a given
+/// integer is a prime number.
+///
+/// ```dart
+/// Future<bool> isPrime(int value) {
+///   return compute(_calculate, value);
+/// }
+///
+/// bool _calculate(int value) {
+///   if (value == 1) {
+///     return false;
+///   }
+///   for (int i = 2; i < value; ++i) {
+///     if (value % i == 0) {
+///       return false;
+///     }
+///   }
+///   return true;
+/// }
+/// ```
+/// {@end-tool}
+///
+/// The function used as `callback` must be one that can be sent to an isolate.
+/// {@template flutter.foundation.compute.callback}
+/// Qualifying functions include:
+///
+///   * top-level functions
+///   * static methods
+///   * closures that only capture objects that can be sent to an isolate
+///
+/// Using closures must be done with care. Due to
+/// [dart-lang/sdk#36983](https://github.com/dart-lang/sdk/issues/36983) a
+/// closure may captures objects that, while not directly used in the closure
+/// itself, may prevent it from being sent to an isolate.
+/// {@endtemplate}
 ///
 /// {@template flutter.foundation.compute.types}
 /// The [compute] method accepts the following parameters:
 ///
 ///  * `Q` is the type of the message that kicks off the computation.
-///
 ///  * `R` is the type of the value returned.
-/// {@endtemplate}
-///
-/// The `callback` must be sendable between isolates i.e. a top-level function,
-/// static method or a closure that only captures objects sendable between
-/// isolates or an instance method which instance properties are also sendable.
-///
-/// {@template flutter.foundation.compute.closure.note}
-/// However, using arbitrary closures should be done with great care because
-/// it may be that the closure captures more variables than initially thought.
-/// See the underlying [dart-lang/sdk#36983](https://github.com/dart-lang/sdk/issues/36983)
-/// issue.
-/// {@endtemplate}
 ///
-/// {@template flutter.foundation.compute.limitations}
 /// There are limitations on the values that can be sent and received to and
 /// from isolates. These limitations constrain the values of `Q` and `R` that
 /// are possible. See the discussion at [SendPort.send].
@@ -56,17 +95,7 @@ typedef ComputeCallback<Q, R> = FutureOr<R> Function(Q message);
 /// The same limitations apply to any errors generated by the computation.
 /// {@endtemplate}
 ///
-/// The `debugLabel` argument can be specified to provide a name to add to the
-/// [Timeline]. This is useful when profiling an application.
-typedef ComputeImpl = Future<R> Function<Q, R>(ComputeCallback<Q, R> callback, Q message, { String? debugLabel });
-
-/// A function that spawns an isolate and runs a callback on that isolate. This
-/// method should be used to execute parallel tasks that reduce the risk of
-/// skipping frames.
-///
-/// {@youtube 560 315 https://www.youtube.com/watch?v=5AxWC49ZMzs}
-///
 /// See also:
 ///
-///   * [ComputeImpl], for function parameters and usage details.
+///   * [ComputeImpl], for the [compute] function's signature.
 const ComputeImpl compute = isolates.compute;