NativeCallable<T extends Function> abstract final

abstract final class NativeCallable<T extends Function>

Annotations: @Since.new('3.1')

A native callable which listens for calls to a native function.

Creates a native function linked to a Dart function, so that calling the native function will call the Dart function in some way, with the arguments converted to Dart values.

Constructors

NativeCallable.isolateGroupBound() factory

factory NativeCallable.isolateGroupBound(
  Function callback, {
  Object? exceptionalReturn,
})

Constructs a NativeCallable that can be invoked from any thread.

When the native code invokes the function nativeFunction, the callback will be executed within the isolate group of the Isolate which originally constructed the callable. Specifically, this means that an attempt to access any static or global field which is not shared between isolates in a group will result in a Error.

If an exception is thrown by the callback, the native function will return the exceptionalReturn, which must be assignable to the return type of the callback.

callback and exceptionalReturn must be trivially shareable.

This callback must be closed when it is no longer needed. An Isolate that created the callback will be kept alive until close is called.

After NativeCallable.close is called, invoking the nativeFunction from native code will cause undefined behavior.

NOTE: This is an experimental feature and may change in the future.

Implementation

factory NativeCallable.isolateGroupBound(
  @DartRepresentationOf("T") Function callback, {
  Object? exceptionalReturn,
}) {
  throw UnsupportedError("NativeCallable cannot be constructed dynamically.");
}

NativeCallable.isolateLocal() factory

factory NativeCallable.isolateLocal(
  Function callback, {
  Object? exceptionalReturn,
})

Constructs a NativeCallable that must be invoked from the same thread that created it.

If an exception is thrown by the callback, the native function will return the exceptionalReturn, which must be assignable to the return type of the callback.

The returned function address can only be invoked on the mutator (main) thread of the current isolate. It will abort the process if invoked on any other thread. Use NativeCallable.listener to create callbacks that can be invoked from any thread.

Unlike Pointer.fromFunction, NativeCallables can be constructed from any Dart function or closure, not just static or top level functions.

This callback must be closed when it is no longer needed. The Isolate that created the callback will be kept alive until close is called. After NativeCallable.close is called, invoking the nativeFunction from native code will cause undefined behavior.

Implementation

factory NativeCallable.isolateLocal(
  @DartRepresentationOf("T") Function callback, {
  Object? exceptionalReturn,
}) {
  throw UnsupportedError("NativeCallable cannot be constructed dynamically.");
}

NativeCallable.listener() factory

factory NativeCallable.listener(Function callback)

Constructs a NativeCallable that can be invoked from any thread.

When the native code invokes the function nativeFunction, the arguments will be sent over a SendPort to the Isolate that created the NativeCallable, and the callback will be invoked.

The native code does not wait for a response from the callback, so only functions returning void are supported.

The callback will be invoked at some time in the future. The native caller cannot assume the callback will be run immediately. Resources passed to the callback (such as pointers to malloc'd memory, or output parameters) must be valid until the call completes.

This callback must be closed when it is no longer needed. The Isolate that created the callback will be kept alive until close is called. After NativeCallable.close is called, invoking the nativeFunction from native code will cause undefined behavior.

For example:

import 'dart:async';
import 'dart:ffi';
import 'package:ffi/ffi.dart';

// Processes a simple HTTP GET request using a native HTTP library that
// processes the request on a background thread.
Future<String> httpGet(String uri) async {
  final uriPointer = uri.toNativeUtf8();

  // Create the NativeCallable.listener.
  final completer = Completer<String>();
  late final NativeCallable<NativeHttpCallback> callback;
  void onResponse(Pointer<Utf8> responsePointer) {
    completer.complete(responsePointer.toDartString());
    calloc.free(responsePointer);
    calloc.free(uriPointer);

    // Remember to close the NativeCallable once the native API is
    // finished with it, otherwise this isolate will stay alive
    // indefinitely.
    callback.close();
  }
  callback = NativeCallable<NativeHttpCallback>.listener(onResponse);

  // Invoke the native HTTP API. Our example HTTP library processes our
  // request on a background thread, and calls the callback on that same
  // thread when it receives the response.
  nativeHttpGet(uriPointer, callback.nativeFunction);

  return completer.future;
}

// Load the native functions from a DynamicLibrary.
final DynamicLibrary dylib = DynamicLibrary.process();
typedef NativeHttpCallback = Void Function(Pointer<Utf8>);

typedef HttpGetFunction = void Function(
    Pointer<Utf8>, Pointer<NativeFunction<NativeHttpCallback>>);
typedef HttpGetNativeFunction = Void Function(
    Pointer<Utf8>, Pointer<NativeFunction<NativeHttpCallback>>);
final nativeHttpGet =
    dylib.lookupFunction<HttpGetNativeFunction, HttpGetFunction>(
        'http_get');

Implementation

factory NativeCallable.listener(
  @DartRepresentationOf("T") Function callback,
) {
  throw UnsupportedError("NativeCallable cannot be constructed dynamically.");
}

Properties

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

external int get hashCode;

keepIsolateAlive read / write

bool get keepIsolateAlive

Whether this NativeCallable keeps its Isolate alive.

By default, NativeCallables keep the Isolate that created them alive until close is called. If keepIsolateAlive is set to false, the isolate may exit even if the NativeCallable isn't closed.

Implementation

external bool get keepIsolateAlive;

external set keepIsolateAlive(bool value);

nativeFunction no setter

Pointer<NativeFunction<T>> get nativeFunction

The native function pointer which can be used to invoke the callback passed to the constructor.

This pointer must not be read after the callable has been closed.

Implementation

Pointer<NativeFunction<T>> get nativeFunction;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

Methods

close()

void close()

Closes this callback and releases its resources.

Further calls to existing nativeFunctions will result in undefined behavior.

Subsequent calls to close will be ignored.

It is safe to call close inside the callback.

Implementation

void close();

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

toString() inherited

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Inherited from Object.

Implementation

external String toString();

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

external bool operator ==(Object other);