Finalizable abstract interface

abstract interface class Finalizable

Annotations: @Since.new('2.17')

Marker interface for objects which should not be finalized too soon.

Any local variable with a static type that includes Finalizable is guaranteed to be alive until execution exits the code block where the variable is in scope.

A type includes Finalizable if either

• the type is a non-Never subtype of Finalizable, or
• the type is T? or FutureOr<T> where T includes Finalizable.

In other words, while an object is referenced by such a variable, it is guaranteed to not be considered unreachable, and the variable itself is considered alive for the entire duration of its scope, even after it is last referenced.

Without this marker interface on the variable's type, a variable's value might be garbage collected before the surrounding scope has been completely executed, as long as the variable is definitely not referenced again. That can, in turn, trigger a NativeFinalizer to perform a callback. When the variable's type includes Finalizable, The NativeFinalizer callback is prevented from running until the current code using that variable is complete.

For example, finalizable is kept alive during the execution of someNativeCall:

void myFunction() {
  final finalizable = MyFinalizable(Pointer.fromAddress(0));
  someNativeCall(finalizable.nativeResource);
}

void someNativeCall(Pointer nativeResource) {
  // ..
}

class MyFinalizable implements Finalizable {
  final Pointer nativeResource;

  MyFinalizable(this.nativeResource);
}

Methods on a class implementing Finalizable keep the this object alive for the duration of the method execution. The this value is treated like a local variable.

For example, this is kept alive during the execution of someNativeCall in myFunction:

class MyFinalizable implements Finalizable {
  final Pointer nativeResource;

  MyFinalizable(this.nativeResource);

  void myFunction() {
    someNativeCall(nativeResource);
  }
}

void someNativeCall(Pointer nativeResource) {
  // ..
}

It is good practise to implement logic involving finalizables as methods on the class that implements Finalizable.

If a closure is created inside the block scope declaring the variable, and that closure contains any reference to the variable, the variable stays alive as long as the closure object does, or as long as the body of such a closure is executing.

For example, finalizable is kept alive by the closure object and until the end of the closure body:

void doSomething() {
  final resourceAction = myFunction();
  resourceAction(); // `finalizable` is alive until this call returns.
}

void Function() myFunction() {
  final finalizable = MyFinalizable(Pointer.fromAddress(0));
  return () {
    someNativeCall(finalizable.nativeResource);
  };
}

void someNativeCall(Pointer nativeResource) {
  // ..
}

class MyFinalizable implements Finalizable {
  final Pointer nativeResource;

  MyFinalizable(this.nativeResource);
}

Only captured variables are kept alive by closures, not all variables.

For example, finalizable is not kept alive by the returned closure object:

void Function() myFunction() {
  final finalizable = MyFinalizable(Pointer.fromAddress(0));
  final nativeResource = finalizable.nativeResource;
  return () {
    someNativeCall(nativeResource);
  };
}

void someNativeCall(Pointer nativeResource) {
  // ..
}

class MyFinalizable implements Finalizable {
  final Pointer nativeResource;

  MyFinalizable(this.nativeResource);
}

It's likely an error if a resource extracted from a finalizable object escapes the scope of the finalizable variable it's taken from.

The behavior of Finalizable variables applies to asynchronous functions too. Such variables are kept alive as long as any code may still execute inside the scope that declared the variable, or in a closure capturing the variable, even if there are asynchronous delays during that execution.

For example, finalizable is kept alive during the await someAsyncCall():

Future<void> myFunction() async {
  final finalizable = MyFinalizable();
  await someAsyncCall();
}

Future<void> someAsyncCall() async {
  // ..
}

class MyFinalizable implements Finalizable {
  // ..
}

Also in asynchronous code it's likely an error if a resource extracted from a finalizable object escapes the scope of the finalizable variable it's taken from. If you have to extract a resource from a Finalizable, you should ensure the scope in which Finalizable is defined outlives the resource by awaiting any asynchronous code that uses the resource.

For example, this is kept alive until resource is not used anymore in useAsync1, but not in useAsync2 and useAsync3:

class MyFinalizable {
  final Pointer<Int8> resource;

  MyFinalizable(this.resource);

  Future<int> useAsync1() async {
    return await useResource(resource);
  }

  Future<int> useAsync2() async {
    return useResource(resource);
  }

  Future<int> useAsync3() {
    return useResource(resource);
  }
}

/// Does not use [resource] after the returned future completes.
Future<int> useResource(Pointer<Int8> resource) async {
  return resource.value;
}

It is possible for an asynchronous function to stall at an await, such that the runtime system can see that there is no possible way for that await to complete. In that case, no code after the await will ever execute, including finally blocks, and the variable may be considered dead along with everything else.

If you're not going to keep a variable alive yourself, make sure to pass the finalizable object to other functions instead of just its resource.

For example, finalizable is not kept alive by myFunction after it has run to the end of its scope, while someAsyncCall could still continue execution. However, finalizable is kept alive by someAsyncCall itself:

void myFunction() {
  final finalizable = MyFinalizable();
  someAsyncCall(finalizable);
}

Future<void> someAsyncCall(MyFinalizable finalizable) async {
  // ..
}

class MyFinalizable implements Finalizable {
  // ..
}

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;

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

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);