engine/src/flutter/lib/ui/plugins.dart

// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

part of dart.ui;

/// A wrapper for a raw callback handle.
///
/// This is the return type for [PluginUtilities.getCallbackHandle].
class CallbackHandle {
  /// Create an instance using a raw callback handle.
  ///
  /// Only values produced by a call to [CallbackHandle.toRawHandle] should be
  /// used, otherwise this object will be an invalid handle.
  CallbackHandle.fromRawHandle(this._handle);

  final int _handle;

  /// Get the raw callback handle to pass over a [MethodChannel] or [SendPort]
  /// (to pass to another [Isolate]).
  int toRawHandle() => _handle;

  @override
  bool operator ==(Object other) {
    if (runtimeType != other.runtimeType) {
      return false;
    }
    return other is CallbackHandle && other._handle == _handle;
  }

  @override
  int get hashCode => _handle.hashCode;
}

/// Functionality for Flutter plugin authors.
///
/// See also:
///
///  * [IsolateNameServer], which provides utilities for dealing with
///    [Isolate]s.
abstract final class PluginUtilities {
  static final Map<Function, CallbackHandle?> _forwardCache = <Function, CallbackHandle?>{};
  static final Map<CallbackHandle, Function?> _backwardCache = <CallbackHandle, Function?>{};

  /// Get a handle to a named top-level or static callback function which can
  /// be easily passed between isolates.
  ///
  /// The `callback` argument must not be null.
  ///
  /// Returns a [CallbackHandle] that can be provided to
  /// [PluginUtilities.getCallbackFromHandle] to retrieve a tear-off of the
  /// original callback. If `callback` is not a top-level or static function,
  /// null is returned.
  static CallbackHandle? getCallbackHandle(Function callback) {
    return _forwardCache.putIfAbsent(callback, () {
      final int? handle = _getCallbackHandle(callback);
      return handle != null ? CallbackHandle.fromRawHandle(handle) : null;
    });
  }

  /// Get a tear-off of a named top-level or static callback represented by a
  /// handle.
  ///
  /// The `handle` argument must not be null.
  ///
  /// If `handle` is not a valid handle returned by
  /// [PluginUtilities.getCallbackHandle], null is returned. Otherwise, a
  /// tear-off of the callback associated with `handle` is returned.
  static Function? getCallbackFromHandle(CallbackHandle handle) {
    return _backwardCache.putIfAbsent(handle, () => _getCallbackFromHandle(handle.toRawHandle()));
  }
}