checkInstance<T extends BindingBase> method Null safety

Implementation

@protected
static T checkInstance<T extends BindingBase>(T? instance) {
  assert(() {
    if (_debugInitializedType == null && instance == null) {
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary('Binding has not yet been initialized.'),
        ErrorDescription('The "instance" getter on the $T binding mixin is only available once that binding has been initialized.'),
        ErrorHint(
          'Typically, this is done by calling "WidgetsFlutterBinding.ensureInitialized()" or "runApp()" (the '
          'latter calls the former). Typically this call is done in the "void main()" method. The "ensureInitialized" method '
          'is idempotent; calling it multiple times is not harmful. After calling that method, the "instance" getter will '
          'return the binding.',
        ),
        ErrorHint(
          'In a test, one can call "TestWidgetsFlutterBinding.ensureInitialized()" as the first line in the test\'s "main()" method '
          'to initialize the binding.',
        ),
        ErrorHint(
          'If $T is a custom binding mixin, there must also be a custom binding class, like WidgetsFlutterBinding, '
          'but that mixes in the selected binding, and that is the class that must be constructed before using the "instance" getter.',
        ),
      ]);
    }
    if (instance == null) {
      assert(_debugInitializedType == null);
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary('Binding mixin instance is null but bindings are already initialized.'),
        ErrorDescription(
          'The "instance" property of the $T binding mixin was accessed, but that binding was not initialized when '
          'the "initInstances()" method was called.',
        ),
        ErrorHint(
          'This probably indicates that the $T mixin was not mixed into the class that was used to initialize the binding. '
          'If this is a custom binding mixin, there must also be a custom binding class, like WidgetsFlutterBinding, '
          'but that mixes in the selected binding. If this is a test binding, check that the binding being initialized '
          'is the same as the one into which the test binding is mixed.',
        ),
        ErrorHint(
          'It is also possible that $T does not implement "initInstances()" to assign a value to "instance". See the '
          'documentation of the BaseBinding class for more details.',
        ),
        ErrorHint(
          'The binding that was initialized was of the type "$_debugInitializedType". '
        ),
      ]);
    }
    try {
      assert(instance != null);
      if (instance._debugConstructed && _debugInitializedType == null) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('Binding initialized without calling initInstances.'),
          ErrorDescription('An instance of $T is non-null, but BindingBase.initInstances() has not yet been called.'),
          ErrorHint(
            'This could happen because a binding mixin was somehow used outside of the normal binding mechanisms, or because '
            'the binding\'s initInstances() method did not call "super.initInstances()".',
          ),
          ErrorHint(
            'This could also happen if some code was invoked that used the binding while the binding was initializing, '
            'for example if the "initInstances" method invokes a callback. Bindings should not invoke callbacks before '
            '"initInstances" has completed.',
          ),
        ]);
      }
      if (!instance._debugConstructed) {
        // The state of _debugInitializedType doesn't matter in this failure mode.
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('Binding did not complete initialization.'),
          ErrorDescription('An instance of $T is non-null, but the BindingBase() constructor has not yet been called.'),
          ErrorHint(
            'This could also happen if some code was invoked that used the binding while the binding was initializing, '
            "for example if the binding's constructor itself invokes a callback. Bindings should not invoke callbacks "
            'before "initInstances" has completed.',
          ),
        ]);
      }
    } on NoSuchMethodError {
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary('Binding does not extend BindingBase'),
        ErrorDescription('An instance of $T was created but the BindingBase constructor was not called.'),
        ErrorHint(
          'This could happen because the binding was implemented using "implements" rather than "extends" or "with". '
          'Concrete binding classes must extend or mix in BindingBase.',
        ),
      ]);
    }
    return true;
  }());
  return instance!;
}