.NET-Community-Toolkit/CommunityToolkit.Mvvm/ComponentModel/ObservableObject.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

// This file is inspired from the MvvmLight library (lbugnion/MvvmLight),
// more info in ThirdPartyNotices.txt in the root of the project.

// ================================== NOTE ==================================
// This file is mirrored in the trimmed-down INotifyPropertyChanged file in
// the source generator project, to be used with the [INotifyPropertyChanged],
// attribute, along with the ObservableObject annotated copy (for debugging info).
// If any changes are made to this file, they should also be appropriately
// ported to that file as well to keep the behavior consistent.
// ==========================================================================

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
using CommunityToolkit.Mvvm.ComponentModel.__Internals;

#pragma warning disable CS0618

namespace CommunityToolkit.Mvvm.ComponentModel;

/// <summary>
/// A base class for objects of which the properties must be observable.
/// </summary>
public abstract class ObservableObject : INotifyPropertyChanged, INotifyPropertyChanging
{
    /// <inheritdoc cref="INotifyPropertyChanged.PropertyChanged"/>
    public event PropertyChangedEventHandler? PropertyChanged;

    /// <inheritdoc cref="INotifyPropertyChanging.PropertyChanging"/>
    public event PropertyChangingEventHandler? PropertyChanging;

    /// <summary>
    /// Raises the <see cref="PropertyChanged"/> event.
    /// </summary>
    /// <param name="e">The input <see cref="PropertyChangedEventArgs"/> instance.</param>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="e"/> is <see langword="null"/>.</exception>
    protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        ArgumentNullException.ThrowIfNull(e);

        PropertyChanged?.Invoke(this, e);
    }

    /// <summary>
    /// Raises the <see cref="PropertyChanging"/> event.
    /// </summary>
    /// <param name="e">The input <see cref="PropertyChangingEventArgs"/> instance.</param>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="e"/> is <see langword="null"/>.</exception>
    protected virtual void OnPropertyChanging(PropertyChangingEventArgs e)
    {
        ArgumentNullException.ThrowIfNull(e);

        PropertyChanging?.Invoke(this, e);
    }

    /// <summary>
    /// Raises the <see cref="PropertyChanged"/> event.
    /// </summary>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    protected void OnPropertyChanged([CallerMemberName] string? propertyName = null)
    {
        OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
    }

    /// <summary>
    /// Raises the <see cref="PropertyChanging"/> event.
    /// </summary>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    protected void OnPropertyChanging([CallerMemberName] string? propertyName = null)
    {
        if (Configuration.IsINotifyPropertyChangingDisabled)
        {
            return;
        }

        OnPropertyChanging(new PropertyChangingEventArgs(propertyName));
    }

    /// <summary>
    /// Compares the current and new values for a given property. If the value has changed,
    /// raises the <see cref="PropertyChanging"/> event, updates the property with the new
    /// value, then raises the <see cref="PropertyChanged"/> event.
    /// </summary>
    /// <typeparam name="T">The type of the property that changed.</typeparam>
    /// <param name="field">The field storing the property's value.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <remarks>
    /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
    /// if the current and new value for the target property are the same.
    /// </remarks>
    protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, [CallerMemberName] string? propertyName = null)
    {
        // We duplicate the code here instead of calling the overload because we can't
        // guarantee that the invoked SetProperty<T> will be inlined, and we need the JIT
        // to be able to see the full EqualityComparer<T>.Default.Equals call, so that
        // it'll use the intrinsics version of it and just replace the whole invocation
        // with a direct comparison when possible (eg. for primitive numeric types).
        // This is the fastest SetProperty<T> overload so we particularly care about
        // the codegen quality here, and the code is small and simple enough so that
        // duplicating it still doesn't make the whole class harder to maintain.
        if (EqualityComparer<T>.Default.Equals(field, newValue))
        {
            return false;
        }

        OnPropertyChanging(propertyName);

        field = newValue;

        OnPropertyChanged(propertyName);

        return true;
    }

    /// <summary>
    /// Compares the current and new values for a given property. If the value has changed,
    /// raises the <see cref="PropertyChanging"/> event, updates the property with the new
    /// value, then raises the <see cref="PropertyChanged"/> event.
    /// See additional notes about this overload in <see cref="SetProperty{T}(ref T,T,string)"/>.
    /// </summary>
    /// <typeparam name="T">The type of the property that changed.</typeparam>
    /// <param name="field">The field storing the property's value.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="comparer">The <see cref="IEqualityComparer{T}"/> instance to use to compare the input values.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="comparer"/> is <see langword="null"/>.</exception>
    protected bool SetProperty<T>([NotNullIfNotNull("newValue")] ref T field, T newValue, IEqualityComparer<T> comparer, [CallerMemberName] string? propertyName = null)
    {
        ArgumentNullException.ThrowIfNull(comparer);

        if (comparer.Equals(field, newValue))
        {
            return false;
        }

        OnPropertyChanging(propertyName);

        field = newValue;

        OnPropertyChanged(propertyName);

        return true;
    }

    /// <summary>
    /// Compares the current and new values for a given property. If the value has changed,
    /// raises the <see cref="PropertyChanging"/> event, updates the property with the new
    /// value, then raises the <see cref="PropertyChanged"/> event.
    /// This overload is much less efficient than <see cref="SetProperty{T}(ref T,T,string)"/> and it
    /// should only be used when the former is not viable (eg. when the target property being
    /// updated does not directly expose a backing field that can be passed by reference).
    /// For performance reasons, it is recommended to use a stateful callback if possible through
    /// the <see cref="SetProperty{TModel,T}(T,T,TModel,Action{TModel,T},string?)"/> whenever possible
    /// instead of this overload, as that will allow the C# compiler to cache the input callback and
    /// reduce the memory allocations. More info on that overload are available in the related XML
    /// docs. This overload is here for completeness and in cases where that is not applicable.
    /// </summary>
    /// <typeparam name="T">The type of the property that changed.</typeparam>
    /// <param name="oldValue">The current property value.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="callback">A callback to invoke to update the property value.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <remarks>
    /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
    /// if the current and new value for the target property are the same.
    /// </remarks>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="callback"/> is <see langword="null"/>.</exception>
    protected bool SetProperty<T>(T oldValue, T newValue, Action<T> callback, [CallerMemberName] string? propertyName = null)
    {
        ArgumentNullException.ThrowIfNull(callback);

        // We avoid calling the overload again to ensure the comparison is inlined
        if (EqualityComparer<T>.Default.Equals(oldValue, newValue))
        {
            return false;
        }

        OnPropertyChanging(propertyName);

        callback(newValue);

        OnPropertyChanged(propertyName);

        return true;
    }

    /// <summary>
    /// Compares the current and new values for a given property. If the value has changed,
    /// raises the <see cref="PropertyChanging"/> event, updates the property with the new
    /// value, then raises the <see cref="PropertyChanged"/> event.
    /// See additional notes about this overload in <see cref="SetProperty{T}(T,T,Action{T},string)"/>.
    /// </summary>
    /// <typeparam name="T">The type of the property that changed.</typeparam>
    /// <param name="oldValue">The current property value.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="comparer">The <see cref="IEqualityComparer{T}"/> instance to use to compare the input values.</param>
    /// <param name="callback">A callback to invoke to update the property value.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="comparer"/> or <paramref name="callback"/> are <see langword="null"/>.</exception>
    protected bool SetProperty<T>(T oldValue, T newValue, IEqualityComparer<T> comparer, Action<T> callback, [CallerMemberName] string? propertyName = null)
    {
        ArgumentNullException.ThrowIfNull(comparer);
        ArgumentNullException.ThrowIfNull(callback);

        if (comparer.Equals(oldValue, newValue))
        {
            return false;
        }

        OnPropertyChanging(propertyName);

        callback(newValue);

        OnPropertyChanged(propertyName);

        return true;
    }

    /// <summary>
    /// Compares the current and new values for a given nested property. If the value has changed,
    /// raises the <see cref="PropertyChanging"/> event, updates the property and then raises the
    /// <see cref="PropertyChanged"/> event. The behavior mirrors that of <see cref="SetProperty{T}(ref T,T,string)"/>,
    /// with the difference being that this method is used to relay properties from a wrapped model in the
    /// current instance. This type is useful when creating wrapping, bindable objects that operate over
    /// models that lack support for notification (eg. for CRUD operations).
    /// Suppose we have this model (eg. for a database row in a table):
    /// <code>
    /// public class Person
    /// {
    ///     public string Name { get; set; }
    /// }
    /// </code>
    /// We can then use a property to wrap instances of this type into our observable model (which supports
    /// notifications), injecting the notification to the properties of that model, like so:
    /// <code>
    /// public class BindablePerson : ObservableObject
    /// {
    ///     public Model { get; }
    ///
    ///     public BindablePerson(Person model)
    ///     {
    ///         Model = model;
    ///     }
    ///
    ///     public string Name
    ///     {
    ///         get => Model.Name;
    ///         set => Set(Model.Name, value, Model, (model, name) => model.Name = name);
    ///     }
    /// }
    /// </code>
    /// This way we can then use the wrapping object in our application, and all those "proxy" properties will
    /// also raise notifications when changed. Note that this method is not meant to be a replacement for
    /// <see cref="SetProperty{T}(ref T,T,string)"/>, and it should only be used when relaying properties to a model that
    /// doesn't support notifications, and only if you can't implement notifications to that model directly (eg. by having
    /// it inherit from <see cref="ObservableObject"/>). The syntax relies on passing the target model and a stateless callback
    /// to allow the C# compiler to cache the function, which results in much better performance and no memory usage.
    /// </summary>
    /// <typeparam name="TModel">The type of model whose property (or field) to set.</typeparam>
    /// <typeparam name="T">The type of property (or field) to set.</typeparam>
    /// <param name="oldValue">The current property value.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="model">The model containing the property being updated.</param>
    /// <param name="callback">The callback to invoke to set the target property value, if a change has occurred.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <remarks>
    /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not
    /// raised if the current and new value for the target property are the same.
    /// </remarks>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="model"/> or <paramref name="callback"/> are <see langword="null"/>.</exception>
    protected bool SetProperty<TModel, T>(T oldValue, T newValue, TModel model, Action<TModel, T> callback, [CallerMemberName] string? propertyName = null)
        where TModel : class
    {
        ArgumentNullException.ThrowIfNull(model);
        ArgumentNullException.ThrowIfNull(callback);

        if (EqualityComparer<T>.Default.Equals(oldValue, newValue))
        {
            return false;
        }

        OnPropertyChanging(propertyName);

        callback(model, newValue);

        OnPropertyChanged(propertyName);

        return true;
    }

    /// <summary>
    /// Compares the current and new values for a given nested property. If the value has changed,
    /// raises the <see cref="PropertyChanging"/> event, updates the property and then raises the
    /// <see cref="PropertyChanged"/> event. The behavior mirrors that of <see cref="SetProperty{T}(ref T,T,string)"/>,
    /// with the difference being that this method is used to relay properties from a wrapped model in the
    /// current instance. See additional notes about this overload in <see cref="SetProperty{TModel,T}(T,T,TModel,Action{TModel,T},string)"/>.
    /// </summary>
    /// <typeparam name="TModel">The type of model whose property (or field) to set.</typeparam>
    /// <typeparam name="T">The type of property (or field) to set.</typeparam>
    /// <param name="oldValue">The current property value.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="comparer">The <see cref="IEqualityComparer{T}"/> instance to use to compare the input values.</param>
    /// <param name="model">The model containing the property being updated.</param>
    /// <param name="callback">The callback to invoke to set the target property value, if a change has occurred.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="comparer"/>, <paramref name="model"/> or <paramref name="callback"/> are <see langword="null"/>.</exception>
    protected bool SetProperty<TModel, T>(T oldValue, T newValue, IEqualityComparer<T> comparer, TModel model, Action<TModel, T> callback, [CallerMemberName] string? propertyName = null)
        where TModel : class
    {
        ArgumentNullException.ThrowIfNull(comparer);
        ArgumentNullException.ThrowIfNull(model);
        ArgumentNullException.ThrowIfNull(callback);

        if (comparer.Equals(oldValue, newValue))
        {
            return false;
        }

        OnPropertyChanging(propertyName);

        callback(model, newValue);

        OnPropertyChanged(propertyName);

        return true;
    }

    /// <summary>
    /// Compares the current and new values for a given field (which should be the backing
    /// field for a property). If the value has changed, raises the <see cref="PropertyChanging"/>
    /// event, updates the field and then raises the <see cref="PropertyChanged"/> event.
    /// The behavior mirrors that of <see cref="SetProperty{T}(ref T,T,string)"/>, with the difference being that
    /// this method will also monitor the new value of the property (a generic <see cref="Task"/>) and will also
    /// raise the <see cref="PropertyChanged"/> again for the target property when it completes.
    /// This can be used to update bindings observing that <see cref="Task"/> or any of its properties.
    /// This method and its overload specifically rely on the <see cref="TaskNotifier"/> type, which needs
    /// to be used in the backing field for the target <see cref="Task"/> property. The field doesn't need to be
    /// initialized, as this method will take care of doing that automatically. The <see cref="TaskNotifier"/>
    /// type also includes an implicit operator, so it can be assigned to any <see cref="Task"/> instance directly.
    /// Here is a sample property declaration using this method:
    /// <code>
    /// private TaskNotifier myTask;
    ///
    /// public Task MyTask
    /// {
    ///     get => myTask;
    ///     private set => SetAndNotifyOnCompletion(ref myTask, value);
    /// }
    /// </code>
    /// </summary>
    /// <param name="taskNotifier">The field notifier to modify.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <remarks>
    /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised if the current
    /// and new value for the target property are the same. The return value being <see langword="true"/> only
    /// indicates that the new value being assigned to <paramref name="taskNotifier"/> is different than the previous one,
    /// and it does not mean the new <see cref="Task"/> instance passed as argument is in any particular state.
    /// </remarks>
    protected bool SetPropertyAndNotifyOnCompletion([NotNull] ref TaskNotifier? taskNotifier, Task? newValue, [CallerMemberName] string? propertyName = null)
    {
        // We invoke the overload with a callback here to avoid code duplication, and simply pass an empty callback.
        // The lambda expression here is transformed by the C# compiler into an empty closure class with a
        // static singleton field containing a closure instance, and another caching the instantiated Action<TTask>
        // instance. This will result in no further allocations after the first time this method is called for a given
        // generic type. We only pay the cost of the virtual call to the delegate, but this is not performance critical
        // code and that overhead would still be much lower than the rest of the method anyway, so that's fine.
        return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier(), newValue, null, propertyName);
    }

    /// <summary>
    /// Compares the current and new values for a given field (which should be the backing
    /// field for a property). If the value has changed, raises the <see cref="PropertyChanging"/>
    /// event, updates the field and then raises the <see cref="PropertyChanged"/> event.
    /// This method is just like <see cref="SetPropertyAndNotifyOnCompletion(ref TaskNotifier,Task,string)"/>,
    /// with the difference being an extra <see cref="Action{T}"/> parameter with a callback being invoked
    /// either immediately, if the new task has already completed or is <see langword="null"/>, or upon completion.
    /// </summary>
    /// <param name="taskNotifier">The field notifier to modify.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="callback">A callback to invoke to update the property value.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <remarks>
    /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
    /// if the current and new value for the target property are the same.
    /// </remarks>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="callback"/> is <see langword="null"/>.</exception>
    protected bool SetPropertyAndNotifyOnCompletion([NotNull] ref TaskNotifier? taskNotifier, Task? newValue, Action<Task?> callback, [CallerMemberName] string? propertyName = null)
    {
        ArgumentNullException.ThrowIfNull(callback);

        return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier(), newValue, callback, propertyName);
    }

    /// <summary>
    /// Compares the current and new values for a given field (which should be the backing
    /// field for a property). If the value has changed, raises the <see cref="PropertyChanging"/>
    /// event, updates the field and then raises the <see cref="PropertyChanged"/> event.
    /// The behavior mirrors that of <see cref="SetProperty{T}(ref T,T,string)"/>, with the difference being that
    /// this method will also monitor the new value of the property (a generic <see cref="Task"/>) and will also
    /// raise the <see cref="PropertyChanged"/> again for the target property when it completes.
    /// This can be used to update bindings observing that <see cref="Task"/> or any of its properties.
    /// This method and its overload specifically rely on the <see cref="TaskNotifier{T}"/> type, which needs
    /// to be used in the backing field for the target <see cref="Task"/> property. The field doesn't need to be
    /// initialized, as this method will take care of doing that automatically. The <see cref="TaskNotifier{T}"/>
    /// type also includes an implicit operator, so it can be assigned to any <see cref="Task"/> instance directly.
    /// Here is a sample property declaration using this method:
    /// <code>
    /// private TaskNotifier&lt;int&gt; myTask;
    ///
    /// public Task&lt;int&gt; MyTask
    /// {
    ///     get => myTask;
    ///     private set => SetAndNotifyOnCompletion(ref myTask, value);
    /// }
    /// </code>
    /// </summary>
    /// <typeparam name="T">The type of result for the <see cref="Task{TResult}"/> to set and monitor.</typeparam>
    /// <param name="taskNotifier">The field notifier to modify.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <remarks>
    /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised if the current
    /// and new value for the target property are the same. The return value being <see langword="true"/> only
    /// indicates that the new value being assigned to <paramref name="taskNotifier"/> is different than the previous one,
    /// and it does not mean the new <see cref="Task{TResult}"/> instance passed as argument is in any particular state.
    /// </remarks>
    protected bool SetPropertyAndNotifyOnCompletion<T>([NotNull] ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, [CallerMemberName] string? propertyName = null)
    {
        return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier<T>(), newValue, null, propertyName);
    }

    /// <summary>
    /// Compares the current and new values for a given field (which should be the backing
    /// field for a property). If the value has changed, raises the <see cref="PropertyChanging"/>
    /// event, updates the field and then raises the <see cref="PropertyChanged"/> event.
    /// This method is just like <see cref="SetPropertyAndNotifyOnCompletion{T}(ref TaskNotifier{T},Task{T},string)"/>,
    /// with the difference being an extra <see cref="Action{T}"/> parameter with a callback being invoked
    /// either immediately, if the new task has already completed or is <see langword="null"/>, or upon completion.
    /// </summary>
    /// <typeparam name="T">The type of result for the <see cref="Task{TResult}"/> to set and monitor.</typeparam>
    /// <param name="taskNotifier">The field notifier to modify.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="callback">A callback to invoke to update the property value.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    /// <remarks>
    /// The <see cref="PropertyChanging"/> and <see cref="PropertyChanged"/> events are not raised
    /// if the current and new value for the target property are the same.
    /// </remarks>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="callback"/> is <see langword="null"/>.</exception>
    protected bool SetPropertyAndNotifyOnCompletion<T>([NotNull] ref TaskNotifier<T>? taskNotifier, Task<T>? newValue, Action<Task<T>?> callback, [CallerMemberName] string? propertyName = null)
    {
        ArgumentNullException.ThrowIfNull(callback);

        return SetPropertyAndNotifyOnCompletion(taskNotifier ??= new TaskNotifier<T>(), newValue, callback, propertyName);
    }

    /// <summary>
    /// Implements the notification logic for the related methods.
    /// </summary>
    /// <typeparam name="TTask">The type of <see cref="Task"/> to set and monitor.</typeparam>
    /// <param name="taskNotifier">The field notifier.</param>
    /// <param name="newValue">The property's value after the change occurred.</param>
    /// <param name="callback">(optional) A callback to invoke to update the property value.</param>
    /// <param name="propertyName">(optional) The name of the property that changed.</param>
    /// <returns><see langword="true"/> if the property was changed, <see langword="false"/> otherwise.</returns>
    private bool SetPropertyAndNotifyOnCompletion<TTask>(ITaskNotifier<TTask> taskNotifier, TTask? newValue, Action<TTask?>? callback, [CallerMemberName] string? propertyName = null)
        where TTask : Task
    {
        if (ReferenceEquals(taskNotifier.Task, newValue))
        {
            return false;
        }

        // Check the status of the new task before assigning it to the
        // target field. This is so that in case the task is either
        // null or already completed, we can avoid the overhead of
        // scheduling the method to monitor its completion.
        bool isAlreadyCompletedOrNull = newValue?.IsCompleted ?? true;

        OnPropertyChanging(propertyName);

        taskNotifier.Task = newValue;

        OnPropertyChanged(propertyName);

        // If the input task is either null or already completed, we don't need to
        // execute the additional logic to monitor its completion, so we can just bypass
        // the rest of the method and return that the field changed here. The return value
        // does not indicate that the task itself has completed, but just that the property
        // value itself has changed (ie. the referenced task instance has changed).
        // This mirrors the return value of all the other synchronous Set methods as well.
        if (isAlreadyCompletedOrNull)
        {
            if (callback is not null)
            {
                callback(newValue);
            }

            return true;
        }

        // We use a local async function here so that the main method can
        // remain synchronous and return a value that can be immediately
        // used by the caller. This mirrors Set<T>(ref T, T, string).
        // We use an async void function instead of a Task-returning function
        // so that if a binding update caused by the property change notification
        // causes a crash, it is immediately reported in the application instead of
        // the exception being ignored (as the returned task wouldn't be awaited),
        // which would result in a confusing behavior for users.
        async void MonitorTask()
        {
            // Await the task and ignore any exceptions
            await newValue!.GetAwaitableWithoutEndValidation();

            // Only notify if the property hasn't changed
            if (ReferenceEquals(taskNotifier.Task, newValue))
            {
                OnPropertyChanged(propertyName);
            }

            if (callback is not null)
            {
                callback(newValue);
            }
        }

        MonitorTask();

        return true;
    }

    /// <summary>
    /// An interface for task notifiers of a specified type.
    /// </summary>
    /// <typeparam name="TTask">The type of value to store.</typeparam>
    private interface ITaskNotifier<TTask>
        where TTask : Task
    {
        /// <summary>
        /// Gets or sets the wrapped <typeparamref name="TTask"/> value.
        /// </summary>
        TTask? Task { get; set; }
    }

    /// <summary>
    /// A wrapping class that can hold a <see cref="Task"/> value.
    /// </summary>
    protected sealed class TaskNotifier : ITaskNotifier<Task>
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="TaskNotifier"/> class.
        /// </summary>
        internal TaskNotifier()
        {
        }

        private Task? task;

        /// <inheritdoc/>
        Task? ITaskNotifier<Task>.Task
        {
            get => this.task;
            set => this.task = value;
        }

        /// <summary>
        /// Unwraps the <see cref="Task"/> value stored in the current instance.
        /// </summary>
        /// <param name="notifier">The input <see cref="TaskNotifier{TTask}"/> instance.</param>
        public static implicit operator Task?(TaskNotifier? notifier)
        {
            return notifier?.task;
        }
    }

    /// <summary>
    /// A wrapping class that can hold a <see cref="Task{T}"/> value.
    /// </summary>
    /// <typeparam name="T">The type of value for the wrapped <see cref="Task{T}"/> instance.</typeparam>
    protected sealed class TaskNotifier<T> : ITaskNotifier<Task<T>>
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="TaskNotifier{TTask}"/> class.
        /// </summary>
        internal TaskNotifier()
        {
        }

        private Task<T>? task;

        /// <inheritdoc/>
        Task<T>? ITaskNotifier<Task<T>>.Task
        {
            get => this.task;
            set => this.task = value;
        }

        /// <summary>
        /// Unwraps the <see cref="Task{T}"/> value stored in the current instance.
        /// </summary>
        /// <param name="notifier">The input <see cref="TaskNotifier{TTask}"/> instance.</param>
        public static implicit operator Task<T>?(TaskNotifier<T>? notifier)
        {
            return notifier?.task;
        }
    }
}