Progress.cs

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using System.Threading;
 
namespace System;
 
public class Progress<T> : IProgress<T>
{
    private readonly SynchronizationContext _synchronizationContext;
 
    private readonly Action<T> _handler;
 
    private readonly SendOrPostCallback _invokeHandlers;
 
    public event EventHandler<T>? ProgressChanged;
 
    public Progress()
    {
        _synchronizationContext = SynchronizationContext.Current ?? ProgressStatics.DefaultContext;
        _invokeHandlers = InvokeHandlers;
    }
 
    public Progress(Action<T> handler)
        : this()
    {
        _handler = handler ?? throw new ArgumentNullException("handler");
    }
 
    protected virtual void OnReport(T value)
    {
        Action<T> handler = _handler;
        EventHandler<T> progressChanged = this.ProgressChanged;
        if (handler != null || progressChanged != null)
        {
            _synchronizationContext.Post(_invokeHandlers, value);
        }
    }
 
    void IProgress<T>.Report(T value)
    {
        OnReport(value);
    }
 
    private void InvokeHandlers(object state)
    {
        T val = (T)state;
        Action<T> handler = _handler;
        EventHandler<T> progressChanged = this.ProgressChanged;
        handler?.Invoke(val);
        progressChanged?.Invoke(this, val);
    }
}