Async task is blocking the UI

Question

Hello esteemed colleagues, I need to save to a text file every time I get a new message from my serial port into my RichTextBox . I'm trying to use async and await it won't block the UI while saving the text file, but unfortunately I'm doing something wrong because it still blocking. Please, what am I doing wrong?

TextChanged event handler from RichTextBox :

private async void rtb_msg_TextChanged(object sender, EventArgs e)
{
    btn_save.Enabled = true;
    rtb_msg.ScrollToCaret();
    rcvFlag = true;
    await SaveFile();
}

Async SaveFile method:

private async Task SaveFile()
{
    if (_serialPort.BytesToRead == 0 && rcvFlag == true)
    {
        for(int i = 0; i < 10; i++)
        {
            Thread.Sleep(1000);
        }
        using (StreamWriter writer = new StreamWriter(reportFolder + reportFile))
        {
            await writer.WriteAsync(rtb_msg.Text);
        }
        rcvFlag = false;
    }
}

Answer 1

This part will definitely block your UI thread:

for(int i = 0; i < 10; i++)
{
    Thread.Sleep(1000);
}

Not sure why it's there, but if you replace it with an async version, it should work:

for(int i = 0; i < 10; i++)
{
    await Task.Delay(1000);
}

Answer 2

Never use Thread.Sleep in async methods. Use Task.Delay and Task.Run instead

private Task SaveFileAsync()
{
    return Task.Run(async () =>
    {
        if (_serialPort.BytesToRead == 0 && rcvFlag == true)
        {
            for (var i = 0; i < 10; i++)
            {
                await Task.Delay(1000);
            }
            using (var writer = new StreamWriter(reportFolder + reportFile))
            {
                await writer.WriteAsync(rtb_msg.Text);
            }
            rcvFlag = false;
        }
    });
}