Automatic reconnection and the IRetryPolicy interface

With ASP.net Core 3 SignalR Core supports automatic reconnection through the IRetryPolicy interface:

public interface IRetryPolicy
{
    /// <summary>
    /// This function must return with a delay time to wait until a new connection attempt, or null to do not do another one.
    /// </summary>
    TimeSpan? GetNextRetryDelay(RetryContext context);
}

GetNextRetryDelay receives a RetryContext object as a parameter and must return a TimeSpan indicating a delay between the next reconnect attempt, or null to do not try to do further attempts and report the connection broken.

RetryContext

RetryContext has the following fields:

• PreviousRetryCount: Previous reconnect attempts. A successful connection sets it back to zero.
• ElapsedTime: Elapsed time since the original connection error.
• RetryReason: String representation of the connection error.

When called, GetNextRetryDelay always receives an up to date RetryContext.

Default implementation

The default IRetryPolicy implementation that the plugin uses by default looks like this:

public sealed class DefaultRetryPolicy : IRetryPolicy
{
    private static TimeSpan?[] DefaultBackoffTimes = new TimeSpan?[]
    {
        TimeSpan.Zero,
        TimeSpan.FromSeconds(2),
        TimeSpan.FromSeconds(10),
        TimeSpan.FromSeconds(30),
        null
    };

    TimeSpan?[] backoffTimes;

    public DefaultRetryPolicy()
    {
        this.backoffTimes = DefaultBackoffTimes;
    }

    public DefaultRetryPolicy(TimeSpan?[] customBackoffTimes)
    {
        this.backoffTimes = customBackoffTimes;
    }

    public TimeSpan? GetNextRetryDelay(RetryContext context)
    {
        if (context.PreviousRetryCount >= this.backoffTimes.Length)
            return null;

        return this.backoffTimes[context.PreviousRetryCount];
    }
}

To use it an instance must be set to the HubConnection's ReconnectPolicy property:

hub = new HubConnection(new Uri("..."), new JsonProtocol(new LitJsonEncoder()), options);
hub.ReconnectPolicy = new DefaultRetryPolicy();

Or with custom backoff times:

hub = new HubConnection(new Uri(base.sampleSelector.BaseURL + this._path), new JsonProtocol(new LitJsonEncoder()), options);
hub.ReconnectPolicy = new DefaultRetryPolicy(new TimeSpan?[] {
    TimeSpan.FromSeconds(5),
    TimeSpan.FromSeconds(15),
    TimeSpan.FromSeconds(45),
    TimeSpan.FromSeconds(90),
    null
});