Creating and Calling Hub methods from Clients

We will see how to call hub methods from clients using SignalR. We will cover the differences between Invoke and Send methods.

Invoke

The Invoke method is used when you need to call a hub method that returns a value. This is useful when you expect a response from the server, such as fetching data or performing a calculation.

• Synchronous-like: It sends a message to the hub and waits for a response
• Promise-based: It opens a promise that resolves when the server acknowledges the method execution and returns a value
• Error Handling: If the server method throws an exception, the promise is rejected, and you can handle the error in a catch block

Example

Let’s say we have GetFullName hub method that concatenates two strings

public string GetFullName(string firstName, string lastName)
{
    return $"{firstName} {lastName}";
}

From the client we can call the above method using Invoke

try
{
    Console.WriteLine("Enter first name:");
    var firstName = Console.ReadLine();
    Console.WriteLine("Enter last name:");
    var lastName = Console.ReadLine();
    var fullName = await connection.InvokeAsync<string>("GetFullName", firstName, lastName);
    Console.WriteLine($"Full name from server: {fullName}");
}
catch (Exception ex)
{
    Console.WriteLine($"Error calling GetFullName: {ex.Message}");
}

The client waits for the response and then prints the full name

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Send

Usually used when you need to call a hub method that does not return a value. This is useful for notifying the server of an event or performing an action that does not require a response.

• Fire-and-forget: Send sends a message to the hub but does not wait for a response.
• Promise-based: It returns a promise that resolves when the request is sent successfully, but it does not wait for the server to acknowledge the method execution.
• No Return Value: Since it does not wait for a response, testing for a return value will yield no result or undefined.

Example

A hub method NotifyWatching that increments a view count and notifies all connected clients

public async Task NotifyWatching()
{
    ViewCount++;
    await Clients.All.SendAsync("UpdateViewCount", ViewCount);
}

call this method from the client without expecting a return value, you use Send

try
{
    await connection.SendAsync("NotifyWatching");
}
catch (Exception ex)
{
    Console.WriteLine($"Error calling NotifyWatching: {ex.Message}");
}

By understanding these differences, you can choose the appropriate method based on whether you need a response from the server or not. This ensures efficient communication between your client and server using SignalR.

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