Ever wondered how a file is attached to an e-mail in web-based mail applications such as Yahoo Mail, Gmail etc.? If you have been using Yahoo Mail for some years now, you might remember that not very long ago, when we used to add a file attachment to an e-mail, we had to keep waiting till the file was uploaded to the server. With the recent UI changes, when we attach a file, it gets added parallely while we perform other tasks on the UI. What has changed? The Active Object Pattern design pattern is working under the hood. If not implemented explicitly then the UI framework must be using it implicitly.