How asynchronous programming works

User Interface

• User Interface is something an end-user uses to interact with the system.
• Using this mechanism the end-user interacts with the mobile and communicates with it to perform certain tasks.
• Once the tasks get completed, we show the result to the end-user.
• If the task that is being performed is taken a lot of time, we need to show feedback to the end-user that a task is in progress, otherwise the user thinks that the application is not responding, or worse it doesn't work at all.

Uploading Data to server

• Once good example is that, say we are uploading a photo to the server and this photo is very large so naturally it takes time to upload the photo.
• Since it is taking a lot of time we need to show some feedback to users that an uploading task is in progress.
• We can do this by showing a loader to the end-user and once the uploading is done, we close the loader.

fun uploadImageToServer(image: Image) {
  showLoader() // --- > This triggers spinning of loader and starts displaying loader
  service.upload(image) // --- > Here the data is sent as bytes to server 
  hideLoader() // --- > This triggers spinning of loader and stops displaying loader
}

• Here in this simple function uploadImageToServer lot of things happen
• showLoader() calls the loader to display, Now let's assume we are calling the android loader, this is not just a call it's an android functionality built that shows an animation. But so it's doing some operation.
• If service.upload need to wait until showLoader() gets completed the service.upload becomes a blocking call.
• Similarly hideLoader() functionality to stop the above loader
• So here the concept of multi-threading comes to the picture, If we have many threads that perform tasks parallel, We can do more things at once.
• In the above example, We show the loader in UI-thread and the service call, we perform in a background thread doing so we need not need to wait for individually things to complete to continue to next instead, We can do things parallel.

How main thread is different than worker threads

• We use the main thread to manage the UI
• Each application has only one thread called main thread to avoid the deadlock.
• We use the many threads to achieve what we called concurrency with the help of multi-threading.
• Understanding how threads communicate is the key to knowing concurrency.
• All threads should be able to communicate between them seamlessly without causing problems.