Learn Spring for Android Application Development

What is a callback?

A callback is a pattern that is used to retrieve the results of an asynchronous task. This approach assumes that we pass a reference to a function that should be invoked when an asynchronous operation is done. 

By synchronous operations, we mean that the tasks are executed one after another. The asynchronous approach assumes that several tasks can be performed in parallel. 

The loadImage function in the following example code uses a callback to return the result:

fun loadImage(callback: (Image) -> Unit) {    executor.submit {        Thread.sleep(3000)        callback(Image())    }}

The preceding code snippet shows the simplest example of how to create an asynchronous function that returns the results using the callback. In our case, the ...