It's possible to write code that wins every performance test in the world, but still sends users in a fiery rage when they try to use the application. These are the applications that aren't responsiVeenough — the ones that feel sluggish, hang or freeze for significant periods, or take too long to process input.
In Android, the system guards against applications that are insufficiently responsive for a period of time by displaying a dialog to the user, called the Application Not Responding (ANR) dialog. The user can choose to let the application continue, but the user won't appreciate having to act on this dialog every time he or she uses your application. So it's important to design responsiveness into your application, so that the system never has cause to display an ANR to the user.
Generally, the system displays an ANR if an application cannot respond to user input. For example, if an application blocks on some I/O operation (frequently a network access), then the main application thread won't be able to process incoming user input events. After a time, the system concludes that the application has hung, and displays the ANR to give the user the option to kill it.
Similarly, if your application spends too much time building an elaborate in-memory structure, or perhaps computing the next move in a game, the system will conclude that your application has hung. It's always important to make sure these computations are efficient using the techniques above, but even the most efficient code still takes time to run.
In both of these cases, the fix is usually to create a child thread, and do most of your work there. This keeps the main thread (which drives the user interface event loop) running, and prevents the system from concluding your code has frozen. Since such threading usually is accomplished at the class level, you can think of responsiveness as a class problem. (Compare this with basic performance, which was described above as a method-level concern.)
Activity Hello, Android (in application Hello, Android) is not responding.
This document discusses how the Android system determines whether an application is not responding and provides guidelines for ensuring that your application is responsive.
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