Android Studio Apply Changes asks if I want to Restart App

Question

"Apply Changes" button was working with another project, but now when I click it I get a popup that says:

"Restart App? The app is already running. Would you like to kill it and restart the session?"

What causes it to want to restart instead of just applying changes?

Answer 1

Here you can find a detailed description of instant run:

https://developer.android.com/studio/run/index.html#instant-run

Answer 2

Normally all changes has to be compiled and republished to the device as new app. When your App is already running on the device this message is displayed. Android Studio has an Instant Run feature, when this is enabled the changes get applied without restart, but this didn't work every time.