SharedPreferences apply vs commit

Was debugging why some preferences weren't being saved and realized I was mixing apply() and commit() without understanding the difference.

commit() is synchronous - it writes to disk immediately and returns a boolean indicating success. apply() is asynchronous - it writes to memory immediately, then schedules a disk write, and returns void.

// Synchronous - blocks until written
val success = prefs.edit().putString("key", "value").commit()

// Asynchronous - returns immediately
prefs.edit().putString("key", "value").apply()

The problem I had was that I was calling apply() and then immediately reading the value, expecting it to be there. But apply() doesn't guarantee the disk write has completed, so if the app crashes or the process is killed, the value might not be persisted.

For most cases, apply() is fine and recommended because it doesn't block the UI thread. But if you need to ensure the write completes (like before starting an activity that depends on that value), use commit().

One thing to watch out for - if you're calling apply() multiple times in quick succession, Android batches them together, which is efficient. But if you need each write to be separate, you might need commit().

Also, apply() can fail silently. If you need to know if the write succeeded, use commit() and check the return value.

Thanks for reading! Check out more posts on the blog if you'd like.