Android retain callback state after configuration change

Question

I understand pretty well about Android lifecycle. I post here because I've observed one weird behavior, anyway this is my own thought.

My case is like this: One activity will use a simple layout with just a single EditText. In the activity onCreate method, i set some default text to the EditText and in later part of the method, assign a TextWatcher to the EditText so whenever user types in anything, i can response in my own way.

Everything is alright until I rotate the screen. The TextWatcher callback starts to react against the code that initialize the EditText.

According to the normal code flow, the TextWatcher is assigned later after initializing text value of the EditText. so it's not supposed to fire because of the text assignment in the onCreate method.

Could anyone here explain this?

Answer 1

Here is why you see this behavior:

1. When 'onCreate' method is called first time, it has no saved state. Which means Bundle savedInstanceState parameter is null.
2. When configuration changed, Android paths non-null value in savedInstanceState parameter.
3. After configuration is changed, and onCreate returns, Android calls onRestoreInstateState.
4. By default, all views that have id are trying to restore their state, EditText restores its state too (actually, that TextView who restores most of it).
5. At some place during state restoration (but after onCreate method is completed) your EditText control calls setText on himself in order to restore text that it had just before configuration changed.
6. Your new TextWatcher that you added in onCreate method is notified about this change.

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Just to make this clear, your old TextWatcher, that you added on first call to onCreate, is not preserved! Its new TextWatcher, that was added on last call to onCreate, which receives the text change notification.

You can examine TextView.onRestoreInstanceState yourself.