Ok, forse sono stato un po 'troppo duro con la documentazione di Android, perché ha alcune informazioni utili, ma purtroppo nessuna di esse è collegata setRetainInstance(). Dalla pagina sui frammenti
Nota: ogni frammento richiede un identificatore univoco che il sistema può utilizzare per ripristinare il frammento se l'attività viene riavviata (e che è possibile utilizzare per acquisire il frammento per eseguire transazioni, ad esempio rimuoverlo). Esistono tre modi per fornire un ID per un frammento:
- Fornisci l'attributo android: id con un ID univoco.
- Fornisci l'attributo android: tag con una stringa univoca.
- Se non si fornisce nessuno dei due precedenti, il sistema utilizza l'ID della visualizzazione contenitore.
Ciò implica fortemente che se lo fai setContentView(R.layout.whatever)in Activity.onCreated()e quel layout contiene un frammento consetRetainInstance(true) , poi, quando l'attività viene ricreata sarà cercata di nuovo usando il suo id o tag.
In secondo luogo, per i frammenti senza interfaccia utente, afferma
Per aggiungere un frammento senza un'interfaccia utente, aggiungi il frammento dall'attività utilizzando add (Fragment, String) (fornendo un "tag" di stringa univoco per il frammento, anziché un ID di visualizzazione). Questo aggiunge il frammento ma, poiché non è associato a una vista nel layout dell'attività, non riceve una chiamata a onCreateView (). Quindi non è necessario implementare quel metodo.
E i documenti rimandano a un ottimo esempio, FragmentRetainInstance.javache ho riprodotto di seguito per comodità. Fa esattamente quello che ho ipotizzato fosse la risposta alla mia domanda (if (...findFragmentByTag() == null) { ... ).
Infine, ho creato la mia attività di test per vedere esattamente quali funzioni vengono chiamate. Emette questo quando inizi in verticale e ruoti in orizzontale. Il codice è sotto.
(Questo è modificato un po 'per renderlo più facile da leggere.)
TestActivity@415a4a30: this()
TestActivity@415a4a30: onCreate()
TestActivity@415a4a30: Existing fragment not found.
TestFragment{41583008}: this() TestFragment{41583008}
TestFragment{41583008}: onAttach(TestActivity@415a4a30)
TestFragment{41583008}: onCreate()
TestFragment{41583008}: onCreateView()
TestFragment{41583008}: onActivityCreated()
TestActivity@415a4a30: onStart()
TestFragment{41583008}: onStart()
TestActivity@415a4a30: onResume()
TestFragment{41583008}: onResume()
<rotate device>
TestFragment{41583008}: onPause()
TestActivity@415a4a30: onPause()
TestFragment{41583008}: onStop()
TestActivity@415a4a30: onStop()
TestFragment{41583008}: onDestroyView()
TestFragment{41583008}: onDetach()
TestActivity@415a4a30: onDestroy()
TestActivity@415a3380: this()
TestFragment{41583008}: onAttach(TestActivity@415a3380)
TestActivity@415a3380: onCreate()
TestActivity@415a3380: Existing fragment found.
TestFragment{41583008}: onCreateView()
TestFragment{41583008}: onActivityCreated()
TestActivity@415a3380: onStart()
TestFragment{41583008}: onStart()
TestActivity@415a3380: onResume()
TestFragment{41583008}: onResume()
Si noti che la documentazione Android è sbagliato: il frammento UI-less non riceve una chiamata a onCreateView(), ma è libero di tornare null.
Codice sorgente per TestActivity/TestFragment
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.concentriclivers.ss.R;
// An activity for understanding Android lifecycle events.
public class TestActivity extends Activity
{
private static final String TAG = TestActivity.class.getSimpleName();
public TestActivity()
{
super();
Log.d(TAG, this + ": this()");
}
protected void finalize() throws Throwable
{
super.finalize();
Log.d(TAG, this + ": finalize()");
}
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
Log.d(TAG, this + ": onCreate()");
TextView tv = new TextView(this);
tv.setText("Hello world");
setContentView(tv);
if (getFragmentManager().findFragmentByTag("test_fragment") == null)
{
Log.d(TAG, this + ": Existing fragment not found.");
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.add(new TestFragment(), "test_fragment").commit();
}
else
{
Log.d(TAG, this + ": Existing fragment found.");
}
}
@Override
public void onStart()
{
super.onStart();
Log.d(TAG, this + ": onStart()");
}
@Override
public void onResume()
{
super.onResume();
Log.d(TAG, this + ": onResume()");
}
@Override
public void onPause()
{
super.onPause();
Log.d(TAG, this + ": onPause()");
}
@Override
public void onStop()
{
super.onStop();
Log.d(TAG, this + ": onStop()");
}
@Override
public void onDestroy()
{
super.onDestroy();
Log.d(TAG, this + ": onDestroy()");
}
public static class TestFragment extends Fragment
{
private static final String TAG = TestFragment.class.getSimpleName();
public TestFragment()
{
super();
Log.d(TAG, this + ": this() " + this);
}
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
Log.d(TAG, this + ": onCreate()");
setRetainInstance(true);
}
@Override
public void onAttach(final Activity activity)
{
super.onAttach(activity);
Log.d(TAG, this + ": onAttach(" + activity + ")");
}
@Override
public void onActivityCreated(Bundle savedInstanceState)
{
super.onActivityCreated(savedInstanceState);
Log.d(TAG, this + ": onActivityCreated()");
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
Log.d(TAG, this + ": onCreateView()");
return null;
}
@Override
public void onViewCreated(View view, Bundle savedInstanceState)
{
super.onViewCreated(view, savedInstanceState);
Log.d(TAG, this + ": onViewCreated()");
}
@Override
public void onDestroyView()
{
super.onDestroyView();
Log.d(TAG, this + ": onDestroyView()");
}
@Override
public void onDetach()
{
super.onDetach();
Log.d(TAG, this + ": onDetach()");
}
@Override
public void onStart()
{
super.onStart();
Log.d(TAG, this + ": onStart()");
}
@Override
public void onResume()
{
super.onResume();
Log.d(TAG, this + ": onResume()");
}
@Override
public void onPause()
{
super.onPause();
Log.d(TAG, this + ": onPause()");
}
@Override
public void onStop()
{
super.onStop();
Log.d(TAG, this + ": onStop()");
}
@Override
public void onDestroy()
{
super.onDestroy();
Log.d(TAG, this + ": onDestroy()");
}
}
}
Codice sorgente per FragmentRetainInstance.java(a partire dall'API 16):
/*
* Copyright (C) 2010 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.apis.app;
import com.example.android.apis.R;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;
/**
* This example shows how you can use a Fragment to easily propagate state
* (such as threads) across activity instances when an activity needs to be
* restarted due to, for example, a configuration change. This is a lot
* easier than using the raw Activity.onRetainNonConfiguratinInstance() API.
*/
public class FragmentRetainInstance extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// First time init, create the UI.
if (savedInstanceState == null) {
getFragmentManager().beginTransaction().add(android.R.id.content,
new UiFragment()).commit();
}
}
/**
* This is a fragment showing UI that will be updated from work done
* in the retained fragment.
*/
public static class UiFragment extends Fragment {
RetainedFragment mWorkFragment;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_retain_instance, container, false);
// Watch for button clicks.
Button button = (Button)v.findViewById(R.id.restart);
button.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
mWorkFragment.restart();
}
});
return v;
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
FragmentManager fm = getFragmentManager();
// Check to see if we have retained the worker fragment.
mWorkFragment = (RetainedFragment)fm.findFragmentByTag("work");
// If not retained (or first time running), we need to create it.
if (mWorkFragment == null) {
mWorkFragment = new RetainedFragment();
// Tell it who it is working with.
mWorkFragment.setTargetFragment(this, 0);
fm.beginTransaction().add(mWorkFragment, "work").commit();
}
}
}
/**
* This is the Fragment implementation that will be retained across
* activity instances. It represents some ongoing work, here a thread
* we have that sits around incrementing a progress indicator.
*/
public static class RetainedFragment extends Fragment {
ProgressBar mProgressBar;
int mPosition;
boolean mReady = false;
boolean mQuiting = false;
/**
* This is the thread that will do our work. It sits in a loop running
* the progress up until it has reached the top, then stops and waits.
*/
final Thread mThread = new Thread() {
@Override
public void run() {
// We'll figure the real value out later.
int max = 10000;
// This thread runs almost forever.
while (true) {
// Update our shared state with the UI.
synchronized (this) {
// Our thread is stopped if the UI is not ready
// or it has completed its work.
while (!mReady || mPosition >= max) {
if (mQuiting) {
return;
}
try {
wait();
} catch (InterruptedException e) {
}
}
// Now update the progress. Note it is important that
// we touch the progress bar with the lock held, so it
// doesn't disappear on us.
mPosition++;
max = mProgressBar.getMax();
mProgressBar.setProgress(mPosition);
}
// Normally we would be doing some work, but put a kludge
// here to pretend like we are.
synchronized (this) {
try {
wait(50);
} catch (InterruptedException e) {
}
}
}
}
};
/**
* Fragment initialization. We way we want to be retained and
* start our thread.
*/
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Tell the framework to try to keep this fragment around
// during a configuration change.
setRetainInstance(true);
// Start up the worker thread.
mThread.start();
}
/**
* This is called when the Fragment's Activity is ready to go, after
* its content view has been installed; it is called both after
* the initial fragment creation and after the fragment is re-attached
* to a new activity.
*/
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
// Retrieve the progress bar from the target's view hierarchy.
mProgressBar = (ProgressBar)getTargetFragment().getView().findViewById(
R.id.progress_horizontal);
// We are ready for our thread to go.
synchronized (mThread) {
mReady = true;
mThread.notify();
}
}
/**
* This is called when the fragment is going away. It is NOT called
* when the fragment is being propagated between activity instances.
*/
@Override
public void onDestroy() {
// Make the thread go away.
synchronized (mThread) {
mReady = false;
mQuiting = true;
mThread.notify();
}
super.onDestroy();
}
/**
* This is called right before the fragment is detached from its
* current activity instance.
*/
@Override
public void onDetach() {
// This fragment is being detached from its activity. We need
// to make sure its thread is not going to touch any activity
// state after returning from this function.
synchronized (mThread) {
mProgressBar = null;
mReady = false;
mThread.notify();
}
super.onDetach();
}
/**
* API for our UI to restart the progress thread.
*/
public void restart() {
synchronized (mThread) {
mPosition = 0;
mThread.notify();
}
}
}
}