ApiDemo/FragmentRetainInstance 解析

/*
* 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.util.Log;
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 {

private static String TAG="FragmentRetainInstance";
@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.
Log.d(TAG, "onDestroy");
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.
Log.d(TAG, "onDetach");
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();
}
}
}
}

这个小样例有两个关键点

1、通过Fragment保存状态。 通常在Activity销毁时和Activity关联的Fragment也会被销毁。当Activity重建时会自己主动创建相关的Fragment。因此常常在Activity的onCreate 函数中判处savedInstanceState 是否为空，（当Activity 有关联的Fragment时。重建Activity时savedInstanceState不为空）来避免反复创建Fragment。重建的Fragment和之前的Fragment是两个不同的对象。

可是假设对Fragment调用setRetainInstance(true)。那么在Activity销毁时（设置改变导致的activity销毁，如横竖屏切换）会保留该Fragment（onDetach会被调用，onDestroy不会被调用），Activity重建时会继续关联该Fragment。即通过FragmentManager
得到的还是之前的Fragment。

能够利用Fragment的这个性质保存Activity的状态。 与通过onSaveInstance或onRetainNonConfiguratinInstance()方法相比，通过Fragment保存状态非常方便。

特别是对于比較大的对象如Bitmap或不easy序列化的 对象（如本例中的线程对象)。用于保存状态的Fragment一般不能有视图（onCreateView 返回null），可是能够设置TargetFragment，能够获取TargetFragment，更新TargetFragment的UI。

2、怎样避免在Activity销毁期间后台线程更新UI。

在本例中activity销毁期间，线程仍在运行，线程运行期间可能会更改进度条。但这时UI已经销毁了。

本例中在onDetach中将mReady 设置为false，来避免更新进度条。

同一时候在更新进度条时获得相互排斥锁，防止更新进度条时UI资源被回收。