/* * 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。