ApiDemo/FragmentRetainInstance 解析，androidapidemo

ApiDemo/FragmentRetainInstance 解析，androidapidemo

/* * 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();            }        }    }}

這個小例子有兩個關鍵點 

1、通過Fragment儲存狀態。 通常在Activity銷毀時和Activity關聯的Fragment也會被銷毀。當Activity重建時會自動建立相關的Fragment。因此經常在Activity的onCreate 函數中判處savedInstanceState 是否為空白，（當Activity 有關聯的Fragment時，重建Activity時savedInstanceState不為空白）來避免重複建立Fragment。重建的Fragment和之前的Fragment是兩個不同的對象。但是如果對Fragment調用setRetainInstance(true)，那麼在Activity銷毀時（設定改變導致的activity銷毀，如橫豎屏切換）會保留該Fragment（onDetach會被調用，onDestroy不會被調用），Activity重建時會繼續關聯該Fragment。即通過FragmentManager 得到的還是之前的Fragment。可以利用Fragment的這個性質儲存Activity的狀態。 與通過onSaveInstance或onRetainNonConfiguratinInstance()方法相比，通過Fragment儲存狀態很方便。特別是對於比較大的對象如Bitmap或不容易序列化的 對象（如本例中的線程對象)。用於儲存狀態的Fragment一般不能有視圖（onCreateView 返回null），但是可以設定TargetFragment，可以擷取TargetFragment，更新TargetFragment的UI。
  2、如何避免在Activity銷毀期間後台線程更新UI。 
 在本例中activity銷毀期間，線程仍在執行，線程執行期間可能會更改進度條。但這時UI已經銷毀了。
 本例中在onDetach中將mReady 設定為false，來避免更新進度條。
 同時在更新進度條時獲得互斥鎖，防止更新進度條時UI資源被回收。