Android中关于Fragment的那些事、那些坑
TaylorFahey
8年前
<h2>引言</h2> <p>在Android3.0开始，Android引入了Fragment。当初最主要的目的是为了给大屏幕手持设备提供更加灵活和动态的UI设计。随着应用功能越来越多，界面越来越复杂，我们会利用Fragment对Activity的界面进行模块化编程。比如一个Activity界面内有多个请求，每个请求得到的结果展示不同的UI片段，那么我们就可以将Activity利用Fragment来完成UI模块化。目前在我目前的项目中，Fragment的地位已经举足轻重了。当然我写篇文章，并不是简单的介绍Fragment的用法，而是结合我在项目中使用Fragment所遇到的问题进行说明。</p> <h2>Fragment的生命周期</h2> <p>Fragment与Activity一样也是具有生命周期的，它的生命周期与Activity息息相关。下面看下Fragment生命周期流程图（摘自Android官方开发文档）</p> <p style="text-align: center;"><img src="https://simg.open-open.com/show/91b2bb0dc9207898818291854066dca2.png" alt="Android中关于Fragment的那些事、那些坑" width="317" height="847"></p> <p style="text-align: center;"> </p> <p><br> 大体上Fragment的生命周期与Activity类似，但是也有自己独有的生命周期方法。每个生命周期的意义大家可以看看<a href="/misc/goto?guid=4959673157382697946">Android Fragment</a>，这里就不做详解了。</p> <h2>Fragment与Activity之间生命周期的关系</h2> <p>每个Fragment实例存在于单独的一个Activity实例中，因此Fragment必须以某种方式与Activity的生命周期合作。Fragment不仅与Activity的生命周期合作，而且他们之间有“密切”联系。</p> <p style="text-align: center;"><img src="https://simg.open-open.com/show/8c5c1e82df783d5eb3b874385a79959c.png" alt="Android中关于Fragment的那些事、那些坑" width="397" height="340"></p> <p style="text-align: center;">Fragment lifecycle, from added to active</p> <p><br> 上图是Fragment创建到激活过程与Activity相关的生命周期图，为了验证该过程，我们写一个demo程序来进行验证。<br> 打印Activity生命周期方法相关方法代码：</p> <pre> <code class="language-java">public class MainActivity extends FragmentActivity { private static final String TAG = "KissonChan"; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.d(TAG, "Activity onCreate"); setContentView(R.layout.activity_main); TestFragment fragment = (TestFragment)getFragmentManager().findFragmentById(R.id.test); } @Override public void onAttachFragment(Fragment fragment) { super.onAttachFragment(fragment); Log.d(TAG, "Activity onAttachFragment"); } @Override protected void onStart() { super.onStart(); Log.d(TAG, "Activity onStart"); } @Override protected void onResume() { super.onResume(); Log.d(TAG, "Activity onResume"); } @Override protected void onPause() { super.onPause(); Log.d(TAG, "Activity onPause"); } @Override protected void onStop() { super.onStop(); Log.d(TAG, "Activity onStop"); } @Override protected void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); Log.d(TAG, "Activity onSaveInstanceState"); } @Override protected void onDestroy() { super.onDestroy(); Log.d(TAG, "Activity onDestroy"); } @Override protected void onRestart() { super.onRestart(); Log.d(TAG, "Activity onRestart"); } }</code></pre> <p>打印Fragment生命周期方法相关方法代码：</p> <pre> <code class="language-java">public class TestFragment extends Fragment { private static final String TAG = "KissonChan"; private View mRootView; @Override public void onAttach(Activity activity) { super.onAttach(activity); Log.e(TAG, "Fragment onAttach"); } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.e(TAG, "Fragment onCreate"); } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.e(TAG, "Fragment onCreateView"); mRootView = inflater.inflate(R.layout.fragment_test, container, false); return mRootView; } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); Log.e(TAG, "Fragment onActivityCreated"); } @Override public void onStart() { super.onStart(); Log.e(TAG, "Fragment onStart"); } @Override public void onResume() { super.onResume(); Log.e(TAG, "Fragment onResume"); } @Override public void onPause() { super.onPause(); Log.e(TAG, "Fragment onPause"); } @Override public void onStop() { super.onStop(); Log.e(TAG, "Fragment onStop"); } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); Log.e(TAG, "Fragment onSaveInstanceState"); } @Override public void onDestroyView() { super.onDestroyView(); Log.e(TAG, "Fragment onDestroyView"); } @Override public void onDetach() { super.onDetach(); Log.e(TAG, "Fragment onDetach"); } @Override public void onDestroy() { super.onDestroy(); Log.e(TAG, "Fragment onDestroy"); } }</code></pre> <p>程序的执行结果是：</p> <pre> <code class="language-java">05-12 11:38:42.901 32590-32590/? D/KissonChan: Activity onCreate 05-12 11:38:42.911 32590-32590/? E/KissonChan: Fragment onAttach 05-12 11:38:42.911 32590-32590/? D/KissonChan: Activity onAttachFragment 05-12 11:38:42.911 32590-32590/? E/KissonChan: Fragment onCreate 05-12 11:38:42.911 32590-32590/? E/KissonChan: Fragment onCreateView 05-12 11:38:42.911 32590-32590/? E/KissonChan: Fragment onActivityCreated 05-12 11:38:42.911 32590-32590/? D/KissonChan: Activity onStart 05-12 11:38:42.911 32590-32590/? E/KissonChan: Fragment onStart 05-12 11:38:42.911 32590-32590/? D/KissonChan: Activity onResume 05-12 11:38:42.911 32590-32590/? E/KissonChan: Fragment onResume</code></pre> <p>在Fragment从激活态到销毁也有类似的生命周期过程，不过与创建过程是相反。当Fragment的生命周期方法调用之后才调用对应的Activity生命周期方法，如下图。</p> <p style="text-align: center;"><img src="https://simg.open-open.com/show/e678e82249ec155e04940d85713bd7da.png" alt="Android中关于Fragment的那些事、那些坑" width="510" height="331"></p> <p style="text-align: center;">Fragment lifecycle, from active to destroy</p> <blockquote> <p>注意：如果正常结束Activity，onSaveInstanceState方法将不被调用。如果用户按下home键或者电源键等相关操作，将Activity置于后台，那么onSaveInstanceState将被调用。</p> </blockquote> <p>正常结束Activity程序运行结果是：</p> <pre> <code class="language-java">05-12 14:10:44.131 8266-8266/? E/KissonChan: Fragment onPause 05-12 14:10:44.131 8266-8266/? D/KissonChan: Activity onPause 05-12 14:10:44.381 8266-8266/? E/KissonChan: Fragment onStop 05-12 14:10:44.381 8266-8266/? D/KissonChan: Activity onStop 05-12 14:10:44.381 8266-8266/? E/KissonChan: Fragment onDestroyView 05-12 14:10:44.381 8266-8266/? E/KissonChan: Fragment onDestroy 05-12 14:10:44.381 8266-8266/? E/KissonChan: Fragment onDetach 05-12 14:10:44.381 8266-8266/? D/KissonChan: Activity onDestroy</code></pre> <p>按下home键程序运行结果是：</p> <pre> <code class="language-java">05-12 14:12:01.781 8266-8266/? E/KissonChan: Fragment onPause 05-12 14:12:01.781 8266-8266/? D/KissonChan: Activity onPause 05-12 14:12:02.041 8266-8266/? E/KissonChan: Fragment onSaveInstanceState 05-12 14:12:02.041 8266-8266/? D/KissonChan: Activity onSaveInstanceState 05-12 14:12:02.041 8266-8266/? E/KissonChan: Fragment onStop 05-12 14:12:02.041 8266-8266/? D/KissonChan: Activity onStop</code></pre> <p>如果对Fragment和Activity的生命周期有深入了解想法的小伙伴，可以自己阅读相关源码。我这里仅仅是抛砖引玉，做个简单介绍，为后续介绍工作过程中碰到的问题做一个铺垫。</p> <h2>Fragment的两种声明方式</h2> <p>Android提供了两种声明Fragment的方式，这两种方式上使用会有不同。同时也有一些需要注意的地方。</p> <h2>在Activity布局中添加Fragment</h2> <p>这种方式数据比较常用，在Activity布局中声明fragment。</p> <pre> <code class="language-xml"> <fragment android:id="@+id/test" android:name="com.dighammer.kisson.testfragment.TestFragment" android:layout_width="match_parent" android:layout_height="wrap_content"/></code></pre> <p>随后，在Activity通过如下方法初始化Fragment。</p> <pre> <code class="language-java">TestFragment fragment = (TestFragment)getFragmentManager().findFragmentById(R.id.test);</code></pre> <h2>在代码中将Fragment添加到已有ViewGroup中</h2> <p>这种方式我自己用的比较多，方法如下：</p> <pre> <code class="language-java">FragmentTransaction transaction = getFragmentManager().beginTransaction(); TestFragment fragment = new TestFragment(); transaction.add(R.id.container,fragment); //transaction.replace(R.id.container,fragment); transaction.commit();</code></pre> <p>这里咱们需要注意FragmentTransaction中add和replace方法的区别。</p> <ol> <li>add方法是把Fragment添加到ViewGroup中，通过该方法可以添加多个Fragment。</li> <li>replace方法会把ViewGroup中的原有Fragment先remove掉然后再add新Fragment。</li> </ol> <p>使用add方法可以避免再次创建Fragment实例，一般情况是建议使用add方法，并配合FragmentTransaction的show和hide方法来使用。</p> <h2>那些年，我踩过的Fragment的坑</h2> <h3>第一个坑</h3> <pre> <code> java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState at android.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1331) at android.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1349) at android.app.BackStackRecord.commitInternal(BackStackRecord.java:735) at android.app.BackStackRecord.commit(BackStackRecord.java:711) at com.dighammer.kisson.testfragment.MainActivity$1.run(MainActivity.java:30) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5273) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)</code></pre> <p>该类型的Crash开始的时候在我们的后台崩溃日志出现很多次。<br> 这里我写了一段代码，运行后可以百分比复现。</p> <pre> <code class="language-java">public class MainActivity extends FragmentActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); new Handler().postDelayed(new Runnable() { @Override public void run() { FragmentTransaction transaction = getFragmentManager().beginTransaction(); TestFragment fragment = new TestFragment(); transaction.add(R.id.container, fragment); transaction.commit(); } }, 5000); }</code></pre> <p>当程序界面显示出来后，按home键，过接近5秒后，就会出现该奔溃日志。<br> 原因是FragmentTransaction中的commit方法必须在onSaveInstanceState之前调用。<br> 那么如何解决了？在FragmentTransaction中提供了commitAllowingStateLoss方法，通过调用该方法就不用关心Activity的状态是否保存。</p> <h3>第二个坑</h3> <p>情景再现，先看代码。</p> <pre> <code class="language-java">public class MainActivity extends FragmentActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); FragmentTransaction transaction = getFragmentManager().beginTransaction(); TestFragment fragment = new TestFragment(); transaction.add(R.id.container, fragment); transaction.commit(); fragment.setText("kisson chan"); } }</code></pre> <p>运行后，会报如下错误。</p> <pre> <code>java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference at com.dighammer.kisson.testfragment.TestFragment.setText(TestFragment.java:53) at com.dighammer.kisson.testfragment.MainActivity.onCreate(MainActivity.java:18) at android.app.Activity.performCreate(Activity.java:6041) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1109) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2283) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2392) at android.app.ActivityThread.access$800(ActivityThread.java:154) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5273) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)</code></pre> <p>这里是空指针异常，TestFragment中的TextView还未被初始化。但是，如果我们通过Activity布局中声明Fragment则不会报错，代码如下。</p> <pre> <code class="language-java">public class MainActivity extends FragmentActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); TestFragment fragment = (TestFragment)getFragmentManager().findFragmentById(R.id.test); fragment.setText("kisson"); } }</code></pre> <p>原因其实很简单，通过Activity布局中声明Fragment，Fragment的初始化以及其布局中控件的初始化都发生在Activity的setContentView方法中。所以在setContentView方法调用之后，我们可以直接操作Fragment中的控件。<br> 但是通过代码将Fragment添加到ViewGroup的方式，并不能在Activity的onCreate方法中去操作Fragment中的控件，只能等到onCreate完成之后调用。<br> 如果必须要在Activity的onCreate方法中操作Fragment中的控件，这里我提供一个参考办法。</p> <pre> <code class="language-java">public class TestFragment extends Fragment{ private View mRootView; private TextView mTextView; private String mTextStr; @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { mRootView = inflater.inflate(R.layout.fragment_test,container,false); return mRootView; } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); mTextView = (TextView)mRootView.findViewById(R.id.text); if (!TextUtils.isEmpty(mTextStr)){ mTextView.setText(mTextStr); } } /** * 如果mTextView为空，即mTextView还未被初始化，那么将text赋值给mTextStr，等到onActivityCreated调用后再setText * 如果mTextView不为空，则直接调用TextView的setText方法进行赋值 * */ public void setText(String text){ if (mTextView == null){ mTextStr = text; } else { mTextView.setText(text); } } }</code></pre> <h3>第三个坑</h3> <p>情景再现，先看代码。</p> <pre> <code class="language-java">public class TestFragment extends Fragment{ private View mRootView; @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { mRootView = inflater.inflate(R.layout.fragment_test,container,false); return mRootView; } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); new Handler().postDelayed(new Runnable() { @Override public void run() { ((MainActivity) (getActivity())).print(); } },5000); } }</code></pre> <p>继续上代码</p> <pre> <code class="language-java">public class MainActivity extends FragmentActivity { private static final String TAG = "KissonChan"; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); FragmentTransaction transaction = getFragmentManager().beginTransaction(); TestFragment fragment = new TestFragment(); transaction.add(R.id.container,fragment); transaction.commit(); } public void print() { Log.d(TAG, "KissonChan"); } }</code></pre> <p>当程序Activity显示出来后，接着按下返回键结束Activity，静待五秒后，出现以下崩溃日志。</p> <pre> <code> Process: com.dighammer.kisson.testfragment, PID: 15870 java.lang.NullPointerException: Attempt to invoke virtual method 'void com.dighammer.kisson.testfragment.MainActivity.print()' on a null object reference at com.dighammer.kisson.testfragment.TestFragment$1.run(TestFragment.java:41) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5273) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)</code></pre> <p>因为MainActivity已经被销毁，五秒钟后在TestFragment实例中会调用MainActivity的print方法，所以抛出空指针异常。<br> 这里很奇怪，Activity已经被销毁了，为什么Fragment还能继续苟活。<br> 这是因为Fragment并不会随着Activity的回收而被系统回收，所有被创建的Fragment都会保存在一个Bundle里面。所以会导致以上崩溃。所以在一些延时操作或者线程操作中调用getActivity最好做下为空判断！</p> <h2>结尾</h2> <p>Fragment作为模块化UI编程类，极大方便了我们代码设计。但是同时我们也要深刻理解其生命周期及其意义。有时间大家可以看看源码，发现其中的奥秘！</p> <p> </p> <p>文／Kisson（简书）<br> via：http://www.jianshu.com/p/e658ffc02211<br> </p>