Fragment篇——面试必问的Fragment的生命周期

Fragment的产生

Android 在 Android 3.0（API 级别 11）中引入了片段，主要是为了给大屏幕（如平板电脑）上更加动态和灵活的 UI 设计提供支持。但是大多数情况下，根据不同的使用情景我们并没有对平板应用使用fragment进行适配，所以更多的是我们把fragment作为一个可重复利用的模块化组件，利用它拥有自身生命周期回调，布局，行为等特性来对功能模块进行分离

Fragment的生命周期

一般来说，Activity管理Fragment已经做得非常好了，比如，锁屏，回到屏幕的时候，Fragment都是跟随Activity发生改变的。Fragment包含与Activity类似的生命周期，比如onCreate，onStart，onResume，onPause，onStop，onDestroy，下面是引用自Google开发者的图片，关于一个Fragment被添加时（动态/静态）的生命周期的变化

我们来看看fragment的生命周期都有什么作用

//系统在向 Activity 添加片段时调用的方法，附加之后才能够通过getActivity()获取Activity的上下文。public void onAttach(Context context)public void onCreate(Bundle savedInstanceState)//返回Fragment的布局public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState)// 在 Activity 的 onCreate() 方法已返回时调用。public void onActivityCreated(@Nullable Bundle savedInstanceState)//在Activity的onStart之后调用public void onStart()//在Activity的onResume之后调用public void onResume()//在Activity的onPause之前调用public void onPause() //在Activity的onStop之前调用public void onStop()// 在移除与片段关联的视图层次结构时调用。public void onDestroyView()// 在Activity的onDestroy之前调用public void onDestroy()// 在取消片段与 Activity 的关联时调用。public void onDetach()

管理片段生命周期与管理 Activity 生命周期很相似。但是因为Fragment依附于Activity，所以很容易想象，关于Fragment创建的生命周期总是在Activity之后调用，关于Fragment销毁的生命周期总是在Activity之前调用。除此之外，在Activity需要重新创建的时候，fragment和activity一样可以通过onSaveInstanceState来保存状态，然后在onCreate，onCreateView或者onActivityCreated期间恢复状态。

对于Fragment的一些操作对生命周期的影响

// replace，FragmentA替换为FragmentB，在创建完B以后会先销毁A，再创建B的视图FragmentB: onAttachFragmentB: onCreate——————————————————————————FragmentA: onPauseFragmentA: onStopFragmentA: onDestroyViewFragmentA: onDestroyFragmentA: onDetach——————————————————————————FragmentB: onCreateViewFragmentB: onActivityCreatedFragmentB: onStartFragmentB: onResume// add，添加FragmentAFragmentA: onAttachFragmentA: onCreateFragmentA: onCreateViewFragmentA: onActivityCreatedFragmentA: onStartFragmentA: onResume//remove，移除FragmentAFragmentA: onPauseFragmentA: onStopFragmentA: onDestroyViewFragmentA: onDestroyFragmentA: onDetach//attach，从detach到attach状态FragmentA: onCreateViewFragmentA: onActivityCreatedFragmentA: onStartFragmentA: onResume//detach，从attach到detach状态FragmentA: onPauseFragmentA: onStopFragmentA: onDestroyView

注意：hide和show不会触发生命周期回调

给Fragment设置参数

使用setArguments，在创建Fragment的时候传递参数,然后在fragment的onCreate方法处获取参数，但是需要注意的是setArguments()方法必须在fragment创建以后，add之前调用。这样的好处是对Fragment的代码进行管理

//流行一种做法是使用静态方法newInstance()的方式来初始化Fragment    public static CircleFragmetn newInstance(String str){        Bundle bundle = new Bundle();        bundle.putString("data", str);        CircleFragmetn fragment = new CircleFragmetn();        fragment.setArguments(bundle);        return fragment;    }

没有UI的Fragment

可以添加没有UI的Fragment，使用add(Fragment,String)向Activity添加片段，由于它不与Activity布局的视图关联，所以不会触发onCreateView。通常没有UI的Fragment都是用于保存Activity的状态，或者借助setTargetFragment来控制其他fragment的UI变化，使用setRetainInstance()这个方法来告诉框架，当配置发生改变的时候，尝试保留这个fragment。SDK的APIdemos提供了范例

/*  * 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();              }          }      }  }  

Fragment通过实现 onCreateOptionsMenu() 向 Activity 的应用栏添加菜单项。不过，为了使此方法能够收到调用，您必须在 onCreate() 期间调用 setHasOptionsMenu()，以指示片段想要向选项菜单添加菜单项（否则，片段将不会收到对 onCreateOptionsMenu() 的调用）。

锁屏带来的问题

最近我发现了一个我们可能在平常的使用中都没有注意到的问题，锁屏界面对生命周期的影响，很多时候我们的Activity都会直接在onResume或者onPause等地方做一些操作，但是大部分手机都存在锁屏界面，如果我们对锁屏的实现比较熟悉的话都知道其实锁屏界面也是一个Activity。如果我们的Activity处于前台的时候，我们进入了锁屏，那么在开启屏幕的时候（出现锁屏的时候），我们的Activity其实经历了一次onRestart—onStart—onResume—onPause—onStop的生命周期变化，这是因为当我们点开屏幕的时候，首先显示的是我们的Activity，然后系统会在我们的Activity上面打开锁屏应用，这样就导致我们在打开锁屏之前，Activity就经历了一次生命周期的变化，这一问题会留到将来学习KeyGuard的时候再进行深入研究