但当我们实例化自定义Fragment时，为什么官方推荐Fragment.setArguments(Bundle bundle)这种方式来传递参数，而不推荐通过构造方法直接来传递参数呢？ 为了弄清这个问题，我们可以做一个测试，分别测试下这两种方式的不同

首先，我们来测试下通过构造方法传递参数的情况

public class FramentTestActivity extends ActionBarActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);    if (savedInstanceState == null) {
      getSupportFragmentManager().beginTransaction()
          .add(R.id.container, new TestFragment("param")).commit();
    }

  }  public static class TestFragment extends Fragment {    private String mArg = "non-param";

    public TestFragment() {
      Log.i("INFO", "TestFragment non-parameter constructor");
    }

    public TestFragment(String arg){
      mArg = arg;
      Log.i("INFO", "TestFragment construct with parameter");
    }    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
      View rootView = inflater.inflate(R.layout.fragment_main, container,
          false);
      TextView tv = (TextView) rootView.findViewById(R.id.tv);
      tv.setText(mArg);
      return rootView;
    }
  }}

可以看到我们传递过来的数据正确的显示了，现在来考虑一个问题，如果设备配置参数发生变化，这里以横竖屏切换来说明问题，显示如下

发生了什么问题呢？我们传递的参数哪去了？为什么会显示默认值？不急着讨论这个问题，接下来我们来看看Fragment.setArguments(Bundle bundle)这种方式的运行情况

public class FramentTest2Activity extends ActionBarActivity {@Overrideprotected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);setContentView(R.layout. activity_main);  if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
         .add(R.id. container, TestFragment.newInstance("param")).commit();}}public static class TestFragment extends Fragment {  private static final String ARG = "arg";  public TestFragment() {
        Log. i("INFO", "TestFragment non-parameter constructor" );}  public static Fragment newInstance(String arg){
        TestFragment fragment = new TestFragment();
        Bundle bundle = new Bundle();
        bundle.putString( ARG, arg);
        fragment.setArguments(bundle);
         return fragment;}  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container,
               Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout. fragment_main, container,
          false);
        TextView tv = (TextView) rootView.findViewById(R.id. tv);
        tv.setText(getArguments().getString( ARG));
         return rootView;}}}

我们再来看看横竖屏切换后的运行情况

protected void onCreate(Bundle savedInstanceState) {
    if (DEBUG_LIFECYCLE ) Slog.v( TAG, "onCreate " + this + ": " + savedInstanceState);
    if (mLastNonConfigurationInstances != null) {
      mAllLoaderManagers = mLastNonConfigurationInstances .loaders ;
    }
    if (mActivityInfo .parentActivityName != null) {
      if (mActionBar == null) {
        mEnableDefaultActionBarUp = true ;
      } else {
        mActionBar .setDefaultDisplayHomeAsUpEnabled( true);
      }
    }
    if (savedInstanceState != null) {
      Parcelable p = savedInstanceState.getParcelable( FRAGMENTS_TAG );
      mFragments .restoreAllState(p, mLastNonConfigurationInstances != null
          ? mLastNonConfigurationInstances .fragments : null);
    }
    mFragments .dispatchCreate();
    getApplication().dispatchActivityCreated( this , savedInstanceState);
    mCalled = true ;
  }

由于我们的Fragment是由FragmentManager来管理，所以可以跟进FragmentManager.restoreAllState()方法，通过对当前活动的Fragmnet找到下面的代码块

for (int i=0; i<fms.mActive.length; i++) {FragmentState fs = fms.mActive[i];if (fs != null) {Fragment f = fs.instantiate(mActivity, mParent);if (DEBUG) Log.v(TAG, "restoreAllState: active #" + i + ": " + f);mActive.add(f);// Now that the fragment is instantiated (or came from being// retained above), clear mInstance in case we end up re-restoring// from this FragmentState again.fs.mInstance = null;} else {mActive.add(null);if (mAvailIndices == null) {mAvailIndices = new ArrayList<Integer>();}if (DEBUG) Log.v(TAG, "restoreAllState: avail #" + i);mAvailIndices.add(i);}
}

接下来我们可以看看FragmentState.instantitate()方法的实现

public Fragment instantiate(Activity activity, Fragment parent) {
    if (mInstance != null) {
      return mInstance ;
    }

    if (mArguments != null) {
      mArguments .setClassLoader(activity.getClassLoader());
    }

    mInstance = Fragment.instantiate(activity, mClassName , mArguments );

    if (mSavedFragmentState != null) {
      mSavedFragmentState .setClassLoader(activity.getClassLoader());
      mInstance .mSavedFragmentState = mSavedFragmentState ;
    }
    mInstance .setIndex(mIndex , parent);
    mInstance .mFromLayout = mFromLayout ;
    mInstance .mRestored = true;
    mInstance .mFragmentId = mFragmentId ;
    mInstance .mContainerId = mContainerId ;
    mInstance .mTag = mTag ;
    mInstance .mRetainInstance = mRetainInstance ;
    mInstance .mDetached = mDetached ;
    mInstance .mFragmentManager = activity.mFragments;
    if (FragmentManagerImpl.DEBUG) Log.v(FragmentManagerImpl.TAG,
        "Instantiated fragment " + mInstance );    return mInstance ;
  }

可以看到最终转入到Fragment. instantitate()方法

public static Fragment instantiate(Context context, String fname, Bundle args) {
    try {
      Class<?> clazz = sClassMap .get(fname);
      if (clazz == null) {
        // Class not found in the cache, see if it's real, and try to add it
        clazz = context.getClassLoader().loadClass(fname);
        sClassMap .put(fname, clazz);
      }
      Fragment f = (Fragment)clazz.newInstance();
      if (args != null) {
        args.setClassLoader(f.getClass().getClassLoader());
        f. mArguments = args;
      }
      return f;
    } catch (ClassNotFoundException e) {
      throw new InstantiationException( "Unable to instantiate fragment " + fname
          + ": make sure class name exists, is public, and has an"
          + " empty constructor that is public" , e);
    } catch (java.lang.InstantiationException e) {
      throw new InstantiationException( "Unable to instantiate fragment " + fname
          + ": make sure class name exists, is public, and has an"
          + " empty constructor that is public" , e);
    } catch (IllegalAccessException e) {
      throw new InstantiationException( "Unable to instantiate fragment " + fname
          + ": make sure class name exists, is public, and has an"
          + " empty constructor that is public" , e);
    }
  }

通过此方法可以看到，最终会通过反射无参构造实例化一个新的Fragment，并且给mArgments初始化为原先的值，而原来的Fragment实例的数据都丢失了，并重新进行了初始化

通过上面的分析，我们可以知道Activity重新创建时，会重新构建它所管理的Fragment，原先的Fragment的字段值将会全部丢失，但是通过 Fragment.setArguments(Bundle bundle)方法设置的bundle会保留下来。所以尽量使用 Fragment.setArguments(Bundle bundle)方式来传递参数