Fragment在Android3.0開始提供，而且在兼容包中也提供了Fragment特性的支持。

Fragment的推出让我们编写和管理用户界面更快捷更方便了。

但当我们实例化自己定义Fragment时，为什么官方推荐Fragment.setArguments(Bundle bundle)这样的方式来传递參数，而不推荐通过构造方法直接来传递參数呢？为了弄清这个问题，我们能够做一个測试。分别測试下这两种方式的不同

首先。我们来測试下通过构造方法传递參数的情况

 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;
         }
     }

 }

能够看到我们传递过来的数据正确的显示了。如今来考虑一个问题。假设设备配置參数发生变化，这里以横竖屏切换来说明问题，显演示样例如以下

发生了什么问题呢？我们传递的參数哪去了？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)这样的方式的执行情况

 public class FramentTest2Activity 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, 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执行情况

看到了吧，我们传递的參数在横竖屏切换的情况下完善保存了下来，正确的显示给用户

那么这究竟是怎么回事呢，我们知道设备横竖屏切换的话，当前展示给用户的Activity默认情况下会又一次创建并展现给用户，那依附于Activity的Fragment会进行怎样处理呢，我们能够通过源代码来查看

先来看看Activity的onCreate(Bundle saveInstance)方法

  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 ;
 }

因为我们的Fragment是由FragmentManager来管理，所以能够跟进FragmentManager.restoreAllState()方法。通过对当前活动的Fragmnet找到以下的代码块

   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);
            }
 }

接下来我们能够看看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 ;
     }

能够看到终于转入到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);
    }

通过此方法能够看到。终于会通过反射无參构造实例化一个新的Fragment，而且给mArgments初始化为原先的值，而原来的Fragment实例的数据都丢失了。并又一次进行了初始化

通过上面的分析，我们能够知道Activity又一次创建时，会又一次构建它所管理的Fragment，原先的Fragment的字段值将会所有丢失，可是通过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)方式来传递參数