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ragment在Android3.0开始提供，并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;		}	} }

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 {               @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运行情况

 

看到了吧，我们传递的参数在横竖屏切换的情况下完好保存了下来，正确的显示给用户

那么这到底是怎么回事呢，我们知道设备横竖屏切换的话，当前展示给用户的Activity默认情况下会重新创建并展现给用户，那依附于Activity的Fragment会进行如何处理呢，我们可以通过源码来查看 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 ;    }

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
   
    ();               }               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 ;    }

总结。使用官方推荐用法横竖屏切换没有问题