Android解惑 - 为什么要用Fragment.setArguments(Bundle bundle)来传递参数

Fragment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;    }  }}

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 {               @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<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