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