Android 中为什么要用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

以上所述是小编给大家介绍的Android 中为什么要用Fragment.setArguments(Bundle bundle)来传递参数，希望对大家有所帮助，如果大家有任何疑问欢迎给我给我留言，小编会及时回复大家的！

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