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();
               }
               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