Android开发之Fragment传递參数的几种方法

Fragment在Android3.0開始提供，而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。

但当我们实例化自己定义Fragment时，为什么官方推荐Fragment.setArguments(Bundle bundle)这样的方式来传递參数，而不推荐通过构造方法直接来传递參数呢？为了弄清这个问题，我们能够做一个測试。分别測试下这两种方式的不同

首先。我们来測试下通过构造方法传递參数的情况

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;
        }
    }

}

能够看到我们传递过来的数据正确的显示了。如今来考虑一个问题。假设设备配置參数发生变化，这里以横竖屏切换来说明问题，显演示样例如以下

发生了什么问题呢？我们传递的參数哪去了？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)这样的方式的执行情况

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执行情况

看到了吧，我们传递的參数在横竖屏切换的情况下完善保存了下来，正确的显示给用户

那么这究竟是怎么回事呢，我们知道设备横竖屏切换的话，当前展示给用户的Activity默认情况下会又一次创建并展现给用户，那依附于Activity的Fragment会进行怎样处理呢，我们能够通过源代码来查看

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 ;
}

因为我们的Fragment是由FragmentManager来管理，所以能够跟进FragmentManager.restoreAllState()方法。通过对当前活动的Fragmnet找到以下的代码块

  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);
           }
}

接下来我们能够看看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 ;
    }

能够看到终于转入到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);
   }

通过此方法能够看到。终于会通过反射无參构造实例化一个新的Fragment，而且给mArgments初始化为原先的值，而原来的Fragment实例的数据都丢失了。并又一次进行了初始化

通过上面的分析，我们能够知道Activity又一次创建时，会又一次构建它所管理的Fragment，原先的Fragment的字段值将会所有丢失，可是通过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)方式来传递參数