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

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

巴特西

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

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