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
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public class FramentTest2Activity extends ActionBarActivity {
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@Override
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protected void onCreate(Bundle savedInstanceState) {
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super.onCreate(savedInstanceState);
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setContentView(R.layout. activity_main);
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if (savedInstanceState == null) {
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getSupportFragmentManager().beginTransaction()
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.add(R.id. container, TestFragment.newInstance("param")).commit();
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}
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}
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public static class TestFragment extends Fragment {
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private static final String ARG = "arg";
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public TestFragment() {
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Log. i("INFO", "TestFragment non-parameter constructor" );
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}
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public static Fragment newInstance(String arg){
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TestFragment fragment = new TestFragment();
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Bundle bundle = new Bundle();
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bundle.putString( ARG, arg);
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fragment.setArguments(bundle);
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return fragment;
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}
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@Override
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public View onCreateView(LayoutInflater inflater, ViewGroup container,
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Bundle savedInstanceState) {
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View rootView = inflater.inflate(R.layout. fragment_main, container,
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false);
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TextView tv = (TextView) rootView.findViewById(R.id. tv);
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tv.setText(getArguments().getString( ARG));
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return rootView;
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}
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}
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}
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
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protected void onCreate(Bundle savedInstanceState) {
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if (DEBUG_LIFECYCLE ) Slog.v( TAG, "onCreate " + this + ": " + savedInstanceState);
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if (mLastNonConfigurationInstances != null) {
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mAllLoaderManagers = mLastNonConfigurationInstances .loaders ;
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}
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if (mActivityInfo .parentActivityName != null) {
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if (mActionBar == null) {
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mEnableDefaultActionBarUp = true ;
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} else {
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mActionBar .setDefaultDisplayHomeAsUpEnabled( true);
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}
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}
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if (savedInstanceState != null) {
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Parcelable p = savedInstanceState.getParcelable( FRAGMENTS_TAG );
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mFragments .restoreAllState(p, mLastNonConfigurationInstances != null
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? mLastNonConfigurationInstances .fragments : null);
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}
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mFragments .dispatchCreate();
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getApplication().dispatchActivityCreated( this , savedInstanceState);
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mCalled = true ;
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}
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
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for (int i=0; i<fms.mActive.length; i++) {
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FragmentState fs = fms.mActive[i];
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if (fs != null) {
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Fragment f = fs.instantiate(mActivity, mParent);
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if (DEBUG) Log.v(TAG, "restoreAllState: active #" + i + ": " + f);
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mActive.add(f);
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// Now that the fragment is instantiated (or came from being
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// retained above), clear mInstance in case we end up re-restoring
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// from this FragmentState again.
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fs.mInstance = null;
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} else {
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mActive.add(null);
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if (mAvailIndices == null) {
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mAvailIndices = new ArrayList<Integer>();
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}
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if (DEBUG) Log.v(TAG, "restoreAllState: avail #" + i);
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mAvailIndices.add(i);
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}
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}
接下来我们可以看看FragmentState.instantitate()方法的实现
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public Fragment instantiate(Activity activity, Fragment parent) {
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if (mInstance != null) {
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return mInstance ;
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}
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if (mArguments != null) {
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mArguments .setClassLoader(activity.getClassLoader());
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}
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mInstance = Fragment.instantiate(activity, mClassName , mArguments );
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if (mSavedFragmentState != null) {
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mSavedFragmentState .setClassLoader(activity.getClassLoader());
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mInstance .mSavedFragmentState = mSavedFragmentState ;
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}
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mInstance .setIndex(mIndex , parent);
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mInstance .mFromLayout = mFromLayout ;
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mInstance .mRestored = true;
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mInstance .mFragmentId = mFragmentId ;
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mInstance .mContainerId = mContainerId ;
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mInstance .mTag = mTag ;
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mInstance .mRetainInstance = mRetainInstance ;
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mInstance .mDetached = mDetached ;
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mInstance .mFragmentManager = activity.mFragments;
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if (FragmentManagerImpl.DEBUG) Log.v(FragmentManagerImpl.TAG,
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"Instantiated fragment " + mInstance );
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return mInstance ;
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}
可以看到最终转入到Fragment.instantitate()方法
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public static Fragment instantiate(Context context, String fname, Bundle args) {
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try {
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Class<?> clazz = sClassMap .get(fname);
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if (clazz == null) {
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clazz = context.getClassLoader().loadClass(fname);
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sClassMap .put(fname, clazz);
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}
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Fragment f = (Fragment)clazz.newInstance();
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if (args != null) {
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args.setClassLoader(f.getClass().getClassLoader());
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f. mArguments = args;
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}
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return f;
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} catch (ClassNotFoundException e) {
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throw new InstantiationException( "Unable to instantiate fragment " + fname
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+ ": make sure class name exists, is public, and has an"
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+ " empty constructor that is public" , e);
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} catch (java.lang.InstantiationException e) {
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throw new InstantiationException( "Unable to instantiate fragment " + fname
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+ ": make sure class name exists, is public, and has an"
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+ " empty constructor that is public" , e);
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} catch (IllegalAccessException e) {
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throw new InstantiationException( "Unable to instantiate fragment " + fname
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+ ": make sure class name exists, is public, and has an"
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+ " empty constructor that is public" , e);
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}
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
