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
