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
1 public class FramentTestActivity extends ActionBarActivity {
2
3 @Override
4 protected void onCreate(Bundle savedInstanceState) {
5 super.onCreate(savedInstanceState);
6 setContentView(R.layout.activity_main);
7
8 if (savedInstanceState == null) {
9 getSupportFragmentManager().beginTransaction()
10 .add(R.id.container, new TestFragment("param")).commit();
11 }
12
13 }
14
15 public static class TestFragment extends Fragment {
16
17 private String mArg = "non-param";
18
19 public TestFragment() {
20 Log.i("INFO", "TestFragment non-parameter constructor");
21 }
22
23 public TestFragment(String arg){
24 mArg = arg;
25 Log.i("INFO", "TestFragment construct with parameter");
26 }
27
28 @Override
29 public View onCreateView(LayoutInflater inflater, ViewGroup container,
30 Bundle savedInstanceState) {
31 View rootView = inflater.inflate(R.layout.fragment_main, container,
32 false);
33 TextView tv = (TextView) rootView.findViewById(R.id.tv);
34 tv.setText(mArg);
35 return rootView;
36 }
37 }
38
39 }
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
1 public class FramentTest2Activity extends ActionBarActivity {
2
3 @Override
4 protected void onCreate(Bundle savedInstanceState) {
5 super.onCreate(savedInstanceState);
6 setContentView(R.layout. activity_main);
7
8 if (savedInstanceState == null) {
9 getSupportFragmentManager().beginTransaction()
10 .add(R.id. container, TestFragment.newInstance("param")).commit();
11 }
12
13 }
14
15 public static class TestFragment extends Fragment {
16
17 private static final String ARG = "arg";
18
19 public TestFragment() {
20 Log. i("INFO", "TestFragment non-parameter constructor" );
21 }
22
23 public static Fragment newInstance(String arg){
24 TestFragment fragment = new TestFragment();
25 Bundle bundle = new Bundle();
26 bundle.putString( ARG, arg);
27 fragment.setArguments(bundle);
28 return fragment;
29 }
30
31 @Override
32 public View onCreateView(LayoutInflater inflater, ViewGroup container,
33 Bundle savedInstanceState) {
34 View rootView = inflater.inflate(R.layout. fragment_main, container,
35 false);
36 TextView tv = (TextView) rootView.findViewById(R.id. tv);
37 tv.setText(getArguments().getString( ARG));
38 return rootView;
39 }
40 }
41
42 }
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
1 protected void onCreate(Bundle savedInstanceState) {
2 if (DEBUG_LIFECYCLE ) Slog.v( TAG, "onCreate " + this + ": " + savedInstanceState);
3 if (mLastNonConfigurationInstances != null) {
4 mAllLoaderManagers = mLastNonConfigurationInstances .loaders ;
5 }
6 if (mActivityInfo .parentActivityName != null) {
7 if (mActionBar == null) {
8 mEnableDefaultActionBarUp = true ;
9 } else {
10 mActionBar .setDefaultDisplayHomeAsUpEnabled( true);
11 }
12 }
13 if (savedInstanceState != null) {
14 Parcelable p = savedInstanceState.getParcelable( FRAGMENTS_TAG );
15 mFragments .restoreAllState(p, mLastNonConfigurationInstances != null
16 ? mLastNonConfigurationInstances .fragments : null);
17 }
18 mFragments .dispatchCreate();
19 getApplication().dispatchActivityCreated( this , savedInstanceState);
20 mCalled = true ;
21 }
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
1 for (int i=0; i<fms.mActive.length; i++) {
2 FragmentState fs = fms.mActive[i];
3 if (fs != null) {
4 Fragment f = fs.instantiate(mActivity, mParent);
5 if (DEBUG) Log.v(TAG, "restoreAllState: active #" + i + ": " + f);
6 mActive.add(f);
7 // Now that the fragment is instantiated (or came from being
8 // retained above), clear mInstance in case we end up re-restoring
9 // from this FragmentState again.
10 fs.mInstance = null;
11 } else {
12 mActive.add(null);
13 if (mAvailIndices == null) {
14 mAvailIndices = new ArrayList<Integer>();
15 }
16 if (DEBUG) Log.v(TAG, "restoreAllState: avail #" + i);
17 mAvailIndices.add(i);
18 }
19 }
接下来我们可以看看FragmentState.instantitate()方法的实现
1 public Fragment instantiate(Activity activity, Fragment parent) {
2 if (mInstance != null) {
3 return mInstance ;
4 }
5
6 if (mArguments != null) {
7 mArguments .setClassLoader(activity.getClassLoader());
8 }
9
10 mInstance = Fragment.instantiate(activity, mClassName , mArguments );
11
12 if (mSavedFragmentState != null) {
13 mSavedFragmentState .setClassLoader(activity.getClassLoader());
14 mInstance .mSavedFragmentState = mSavedFragmentState ;
15 }
16 mInstance .setIndex(mIndex , parent);
17 mInstance .mFromLayout = mFromLayout ;
18 mInstance .mRestored = true;
19 mInstance .mFragmentId = mFragmentId ;
20 mInstance .mContainerId = mContainerId ;
21 mInstance .mTag = mTag ;
22 mInstance .mRetainInstance = mRetainInstance ;
23 mInstance .mDetached = mDetached ;
24 mInstance .mFragmentManager = activity.mFragments;
25 if (FragmentManagerImpl.DEBUG) Log.v(FragmentManagerImpl.TAG,
26 "Instantiated fragment " + mInstance );
27
28 return mInstance ;
29 }
可以看到最终转入到Fragment. instantitate()方法
1 public static Fragment instantiate(Context context, String fname, Bundle args) {
2 try {
3 Class<?> clazz = sClassMap .get(fname);
4 if (clazz == null) {
5 // Class not found in the cache, see if it‘s real, and try to add it
6 clazz = context.getClassLoader().loadClass(fname);
7 sClassMap .put(fname, clazz);
8 }
9 Fragment f = (Fragment)clazz.newInstance();
10 if (args != null) {
11 args.setClassLoader(f.getClass().getClassLoader());
12 f. mArguments = args;
13 }
14 return f;
15 } catch (ClassNotFoundException e) {
16 throw new InstantiationException( "Unable to instantiate fragment " + fname
17 + ": make sure class name exists, is public, and has an"
18 + " empty constructor that is public" , e);
19 } catch (java.lang.InstantiationException e) {
20 throw new InstantiationException( "Unable to instantiate fragment " + fname
21 + ": make sure class name exists, is public, and has an"
22 + " empty constructor that is public" , e);
23 } catch (IllegalAccessException e) {
24 throw new InstantiationException( "Unable to instantiate fragment " + fname
25 + ": make sure class name exists, is public, and has an"
26 + " empty constructor that is public" , e);
27 }
28 }
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
时间： 2024-11-02 01:27:29