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