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

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

博客时间：2014-04-19 23:05

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。