Fragment.setArguments()方法向fragment对象传递数据的重要作用

/** * Supply the construction arguments for this fragment.  This can only * be called before the fragment has been attached to its activity; that * is, you should call it immediately after constructing the fragment.  The * arguments supplied here will be retained across fragment destroy and * creation. */public void setArguments(Bundle args) {    if (mIndex >= 0) {        throw new IllegalStateException("Fragment already active");    }    mArguments = args;}大意就是这个方法为fragment提供结构参数。如果调用这个方法只能在fragment对象attach到activity完成之前，应该在fragment对方new出之后立即调用。传递的参数会存在于从fragment对象创建到destroy之间的整个过程。


对于开发中一般会遇到的问题是activity状态改变后，比如屏幕旋转后重新显示activity和其中的fragment，结果fragment显示的并不是旋转前的样子，有时还会有异常抛出。
其实很多时候是因为不知道fragment重新创建时默认调用Fragment无参的构造方法，所以通过添加有参构造方法自定义Fragment来传递数据的时候往往会在后续使用中出现问题。


具体的原理借用网上文章记录如下：
    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 {@Overrideprotected 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");}@Overridepublic 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;             }             @Overridepublic 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 itclazz = 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