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

2019-12-12 04:05:30

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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

以上所述是小编给大家介绍的Android 中为什么要用Fragment.setArguments(Bundle bundle)来传递参数，希望对大家有所帮助，如果大家有任何疑问欢迎给我给我留言，小编会及时回复大家的！

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