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名师互学网 > IT > 软件开发 > 移动开发 > Android

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

Android 更新时间: 发布时间: IT归档 最新发布 模块sitemap

Android 中为什么要用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

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(); 
 } 
 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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