Android 中為什麼要用Fragment.setArguments(Bundle bundle)來傳遞參數

Fragment在Android3.0開始提供，並且在兼容包中也提供了Fragment特性的支持。Fragment的推出讓我們編寫和管理用戶界面更快捷更方便了。

但當我們實例化自定義Fragment時，為什麼官方推薦Fragment.setArguments(Bundle bundle)這種方式來傳遞參數，而不推薦通過構造方法直接來傳遞參數呢？為了弄清這個問題，我們可以做一個測試，分別測試下這兩種方式的不同

首先，我們來測試下通過構造方法傳遞參數的情況

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; 
    } 
  } 
} 

可以看到我們傳遞過來的數據正確的顯示了，現在來考慮一個問題，如果設備配置參數發生變化，這裡以橫豎屏切換來說明問題，顯示如下

發生了什麼問題呢？我們傳遞的參數哪去了？為什麼會顯示默認值？不急著討論這個問題，接下來我們來看看Fragment.setArguments(Bundle bundle)這種方式的運行情況

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; 
       } 
    } 
} 

我們再來看看橫豎屏切換後的運行情況

看到了吧，我們傳遞的參數在橫豎屏切換的情況下完好保存了下來，正確的顯示給用戶
那麼這到底是怎麼回事呢，我們知道設備橫豎屏切換的話，當前展示給用戶的Activity默認情況下會重新創建並展現給用戶，那依附於Activity的Fragment會進行如何處理呢，我們可以通過源碼來查看

先來看看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 ; 
} 

由於我們的Fragment是由FragmentManager來管理，所以可以跟進FragmentManager.restoreAllState()方法，通過對當前活動的Fragmnet找到下面的代碼塊

 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); 
      } 
} 

接下來我們可以看看FragmentState.instantitate()方法的實現

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 ; 
  } 

可以看到最終轉入到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); 
  } 

通過此方法可以看到，最終會通過反射無參構造實例化一個新的Fragment，並且給mArgments初始化為原先的值，而原來的Fragment實例的數據都丟失了，並重新進行了初始化

通過上面的分析，我們可以知道Activity重新創建時，會重新構建它所管理的Fragment，原先的Fragment的字段值將會全部丟失，但是通過Fragment.setArguments(Bundle bundle)方法設置的bundle會保留下來。所以盡量使用Fragment.setArguments(Bundle bundle)方式來傳遞參數

以上所述是小編給大家介紹的Android 中為什麼要用Fragment.setArguments(Bundle bundle)來傳遞參數，希望對大家有所幫助，如果大家有任何疑問歡迎給我給我留言，小編會及時回復大家的！