19 原 Android解惑 - 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

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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;  
        }  
    }  
  
}  

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

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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;  
             }  
       }  
  
}  

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

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 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 ;  
}  

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

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  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);  
           }  
}  

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

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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 ;  
    }  

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

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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);  
   }  

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

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