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;

}

}

}



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;

}

}

}



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 ;

}

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);

}

}

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 ;

}

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);

}

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

原文地址：http://blog.csdn.net/tu_bingbing/article/details/24143249