Android解惑 - 为什么要用Fragment.setArguments(Bundle bundle)来传递参数
博客源址：Android解惑 - 为什么要用Fragment.setArguments(Bundle bundle)来传递参数
博客时间：2014-04-19 23:05
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
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
我们再来看看横竖屏切换后的运行情况
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。