片段真的需要一个空构造函数吗?

是的，正如您所看到的，支持包也实例化了片段(当它们被销毁并重新打开时)。你的Fragment子类需要一个公共空构造函数，因为这是由框架调用的。

是的，他们有。

你不应该重写构造函数。你应该定义一个newInstance()静态方法，并通过参数(bundle)传递任何参数

例如:

public static final MyFragment newInstance(int title, String message) {
MyFragment f = new MyFragment();
Bundle bdl = new Bundle(2);
bdl.putInt(EXTRA_TITLE, title);
bdl.putString(EXTRA_MESSAGE, message);
f.setArguments(bdl);
return f;
}

当然，用这种方式获取参数:

@Override
public void onCreate(Bundle savedInstanceState) {
title = getArguments().getInt(EXTRA_TITLE);
message = getArguments().getString(EXTRA_MESSAGE);


//...
//etc
//...
}

然后你可以从你的片段管理器实例化，如下所示:

@Override
public void onCreate(Bundle savedInstanceState) {
if (savedInstanceState == null){
getSupportFragmentManager()
.beginTransaction()
.replace(R.id.content, MyFragment.newInstance(
R.string.alert_title,
"Oh no, an error occurred!")
)
.commit();
}
}

这样，如果分离和重新附加对象状态，可以通过参数存储。很像连接到intent的bundle。

原因-额外阅读

我想我可以解释给那些想知道为什么的人。

如果你检查:https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/app/Fragment.java

你将看到Fragment类中的instantiate(..)方法调用newInstance方法:

public static Fragment instantiate(Context context, String fname, @Nullable 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);
if (!Fragment.class.isAssignableFrom(clazz)) {
throw new InstantiationException("Trying to instantiate a class " + fname
+ " that is not a Fragment", new ClassCastException());
}
sClassMap.put(fname, clazz);
}
Fragment f = (Fragment) clazz.getConstructor().newInstance();
if (args != null) {
args.setClassLoader(f.getClass().getClassLoader());
f.setArguments(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);
} catch (NoSuchMethodException e) {
throw new InstantiationException("Unable to instantiate fragment " + fname
+ ": could not find Fragment constructor", e);
} catch (InvocationTargetException e) {
throw new InstantiationException("Unable to instantiate fragment " + fname
+ ": calling Fragment constructor caused an exception", e);
}
}

解释为什么，在实例化时，它检查访问器是public，并且类装入器允许访问它。

总的来说，这是一个非常讨厌的方法，但它允许FragmentManger杀死并重新创建带有状态的Fragments。(Android子系统对Activities做类似的事情)。

示例类

我被问到很多关于调用newInstance的问题。不要将其与类方法混淆。这个完整的类示例应该展示其用法。

/**
* Created by chris on 21/11/2013
*/
public class StationInfoAccessibilityFragment extends BaseFragment implements JourneyProviderListener {


public static final StationInfoAccessibilityFragment newInstance(String crsCode) {
StationInfoAccessibilityFragment fragment = new StationInfoAccessibilityFragment();


final Bundle args = new Bundle(1);
args.putString(EXTRA_CRS_CODE, crsCode);
fragment.setArguments(args);


return fragment;
}


// Views
LinearLayout mLinearLayout;


/**
* Layout Inflater
*/
private LayoutInflater mInflater;
/**
* Station Crs Code
*/
private String mCrsCode;


@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mCrsCode = getArguments().getString(EXTRA_CRS_CODE);
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
mInflater = inflater;
return inflater.inflate(R.layout.fragment_station_accessibility, container, false);
}


@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
mLinearLayout = (LinearLayout)view.findViewBy(R.id.station_info_accessibility_linear);
//Do stuff
}


@Override
public void onResume() {
super.onResume();
getActivity().getSupportActionBar().setTitle(R.string.station_info_access_mobility_title);
}


// Other methods etc...
}

正如CommonsWare在这个问题https://stackoverflow.com/a/16064418/1319061中所指出的，如果你正在创建Fragment的匿名子类，这个错误也会发生，因为匿名类不能有构造函数。

不要创建Fragment的匿名子类:-)

以下是我的简单解决方案:

定义你的片段

public class MyFragment extends Fragment {


private String parameter;


public MyFragment() {
}


public void setParameter(String parameter) {
this.parameter = parameter;
}
}

2 -创建新的片段并填充参数

    myfragment = new MyFragment();
myfragment.setParameter("here the value of my parameter");

3 -享受它!

看一下官方文档:Fragment: https://developer.android.com/reference/android/app/Fragment

Fragment的所有子类都必须包含一个公共的无参数构造函数。框架通常会在需要时重新实例化一个片段类，特别是在状态恢复期间，并且需要能够找到这个构造函数来实例化它。如果无参数构造函数不可用，在某些情况下，在状态恢复期间将发生运行时异常。