片段真的需要一个空的构造函数吗? [英] Do fragments really need an empty constructor?
问题描述
我有一个带有多个参数的构造函数的 Fragment.我的应用在开发过程中运行良好，但在生产中，我的用户有时会看到此崩溃:
android.support.v4.app.Fragment$InstantiationException:无法实例化片段确保类名存在，是公共的，并且有一个公共的空构造函数我可以按照此错误消息的提示创建一个空的构造函数，但这对我来说没有意义，因为那时我将不得不调用一个单独的方法来完成 Fragment 的设置.
我很好奇为什么这种崩溃只是偶尔发生.也许我使用的 ViewPager 不正确?我自己实例化了所有 Fragment 并将它们保存在 Activity 内的列表中.我不使用 FragmentManager 事务，因为我看到的 ViewPager 示例不需要它，而且在开发过程中一切似乎都在工作.
是的.
无论如何你都不应该重写构造函数.您应该定义一个 newInstance() 静态方法并通过参数(包)传递任何参数
例如:
public static final MyFragment newInstance(int title, String message) {MyFragment f = new MyFragment();捆绑 bdl = 新捆绑 (2);bdl.putInt(EXTRA_TITLE, 标题);bdl.putString(EXTRA_MESSAGE, message);f.setArguments(bdl);返回 f;}当然也可以这样获取参数:
@Overridepublic void onCreate(Bundle savedInstanceState) {标题 = getArguments().getInt(EXTRA_TITLE);消息 = getArguments().getString(EXTRA_MESSAGE);//...//等等//...}然后你会像这样从你的片段管理器实例化:
@Overridepublic void onCreate(Bundle savedInstanceState) {if (savedInstanceState == null){getSupportFragmentManager().beginTransaction().replace(R.id.content, MyFragment.newInstance(R.string.alert_title,哦不，发生错误！")).犯罪();}}这种方式如果分离和重新附加对象状态可以通过参数存储.很像附加到 Intent 的捆绑包.
原因 - 额外阅读
我想我会解释为什么人们想知道为什么.
您将看到 Fragment 类中的 instantiate(..) 方法调用了 newInstance 方法:
public static Fragment instanceiate(Context context, String fname, @Nullable Bundle args) {尝试 {类clazz = sClassMap.get(fname);if (clazz == null) {//缓存中没有找到类，看看是不是真的，尝试添加clazz = context.getClassLoader().loadClass(fname);如果 (!Fragment.class.isAssignableFrom(clazz)) {throw new InstantiationException("试图实例化一个类" + fname+ " 那不是 Fragment", new ClassCastException());}sClassMap.put(fname, clazz);}Fragment f = (Fragment) clazz.getConstructor().newInstance();如果(参数！= null){args.setClassLoader(f.getClass().getClassLoader());f.setArguments(args);}返回 f;} catch (ClassNotFoundException e) {throw new InstantiationException("无法实例化片段" + fname+ ": 确保类名存在，是公开的，并且有一个"+ " 公共的空构造函数", e);} catch (java.lang.InstantiationException e) {throw new InstantiationException("无法实例化片段" + fname+ ": 确保类名存在，是公开的，并且有一个"+ " 公共的空构造函数", e);} catch (IllegalAccessException e) {throw new InstantiationException("无法实例化片段" + fname+ ": 确保类名存在，是公开的，并且有一个"+ " 公共的空构造函数", e);} catch (NoSuchMethodException e) {throw new InstantiationException("无法实例化片段" + fname+ ": 找不到 Fragment 构造函数", e);} catch (InvocationTargetException e) {throw new InstantiationException("无法实例化片段" + fname+ ": 调用 Fragment 构造函数导致异常", e);}}http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#newInstance() 解释为什么，在实例化时它会检查访问器是 public 和那个类loader 允许访问它.
总而言之，这是一个非常讨厌的方法，但它允许 FragmentManger 使用状态杀死和重新创建 Fragment.(Android 子系统对 Activities 做了类似的事情).
示例类
我经常被问到关于调用 newInstance 的问题.不要将此与类方法混淆.整个类示例应该显示用法.
/*** 由 Chris 于 21/11/2013 创建*/公共类 StationInfoAccessibilityFragment 扩展 BaseFragment 实现 JourneyProviderListener {public static final StationInfoAccessibilityFragment newInstance(String crsCode) {StationInfoAccessibilityFragment fragment = new StationInfoAccessibilityFragment();最终捆绑包 args = 新捆绑包(1)；args.putString(EXTRA_CRS_CODE, crsCode);fragment.setArguments(args);返回片段；}//视图LinearLayout mLinearLayout;/*** 布局充气机*/私人 LayoutInflater mInflater；/*** 站 Crs 代码*/私人字符串 mCrsCode；@覆盖public void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);mCrsCode = getArguments().getString(EXTRA_CRS_CODE);}@覆盖公共视图 onCreateView(LayoutInflater inflater，ViewGroup 容器，Bundle savedInstanceState) {mInflater = 充气机；返回 inflater.inflate(R.layout.fragment_station_accessibility, container, false);}@覆盖public void onViewCreated(View view, Bundle savedInstanceState) {super.onViewCreated(view,savedInstanceState);mLinearLayout = (LinearLayout)view.findViewBy(R.id.station_info_accessibility_linear);//做东西}@覆盖公共无效 onResume() {super.onResume();getActivity().getSupportActionBar().setTitle(R.string.station_info_access_mobility_title);}//其他方法等...}I have a Fragment with a constructor that takes multiple arguments. My app worked fine during development, but in production my users sometimes see this crash:
android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment
make sure class name exists, is public, and has an empty constructor that is public
I could make an empty constructor as this error message suggests, but that doesn't make sense to me since then I would have to call a separate method to finish setting up the Fragment.
I'm curious as to why this crash only happens occasionally. Maybe I'm using the ViewPager incorrectly? I instantiate all the Fragments myself and save them in a list inside the Activity. I don't use FragmentManager transactions, since the ViewPager examples I have seen did not require it and everything seemed to be working during development.
Yes they do.
You shouldn't really be overriding the constructor anyway. You should have a newInstance() static method defined and pass any parameters via arguments (bundle)
For example:
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;
}
And of course grabbing the args this way:
@Override
public void onCreate(Bundle savedInstanceState) {
title = getArguments().getInt(EXTRA_TITLE);
message = getArguments().getString(EXTRA_MESSAGE);
//...
//etc
//...
}
Then you would instantiate from your fragment manager like so:
@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();
}
}
This way if detached and re-attached the object state can be stored through the arguments. Much like bundles attached to Intents.
Reason - Extra reading
I thought I would explain why for people wondering why.
If you check: https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/app/Fragment.java
You will see the instantiate(..) method in the Fragment class calls the newInstance method:
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);
}
}
http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#newInstance() Explains why, upon instantiation it checks that the accessor is public and that that class loader allows access to it.
It's a pretty nasty method all in all, but it allows the FragmentManger to kill and recreate Fragments with states. (The Android subsystem does similar things with Activities).
Example Class
I get asked a lot about calling newInstance. Do not confuse this with the class method. This whole class example should show the usage.
/**
* 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...
}
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