Fragment不推荐使用自定义带参的构造函数

Fragment不推荐使用自定义带参的构造函数

相信大家在使用Fragment会遇到如下异常

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.。/com...Activity}: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.aliyun.easylauncher.dialpad.DialFragment: make sure class name exists, is public, and has an empty constructor that is public

正常的使用一切都是正常，Google后发现，Fragment有下面的注意事项：

public Fragment

Default c

onstructor. Every fragment must have an empty constructor, so it can be instantiated when restoring its activity’s state. It is strongly recommended that subclasses do not have other constructors with parameters, since these constructors will not be called when the fragment is re-instantiated; instead, arguments can be supplied by the caller with setArguments(Bundle) and later retrieved by the Fragment withgetArguments().

Applications should generally not implement a constructor. The first place application code an run where the fragment is ready to be used is in onAttach(Activity), the point where the fragment is actually associated with its activity. Some applications may also want to implementonInflate(Activity, AttributeSet, Bundle) to retrieve attributes from a layout resource, though should take care here because this happens for the fragment is attached to its activity.

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(); //问题出在这里
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);
}

解决方案来自：stackoverflow

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 AlertFragment newInstance(int title, String message)
{
AlertFragment f = new AlertFragment();
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:

public onCreate(Bundle savedInstanceState) {
if(savedInstanceState == null){
getSupportFragmentManager()
.beginTransaction()
.replace(R.id.content,AlertFragment.newInstance(
R.string.alert_title,
"Oh noes an error occured!")
)
.commit();
}
}

This way if detached and re-attached the object state can be stored through the arguments. Much like bundles attached to Intents.