1. Fragment 使用时要有一个无参构造函数
如果没有无参构造函数，而是像按照普通类来使用，只创建有参构造函数，则会出现 android.support.v4.app.Fragment$InstantiationException 错误。
原因：Fragment 和 Activity 都是生命周期的组件，不能看做一般的类。如果非要使用有参构造函数，可能在使用的时候第一次传参没有问题，但是大概率在后面使用的时候出现问题。因为Fragment的什么周期依附在Activity中，如果Activity为null，那么Fragment肯定不能够正常使用了，比如手机屏幕的横竖屏切换导致Activity重建了。
至于为什么是这样的呢？看下Fragment初始化的源码,有这么一段:
/**
* Create a new instance of a Fragment with the given class name. This is
* the same as calling its empty constructor.
*
* @param context The calling context being used to instantiate the fragment.
* This is currently just used to get its ClassLoader.
* @param fname The class name of the fragment to instantiate.
* @param args Bundle of arguments to supply to the fragment, which it
* can retrieve with {@link #getArguments()}. May be null.
* @return Returns a new fragment instance.
* @throws InstantiationException If there is a failure in instantiating
* the given fragment class. This is a runtime exception; it is not
* normally expected to happen.
*/
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);
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);
}
}
整个过程中,Fragment的创建其实也是利用了无参数的构造方法去实例化.但关键的是,它将Bundle传类新建的Fragment,这样旧的Fragment和新的Fragment就能拥有一样的Bundle,从而达到利用Bundle传递参数的目的.
查看Android的SDK文档，也给出来相关的说法：
2. 给 Fragment 传递参数
一定要使用 Bundle 方式传递参数，而不是通过重载构造函数传递参数。
public static VechileFrag newInstance(Vehicle vehicle, String userId, boolean isAdd) {
VechileFrag mf = new VechileFrag();
Bundle args = new Bundle();
args.putString("userId", userId);
args.putBoolean("isAdd", isAdd);
args.putParcelable("vehicle", vehicle);
mf.setArguments(args);
return mf;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Bundle args = getArguments();
if (args != null) {
userId = args.getString("userId");
isAdd = args.getBoolean("isAdd");
vehicle = args.getParcelable("vehicle");
if (vehicle == null) {
vehicle = new Vehicle();
}
}
}
3. Fragment 与 Activity 通信
在 Fragment 中定义一个接口和要回调的方法， Activity实现Fragment接口，需要时回调 Fragment 方法。
public IVechile mIVechile;
public interface IVechile {
public void submitCarSuccess(String carId, String plateNo);
}
@Override
public void onAttach(Activity activity) {
fueltypes = FuelType.getList(activity);
try {
mIVechile = (IVechile) activity;
}
catch (Exception e) {
// TODO: handle exception }
}
}