如果Fragment通過XML創建，為什么在旋轉期間Fragment無法接收到非null的savedInstanceState包？

Question

我有以下非常簡單的片段。

UiFragment.java

package com.example.test;

import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.actionbarsherlock.app.SherlockFragment;

/**
 * This is a fragment showing UI that will be updated from work done
 * in the retained fragment.
 */
public class UiFragment extends SherlockFragment {


    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        Log.i("CHEOK", "UiFragment onActivityCreated");

        super.onActivityCreated(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        Log.i("CHEOK", "UiFragment onCreateView");

        View v = inflater.inflate(R.layout.uifragment, container, false);

        return v;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        Log.i("CHEOK", "UiFragment onCreate");

        super.onCreate(savedInstanceState);

        if (savedInstanceState != null) {
            Log.i("CHEOK", "--> UiFragment able get savedInstanceState's data : " + savedInstanceState.getInt("value"));
        }       
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("value", 111222);        
    }    
}

uifragment.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="8dp">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="fragment_retain_instance_msg" />

</LinearLayout>

我意識到是否可以通過Java代碼將其附加到FragmentActivity。

MainActivity.java

package com.example.test;

import android.os.Bundle;
import android.util.Log;

import com.actionbarsherlock.app.SherlockFragmentActivity;

public class MainActivity extends SherlockFragmentActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Log.i("CHEOK", "MainActivity onCreate");

        super.onCreate(savedInstanceState);

        if (savedInstanceState != null) {
            Log.i("CHEOK", "--> MainActivity able get savedInstanceState's data : " + savedInstanceState.getInt("value"));
        }

        // ???
        //setContentView(R.layout.activity_main);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction().add(android.R.id.content,
                new UiFragment()).commit();
        }       
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("value", 888999);        
    }   
}

每次執行旋轉時，我的Fragment都能在onCreate期間獲取非null savedInstanceState

02-26 00:36:48.777: I/CHEOK(17740): MainActivity onCreate
02-26 00:36:48.812: I/CHEOK(17740): UiFragment onCreate
02-26 00:36:49.031: I/CHEOK(17740): UiFragment onCreateView
02-26 00:36:49.035: I/CHEOK(17740): UiFragment onActivityCreated
[Rotation happens here]
02-26 00:37:37.472: I/CHEOK(17740): MainActivity onCreate
02-26 00:37:37.472: I/CHEOK(17740): UiFragment onCreate
02-26 00:37:37.472: I/CHEOK(17740): --> UiFragment able get savedInstanceState's data : 111222
02-26 00:37:37.472: I/CHEOK(17740): --> MainActivity able get savedInstanceState's data : 888999
02-26 00:37:37.511: I/CHEOK(17740): UiFragment onCreateView
02-26 00:37:37.511: I/CHEOK(17740): UiFragment onActivityCreated

但是，如果我替換了MainActivity的onCreate的代碼

if (savedInstanceState == null) {
    getSupportFragmentManager().beginTransaction().add(android.R.id.content,
        new UiFragment()).commit();
}

通過XML創建片段

setContentView(R.layout.activity_main);

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <fragment class="com.example.test.UiFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>

執行輪換時，我將獲得以下日志。

02-26 00:39:23.437: I/CHEOK(18055): MainActivity onCreate
02-26 00:39:23.734: I/CHEOK(18055): UiFragment onCreate
02-26 00:39:23.734: I/CHEOK(18055): UiFragment onCreateView
02-26 00:39:23.754: I/CHEOK(18055): UiFragment onActivityCreated
[Rotation happens here]
02-26 00:39:27.703: I/CHEOK(18055): MainActivity onCreate
02-26 00:39:27.703: I/CHEOK(18055): --> MainActivity able get savedInstanceState's data : 888999
02-26 00:39:27.746: I/CHEOK(18055): UiFragment onCreate
02-26 00:39:27.746: I/CHEOK(18055): UiFragment onCreateView
02-26 00:39:27.750: I/CHEOK(18055): UiFragment onActivityCreated

為什么Fragment行為有如此不同？ 如果Fragment通過XML創建， 為什么在旋轉期間Fragment無法接收到非null的savedInstanceState包 ？

Answer 1

要實例化的片段必須具有某種唯一標識符，以便在需要銷毀並重新創建父活動時可以將其與先前的實例重新關聯。 可以通過以下方式提供：

提供ID以片段化XML標簽可解決此問題。

<fragment class="com.example.test.UiFragment"
    android:id="@+id/myFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />