android fragment保存數據

不能用 replace 應該是 add

replace()的調用會導致Fragment的onCreteView()被調用，所以切換界面時會無法保存當前的狀態。因此一般採用add()、hide()與show()配合，來達到保存Fragment的狀態。

1  推薦使用，考慮到回棧因素，效率更好 可以把一個fragment保存到返回棧中

通過顯示隱藏來處理：
            int tabIndex=0,tag=0;
            fragment[] fragments=new fragment[]{A.class,B.class,C.class,D.class};
            Fragment newFragment = fragments[tag];
            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
            if (!getSupportFragmentManager().getFragments().contains(fragments[tag])) {
                transaction.add(R.id.main_content, newFragment);
            }
            transaction.addToBackStack(null);
            
            transaction.hide(fragments[tabIndex]);
            transaction.show(newFragment);
            transaction.commit();
            tabIndex = tag;

2  相對來說比較複雜，這個更適合保存數據的，意外等適合這個

FragmentPagerAdapter可以將很多個Fragment保存起來以供取出，並且封裝了取出的接口，其實還是通過FragmentTransaction來實現的。

package com.nmbb.sample.fragmentswitch;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentPagerAdapter;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.CompoundButton;
import android.widget.CompoundButton.OnCheckedChangeListener;
import android.widget.FrameLayout;
import android.widget.RadioButton;
public class MainActivity extends FragmentActivity implements
        OnCheckedChangeListener, OnClickListener {
    private RadioButton mTab1;
    private RadioButton mTab2;
    private RadioButton mTab3;
    private RadioButton mTab4;
    private RadioButton mTab5;
    private FrameLayout mContainer;
    public CompoundButton currentButtonView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTab1 = (RadioButton) findViewById(R.id.radio_button0);
        mTab2 = (RadioButton) findViewById(R.id.radio_button1);
        mTab3 = (RadioButton) findViewById(R.id.radio_button2);
        mTab4 = (RadioButton) findViewById(R.id.radio_button3);
        mTab5 = (RadioButton) findViewById(R.id.radio_button4);
        mContainer = (FrameLayout) findViewById(R.id.container);
        mTab1.setOnCheckedChangeListener(this);
        mTab2.setOnCheckedChangeListener(this);
        mTab3.setOnCheckedChangeListener(this);
        mTab4.setOnCheckedChangeListener(this);
        mTab5.setOnCheckedChangeListener(this);
        mTab1.setOnClickListener(this);
        mTab2.setOnClickListener(this);
        mTab3.setOnClickListener(this);
        mTab4.setOnClickListener(this);
        mTab5.setOnClickListener(this);
        mTab1.performClick();
    }
    @Override
    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
        if (isChecked) {
            Fragment fragment = (Fragment) mFragmentPagerAdapter
                    .instantiateItem(mContainer, buttonView.getId());
            mFragmentPagerAdapter.setPrimaryItem(mContainer, 0, fragment);
            mFragmentPagerAdapter.finishUpdate(mContainer);
        }
    }
    private FragmentPagerAdapter mFragmentPagerAdapter = new FragmentPagerAdapter(
            getSupportFragmentManager()) {
        @Override
        public Fragment getItem(int position) {
            switch (position) {
            case R.id.radio_button1:
                return FragmentTest.instantiation(2);
            case R.id.radio_button2:
                return FragmentTest.instantiation(3);
            case R.id.radio_button3:
                return FragmentTest.instantiation(4);
            case R.id.radio_button4:
                return FragmentTest.instantiation(5);
            case R.id.radio_button0:
            default:
                return FragmentTest.instantiation(1);
            }
        }
        @Override
        public int getCount() {
            return 5;
        }
    };
    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
    }
}
instantiateItem從FragmentManager中查找Fragment，找不到就getItem新建一個，setPrimaryItem設置隱藏和顯示，最後finishUpdate提交事務。
mContainer就是xml中的FrameLayout。

package com.nmbb.sample.fragmentswitch;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
public class FragmentTest extends Fragment {
    public static FragmentTest instantiation(int position) {
        FragmentTest fragment = new FragmentTest();
        Bundle args = new Bundle();
        args.putInt("position", position);
        fragment.setArguments(args);
        return fragment;
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_test, container, false);
    }
    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        TextView text1 = (TextView) view.findViewById(android.R.id.text1);
        text1.setText("Fragment " + getArguments().getInt("position", 1));
    }
                                                                                                                                                                                                               
    @Override
    public void setMenuVisibility(boolean menuVisible) {
        super.setMenuVisibility(menuVisible);
        if (this.getView() != null)
            this.getView().setVisibility(menuVisible ? View.VISIBLE : View.GONE);
    }
}
其中,下面的代碼很關鍵,沒有下面的代碼會出現切換tab的時候重影現象：
@Override
public void setMenuVisibility(boolean menuVisible) {
    super.setMenuVisibility(menuVisible);
    if (this.getView() != null)
        this.getView().setVisibility(menuVisible ? View.VISIBLE : View.GONE);
}

問題的出現正是因爲使用了Fragment的狀態保存，當系統內存不足，Fragment的宿主Activity回收的時候，Fragment的實例並沒有隨之被回收。Activity被系統回收時，會主動調用onSaveInstance()方法來保存視圖層（View Hierarchy），所以當Activity通過導航再次被重建時，之前被實例化過的Fragment依然會出現在Activity中，然而從上述代碼中可以明顯看出，再次重建了新的Fragment，綜上這些因素導致了多個Fragment重疊在一起。
我嘗試了很多種方法去解決這個問題，比如：
在onSaveInstance()裏面去remove()所有非空的Fragment，然後在onRestoreInstanceState()中去再次按照問題一的方式創建Activity。當我處於打開“不保留活動”的時候，效果非常令人滿意，然而當我關閉“不保留活動”的時候，問題卻出現了。當轉跳到其他Activity、打開多任務窗口、使用Home回到主屏幕再返回時，發現根本沒有Fragment了，一篇空白。
於是跟蹤下去，我調查了onSaveInstanceState()與onRestoreInstanceState()這兩個方法。原本以爲只有在系統因爲內存回收Activity時纔會調用的onSaveInstanceState()，居然在轉跳到其他Activity、打開多任務窗口、使用Home回到主屏幕這些操作中也被調用，然而onRestoreInstanceState()並沒有在再次回到Activity時被調用。而且我在onResume()發現之前的Fragment只是被移除，並不是空，所以就算你在onResume()中執行問題一中創建的Fragment的方法，同樣無濟於事。所以通過remove()宣告失敗。
接着通過調查資料發現Activity中的onSaveInstanceState()裏面有一句super.onRestoreInstanceState(savedInstanceState)，Google對於這句話的解釋是“Always call the superclass so it can save the view hierarchy state”，大概意思是“總是執行這句代碼來調用父類去保存視圖層的狀態”。其實到這裏大家也就明白了，就是因爲這句話導致了重影的出現，於是我刪除了這句話，然後onCreate()與onRestoreInstanceState()中同時使用問題一中的創建Fragment方法，然後再通過保存切換的狀態，發現結果非常完美。代碼如下：
//記錄Fragment的位置
private int position = 0;
@Override
protected void onCreate(Bundle savedInstanceState) {
	super.onCreate(savedInstanceState);
	setContentView(R.layout.activity_index);
	setTabSelection(position);
}
@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
	position = savedInstanceState.getInt("position");
	setTabSelection(position);
	super.onRestoreInstanceState(savedInstanceState);
}
@Override
protected void onSaveInstanceState(Bundle outState) {
	//記錄當前的position
	outState.putInt("position", position);
}

3 最簡單的使用

  /**
     * 切換選中的item
     * Add Show
     *
     * @param mToTab
     */
    private void setCurrentItem(int mToTab) {
        // 隱藏所有Fragment
        FragmentTransaction transaction = manager.beginTransaction();
        hideFragments(transaction);
        switch (mToTab) {
            case TAB_HOME:
                if (null == mHomeFragment) {
                    mHomeFragment = new HomeFragment();
                    transaction.add(R.id.content, mHomeFragment);
                } else {
                    transaction.show(mHomeFragment);
                }
                break;
//            case TAB_ORDER:
//                if (null == mOrderFragment) {
//                    mOrderFragment = new OrderFragment();
//                    transaction.add(R.id.content, mOrderFragment);
//                } else {
//                    transaction.show(mOrderFragment);
//                }
//                break;
//            case TAB_SHOP:
//                if (null == mShoppingFragment) {
//                    mShoppingFragment = new ShoppingFragment();
//                    transaction.add(R.id.content, mShoppingFragment);
//                } else {
//                    transaction.show(mShoppingFragment);
//                }
//                break;
            case TAB_SELF:
                if (null == mPersonalCenterFragment) {
                    mPersonalCenterFragment = new PersonalCenterFragment();
                    transaction.add(R.id.content, mPersonalCenterFragment);
                } else {
                    transaction.show(mPersonalCenterFragment);
                }
                break;
            case -1:
                new RuntimeException();
                break;
        }
        transaction.commitAllowingStateLoss();
    }
    /**
     * 隱藏所有的fragment避免數據錯亂或者不見
     *
     * @param transaction
     */
    private void hideFragments(FragmentTransaction transaction) {
        if (null != mHomeFragment) {
            transaction.hide(mHomeFragment);
        }
//        if (null != mOrderFragment) {
//        //    transaction.hide(mOrderFragment);
//        }
//        if (null != mShoppingFragment) {
//         //   transaction.hide(mShoppingFragment);
//        }
        if (null != mPersonalCenterFragment) {
            transaction.hide(mPersonalCenterFragment);
        }
    }