FragmentTabHost中切换Fragment保存状态的2种方法

方法1.

重写FragmentTabHost类

FragmentTabHost不保存状态时因为切换Fragment时使用detach和attach来隐藏和显示Fragment的，所以导致每次切换都会重新CreateView，我们可以重写FragmentTabHost类，将其中的detach和attach方法替换为hide和show，这样就可以保存状态了。

FragmentTabHost类：

 import java.util.ArrayList;
 import android.content.Context;
 import android.content.res.TypedArray;
 import android.os.Bundle;
 import android.os.Parcel;
 import android.os.Parcelable;
 import android.support.v4.app.Fragment;
 import android.support.v4.app.FragmentManager;
 import android.support.v4.app.FragmentTransaction;
 import android.util.AttributeSet;
 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.FrameLayout;
 import android.widget.LinearLayout;
 import android.widget.TabHost;
 import android.widget.TabWidget;

 /**
  * Special TabHost that allows the use of {@link Fragment} objects for its tab
  * content. When placing this in a view hierarchy, after inflating the hierarchy
  * you must call {@link #setup(Context, FragmentManager, int)} to complete the
  * initialization of the tab host.
  *
  *
  * Here is a simple example of using a FragmentTabHost in an Activity:
  *
  * {@sample
  * development/samples/Support4Demos/src/com/example/android/supportv4/app/
  * FragmentTabs.java complete}
  *
  *
  * This can also be used inside of a fragment through fragment nesting:
  *
  * {@sample
  * development/samples/Support4Demos/src/com/example/android/supportv4/app/
  * FragmentTabsFragmentSupport.java complete}
  */
 public class FragmentTabHost extends TabHost implements
         TabHost.OnTabChangeListener {
     private final ArrayList mTabs = new ArrayList();
     private FrameLayout mRealTabContent;
     private Context mContext;
     private FragmentManager mFragmentManager;
     private int mContainerId;
     private TabHost.OnTabChangeListener mOnTabChangeListener;
     private TabInfo mLastTab;
     private boolean mAttached;

     static final class TabInfo {
         private final String tag;
         private final Class clss;
         private final Bundle args;
         private Fragment fragment;

         TabInfo(String _tag, Class _class, Bundle _args) {
             tag = _tag;
             clss = _class;
             args = _args;
         }
     }

     static class DummyTabFactory implements TabHost.TabContentFactory {
         private final Context mContext;

         public DummyTabFactory(Context context) {
             mContext = context;
         }

         @Override
         public View createTabContent(String tag) {
             View v = new View(mContext);
             v.setMinimumWidth(0);
             v.setMinimumHeight(0);
             return v;
         }
     }

     static class SavedState extends BaseSavedState {
         String curTab;

         SavedState(Parcelable superState) {
             super(superState);
         }

         private SavedState(Parcel in) {
             super(in);
             curTab = in.readString();
         }

         @Override
         public void writeToParcel(Parcel out, int flags) {
             super.writeToParcel(out, flags);
             out.writeString(curTab);
         }

         @Override
         public String toString() {
             return "FragmentTabHost.SavedState{"
                     + Integer.toHexString(System.identityHashCode(this))
                     + " curTab=" + curTab + "}";
         }

         public static final Parcelable.Creator CREATOR = new Parcelable.Creator() {
             public SavedState createFromParcel(Parcel in) {
                 return new SavedState(in);
             }

             public SavedState[] newArray(int size) {
                 return new SavedState[size];
             }
         };
     }

     public FragmentTabHost(Context context) {
         // Note that we call through to the version that takes an AttributeSet,
         // because the simple Context construct can result in a broken object!
         super(context, null);
         initFragmentTabHost(context, null);
     }

     public FragmentTabHost(Context context, AttributeSet attrs) {
         super(context, attrs);
         initFragmentTabHost(context, attrs);
     }

     private void initFragmentTabHost(Context context, AttributeSet attrs) {
         TypedArray a = context.obtainStyledAttributes(attrs,
                 new int[] { android.R.attr.inflatedId }, 0, 0);
         mContainerId = a.getResourceId(0, 0);
         a.recycle();

         super.setOnTabChangedListener(this);
     }

     private void ensureHierarchy(Context context) {
         // If owner hasn't made its own view hierarchy, then as a convenience
         // we will construct a standard one here.
         if (findViewById(android.R.id.tabs) == null) {
             LinearLayout ll = new LinearLayout(context);
             ll.setOrientation(LinearLayout.VERTICAL);
             addView(ll, new FrameLayout.LayoutParams(
                     ViewGroup.LayoutParams.MATCH_PARENT,
                     ViewGroup.LayoutParams.MATCH_PARENT));

             TabWidget tw = new TabWidget(context);
             tw.setId(android.R.id.tabs);
             tw.setOrientation(TabWidget.HORIZONTAL);
             ll.addView(tw, new LinearLayout.LayoutParams(
                     ViewGroup.LayoutParams.MATCH_PARENT,
                     ViewGroup.LayoutParams.WRAP_CONTENT, 0));

             FrameLayout fl = new FrameLayout(context);
             fl.setId(android.R.id.tabcontent);
             ll.addView(fl, new LinearLayout.LayoutParams(0, 0, 0));

             mRealTabContent = fl = new FrameLayout(context);
             mRealTabContent.setId(mContainerId);
             ll.addView(fl, new LinearLayout.LayoutParams(
                     LinearLayout.LayoutParams.MATCH_PARENT, 0, 1));
         }
     }

     /**
      * @deprecated Don't call the original TabHost setup, you must instead call
      *             {@link #setup(Context, FragmentManager)} or
      *             {@link #setup(Context, FragmentManager, int)}.
      */
     @Override
     @Deprecated
     public void setup() {
         throw new IllegalStateException(
                 "Must call setup() that takes a Context and FragmentManager");
     }

     public void setup(Context context, FragmentManager manager) {
         ensureHierarchy(context); // Ensure views required by super.setup()
         super.setup();
         mContext = context;
         mFragmentManager = manager;
         ensureContent();
     }

     public void setup(Context context, FragmentManager manager, int containerId) {
         ensureHierarchy(context); // Ensure views required by super.setup()
         super.setup();
         mContext = context;
         mFragmentManager = manager;
         mContainerId = containerId;
         ensureContent();
         mRealTabContent.setId(containerId);

         // We must have an ID to be able to save/restore our state. If
         // the owner hasn't set one at this point, we will set it ourself.
         if (getId() == View.NO_ID) {
             setId(android.R.id.tabhost);
         }
     }

     private void ensureContent() {
         if (mRealTabContent == null) {
             mRealTabContent = (FrameLayout) findViewById(mContainerId);
             if (mRealTabContent == null) {
                 throw new IllegalStateException(
                         "No tab content FrameLayout found for id "
                                 + mContainerId);
             }
         }
     }

     @Override
     public void setOnTabChangedListener(OnTabChangeListener l) {
         mOnTabChangeListener = l;
     }

     public void addTab(TabHost.TabSpec tabSpec, Class clss, Bundle args) {
         tabSpec.setContent(new DummyTabFactory(mContext));
         String tag = tabSpec.getTag();

         TabInfo info = new TabInfo(tag, clss, args);

         if (mAttached) {
             // If we are already attached to the window, then check to make
             // sure this tab's fragment is inactive if it exists. This shouldn't
             // normally happen.
             info.fragment = mFragmentManager.findFragmentByTag(tag);
             if (info.fragment != null && !info.fragment.isDetached()) {
                 FragmentTransaction ft = mFragmentManager.beginTransaction();
 //              ft.detach(info.fragment);
                 ft.hide(info.fragment);
                 ft.commit();
             }
         }

         mTabs.add(info);
         addTab(tabSpec);
     }

     @Override
     protected void onAttachedToWindow() {
         super.onAttachedToWindow();

         String currentTab = getCurrentTabTag();

         // Go through all tabs and make sure their fragments match
         // the correct state.
         FragmentTransaction ft = null;
         for (int i = 0; i < mTabs.size(); i++) {
             TabInfo tab = mTabs.get(i);
             tab.fragment = mFragmentManager.findFragmentByTag(tab.tag);
 //          if (tab.fragment != null && !tab.fragment.isDetached()) {
             if (tab.fragment != null) {
                 if (tab.tag.equals(currentTab)) {
                     // The fragment for this tab is already there and
                     // active, and it is what we really want to have
                     // as the current tab. Nothing to do.
                     mLastTab = tab;
                 } else {
                     // This fragment was restored in the active state,
                     // but is not the current tab. Deactivate it.
                     if (ft == null) {
                         ft = mFragmentManager.beginTransaction();
                     }
 //                  ft.detach(tab.fragment);
                     ft.hide(tab.fragment);
                 }
             }
         }

         // We are now ready to go. Make sure we are switched to the
         // correct tab.
         mAttached = true;
         ft = doTabChanged(currentTab, ft);
         if (ft != null) {
             ft.commit();
             mFragmentManager.executePendingTransactions();
         }
     }

     @Override
     protected void onDetachedFromWindow() {
         super.onDetachedFromWindow();
         mAttached = false;
     }

     @Override
     protected Parcelable onSaveInstanceState() {
         Parcelable superState = super.onSaveInstanceState();
         SavedState ss = new SavedState(superState);
         ss.curTab = getCurrentTabTag();
         return ss;
     }

     @Override
     protected void onRestoreInstanceState(Parcelable state) {
         SavedState ss = (SavedState) state;
         super.onRestoreInstanceState(ss.getSuperState());
         setCurrentTabByTag(ss.curTab);
     }

     @Override
     public void onTabChanged(String tabId) {
         if (mAttached) {
             FragmentTransaction ft = doTabChanged(tabId, null);
             if (ft != null) {
                 ft.commit();
             }
         }
         if (mOnTabChangeListener != null) {
             mOnTabChangeListener.onTabChanged(tabId);
         }
     }

     private FragmentTransaction doTabChanged(String tabId,
             FragmentTransaction ft) {
         TabInfo newTab = null;
         for (int i = 0; i < mTabs.size(); i++) {
             TabInfo tab = mTabs.get(i);
             if (tab.tag.equals(tabId)) {
                 newTab = tab;
             }
         }
         if (newTab == null) {
             throw new IllegalStateException("No tab known for tag " + tabId);
         }
         if (mLastTab != newTab) {
             if (ft == null) {
                 ft = mFragmentManager.beginTransaction();
             }
             if (mLastTab != null) {
                 if (mLastTab.fragment != null) {
                     //将detach替换为hide，隐藏Fragment
 //                  ft.detach(mLastTab.fragment);
                     ft.hide(mLastTab.fragment);
                 }
             }
             if (newTab != null) {
                 if (newTab.fragment == null) {
                     newTab.fragment = Fragment.instantiate(mContext,
                             newTab.clss.getName(), newTab.args);
                     ft.add(mContainerId, newTab.fragment, newTab.tag);
                 } else {
                     //将attach替换为show，显示Fragment
 //                  ft.attach(newTab.fragment);
                     ft.show(newTab.fragment);
                 }
             }

             mLastTab = newTab;
         }
         return ft;
     }
 }

方法2.缓存Fragment中的View

在Fragment子类中加入一个成员变量，用来保存onCreateView中将要返回的View，当每次重绘时判断此成员变量是否为空，为空的时候重绘此页面，否则返回此成员变量（这个办法可以选择性地保存不同Fragment切换状态）。

 View mView;
 @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

         if(mView == null) {
             mView = inflater.inflate(R.layout.fragment_mine, container, false);
             init();
         }
         //判断是否已经被加过parent， 如果有parent需要从parent删除，要不然会发生这个rootview已经有parent的错误。
         ViewGroup parent = (ViewGroup)mView.getParent();
         if(parent != null) {
             parent.removeView(mView);
         }

         return mView;
     }