解决重写FragmentTabHost后，屏幕切换时Fragment显示空白的问题

问题背景：

公司的App首页架构采用了FragmentTabHost + Fragment的结构。之前为了解决每次切换 tab 时，Fragment不重新加载的问题，重写了FragmentTabHost类，重写的类核心代码如下：

 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);
        }

        // 防止画面被回收后Fragment白屏
        if (isRestored) {
            if (mTabs != null && mTabs.size() > 0) {
                for (int i=0; i<mTabs.size(); i++) {
                    TabInfo tab = mTabs.get(i);
                    if (!tab.tag.equals(mLastTab.tag)) {
                        tab.fragment = null;
                    }
                }
            }
            isRestored = false;
        }


        if (mLastTab != newTab) {
            if (ft == null) {
                ft = mFragmentManager.beginTransaction();
            }
            if (mLastTab != null) {
                if (mLastTab.fragment != null) {
//                    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 {
//                    ft.attach(newTab.fragment);
                    ft.show(newTab.fragment);

                }
            }

            mLastTab = newTab;
        }
        return ft;
    }

其实质就是将原FragmentTabHost中的  ft.detach、ft.attach 替换为 ft.hide、ft.show。

这样做确实达到了切换tab时不用重新走Fragment生命周期的目标。但是相关的副作用也出现了：那就是在宿主Activity被回收之后，重新打开App，宿主Activity重新创建后，切换tab时，会出现Fragment白屏的现象。

问题的原因就在于，在调用ft.show方法时，不会重新走Fragment的onCreateView方法。而在整体Activity被回收后，相应的Fragment中的画面元素也被回收，要想重建画面元素，必须调用相应Fragment的onCreateView方法。

那么如何调用onCreateView方法呢？

经过仔细的研究我们重写的FragmentTabHost中的代码之后发现，主要是两种思路：

第一种是在调用 ft.show 时加入判断，如果是被回收后并再次显示的时候，调用 ft.attch，但是这样做有一个问题，那就是如果tab比较多的情况下，需要标记本tabHost中哪些Fragment已经Attch过，哪些没有Attach过，逻辑比较复杂。

还有一种方式就是当我们的宿主Activity已经被回收重建之后，告诉 tabHost 清除之前缓存的Fragment地址。这样在我们切换tab时就会重新创建Fragment，问题自然得到解决。

最后经过权衡对比，我决定使用第二种方式来解决问题。

实现的思路：

1.在我们自己重写的FragmentTabHost类中加入布尔字段：isRestored，

2.在画面回收重建方法中将此字段置为true

3.在doTabChanged方法中利用此字段进行判断，如果为true，则清空 tabHost类中的除要显示的tab外的其他tab的fragment对象缓存。

核心代码如下：

    /**
     * 防止画面回收后重启白屏
     */
    private boolean isRestored;

    @Override
    protected void onRestoreInstanceState(Parcelable state) {
        SavedState ss = (SavedState)state;
        super.onRestoreInstanceState(ss.getSuperState());
        setCurrentTabByTag(ss.curTab);
        // 防止画面被回收后Fragment白屏
        isRestored = true;
    }

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);
        }

        // 防止画面被回收后Fragment白屏
        if (isRestored) {
            if (mTabs != null && mTabs.size() > 0) {
                for (int i=0; i<mTabs.size(); i++) {
                    TabInfo tab = mTabs.get(i);
                    if (!tab.tag.equals(mLastTab.tag)) {
                        tab.fragment = null;
                    }
                }
            }
            isRestored = false;
        }


        if (mLastTab != newTab) {
            if (ft == null) {
                ft = mFragmentManager.beginTransaction();
            }
            if (mLastTab != null) {
                if (mLastTab.fragment != null) {
//                    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 {
//                    ft.attach(newTab.fragment);
                    ft.show(newTab.fragment);

                }
            }

            mLastTab = newTab;
        }
        return ft;
    }

完整的类的代码如下：

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;

import net.proding.module.base.BaseFragment;

import java.util.ArrayList;
import java.util.List;

/**
 * 用于改善切换tab时不用重新加载
 */
public class CommonFragmentTabHost extends TabHost
        implements TabHost.OnTabChangeListener {
    private final ArrayList<TabInfo> mTabs = new ArrayList<TabInfo>();
    private FrameLayout mRealTabContent;
    private Context mContext;
    private FragmentManager mFragmentManager;
    private int mContainerId;
    private OnTabChangeListener mOnTabChangeListener;
    private TabInfo mLastTab;
    private boolean mAttached;
    /**
     * 防止画面回收后重启白屏
     */
    private boolean isRestored;

    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 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 Creator<SavedState> CREATOR
                = new Creator<SavedState>() {
            public SavedState createFromParcel(Parcel in) {
                return new SavedState(in);
            }

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

    public CommonFragmentTabHost(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 CommonFragmentTabHost(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 LayoutParams(
                    ViewGroup.LayoutParams.FILL_PARENT,
                    ViewGroup.LayoutParams.FILL_PARENT));

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

        // 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);
        // 防止画面被回收后Fragment白屏
        isRestored = true;
    }

    @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);
        }

        // 防止画面被回收后Fragment白屏
        if (isRestored) {
            if (mTabs != null && mTabs.size() > 0) {
                for (int i=0; i<mTabs.size(); i++) {
                    TabInfo tab = mTabs.get(i);
                    if (!tab.tag.equals(mLastTab.tag)) {
                        tab.fragment = null;
                    }
                }
            }
            isRestored = false;
        }


        if (mLastTab != newTab) {
            if (ft == null) {
                ft = mFragmentManager.beginTransaction();
            }
            if (mLastTab != null) {
                if (mLastTab.fragment != null) {
//                    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 {
//                    ft.attach(newTab.fragment);
                    ft.show(newTab.fragment);

                }
            }

            mLastTab = newTab;
        }
        return ft;
    }

    public void reloadTabFragment(String tabId) {
        List<Fragment> fragments = mFragmentManager.getFragments();

        if(fragments == null) return;

        for (Fragment f : fragments){

            if(f != null && f.getArguments() != null && tabId.equals(f.getArguments().getString("tabId"))){
                ((BaseFragment) f).reload();
            }
        }
    }

    public Fragment getFragment(String tabId) {
        List<Fragment> fragments = mFragmentManager.getFragments();

        Fragment fragment = null;
        if(fragments == null) return fragment;

        for (Fragment f : fragments){

            if(f != null && f.getArguments() != null && tabId.equals(f.getArguments().getString("tabId"))){
                fragment = f;
                break;
            }
        }

        return fragment;
    }
}

亲测问题已经解决。时间紧迫的小伙伴可以直接将拿来主义进行到底。

谢谢！