用FragmentTabHost管理Fragment，实现页面切换

现在大多数的应用的主页面都有点类似新浪微博的主页面，下方是一个导航栏，点击导航栏会进行页面的切换。从技术实现上来说，这并不复杂，可以有很多方法来实现，比如用radiogroup和fragment配合就可以。可是这样我们并不是我们想要的，因为这样写需要我们自己写不少逻辑去控制。俗话说不会偷懒的程序员不是好程序员，所以我们希望系统给我们提供一个可以对Fragment管理的类，我们不用关心太多的逻辑，只需要做好相应Fragment的显示，那样是不是很爽。值得高兴的是系统真的提供了这样一个类：FragmentTabHost。可是它存在一个缺陷，到后面再说，我们先简单看一下它的用法。
它的用法和别的控件一样需要我们先在布局文件中声明，如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical">
    <!--fragment容器-->
    <FrameLayout
        android:id="@+id/container_fragment"
        android:layout_width="fill_parent"
        android:layout_height="0dip"
        android:layout_weight="1" />
    <!--分割线-->
    <View
        android:layout_width="match_parent"
        android:layout_height="1px"
        android:background="#dcdcdc"
        />
    <!--下方导航栏-->
    <android.support.v4.app.FragmentTabHost
        android:id="@android:id/tabhost"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content">

        <FrameLayout
            android:id="@+id/container_tab"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_weight="0" />
    </android.support.v4.app.FragmentTabHost>
</LinearLayout>
 来自CODE的代码片 activity_main.xml
接下来我们就需要创建导航栏单个item的通用布局，如下：

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

    <ImageView
        android:id="@+id/tab_icon"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:focusable="false"
        android:padding="3dp"/>

    <TextView
        android:id="@+id/tab_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text=""
        android:textColor="@drawable/selector_text"
        android:textSize="15sp"/>


</LinearLayout>
 来自CODE的代码片 tab_item_view.xml 这里只是简单的演示一下用法，Fragment布局里面就一个TextView用于在切换的时候区分，这里就不贴代码了。
接下来我们在activity里面进行设置就可以了，代码如下：

package com.demo.zhangs.demo_fragmenttabhost;

import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTabHost;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.ImageView;
import android.widget.TabHost;
import android.widget.TextView;

/**
 * Created by Admin on 2016/5/18.
 */
public class MainActivity extends FragmentActivity {
    private static final String TAG = "MainActivity";

    //定义FragmentTabHost对象
    private FragmentTabHost mTabHost;

    //定义一个布局
    private LayoutInflater layoutInflater;

    //定义数组来存放Fragment界面
    private Class fragmentArray[] = {Fragment_home.class, Fragment_quotation.class, Fragment_me.class};

    //定义数组来存放按钮图片
    private int mImageViewArray[] = {R.drawable.selector_home, R.drawable.selector_quotation, R.drawable.selector_me};
    //Tab选项卡的文字
    private String mTextviewArray[] = {"首页", "报价单", "我的"};

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();

    }

    /**
     * 初始化组件
     */
    private void initView() {
        //实例化布局对象
        layoutInflater = LayoutInflater.from(this);

        //实例化TabHost对象，得到TabHost
        mTabHost = (FragmentTabHost) findViewById(android.R.id.tabhost);
        mTabHost.setup(this, getSupportFragmentManager(), R.id.container_fragment);

        //得到fragment的个数
        int count = fragmentArray.length;

        for (int i = 0; i < count; i++) {
            //为每一个Tab按钮设置图标、文字和内容
            TabHost.TabSpec tabSpec = mTabHost.newTabSpec(mTextviewArray[i]).setIndicator(getTabItemView(i));
            //将Tab按钮添加进Tab选项卡中
            mTabHost.addTab(tabSpec, fragmentArray[i], null);
            //可以根据自己的需求设置Tab按钮的背景
//            mTabHost.getTabWidget().getChildAt(i).setBackgroundResource(R.drawable.selector_tab_background);
        }
    }

    /**
     * 给Tab按钮设置图标和文字
     */
    private View getTabItemView(int index) {
        View view = layoutInflater.inflate(R.layout.tab_item_view, null);

        ImageView imageView = (ImageView) view.findViewById(R.id.tab_icon);
        imageView.setImageResource(mImageViewArray[index]);

        TextView textView = (TextView) view.findViewById(R.id.tab_text);
        textView.setText(mTextviewArray[index]);

        return view;
    }
}
 来自CODE的代码片 MainActivity.java 简单看一下效果：

ok~效果出来了  上面说过它存在一个缺陷，缺陷就是每次FragmentTabHost切换fragment时会重新调用onCreateView()重新绘制UI。假如我们在onCreateView()中有网络操作，在切换的时候也会重复进行，这样当然不是我们希望的。当然也有解决方法，我们可以在onCreateView()方法里面做一些处理：
private View rootView;// 缓存Fragment view

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

    if (rootView == null)
    {
      rootView = inflater.inflate(R.layout.fragment_home, null);
      //假如有网络操作建议放在这里面，避免重复加载
   }
    // 缓存的rootView需要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) rootView.getParent();
    if (parent != null)
    {
      parent.removeView(rootView);
    }
    return rootView;
  }
到这也算是比较圆满了，可是大家想过没有，假如它内部能够通过hide和show来管理fragment多好，这样我们就不用做以上处理，而且在效率上也会高了很多。下面我就贴一个重写了的FragmentTabHost它的内部就是通过hide和show来隐藏和显示Fragment的，而且它的用法和系统的完全一样。大家可以导入到自己的项目中，直接使用。
  
/*
 * Copyright (C) 2012 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.demo.zhangs.demo_fragmenttabhost;

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 java.util.ArrayList;

/**
 * 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.
 * 
 * <p>
 * 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}
 * 
 * <p>
 * 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<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;

    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 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 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(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) {
//                    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;
    }
}