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：

接下来我们就需要创建导航栏单个item的通用布局，如下：

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;
}
}

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.
*
*
* 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 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 CREATOR = new 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 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;
}
}

最后我们来想一想，用它来管理Fragment的好处是什么。比如说，需求发生了改变，我们需要添加或删除Fragment的时候，我们只需要，在Class数组添加或删除相应的Fragment的calss，别的逻辑都不需要改变就可以达到我们想要的效果。ok~就写这么多了。