一、常见的底部菜单（底部导航栏）的实现方式
常见的实现方式有：
- TabHost+Activity：资源开销比较大，官方已经不推荐使用。
- RadioButton（RadioGroup）+Fragment：实现起来比较麻烦。
- FragmentTabHost+Fragment：实现简单，资源开销小，推荐使用。
二、FragmentTabHost介绍
如下图所示，整一个底部导航栏是一个FragmentTabHost，里面包含的每一个“小按钮”我们称之为TabSpec，也就是每一个分页。TabSpec里面需要有指示器Indicator，用来指示用户选中了哪一个，里面一般包含一张图片和文字描述。
三、FragmentTabHost具体实现方法
核心的实现步骤以及注意点有：
- 所用的Activity必须要继承FragmentActivity，不然项目就会崩溃。
- 调用FragmentTabHost的setup()方法，设置FragmentManager，以及指定用于装载Fragment的布局容器。
- 调用FragmentTabHost的addTab()方法添加分页。
四、代码
要使用FragmentTabHost，首先需要布局中添加进来，这里我们并没有使用官方提供的v4支持包中的FragmentTabHost，而是使用了我们自定义的FragmentTabHost，主要是因为官方提供的FragmentTabHost并没有进行优化，每次都会重新初始化一次Fragment。自定义FragmentTabHost的代码会在附件给出。
第一个FrameLayout是用于装载Fragment的，第二个Fragment只是官方文档要求我们这样写的而已，官方要求我们将Fragment放在导航栏之下，与我们的需求刚好相反了。
再仔细看布局文件，主要是通过LinearLayout的weight属性来控制整个竖直方向的分配，具体不再赘述。
<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"
tools:context=".ui.activity.MainActivity">
<FrameLayout
android:id="@+id/realtabcontent"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:background="@color/bg_color"/>
<com.nan.cnshop.widget.FragmentTabHost
android:id="@android:id/tabhost"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@color/white">
<FrameLayout
android:id="@android:id/tabcontent"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_weight="0"/>
</com.nan.cnshop.widget.FragmentTabHost>
</LinearLayout>
在代码当中，先通过ID找到FragmentTabHost，然后就可以进行配置了，具体看代码的注释。
public class MainActivity extends BaseActivity {
private FragmentTabHost tabHost;
//用于装载每一个分页的Tab
private List<Tab> tabs = null;
@Override
public void initView() {
setContentView(R.layout.activity_main);
tabHost = (FragmentTabHost) findViewById(android.R.id.tabhost);
}
@Override
public void initData() {
initTabs();
}
private void initTabs() {
//调用setup()方法，设置FragmentManager，以及指定用于装载Fragment的布局容器
tabHost.setup(this, getSupportFragmentManager(), R.id.realtabcontent);
//新建5个分页，并且添加到List当中，便于管理，其中的图标我们使用了selector进行状态选择，即选中的时候会变色。
Tab home = new Tab(HomeFragment.class, R.string.home, R.drawable.selector_icon_home);
Tab hot = new Tab(HotFragment.class, R.string.hot, R.drawable.selector_icon_hot);
Tab cate = new Tab(CategoryFragment.class, R.string.catagory, R.drawable.selector_icon_category);
Tab cart = new Tab(CartFragment.class, R.string.cart, R.drawable.selector_icon_cart);
Tab mine = new Tab(MineFragment.class, R.string.mine, R.drawable.selector_icon_mine);
tabs = new ArrayList<>();
tabs.add(home);
tabs.add(hot);
tabs.add(cate);
tabs.add(cart);
tabs.add(mine);
for (Tab tab : tabs) {
//新建5个TabSpec，并且设置好它的Indicator
TabHost.TabSpec tabSpec = tabHost.newTabSpec(getString(tab.getTitle()));
View view = View.inflate(this, R.layout.tab_indicator, null);
TextView tv_tab_txt = (TextView) view.findViewById(R.id.txt_indicator);
ImageView iv_tab_icon = (ImageView) view.findViewById(R.id.icon_tab);
tv_tab_txt.setText(tab.getTitle());
iv_tab_icon.setImageResource(tab.getIcon());
tabSpec.setIndicator(view);
//把每个TabSpec添加到FragmentTabHost里面
tabHost.addTab(tabSpec, tab.getFragment(), null);
}
//去掉分隔线
tabHost.getTabWidget().setShowDividers(LinearLayout.SHOW_DIVIDER_NONE);
//设置当前默认的分页为第一页
tabHost.setCurrentTab(0);
}
@Override
public void initListener() {
}
@Override
public void processClick(View v) {
}
}
需要注意的是，BaseActivity已经继承了FragmentActivity了。每一个TabSpec中Indicator的布局文件只有一个ImageView和一个TextView，具体布局代码如下：
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_gravity="center"
android:gravity="center"
android:orientation="vertical"
android:paddingBottom="3dp"
android:paddingTop="3dp">
<ImageView
android:id="@+id/icon_tab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
<TextView
android:id="@+id/txt_indicator"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="2dp"
android:textColor="@color/selector_tab_text"/>
</LinearLayout>
最需要注意的是我们的TextView的文字颜色是通过selector进行状态选择的。需要注意的是，这并不是图片，只是颜色，不能放在drawable目录下，而应该放在color目录下。
六、效果
七、附件-FragmentTabHost的源代码
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;
/**
* 功能描述：修改过的FragmentTabHost,保存fragment实例不销毁
*/
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.commitAllowingStateLoss();
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;
}
}
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