Android FragmentTabHost分析

Android

WHAT

FragmentTabHost extends TabHost

基本功能继承了TabHost,Content使用Fragment展示。实现底部、顶部Tab，其他区域展示内容的效果。

// ContentView
private FrameLayout mRealTabContent;
private Context mContext;
private FragmentManager mFragmentManager;
// ContentView的ID
private int mContainerId;
private TabHost.OnTabChangeListener mOnTabChangeListener;
// 上次切换的Tab信息
private TabInfo mLastTab;
private boolean mAttached;

WHY

实现了TabHost.TabContentFactoryFactory

static class DummyTabFactory implements TabHost.TabContentFactory {
    private final Context mContext;
    public DummyTabFactory(Context context) {
        mContext = context;
    }
    @Override
    public View createTabContent(String tag) {
        // 这个貌似没有用到，真实的ContentView是mRealTabContent,其ID就是mContainerId
        View v = new View(mContext);
        v.setMinimumWidth(0);
        v.setMinimumHeight(0);
        return v;
    }
}

初始化

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

确保有TabWidget和ConentView

// 自动生成ViewTree，上面是TabWidget,下面是Content
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));
    }
}
// 确保必须有指定的mContainerId
private void ensureContent() {
    if (mRealTabContent == null) {
        mRealTabContent = (FrameLayout)findViewById(mContainerId);
        if (mRealTabContent == null) {
            throw new IllegalStateException(
                    "No tab content FrameLayout found for id " + mContainerId);
        }
    }
}

如果布局中的FragmentTabHost有指定android:inflatedId则使用这个方法初始化，一般是使用布局设置的情况。

setup(Context context, FragmentManager manager)

或者指定一个containerId，一般用在java代码中写布局的情况。

setup(Context context, FragmentManager manager, int containerId)

切换Tab,通过FragmentManager的事物来管理的

public void onTabChanged(String tabId) {
    if (mAttached) {
        final FragmentTransaction ft = doTabChanged(tabId, null);
        if (ft != null) {
            ft.commit();
        }
    }
    if (mOnTabChangeListener != null) {
        mOnTabChangeListener.onTabChanged(tabId);
    }
}
@Nullable
private FragmentTransaction doTabChanged(@Nullable String tag,
            @Nullable FragmentTransaction ft) {
        final TabInfo newTab = getTabInfoForTag(tag);
        if (mLastTab != newTab) {
            if (ft == null) {
            ft = mFragmentManager.beginTransaction();
        }
        if (mLastTab != null) {
            if (mLastTab.fragment != null) {
                ft.detach(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);
            }
        }
        mLastTab = newTab;
    }
    return ft;
}

每次新旧的切换是使用detach和attach，所以会造成Fragment的生命周期（onCreatedView）重复执行，导致每次都会重新加载布局等问题。
解决方案有2个：
1. 重写doTabChanged方法，使用FragmentTransaction的show/hide来切换显示、隐藏。
2. 在Fragment的onCreateView中判断这个rootView是否已经存在，不存在才加载View。

HOW TO USE

布局中使用,基本上和TabHost一样，只是把TabHost控件换成了FragmentTabHost

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.app.FragmentTabHost xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
        <FrameLayout
            android:id="@android:id/tabcontent"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="1" />
        <TabWidget
            android:id="@android:id/tabs"
            android:layout_width="match_parent"
            android:layout_height="48dp" />
    </LinearLayout>
</android.support.v4.app.FragmentTabHost>

在Java代码中设置

FragmentTabHost tabHost = (FragmentTabHost) findViewById(android.R.id.tabhost);
        // 指定content id
//        tabHost.setup(this, getSupportFragmentManager(), android.R.id.tabcontent);
        // content id = xml 中设置的inflatedId
        tabHost.setup(this, getSupportFragmentManager());
        TabHost.TabSpec tabSpec1 = tabHost.newTabSpec("TabA").setIndicator("TabA");
        TabHost.TabSpec tabSpec2 = tabHost.newTabSpec("TabB").setIndicator("TabB");
        tabHost.addTab(tabSpec1, Tab1Fragment.class, null);
        tabHost.addTab(tabSpec2, Tab2Fragment.class, null);