Android中的FragmentManager的问题

    Fragment是构成灵活UI的重要部分。最近学习到Fragment的使用时候，碰到一个问题。主程序程序布局非常简单：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.SlidingPaneLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/slidingPanelLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <fragment
        android:id="@+id/bookmarkFragment"
        android:name="com.hnb.zzk.slidingpanellayouttest.BookMarkerFragment"
        android:layout_width="100dp"
        android:layout_height="match_parent"
        android:layout_gravity="left"/>

    <fragment
        android:id="@+id/browserFragment"
        android:name="com.hnb.zzk.slidingpanellayouttest.BrowserFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_gravity="right"
        android:layout_weight="1"/>


</android.support.v4.widget.SlidingPaneLayout>

　　就是两个Fragment构成主要布局，然后在Java代码中获取Fragment：

SlidingPaneLayout spl = (SlidingPaneLayout) this.findViewById(R.id.slidingPanelLayout);
spl.setPanelSlideListener(new SlidingPaneLayout.PanelSlideListener() {
    @Override
    public void onPanelSlide(View panel, float slideOffset) {

    }

    @Override
    public void onPanelOpened(View panel) {
        MainActivity.this.getFragmentManager().findFragmentById(R.id.bookmarkFragment).setHasOptionsMenu(true);
    }

    @Override
    public void onPanelClosed(View panel) {
        MainActivity.this.getFragmentManager().findFragmentById(R.id.bookmarkFragment).setHasOptionsMenu(false);
    }
});

　　初步看起来似乎一切顺利，代码也是精简的一塌糊涂。于是乎运行起来看看，在模拟器中却直接奔溃，赶紧加上异常捕捉代码，得到如下结果：

java.lang.NullPointerException
at com.hnb.zzk.slidingpanellayouttest.MainActivity$2.onPanelOpened(MainActivity.java:44)
at android.support.v4.widget.SlidingPaneLayout.dispatchOnPanelOpened(SlidingPaneLayout.java:343)
at android.support.v4.widget.SlidingPaneLayout$DragHelperCallback.onViewDragStateChanged(SlidingPaneLayout.java:1318)
at android.support.v4.widget.ViewDragHelper.setDragState(ViewDragHelper.java:874)
at android.support.v4.widget.ViewDragHelper.forceSettleCapturedViewAt(ViewDragHelper.java:592)
at android.support.v4.widget.ViewDragHelper.settleCapturedViewAt(ViewDragHelper.java:569)
at android.support.v4.widget.SlidingPaneLayout$DragHelperCallback.onViewReleased(SlidingPaneLayout.java:1354)
at android.support.v4.widget.ViewDragHelper.dispatchViewReleased(ViewDragHelper.java:767)
at android.support.v4.widget.ViewDragHelper.releaseViewForPointerUp(ViewDragHelper.java:1395)
at android.support.v4.widget.ViewDragHelper.processTouchEvent(ViewDragHelper.java:1213)
at android.support.v4.widget.SlidingPaneLayout.onTouchEvent(SlidingPaneLayout.java:826)

　　在onPanelOpend()方法里面出现了空指针异常。可能出现空指针异常的就是getFragmentManager()和findFragmentById()两个调用。findFragmentById()应该不太可能出现空指针异常，布局文件中两个fragment明摆着。那么只能说是getFragmentManager()这个调用出现了指针。在Fragment的开发文档中明确指出，Fragment的管理都是通过FragmentManager来实现的，那么问题出现在哪呢？在网上搜了一番终于发现了症结所在。我们引用的是v4兼容包中的Fragment组件，而getFragmentManager()是针对v11+的Fragment的，因此将getFragmentManager()替换成getSupportFragmentManager()就好了。