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Android中的FragmentManager的问题

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11/25
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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。

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