記錄日常BUG

一，在使用ViewPager+Fragment加載多個fragment時，報出異常java.lang.IllegalStateException Fragment already added，現將解決方法記錄如下

Process: com.shgbit.lawwisdom, PID: 10033
    java.lang.IllegalStateException: Fragment already added
        at androidx.fragment.app.Fragment.setInitialSavedState(Fragment.java:709)
        at androidx.fragment.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:183)
        at androidx.viewpager.widget.ViewPager.addNewItem(ViewPager.java:1010)
        at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1158)
        at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1092)
        at androidx.viewpager.widget.ViewPager.onMeasure(ViewPager.java:1622)
        at android.view.View.measure(View.java:24772)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6903)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1562)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:849)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:728)
        at android.view.View.measure(View.java:24772)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6903)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.view.View.measure(View.java:24772)
        at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:735)
        at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:481)
        at android.view.View.measure(View.java:24772)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6903)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.view.View.measure(View.java:24772)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6903)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1562)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:849)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:728)
        at android.view.View.measure(View.java:24772)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6903)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at com.android.internal.policy.DecorView.onMeasure(DecorView.java:930)
        at android.view.View.measure(View.java:24772)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3224)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1987)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2308)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1870)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:8128)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1057)
        at android.view.Choreographer.doCallbacks(Choreographer.java:875)
        at android.view.Choreographer.doFrame(Choreographer.java:776)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1042)
        at android.os.Handler.handleCallback(Handler.java:894)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:213)
        at android.app.ActivityThread.main(ActivityThread.java:8210)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1101)

錯誤的代碼

viewPager.setAdapter(adapter);
viewPager.setOffscreenPageLimit(VIEW_PAGER_DEFAULT_PAGE_NUMBER);

錯誤原因
正是這兩句代碼的順序導致了異常的拋出。原因是當調用setAdapter時，ViewPager調用了populate()去刷新UI，但是是按照默認的兩頁預加載的數量進行刷新，刷新完畢後調用了finishUpdate(),其中調用了commitNowAllowingStateLoss()方法；這之後調用的setOffscreenPageLimit()方法，其中也調用了populate()方法，這樣我們就調用了兩次populate()，也就間接調用了兩次commitNowAllowingStateLoss()方法。

解決方法就是按照正確的順序編寫代碼，先setOffscreenPageLimit(),再setAdapter();

viewPager.setOffscreenPageLimit(VIEW_PAGER_DEFAULT_PAGE_NUMBER);
viewPager.setAdapter(adapter);

2，使用 ScrollView +RecyclerView 時 當切換fragment時 回到 次頁面後，RecyclerView 刷新頁面會定位到屏幕底部
問題：RecyclerView 搶佔焦點
解決辦法： android:descendantFocusability=“blocksDescendants”