安卓:启动应用程序时无法从片段中的活动中获取视图

Question

我正在使用NavHostFragment来显示片段。当我试图在碎片应用程序崩溃中获取活动的FloatingActionButton时。

主活动.kt

class MainActivity : AppCompatActivity(), ImageDrawerListDialogFragment.OnImageClickListener {
...
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main) // <-- 
...
}
}

SearchFragment.kt

class SearchFragment : Fragment(), ImageDrawerListDialogFragment.OnImageClickListener {

private lateinit var fab: FloatingActionButton
...
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?): View {
_binding = FragmentSearchBinding.inflate(inflater, container, false)
return binding.root
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
fab = (requireActivity() as MainActivity).findViewById(R.id.floatingActionButton) // <-- 
fab.setOnSafeClickListener { requestPermission() }
...
}

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<com.google.android.material.appbar.AppBarLayout
android:id="@+id/appBar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="@style/Theme.AppName.AppBarOverlay">
<androidx.appcompat.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:layout_scrollFlags="scroll|enterAlways"
app:popupTheme="@style/Theme.AppName.PopupOverlay">
<androidx.appcompat.widget.SearchView
android:id="@+id/searchView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:focusable="true"
android:imeOptions="actionDone"
android:inputType="text"
app:iconifiedByDefault="false" />
</androidx.appcompat.widget.Toolbar>
</com.google.android.material.appbar.AppBarLayout>
<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="@+id/floatingActionButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="bottom|end"
android:layout_margin="@dimen/fab_margin"
android:contentDescription="create new search request"
app:layout_anchorGravity="bottom|end"
app:srcCompat="@android:drawable/ic_input_add" />

<include layout="@layout/content_main" />

</androidx.coordinatorlayout.widget.CoordinatorLayout>

content_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior">
<fragment
android:id="@+id/nav_host_fragment"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="0dp"
android:layout_height="0dp"
app:defaultNavHost="true"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/nav_graph" />
</androidx.constraintlayout.widget.ConstraintLayout>

nav_graph.xml

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/nav_graph"
app:startDestination="@id/SearchFragment">
...
</navigation>

错误

引起原因：android.view.InflateException：com.dmytroa中的二进制XML文件行#36。appName:layout/activity_main:com.dmytroa中的二进制XML文件行#18。appName:layout/content_main:膨胀类片段时出错引起原因：android.view.FonflateException：com.ibm.dmytroa.appName:layout/content_min:膨胀类碎片时出错引起因素：java.lang.NullPointerException：requireActivity((作为MainActivity(.findViewById(R.id.floatingActionButton(不得为空

Answer 1

fab=(requireActivity((asMainActivity(.findViewById(R.id.floatingActionButton(//&lt--fab.setOnSafeClickListener｛requestPermission((｝

我建议你不要"向上伸展"；从Fragment到Activity，以对其布局中存在的视图进行操作。这会在您的片段中创建对该"活动"的显式依赖，如果您每次尝试在其他地方使用该片段，该依赖就会中断。

执行对"活动"本身中的"活动"视图进行操作的逻辑。

主要活动：

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main) 

fab = findViewById(R.id.floatingActionButton)
fab.setOnSafeClickListener { requestPermission() }
...
}