[LinearLayout中的getLayoutParams（）返回null

Question

我想在活动屏幕上实现BottomSheetBehavoir组件，但是当我想通过BottomSheetBehavoir.from(view)初始化该组件时，getLayoutParams（）方法会引发空指针异常。

这是我在onCreate（）中的初始化代码：

LinearLayout llBottomSheet = findViewById(R.id.bottom_sheet_live_tracking_list_container);
BottomSheetBehavior bottomSheetBehavior = BottomSheetBehavior.from(llBottomSheet);

这是我的xml布局：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/bottom_sheet_live_tracking_list_container"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@color/white"
app:behavior_hideable="false"
app:behavior_peekHeight="40dp"
app:layout_behavior="com.google.android.material.bottomsheet.BottomSheetBehavior">

<LinearLayout
    android:id="@+id/linearLayout2"
    android:layout_width="match_parent"
    android:layout_height="40dp"
    android:orientation="horizontal"
    android:background="@drawable/rounded_corners_gray">


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:text="@string/show_more"
        android:textColor="@color/white"
        android:layout_gravity="center"
        android:gravity="center"
        android:layout_weight="1"
        android:id="@+id/activity_live_tracking_show_more_tv" />

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_weight="0.1"
        android:src="@drawable/ic_filter_list"
        android:visibility="visible" />
</LinearLayout>

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/activity_live_tracking_position_list_rv"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

我通过include标记将此布局包含在我的根xml布局中。

错误代码：

2020-02-21 12:34:05.097 8419-8419/de.app.haveltec.ilockit E/AndroidRuntime: FATAL EXCEPTION: main
Process: de.app.haveltec.ilockit, PID: 8419
java.lang.RuntimeException: Unable to start activity ComponentInfo{de.app.haveltec.ilockit/de.app.haveltec.ilockit.screens.livetracking.LiveTrackingActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewGroup$LayoutParams android.view.View.getLayoutParams()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2984)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3119)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1839)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:201)
    at android.app.ActivityThread.main(ActivityThread.java:6861)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.ViewGroup$LayoutParams android.view.View.getLayoutParams()' on a null object reference
    at com.google.android.material.bottomsheet.BottomSheetBehavior.from(BottomSheetBehavior.java:981)
    at de.app.haveltec.ilockit.screens.livetracking.LiveTrackingActivity.onCreate(LiveTrackingActivity.java:147)
    at android.app.Activity.performCreate(Activity.java:7232)
    at android.app.Activity.performCreate(Activity.java:7221)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2964)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3119) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1839) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:201) 
    at android.app.ActivityThread.main(ActivityThread.java:6861) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 

Answer 1

如documentation BottomSheetBehaviour中所述。如果仍要使用它，请确保它是CoordinatorLayout的子级，因为这是根据文档使用它的先决条件。宁愿将此答案添加为评论，但声望不足...