blocks|key|4768216|text|我已经找到了解决办法，但我还是不知道为什么会这样。解决方案是在活动运行后将这最后一行直接放到用户调用中。例:在contextMenu回调或任何OnClickListener中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4768217|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I've found a solution but I still don't known why this happen. The solution is put this last line to user call directly after the activity is running. Ex: in a contextMenu callback or in any OnClickListener.

blocks|key|72302|text|虽然Sanf0rds答案是正确的，但它不允许默认情况下将BottomSheet定义为扩展。该问题是由直到最后一行WeakReference才设置的onLayoutChild引起的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|72303|解决方案是提供我们自己的类来扩展BottomSheetBehavior，但是在覆盖的onLayoutChild中设置状态。代码如下所示。|72304|uk/ac/qub/quibe/misc/ExpandedBottomSheetBehavior.java|72305|package+uk.ac.qub.quibe.misc;

import+android.content.Context;
import+android.support.design.widget.CoordinatorLayout;
import+android.util.AttributeSet;
import+android.view.View;

/**
+*+Created+by+mcp+on+15/03/16.
+*/
public+class+ExpandedBottomSheetBehavior<V+extends+View>+extends+android.support.design.widget.BottomSheetBehavior<V>+{

++++public+ExpandedBottomSheetBehavior(Context+context,+AttributeSet+attrs)+{
++++++++super(context,+attrs);
++++}

++++@Override
++++public+boolean+onLayoutChild(final+CoordinatorLayout+parent,+final+V+child,+final+int+layoutDirection)+{
++++++++SavedState+dummySavedState+=+new+SavedState(super.onSaveInstanceState(parent,+child),+STATE_EXPANDED);
++++++++super.onRestoreInstanceState(parent,+child,+dummySavedState);
++++++++return+super.onLayoutChild(parent,+child,+layoutDirection);
++++++++/*
++++++++++++Unfortunately+its+not+good+enough+to+just+call+setState(STATE_EXPANDED);+after+super.onLayoutChild
++++++++++++The+reason+is+that+an+animation+plays+after+calling+setState.+This+can+cause+some+graphical+issues+with+other+layouts
++++++++++++Instead+we+need+to+use+setInternalState,+however+this+is+a+private+method.
++++++++++++The+trick+is+to+utilise+onRestoreInstance+to+call+setInternalState+immediately+and+indirectly
+++++++++*/
++++}

}|code-block|syntax|javascript|72306|在布局文件引用中引用新的自定义行为。|72307|变化|72308|app:layout_behavior="android.support.design.widget.BottomSheetBehavior"|72309|至|72310|app:layout_behavior="uk.ac.qub.quibe.misc.ExpandedBottomSheetBehavior"|72311|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Y|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|Z|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|H|7|12|8|@]|9|@]|A|$I|J]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|T|5|H|7|14|8|@]|9|@]|A|$I|J]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

While Sanf0rds answer is correct, it doesn't allow the ability to define the BottomSheet as expanded by default. The issue is caused by the WeakReference not being set until the last line of onLayoutChild.

The solution is to provide our own class which extends BottomSheetBehavior, but setting the state inside an overridden onLayoutChild. The code is provided below.

uk/ac/qub/quibe/misc/ExpandedBottomSheetBehavior.java

<pre><code>package uk.ac.qub.quibe.misc;

import android.content.Context;
import android.support.design.widget.CoordinatorLayout;
import android.util.AttributeSet;
import android.view.View;

/**
 * Created by mcp on 15/03/16.
 */
public class ExpandedBottomSheetBehavior&lt;V extends View&gt; extends android.support.design.widget.BottomSheetBehavior&lt;V&gt; {

 public ExpandedBottomSheetBehavior(Context context, AttributeSet attrs) {
 super(context, attrs);
 }

 @Override
 public boolean onLayoutChild(final CoordinatorLayout parent, final V child, final int layoutDirection) {
 SavedState dummySavedState = new SavedState(super.onSaveInstanceState(parent, child), STATE_EXPANDED);
 super.onRestoreInstanceState(parent, child, dummySavedState);
 return super.onLayoutChild(parent, child, layoutDirection);
 /*
 Unfortunately its not good enough to just call setState(STATE_EXPANDED); after super.onLayoutChild
 The reason is that an animation plays after calling setState. This can cause some graphical issues with other layouts
 Instead we need to use setInternalState, however this is a private method.
 The trick is to utilise onRestoreInstance to call setInternalState immediately and indirectly
 */
 }

}
</code></pre>

In the layout file reference reference your new custom behavior.

Change

<pre><code>app:layout_behavior="android.support.design.widget.BottomSheetBehavior"
</code></pre>

To

<pre><code>app:layout_behavior="uk.ac.qub.quibe.misc.ExpandedBottomSheetBehavior"
</code></pre>

blocks|key|72342|text|代码的问题是，您试图直接在setState中调用onCreate方法。这将引发一个nullPointer，因为WeakReference尚未初始化。当协调员布局即将放置其子视图时，它将被初始化。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|72343|onLayoutChild(CoordinatorLayout+parent，V子，int+layoutDirection)
当父CoordinatorLayout与布局给定的子视图有关时调用。|blockquote|72344|因此，最好的方法是将peek高度设置为0，并在onItemClick侦听器中显示/隐藏。|72345|我在这里回答了这个问题：https://stackoverflow.com/a/36236743/1314796|offset|length|72346|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/36236743/1314796^0|0|0|0|C|18|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|U|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|V|8|@]|9|@$I|W|J|X|1|Y]]|A|$]]|$1|K|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

The issue with your code is you are trying to call the setState method directly inside onCreate. This is will throw a nullPointer because the WeakReference is not initialized yet. It will get initialized when the Coordinator layout is about to lay its child view.

<blockquote>
 onLayoutChild(CoordinatorLayout parent, V child, int layoutDirection)
 
 Called when the parent CoordinatorLayout is about the lay out the
 given child view.
</blockquote>

So the best approach is set the peek height to 0 and show/hide inside the onItemClick listener. 

I have answered this question here: 
<a href="https://stackoverflow.com/a/36236743/1314796">https://stackoverflow.com/a/36236743/1314796</a>

blocks|key|1271518|text|您还可以考虑侦听全局布局事件，这样您就可以确保在设置折叠状态时已经布置了底部工作表。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1271519|final+View+bottomSheet+=+findViewById(R.id.bottom_sheet);
bottomSheet.getViewTreeObserver().addOnGlobalLayoutListener(new+ViewTreeObserver.OnGlobalLayoutListener()+{
++++++++@Override
++++++++public+void+onGlobalLayout()+{
++++++++++++bottomSheet.getViewTreeObserver().removeOnGlobalLayoutListener(this);
++++++++++++bottomSheetBehavior+=+BottomSheetBehavior.from(bottomSheet);
++++++++++++bottomSheetBehavior.setPeekHeight(300);
++++++++++++bottomSheetBehavior.setState(BottomSheetBehavior.STATE_COLLAPSED);
++++++++}
++++});|code-block|syntax|javascript|1271520|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can also consider listening to the global layout event, this way you'll be sure that the bottomsheet has been laid out when setting the collapsed state. 

<pre><code>final View bottomSheet = findViewById(R.id.bottom_sheet);
bottomSheet.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
 @Override
 public void onGlobalLayout() {
 bottomSheet.getViewTreeObserver().removeOnGlobalLayoutListener(this);
 bottomSheetBehavior = BottomSheetBehavior.from(bottomSheet);
 bottomSheetBehavior.setPeekHeight(300);
 bottomSheetBehavior.setState(BottomSheetBehavior.STATE_COLLAPSED);
 }
 });
</code></pre>

blocks|key|4768290|text|public+class+ExpandedBottomSheetBehavior<V+extends+View>+extends
++++android.support.design.widget.BottomSheetBehavior<V>+{

++public+ExpandedBottomSheetBehavior(Context+context,+AttributeSet+attrs)+{
++++super(context,+attrs);
}

++@Override
++public+boolean+onLayoutChild(final+CoordinatorLayout+parent,+final+V+child,+final+int+layoutDirection)+{
++++return+super.onLayoutChild(parent,+child,+layoutDirection);
++}

++@Override
++public+boolean+onInterceptTouchEvent(CoordinatorLayout+parent,+V+child,+MotionEvent+event)+{
++++try+{
++++++return+super.onInterceptTouchEvent(parent,+child,+event);
++++}+catch+(NullPointerException+ignored)+{
++++++return+false;
++++}
++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|4768291|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>public class ExpandedBottomSheetBehavior&lt;V extends View&gt; extends
 android.support.design.widget.BottomSheetBehavior&lt;V&gt; {

 public ExpandedBottomSheetBehavior(Context context, AttributeSet attrs) {
 super(context, attrs);
}

 @Override
 public boolean onLayoutChild(final CoordinatorLayout parent, final V child, final int layoutDirection) {
 return super.onLayoutChild(parent, child, layoutDirection);
 }

 @Override
 public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
 try {
 return super.onInterceptTouchEvent(parent, child, event);
 } catch (NullPointerException ignored) {
 return false;
 }
 }
}
</code></pre>

<pre><code>LinearLayout bottomSheetViewgroup = (LinearLayout) findViewById(R.id.bottomSheet);

bottomSheetBehavior = BottomSheetBehavior.from(bottomSheetViewgroup);

bottomSheetBehavior.setState(BottomSheetBehavior.STATE_EXPANDED); //this line
</code></pre>

I have this code within my activity's <code>onCreate()</code> method and I'm getting the below NPE exception when the last line is executed:

<blockquote>
 Caused by: java.lang.NullPointerException:
 Attempt to invoke virtual method 'java.lang.Object java.lang.ref.WeakReference.get()' on a null object reference 
 at android.support.design.widget.BottomSheetBehavior.setState(BottomSheetBehavior.java:440)
</blockquote>

NullPointerException with AppCompat BottomSheets

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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LinearLayout bottomSheetViewgroup = (LinearLayout) findViewById(R.id.bottomSheet);bottomSheetBehavior = BottomSheetBehavior.from(bottomSheetViewgroup);bottomShe...

问NullPointerException与AppCompat BottomSheets
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