android - NullPointerException with AppCompat BottomSheets

Question

LinearLayout bottomSheetViewgroup = (LinearLayout) findViewById(R.id.bottomSheet);

bottomSheetBehavior = BottomSheetBehavior.from(bottomSheetViewgroup);

bottomSheetBehavior.setState(BottomSheetBehavior.STATE_EXPANDED); //this line

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

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)

Answer 1

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

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
         */
    }

}

In the layout file reference reference your new custom behavior.

Change

app:layout_behavior="android.support.design.widget.BottomSheetBehavior"

To

app:layout_behavior="uk.ac.qub.quibe.misc.ExpandedBottomSheetBehavior"