attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
	
    <declare-styleable name="ExpandablePanel">
        <attr name="handle" format="reference" />
        <attr name="content" format="reference" />
        <attr name="contentContainer" format="reference" />
        <attr name="animationDuration" format="integer"/>
    </declare-styleable>

</resources>

ExpandablePanel.java

package some.awesome.package;

import android.content.Context;
import android.content.res.TypedArray;
import android.util.AttributeSet;
import android.view.View;
import android.view.animation.Animation;
import android.view.animation.Animation.AnimationListener;
import android.view.animation.Transformation;
import android.widget.RelativeLayout;

public class ExpandablePanel extends RelativeLayout {

	private static final int DEFAULT_ANIM_DURATION = 500;

	private final int mHandleId;
	private final int mContentContainerId;
	private final int mContentId;

	private View mHandle;
	private View mContentContainer;
	private View mContent;

	private boolean mExpanded = false;
	private boolean mFirstOpen = true;

	private int mCollapsedHeight;
	private int mContentHeight;
	private int mContentWidth;
	private int mAnimationDuration = 0;

	private OnExpandListener mListener;

	public ExpandablePanel(Context context) {
		this(context, null);
	}

	public ExpandablePanel(Context context, AttributeSet attrs) {
		super(context, attrs);

		TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ExpandablePanel, 0, 0);

		mAnimationDuration = a.getInteger(R.styleable.ExpandablePanel_animationDuration, DEFAULT_ANIM_DURATION);

		int handleId = a.getResourceId(R.styleable.ExpandablePanel_handle, 0);
		if (handleId == 0) {
			throw new IllegalArgumentException(
				"The handle attribute is required and must refer to a valid child.");
		}

		int contentContainerId = a.getResourceId(R.styleable.ExpandablePanel_contentContainer, 0);
		if (contentContainerId == 0) {
			throw new IllegalArgumentException("The content attribute is required and must refer to a valid child.");
		}

		int contentId = a.getResourceId(R.styleable.ExpandablePanel_content, 0);
		if (contentId == 0) {
			throw new IllegalArgumentException("The content attribute is required and must refer to a valid child.");
		}

		mHandleId = handleId;
		mContentContainerId = contentContainerId;
		mContentId = contentId;

		a.recycle();
	}

	public void setOnExpandListener(OnExpandListener listener) {
		mListener = listener;
	}

	public void setAnimationDuration(int animationDuration) {
		mAnimationDuration = animationDuration;
	}

	@Override
	protected void onFinishInflate() {
		super.onFinishInflate();

		mHandle = findViewById(mHandleId);
		if (mHandle == null) {
			throw new IllegalArgumentException("The handle attribute is must refer to an existing child.");
		}

		mContentContainer = findViewById(mContentContainerId);
		if (mContentContainer == null) {
			throw new IllegalArgumentException("The content container attribute must refer to an existing child.");
		}

		mContent = findViewById(mContentId);
		if (mContentContainer == null) {
			throw new IllegalArgumentException("The content attribute must refer to an existing child.");
		}

		mContent.setVisibility(View.INVISIBLE);

		mHandle.setOnClickListener(new PanelToggler());
	}

	@Override
	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
		mContentContainer.measure(MeasureSpec.UNSPECIFIED, MeasureSpec.UNSPECIFIED);
		mHandle.measure(MeasureSpec.UNSPECIFIED, heightMeasureSpec);
		mCollapsedHeight = mHandle.getMeasuredHeight();
		mContentWidth = mContentContainer.getMeasuredWidth();
		mContentHeight = mContentContainer.getMeasuredHeight();

		super.onMeasure(widthMeasureSpec, heightMeasureSpec);

		if (mFirstOpen) {
			mContentContainer.getLayoutParams().width = 0;
			mContentContainer.getLayoutParams().height = mCollapsedHeight;
			mFirstOpen = false;
		}
		
		int width = mHandle.getMeasuredWidth()
			+ mContentContainer.getMeasuredWidth()
			+ mContentContainer.getPaddingRight();
		int height = mContentContainer.getMeasuredHeight() + mContentContainer.getPaddingBottom();

		setMeasuredDimension(width, height);
	}

	private class PanelToggler implements OnClickListener {
		@Override
		public void onClick(View v) {
			Animation animation;

			if (mExpanded) {
				mContent.setVisibility(View.INVISIBLE);
				animation = new ExpandAnimation(mContentWidth, 0, mContentHeight, mCollapsedHeight);
				if (mListener != null) {
					mListener.onCollapse(mHandle, mContentContainer);
				}
			} else {
                                ExpandablePanel.this.invalidate();
				animation = new ExpandAnimation(0, mContentWidth, mCollapsedHeight, mContentHeight);
				if (mListener != null) {
					mListener.onExpand(mHandle, mContentContainer);
				}
			}

			animation.setDuration(mAnimationDuration);
			animation.setAnimationListener(new AnimationListener() {
				@Override
				public void onAnimationStart(Animation animation) {
				}
				@Override
				public void onAnimationRepeat(Animation animation) {
				}
				@Override
				public void onAnimationEnd(Animation animation) {
					mExpanded = !mExpanded;
					if (mExpanded) {
						mContent.setVisibility(View.VISIBLE);
					}
				}
			});

			mContentContainer.startAnimation(animation);
		}
	}

	private class ExpandAnimation extends Animation {

		private final int mStartWidth;
		private final int mDeltaWidth;
		private final int mStartHeight;
		private final int mDeltaHeight;

		public ExpandAnimation(int startWidth, int endWidth, int startHeight, int endHeight) {
			mStartWidth = startWidth;
			mDeltaWidth = endWidth - startWidth;
			mStartHeight = startHeight;
			mDeltaHeight = endHeight - startHeight;
		}

		@Override
		protected void applyTransformation(float interpolatedTime, Transformation t) {
			android.view.ViewGroup.LayoutParams lp = mContentContainer.getLayoutParams();
			lp.width = (int) (mStartWidth + mDeltaWidth * interpolatedTime);
			lp.height = (int) (mStartHeight + mDeltaHeight * interpolatedTime);
			mContentContainer.setLayoutParams(lp);
		}

		@Override
		public boolean willChangeBounds() {
			return true;
		}
	}

	public interface OnExpandListener {

		public void onExpand(View handle, View content);
		public void onCollapse(View handle, View content);

	}
}

layout.xml

<?xml version="1.0" encoding="utf-8"?>
<some.awesome.package.ExpandablePanel
	xmlns:android="http://schemas.android.com/apk/res/android"
	xmlns:panel="http://schemas.android.com/apk/res/com.digitalfootsteps.android"
	android:id="@+id/expandable_panel"
	android:layout_width="wrap_content"
	android:layout_height="wrap_content"
	android:layout_alignParentRight="true"
	android:layout_marginRight="10dp"
	android:layout_marginTop="10dp"
	android:background="@drawable/some_awesome_background"
	panel:content="@+id/value"
	panel:contentContainer="@+id/content_container"
	panel:handle="@+id/expand" >

	<FrameLayout
		android:id="@id/content_container"
		android:layout_width="wrap_content"
		android:layout_height="wrap_content"
		android:layout_alignParentTop="true"
		android:layout_toLeftOf="@id/expand"
		android:paddingBottom="5dp"
		android:paddingRight="15dp" >

		<some.awesome.package.SomeAwesomeContentView
			android:id="@id/value"
			android:layout_width="match_parent"
			android:layout_height="match_parent" />
	</FrameLayout>

	<Button
		android:id="@id/expand"
		android:layout_width="wrap_content"
		android:layout_height="wrap_content"
		android:layout_alignParentRight="true"
		android:layout_alignParentTop="true" />
</some.awesome.package.ExpandablePanel>