How to implement expandable panels in Android?

Thanks very much OP! For anyone interested I took OP's solution and refined it a bit.

• Handle only displays if there is overflow
• Added ability to specify animation duration via 'animationDuration' attribute
• Added ability to attach event listeners that get fired onExpand and onCollapse (this is useful for e.g changing the text of the "More" button to "Less"
• Collapsed by default
• Content can be modified programmatically (same with attributes)

Here's the updated code:

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.Transformation;import android.widget.LinearLayout;public class ExpandablePanel extends LinearLayout {    private final int mHandleId;    private final int mContentId;    private View mHandle;    private View mContent;    private boolean mExpanded = false;    private int mCollapsedHeight = 0;    private int mContentHeight = 0;    private int mAnimationDuration = 0;    private OnExpandListener mListener;    public ExpandablePanel(Context context) {        this(context, null);    }    public ExpandablePanel(Context context, AttributeSet attrs) {        super(context, attrs);        mListener = new DefaultOnExpandListener();        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ExpandablePanel, 0, 0);        // How high the content should be in "collapsed" state        mCollapsedHeight = (int) a.getDimension(R.styleable.ExpandablePanel_collapsedHeight, 0.0f);        // How long the animation should take        mAnimationDuration = a.getInteger(R.styleable.ExpandablePanel_animationDuration, 500);        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 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;        mContentId = contentId;        a.recycle();    }    public void setOnExpandListener(OnExpandListener listener) {        mListener = listener;     }    public void setCollapsedHeight(int collapsedHeight) {        mCollapsedHeight = collapsedHeight;    }    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.");        }        mContent = findViewById(mContentId);        if (mContent == null) {            throw new IllegalArgumentException(                "The content attribute must refer to an"                    + " existing child.");        }        android.view.ViewGroup.LayoutParams lp = mContent.getLayoutParams();        lp.height = mCollapsedHeight;        mContent.setLayoutParams(lp);        mHandle.setOnClickListener(new PanelToggler());    }    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        // First, measure how high content wants to be        mContent.measure(widthMeasureSpec, MeasureSpec.UNSPECIFIED);        mContentHeight = mContent.getMeasuredHeight();        if (mContentHeight < mCollapsedHeight) {            mHandle.setVisibility(View.GONE);        } else {            mHandle.setVisibility(View.VISIBLE);        }        // Then let the usual thing happen        super.onMeasure(widthMeasureSpec, heightMeasureSpec);    }    private class PanelToggler implements OnClickListener {        public void onClick(View v) {            Animation a;            if (mExpanded) {                a = new ExpandAnimation(mContentHeight, mCollapsedHeight);                mListener.onCollapse(mHandle, mContent);            } else {                a = new ExpandAnimation(mCollapsedHeight, mContentHeight);                mListener.onExpand(mHandle, mContent);            }            a.setDuration(mAnimationDuration);            mContent.startAnimation(a);            mExpanded = !mExpanded;        }    }    private class ExpandAnimation extends Animation {        private final int mStartHeight;        private final int mDeltaHeight;        public ExpandAnimation(int startHeight, int endHeight) {            mStartHeight = startHeight;            mDeltaHeight = endHeight - startHeight;        }        @Override        protected void applyTransformation(float interpolatedTime, Transformation t) {            android.view.ViewGroup.LayoutParams lp = mContent.getLayoutParams();            lp.height = (int) (mStartHeight + mDeltaHeight * interpolatedTime);            mContent.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);    }    private class DefaultOnExpandListener implements OnExpandListener {        public void onCollapse(View handle, View content) {}        public void onExpand(View handle, View content) {}    }}

And don't forget the 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="collapsedHeight" format="dimension"/>        <attr name="animationDuration" format="integer"/>    </declare-styleable></resources>

See OP's example usage for the XML layout above. Here's an example for the listeners:

// Set expandable panel listenerExpandablePanel panel = (ExpandablePanel)view.findViewById(R.id.foo);panel.setOnExpandListener(new ExpandablePanel.OnExpandListener() {    public void onCollapse(View handle, View content) {        Button btn = (Button)handle;        btn.setText("More");    }    public void onExpand(View handle, View content) {        Button btn = (Button)handle;        btn.setText("Less");    }});

Have you tried having a ScrollView at a set size that you make not clickable and not focusable? Then, when you expand it, you could just animate it getting bigger.

I know this is an old question but for those who are interested, I made additions to what ahal and Pēteris Caune did.

Additions

1. Included a layout to contain the horizontal view and the more button(refer to image from Pēteris Caune's question)
2. The layout, instead of just the button is removed when there is no overflow
3. Hidden text is shown or hidden depending on state of button

Updated Code

ExpandablePanel Class

package com.example.myandroidhustles;import com.example.myandroidhustles.R;import android.content.Context;import android.content.res.TypedArray;import android.util.AttributeSet;import android.view.View;import android.view.ViewGroup;import android.view.animation.Animation;import android.view.animation.Transformation;import android.widget.LinearLayout;public class ExpandablePanel extends LinearLayout {    private final int mHandleId;    private final int mContentId;    private final int mViewGroupId;    private final boolean isViewGroup;    private View mHandle;    private View mContent;    private ViewGroup viewGroup;    private boolean mExpanded = false;    private int mCollapsedHeight = 0;    private int mContentHeight = 0;    private int mAnimationDuration = 0;    private OnExpandListener mListener;    public ExpandablePanel(Context context) {        this(context, null);    }    public ExpandablePanel(Context context, AttributeSet attrs) {        super(context, attrs);        mListener = new DefaultOnExpandListener();        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ExpandablePanel, 0, 0);        // How high the content should be in "collapsed" state        mCollapsedHeight = (int) a.getDimension(R.styleable.ExpandablePanel_collapsedHeight, 0.0f);        // How long the animation should take        mAnimationDuration = a.getInteger(R.styleable.ExpandablePanel_animationDuration, 500);        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 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.");        }        int isViewGroupId = a.getResourceId(R.styleable.ExpandablePanel_isviewgroup, 0);        int viewGroupId = a.getResourceId(R.styleable.ExpandablePanel_viewgroup, 0);//        isViewGroup = findViewById(isViewGroupId);        isViewGroup = a.getBoolean(R.styleable.ExpandablePanel_isviewgroup, false);        if (isViewGroup) {            mViewGroupId = viewGroupId;        }        else {            mViewGroupId = 0;        }        mHandleId = handleId;        mContentId = contentId;                a.recycle();    }    public void setOnExpandListener(OnExpandListener listener) {        mListener = listener;     }    public void setCollapsedHeight(int collapsedHeight) {        mCollapsedHeight = collapsedHeight;    }    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.");        }        if(mViewGroupId != 0) {            viewGroup = (ViewGroup) findViewById(mViewGroupId);        }        mContent = findViewById(mContentId);        if (mContent == null) {            throw new IllegalArgumentException(                "The content attribute must refer to an"                    + " existing child.");        }        android.view.ViewGroup.LayoutParams lp = mContent.getLayoutParams();        lp.height = mCollapsedHeight;        mContent.setLayoutParams(lp);        mHandle.setOnClickListener(new PanelToggler());    }    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        // First, measure how high content wants to be        mContent.measure(widthMeasureSpec, MeasureSpec.UNSPECIFIED);        mContentHeight = mContent.getMeasuredHeight();        if (mContentHeight < mCollapsedHeight) {            viewGroup.setVisibility(View.GONE);//            mHandle.setVisibility(View.GONE);        } else {            viewGroup.setVisibility(View.VISIBLE);//            mHandle.setVisibility(View.VISIBLE);        }        // Then let the usual thing happen        super.onMeasure(widthMeasureSpec, heightMeasureSpec);    }    private class PanelToggler implements OnClickListener {        public void onClick(View v) {            Animation a;            if (mExpanded) {                a = new ExpandAnimation(mContentHeight, mCollapsedHeight);                mListener.onCollapse(mHandle, mContent);            } else {                a = new ExpandAnimation(mCollapsedHeight, mContentHeight);                mListener.onExpand(mHandle, mContent);            }            a.setDuration(mAnimationDuration);            if(mContent.getLayoutParams().height == 0) //Need to do this or else the animation will not play if the height is 0           {            android.view.ViewGroup.LayoutParams lp = mContent.getLayoutParams();            lp.height = 1;            mContent.setLayoutParams(lp);            mContent.requestLayout();          }            mContent.startAnimation(a);            mExpanded = !mExpanded;        }    }    private class ExpandAnimation extends Animation {        private final int mStartHeight;        private final int mDeltaHeight;        public ExpandAnimation(int startHeight, int endHeight) {            mStartHeight = startHeight;            mDeltaHeight = endHeight - startHeight;        }        @Override        protected void applyTransformation(float interpolatedTime, Transformation t) {            android.view.ViewGroup.LayoutParams lp = mContent.getLayoutParams();            lp.height = (int) (mStartHeight + mDeltaHeight * interpolatedTime);            mContent.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);    }    private class DefaultOnExpandListener implements OnExpandListener {        public void onCollapse(View handle, View content) {}        public void onExpand(View handle, View content) {}    }}

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="viewgroup" format="reference"/>        <attr name="isviewgroup" format="boolean"/>        <attr name="collapsedHeight" format="dimension"/>        <attr name="animationDuration" format="integer"/>    </declare-styleable></resources>

Layout: tryExpandablePanel.xml

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:example="http://schemas.android.com/apk/res/com.example.myandroidhustles"    android:layout_width="fill_parent"    android:layout_height="match_parent" >    <com.example.myandroidhustles.ExpandablePanel        android:id="@+id/expandablePanel"        android:layout_width="fill_parent"        android:layout_height="fill_parent"        android:orientation="vertical"        example:collapsedHeight="50dip"        example:content="@+id/value"        example:handle="@+id/expand"        example:isviewgroup="true"        example:viewgroup="@+id/expandL" >        <TextView            android:id="@+id/value"            android:layout_width="fill_parent"            android:layout_height="wrap_content"            android:maxHeight="100dip" />        <LinearLayout            android:id="@+id/expandL"            android:layout_width="fill_parent"            android:layout_height="wrap_content"            android:orientation="horizontal"            android:paddingLeft="10dp"            android:weightSum="100" >            <View                android:id="@+id/view"                android:layout_width="fill_parent"                android:layout_height="1dp"                android:layout_gravity="center_vertical|left"                android:layout_weight="30"                android:background="@android:color/darker_gray" />            <Button                android:id="@+id/expand"                android:layout_width="fill_parent"                android:layout_height="wrap_content"                android:layout_gravity="right"                android:layout_weight="70"                android:text="More" />        </LinearLayout>    </com.example.myandroidhustles.ExpandablePanel></LinearLayout>

Implementation: ExpandablePanelImplementation Class

package com.example.myandroidhustles;import android.app.Activity;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.TextView;public class ExpandablePanelImplementation extends Activity {    ExpandablePanel panel;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.tryexpandable);        TextView text;        text = (TextView)findViewById(R.id.value);            text.setText("ksaflfsklafjsfj sdfjklds fj asklfjklasfjskladf fjslkafjf" +                    "asfkdaslfjsf;sjdaflkadsjflkdsajfkldsajflkdsanfvsjvfdskljflkdnjdsadf" +                    "askfvdsklfjvsdlkfjdsklvdkjkdsadsj;lkasjdfklvsddsjkdsljskldfj");      panel = (ExpandablePanel)findViewById(R.id.expandablePanel);      panel.setOnExpandListener(new ExpandablePanel.OnExpandListener() {          public void onCollapse(View handle, View content) {              Button btn = (Button)handle;              btn.setText("More");              panel.setCollapsedHeight(100);          }          public void onExpand(View handle, View content) {              Button btn = (Button)handle;              panel.setCollapsedHeight(50);              btn.setText("Less");          }      });    }}