Expandable Panel Android widget -- WIP

ExpandablePanel.java

package com.example.myapp.widgets;

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;

import com.example.myapp.R;

public class ExpandablePanel extends LinearLayout {

    private final int mHandleId;
    private final int mContentId;

    private View mHandle;
    private View mContent;

    private boolean mExpanded = true;
    private int mCollapsedHeight = 0;
    private int mContentHeight = 0;

    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);

	// How high the content should be in "collapsed" state
	mCollapsedHeight = (int) a.getDimension(
		R.styleable.ExpandablePanel_collapsedHeight, 0.0f);

	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();
    }

    @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 is must refer to an"
			    + " existing child.");
	}

	mHandle.setOnClickListener(new PanelToggler());
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
	if (mContentHeight == 0) {
	    // First, measure how high content wants to be
	    mContent.measure(widthMeasureSpec, MeasureSpec.UNSPECIFIED);
	    mContentHeight = mContent.getMeasuredHeight();
	}

	// 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);
	    } else {
		a = new ExpandAnimation(mCollapsedHeight, mContentHeight);
	    }
	    a.setDuration(500);
	    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() {
	    // TODO Auto-generated method stub
	    return true;
	}

    }
}