FadableLinearLayout.java

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Bitmap.Config;
import android.util.AttributeSet;
import android.widget.LinearLayout;

public class FadableLinearLayout extends LinearLayout {
	
	protected float mAlpha = 1f;
	protected Bitmap offScreen;

	public FadableLinearLayout(Context context) {
		super(context);
		myInit();
	}

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

	public FadableLinearLayout(Context context, AttributeSet attrs, int defStyle) {
		super(context, attrs, defStyle);
		myInit();
	}
	
	protected void myInit() {
		setWillNotDraw(false);
		setDrawingCacheEnabled(true);
	}
	
	public void changeAlpha( float alpha ) {
		mAlpha = alpha;
		int x = 0, y = 0, w = getMeasuredWidth(), h = getMeasuredHeight();
		invalidate(x,y,w,h);
	}

	protected Paint p = new Paint();
	
	@Override
	public void draw(Canvas canvas) {
		if( mAlpha != 1f ) {
			int x = 0, y = 0, w = getMeasuredWidth(), h = getMeasuredHeight();
			int a = (int)(mAlpha * 255);
			
			if( offScreen == null ) {
				offScreen = Bitmap.createBitmap(w, h, Config.ARGB_8888);
				Canvas c = new Canvas(offScreen);
				super.draw(c);
			}
			
			p.setAlpha(a);
			canvas.drawBitmap(offScreen, x, y, p);
		}
		else {
			if( offScreen != null ) {
				offScreen.recycle();
				offScreen = null;
			}
			super.draw(canvas);
		}
	}
}