Android应用中设置alpha值来制作透明与渐变效果的实例

  private class MyView extends View
  {
 
    private Bitmap bitmap;
 
    public MyView(Context context)
    {
      super(context);
      InputStream is =getResources().openRawResource(R.drawable.image);
      bitmap = BitmapFactory.decodeStream(is);
      setBackgroundColor(Color.WHITE);
    }
 
    @Override
    protected void onDraw(Canvas canvas)
    {
      Paint paint = new Paint();
      paint.setAlpha(alpha);
 
      canvas.drawBitmap(bitmap, new Rect(0, 0, bitmap.getWidth(), bitmap
         .getHeight()), new Rect(10, 10, 310, 235), paint);
 
    }
  }

public void onProgressChanged(SeekBar seekBar, int progress,
      boolean fromUser)
  {
    alpha = progress;
    setTitle("alpha:" + progress);
    myView.invalidate();
  }

<?xml
version="1.0"
encoding="utf-8"?>
<alpha
xmlns:android="http://schemas.android.com/apk/res/android"
  android:fromAlpha="1.0"
  android:toAlpha="0.0"
  android:repeatCount="infinite"
  android:duration="2000">
</alpha>

import android.app.Activity; 
import android.os.Bundle; 
import android.view.animation.Animation; 
import android.view.animation.AnimationUtils; 
import android.widget.ImageView; 

public
class AlphaActivity extends Activity { 
  /**显示动画的ImageView**/
  ImageView mImageView = null; 

  /**透明动画**/
  Animation mAnimation = null; 
  @Override
  public
void onCreate(Bundle savedInstanceState) { 
  super.onCreate(savedInstanceState); 
  setContentView(R.layout.translate); 

  /**拿到ImageView对象**/
  mImageView = (ImageView)findViewById(R.id.imageView); 

  /**加载透明动画**/
  mAnimation = AnimationUtils.loadAnimation(this, R.anim.alpha); 

  /**播放透明动画**/
  mImageView.startAnimation(mAnimation); 
  } 
}