UI:使View背景逐渐变暗的方法

setContentView(R.layout.activity_alpha_view);

SeekBar seekBar = (SeekBar) findViewById(R.id.seekbar);
seekBar.setMax(100);
final LinearLayout ll = (LinearLayout) findViewById(R.id.rl);

seekBar.setOnSeekBarChangeListener(new OnSeekBarChangeListener() {

@Override
public void onStopTrackingTouch(SeekBar seekBar) {

}

@Override
public void onStartTrackingTouch(SeekBar seekBar) {

}

@Override
public void onProgressChanged(SeekBar seekBar, int progress,
boolean fromUser) {
ll.setAlpha(progress * 1.0f / 100);
}
});

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:background="#333333">

<SeekBar
android:id="@+id/seekbar"
android:layout_width="match_parent"
android:layout_height="wrap_content"/>

<LinearLayout
android:id="@+id/rl"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:background="#f0f0f0">
</LinearLayout>

</LinearLayout>