UI:使View背景逐渐变暗的方法

使用View 的 setAlpha(int alpha);

代码

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);
		}
	});

xml:

<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>

其实就是将当前view 的透明度放低, 从而显示出ViewGroup的背景颜色