实现activity变暗的效果

https://github.com/android-cn/android-discuss/issues/147

/**
     * 调整窗口的透明度
     * @param from>=0&&from<=1.0f
     * @param to>=0&&to<=1.0f
     * 
     * */
    private void dimBackground(final float from, final float to) {
        final Window window = getWindow();
        ValueAnimator valueAnimator = ValueAnimator.ofFloat(from, to);
        valueAnimator.setDuration(500);
        valueAnimator.addUpdateListener(new AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                WindowManager.LayoutParams params = window.getAttributes();
                params.alpha = (Float) animation.getAnimatedValue();
                window.setAttributes(params);
            }
        });

        valueAnimator.start();
    }

然后这样调用：

/** 窗口背景变暗*/
    dimBackground(1.0f,0.5f);


    /** 窗口背景变亮*/
    dimBackground(0.5f,1.0f);