折叠式Toolbar的沉浸式状态栏实现

首先使用这段代码让状态栏透明：

    /**
     * 使状态栏透明并且兼容有虚拟按键的手机
     */
    public static void setStatusBarTransparent(Activity activity) {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            return;
        }
        // 实现功能
        final Window window = activity.getWindow();
        window.clearFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS
                | WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION);
        window.getDecorView().setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                | View.SYSTEM_UI_FLAG_LAYOUT_STABLE);
        window.addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS);
        statusBarColor = window.getStatusBarColor();
        window.setStatusBarColor(Color.TRANSPARENT);
    }

然后绘制一个和状态栏一样高的View，并添加到decorView中：

    public static View createStatusView(Activity activity, int color) {
        if (activity == null) {
            return null;
        }
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            return null;
        }
        View statusView = new View(activity);
        LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT,
                TRStatusBarUtil.getStatusBarHeight(activity));
        statusView.setLayoutParams(params);
        // 设置背景色
        statusView.setBackgroundColor(color);
        // 添加 statusView 到布局中
        ViewGroup decorView = (ViewGroup) activity.getWindow().getDecorView();
        decorView.addView(statusView);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            int systemUiVisibility = decorView.getSystemUiVisibility();
            activity.getWindow().getDecorView().
                    setSystemUiVisibility( systemUiVisibility | View.SYSTEM_UI_FLAG_LIGHT_STATUS_BAR);
        }
        return statusView;
    }

在Toolbar折叠的时候，需要通过AppBarLayout的OnOffsetChangedListener回调监听滑动的距离，从而设置statusBar的alpha值：

    @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
        int abOffset = Math.abs(verticalOffset);

        float alpha = abOffset / (float) appBarLayout.getTotalScrollRange();//796
        if(statusBar != null){
            statusBar.setAlpha(alpha);
        }
    }

这样当Toolbar上滑的时候状态栏alpha从0到1，下滑的时候从1到0，如此便实现了想要的效果，代码链接地址：
https://github.com/leihupqrst/status_bar2