Android 沉浸式状态栏，上滑页面StatusBar透明度发生变化

参考资料：http://blog.cgsdream.org/2017/03/16/android-translcent-statusbar/
https://blog.csdn.net/coderder/article/details/78294777

开发过程中，首页顶部是一张图片，要求占据全屏幕，状态栏覆盖在图片之上。

首先，activity的theme为：

    <style name="TestAppTheme" parent="Theme.AppCompat.Light.NoActionBar">
        <item name="colorPrimaryDark">#4D0F294D</item>
        <item name="colorPrimary">@color/color_main</item>
        <item name="android:windowBackground">@color/white</item>
    </style>

colorPrimaryDark 表示状态栏背景颜色。

怎么实现沉浸式状态栏：

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            getWindow().getDecorView().setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                    | View.SYSTEM_UI_FLAG_LAYOUT_STABLE);
        }
        //这里只对api21以上做处理

如果想实现MD，页面上滑，toolbar显示出来，并且statusbar逐渐不透明。
需要给statusbar一个占位view：
activity的XML文件为：

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">


    <android.support.design.widget.AppBarLayout
        android:id="@+id/app_bar_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <android.support.design.widget.CollapsingToolbarLayout
            android:id="@+id/collapsing_toolbar_layout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:layout_scrollFlags="scroll|exitUntilCollapsed|snap">

            <android.support.v7.widget.AppCompatImageView
                android:id="@+id/background_iv"
                android:layout_width="match_parent"
                android:layout_height="138dp"
                android:scaleType="centerCrop"
                android:src="@drawable/train_pic_brand" />

            <LinearLayout
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginStart="16dp"
                android:layout_marginBottom="24dp"
                android:layout_gravity="bottom"
                android:orientation="horizontal"
                app:layout_collapseMode="pin">

                <com.ctrip.ibu.train.widget.TrainI18nTextView
                    style="@style/TextAppearance.Trip.Bold"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="Train"
                    android:textColor="@color/color_white"
                    android:textAppearance="@style/TextAppearance.Trip.Medium"
                    android:textSize="28dp" />

                <View
                    android:layout_width="6dp"
                    android:layout_height="6dp"
                    android:layout_marginBottom="7dp"
                    android:layout_marginStart="4dp"
                    android:layout_gravity="bottom"
                    android:background="@drawable/train_circle_ff9500" />

            </LinearLayout>
            
            //占位statusbar
            <View
                android:id="@+id/status_bar"
                android:layout_width="match_parent"
                android:layout_height="24dp"
                android:background="@drawable/train_bg_ffffff"
                app:layout_collapseMode="pin" />

            <com.ctrip.ibu.train.widget.TrainNewToolbar
                android:id="@+id/train_tool_bar"
                android:layout_width="match_parent"
                android:layout_height="?android:actionBarSize"
                android:layout_marginTop="24dp"
                android:background="@drawable/train_bg_ffffff"
                app:layout_collapseMode="pin" />

            <View
                android:layout_width="match_parent"
                android:layout_height="@dimen/dimen_12dp"
                android:layout_gravity="bottom"
                android:background="@drawable/train_bg_main_search_view"
                />
        </android.support.design.widget.CollapsingToolbarLayout>

    </android.support.design.widget.AppBarLayout>

    <android.support.v4.widget.NestedScrollView
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:background="@color/color_page_bg"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"
        >

        <LinearLayout
            android:id="@+id/ll_content"
            android:orientation="vertical"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </android.support.v4.widget.NestedScrollView>

</android.support.design.widget.CoordinatorLayout>

java代码中：

        mAppBarLayout.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
            @Override
            public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
                mToolbar.setTitleColor(R.color.color_train_main_text);
                int totalScrollRange = appBarLayout.getTotalScrollRange();
                float offset = (-verticalOffset) / (float) totalScrollRange;
                //toolbar变化
                mStatusBar.setAlpha(offset); //statusbar透明度变化
                mToolbar.getBackground().setAlpha((int) (offset * 255)); //toolbar透明度变化
                mToolbar.getTitleTv().setAlpha(offset);
        });