设备在解锁状态下的顶部状态栏：

问题一： status_bar.xml是谁的孩子？

CollapsedStatusBarFragment的内容。

问题二：何时创建CollapsedStatusBarFragment？

在StatusBar的makeStatusBarView方法中有如下代码：

.getFragmentManager()
                .beginTransaction()
                .replace(R.id.status_bar_container, new CollapsedStatusBarFragment(),
                        CollapsedStatusBarFragment.TAG)
                .commit();

StatusBar的createAndAddWindows方法会将StatusBarWindowView添加到mWindowManager中.

public void createAndAddWindows(@Nullable RegisterStatusBarResult result) {
    makeStatusBarView(result);
    mNotificationShadeWindowController.attach();
    mStatusBarWindowController.attach();
}

3个窗口：

R.layout.super_notification_shade, 类为： NotificationShadeWindowView， include 了 status_bar_expanded.xml 。
R.layout.navigation_bar_window ： NavigationBarFrame
R.layout.super_status_bar ： StatusBarWindowView

开机时执行以下3个：

mWindowManager.addView(mNotificationShadeView, mLp);

 mWindowManager.addView(mStatusBarView, mLp);

context.getSystemService(WindowManager.class).addView(navigationBarView, lp);

notification_icon_area的功能

@layout/system_icons

status_bar.xml内容

<?xml version="1.0" encoding="utf-8"?>
<!--
**
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/
-->

<!--    android:background="@drawable/status_bar_closed_default_background" -->
<com.android.systemui.statusbar.phone.PhoneStatusBarView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui"
    android:layout_width="match_parent"
    android:layout_height="@dimen/status_bar_height"
    android:id="@+id/status_bar"
    android:orientation="vertical"
    android:focusable="false"
    android:descendantFocusability="afterDescendants"
    android:accessibilityPaneTitle="@string/status_bar"
    >

    <ImageView
        android:id="@+id/notification_lights_out"
        android:layout_width="@dimen/status_bar_icon_size_sp"
        android:layout_height="match_parent"
        android:paddingStart="@dimen/status_bar_padding_start"
        android:paddingBottom="2dip"
        android:src="@drawable/ic_sysbar_lights_out_dot_small"
        android:scaleType="center"
        android:visibility="gone"
        />

    <LinearLayout android:id="@+id/status_bar_contents"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:clipChildren="false"
        android:clipToPadding="false"
        android:paddingStart="@dimen/status_bar_padding_start"
        android:paddingEnd="@dimen/status_bar_padding_end"
        android:paddingTop="@dimen/status_bar_padding_top"
        android:orientation="horizontal">

        <!-- Container for the entire start half of the status bar. It will always use the same
             width, independent of the number of visible children and sub-children. -->
        <FrameLayout
            android:id="@+id/status_bar_start_side_container"
            android:layout_height="match_parent"
            android:layout_width="0dp"
            android:clipChildren="false"
            android:layout_weight="1">

            <!-- Container that is wrapped around the views on the start half of the status bar.
                 Its width will change with the number of visible children and sub-children.
                 It is useful when we want to know the visible bounds of the content. -->
            <FrameLayout
                android:id="@+id/status_bar_start_side_content"
                android:layout_width="wrap_content"
                android:layout_height="match_parent"
                android:layout_gravity="center_vertical|start"
                android:clipChildren="false">

                <include layout="@layout/heads_up_status_bar_layout" />

                <!-- The alpha of the start side is controlled by PhoneStatusBarTransitions, and the
                     individual views are controlled by StatusBarManager disable flags DISABLE_CLOCK
                     and DISABLE_NOTIFICATION_ICONS, respectively -->
                <LinearLayout
                    android:id="@+id/status_bar_start_side_except_heads_up"
                    android:layout_height="match_parent"
                    android:layout_width="match_parent"
                    android:layout_gravity="center_vertical|start"
                    android:clipChildren="false">
                    <ViewStub
                        android:id="@+id/operator_name_stub"
                        android:layout_width="wrap_content"
                        android:layout_height="match_parent"
                        android:layout="@layout/operator_name" />

                    <com.android.systemui.statusbar.policy.Clock
                        android:id="@+id/clock"
                        android:layout_width="wrap_content"
                        android:layout_height="@dimen/status_bar_system_icons_height"
                        android:layout_gravity="center_vertical"
                        android:textAppearance="@style/TextAppearance.StatusBar.Clock"
                        android:singleLine="true"
                        android:paddingStart="@dimen/status_bar_left_clock_starting_padding"
                        android:paddingEnd="@dimen/status_bar_left_clock_end_padding"
                        android:gravity="center_vertical|start"
                    />

                    <include layout="@layout/ongoing_call_chip" />

                    <com.android.systemui.statusbar.AlphaOptimizedFrameLayout
                        android:id="@+id/notification_icon_area"
                        android:layout_width="wrap_content"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:clipChildren="false"/>

                </LinearLayout>
            </FrameLayout>
        </FrameLayout>

        <!-- Space should cover the notch (if it exists) and let other views lay out around it -->
        <android.widget.Space
            android:id="@+id/cutout_space_view"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:gravity="center_horizontal|center_vertical"
        />

        <!-- Container for the entire end half of the status bar. It will always use the same
             width, independent of the number of visible children and sub-children. -->
        <FrameLayout
            android:id="@+id/status_bar_end_side_container"
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:layout_weight="1"
            android:clipChildren="false">

            <!-- Container that is wrapped around the views on the end half of the
                 status bar. Its width will change with the number of visible children and
                 sub-children.
                 It is useful when we want know the visible bounds of the content.-->
            <com.android.keyguard.AlphaOptimizedLinearLayout
                android:id="@+id/status_bar_end_side_content"
                android:layout_width="wrap_content"
                android:layout_height="match_parent"
                android:layout_gravity="end"
                android:orientation="horizontal"
                android:gravity="center_vertical|end"
                android:clipChildren="false">

                <include
                    android:id="@+id/user_switcher_container"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_marginEnd="@dimen/status_bar_user_chip_end_margin"
                    layout="@layout/status_bar_user_chip_container" />

                <include layout="@layout/system_icons"
                     android:layout_gravity="center_vertical"
                     android:layout_width="wrap_content"
                     android:layout_height="@dimen/status_bar_system_icons_height" />
            </com.android.keyguard.AlphaOptimizedLinearLayout>
        </FrameLayout>
    </LinearLayout>

</com.android.systemui.statusbar.phone.PhoneStatusBarView>