SystemUI中StatusBar的图标控制器实现类为StatusBarIconControllerImpl，其继承了StatusBarIconController的接口，用于跟踪所有图标的状态，并将对应的状态发送给注册的图标管理器(IconManagers)。当我们在StatusBar中获取到它的实例后，还会将它传给PhoneStatusBarPolicy和StatusBarSignalPolicy对象。PhoneStatusBarPolicy控制启动时装载哪些图标（蓝牙，定位等），而StatusBarSignalPolicy控制网络信号图标（移动网络，WiFi，以太网）的变化。
一起来看 StatuBar 的 start() 方法：
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBarSignalPolicy.java

@Override
public void start() {
    
    // 省略部分代码......
    // 创建整个SystemUI视图并添加到WindowManager中
    createAndAddWindows();//这个重点方法，创建相关的视图
    // 省略部分代码......
    // Lastly, call to the icon policy to install/update all the icons.
    mIconPolicy.init();
    mSignalPolicy = new StatusBarSignalPolicy(mContext, mIconController);
    // 省略部分代码......
}

这里的 mIconPolicy 就是 PhoneStatusBarPolicy对象，mSignalPolicy 就是 StatusBarSignalPolicy 对象。我们这里以 StatusBarSignalPolicy 为例去研究。
StatusBarSignalPolicy实现了NetworkControllerImpl.SignalCallback接口，SignalCallback接口定义在NetworkControllerImpl实现的接口NetworkController中。
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/policy/NetworkControllerImpl.java

    @Inject
    public NetworkControllerImpl(Context context, @Background Looper bgLooper,
            DeviceProvisionedController deviceProvisionedController,
            BroadcastDispatcher broadcastDispatcher, ConnectivityManager connectivityManager,
            TelephonyManager telephonyManager, WifiManager wifiManager,
            NetworkScoreManager networkScoreManager) {
        this(context, connectivityManager,
                telephonyManager,
                wifiManager,
                networkScoreManager,
                SubscriptionManager.from(context), Config.readConfig(context), bgLooper,
                new CallbackHandler(),
                new AccessPointControllerImpl(context),
                new DataUsageController(context),
                new SubscriptionDefaults(),
                deviceProvisionedController,
                broadcastDispatcher);
        mReceiverHandler.post(mRegisterListeners);
    }
    private final Runnable mRegisterListeners = new Runnable() {
        @Override
        public void run() {
            registerListeners();
        }
    };
    void registerListeners() {
        for (int i = 0; i < mMobileSignalControllers.size(); i++) {
            MobileSignalController mobileSignalController = mMobileSignalControllers.valueAt(i);
            mobileSignalController.registerListener();
        }
        if (mSubscriptionListener == null) {
            mSubscriptionListener = new SubListener();
        }
        mSubscriptionManager.addOnSubscriptionsChangedListener(mSubscriptionListener);
        mPhone.listen(mPhoneStateListener, LISTEN_ACTIVE_DATA_SUBSCRIPTION_ID_CHANGE);
        // broadcasts
        IntentFilter filter = new IntentFilter();
        // wifi相关
        // wifi信号强度广播
        filter.addAction(WifiManager.RSSI_CHANGED_ACTION);
        // wifi状态变化广播
        filter.addAction(WifiManager.WIFI_STATE_CHANGED_ACTION);
        // wifi连接状态改变
        filter.addAction(WifiManager.NETWORK_STATE_CHANGED_ACTION);
        
        // 移动网络相关
        // SIM卡状态改变
        filter.addAction(TelephonyIntents.ACTION_SIM_STATE_CHANGED);
        // 数据语音订阅修改
        filter.addAction(TelephonyIntents.ACTION_DEFAULT_DATA_SUBSCRIPTION_CHANGED);
        filter.addAction(TelephonyIntents.ACTION_DEFAULT_VOICE_SUBSCRIPTION_CHANGED);
        filter.addAction(TelephonyIntents.ACTION_SERVICE_STATE_CHANGED);
        filter.addAction(TelephonyIntents.SPN_STRINGS_UPDATED_ACTION);
        // 连接状态相关
        // 网络连接状态发生变化
        filter.addAction(ConnectivityManager.CONNECTIVITY_ACTION);
        // 网络连接可能不好
        filter.addAction(ConnectivityManager.INET_CONDITION_ACTION);
        // 切换飞行模式时
        filter.addAction(Intent.ACTION_AIRPLANE_MODE_CHANGED);
        filter.addAction(CarrierConfigManager.ACTION_CARRIER_CONFIG_CHANGED);
        mContext.registerReceiver(this, filter, null, mReceiverHandler);
        mListening = true;
        // 省略部分代码......
        // 4.更新移动网络控制器
        updateMobileControllers();
    }

在NetworkControllerImpl 的构造方法里，最终会调用到：registerListeners() 方法进行广播的注册。
广播处理：
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/policy/NetworkControllerImpl.java

@Override
public void onReceive(Context context, Intent intent) {
    if (CHATTY) {
        Log.d(TAG, "onReceive: intent=" + intent);
    }
    final String action = intent.getAction();
    switch (action) {
        case ConnectivityManager.CONNECTIVITY_ACTION:
        case ConnectivityManager.INET_CONDITION_ACTION:
            // 省略部分代码......
            break;
        case Intent.ACTION_AIRPLANE_MODE_CHANGED:
                // 省略部分代码......
            break;
        case TelephonyManager.ACTION_DEFAULT_VOICE_SUBSCRIPTION_CHANGED:
            // We are using different subs now, we might be able to make calls.
                // 省略部分代码......
            break;
        case TelephonyManager.ACTION_DEFAULT_DATA_SUBSCRIPTION_CHANGED:
            // Notify every MobileSignalController so they can know whether they are the
            // data sim or not.
            // 省略部分代码......
            break;
        case Intent.ACTION_SIM_STATE_CHANGED:
            // Avoid rebroadcast because SysUI is direct boot aware.
            // 省略部分代码......
            break;
        case Intent.ACTION_SERVICE_STATE:
            // 省略部分代码......
            break;
        case CarrierConfigManager.ACTION_CARRIER_CONFIG_CHANGED:
            // 省略部分代码......
            break;
        case ImsManager.ACTION_IMS_SERVICE_UP:
        case ImsManager.ACTION_IMS_SERVICE_DOWN:
            // 省略部分代码......
            break;
        case ACTION_HIGH_DEF_AUDIO_SUPPORT:
            // 省略部分代码......
            break;
        case ACTION_MODEM_CHANGE:
            // 省略部分代码......
            break;
        default:
            int subId = intent.getIntExtra(SubscriptionManager.EXTRA_SUBSCRIPTION_INDEX,
                    SubscriptionManager.INVALID_SUBSCRIPTION_ID);
            if (SubscriptionManager.isValidSubscriptionId(subId)) {
                if (mMobileSignalControllers.indexOfKey(subId) >= 0) {
                    mMobileSignalControllers.get(subId).handleBroadcast(intent);
                } else {
                    // Can't find this subscription...  We must be out of date.
                    updateMobileControllers();
                }
            } else {
                    // wifi状态图标处理
                // No sub id, must be for the wifi.
                mWifiSignalController.handleBroadcast(intent);
            }
            break;
    }
}

这里以 wifi状态图标处理 为例；接下来看WifiSignalController#handleBroadcast():
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/policy/WifiSignalController.java

public void handleBroadcast(Intent intent) {
    mWifiTracker.handleBroadcast(intent);
    mCurrentState.enabled = mWifiTracker.enabled;
    mCurrentState.connected = mWifiTracker.connected;
    mCurrentState.ssid = mWifiTracker.ssid;
    mCurrentState.rssi = mWifiTracker.rssi;
    mCurrentState.level = mWifiTracker.level;
    mCurrentState.statusLabel = mWifiTracker.statusLabel;
    notifyListenersIfNecessary();
}

在WifiSignalController#handleBroadcast()方法中，就两个实现，一个是获取 WiFi 的状态，一个是通知更新状态。
我们直接看通知SignalController# notifyListenersIfNecessary() :
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/policy/SignalController.java

public void notifyListenersIfNecessary() {
    if (isDirty()) {
        saveLastState();    // 保持此时的状态
        notifyListeners();    // 通知监听器
    }
}
public final void notifyListeners() {
    notifyListeners(mCallbackHandler);
}
public abstract void notifyListeners(SignalCallback callback);

notifyListener()方法的实现在WifiSignalController类中：
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/policy/WifiSignalController.java

@Override
public void notifyListeners(SignalCallback callback) {
    // only show wifi in the cluster if connected or if wifi-only
    boolean visibleWhenEnabled = mContext.getResources().getBoolean(
            R.bool.config_showWifiIndicatorWhenEnabled);
    boolean wifiVisible = mCurrentState.enabled && (
            (mCurrentState.connected && mCurrentState.inetCondition == 1)
                    || !mHasMobileDataFeature || mWifiTracker.isDefaultNetwork
                    || visibleWhenEnabled);
    String wifiDesc = mCurrentState.connected ? mCurrentState.ssid : null;
    boolean ssidPresent = wifiVisible && mCurrentState.ssid != null;
    String contentDescription = getTextIfExists(getContentDescription()).toString();
    if (mCurrentState.inetCondition == 0) {
        contentDescription += ("," + mContext.getString(R.string.data_connection_no_internet));
    }
    IconState statusIcon = new IconState(wifiVisible, getCurrentIconId(), contentDescription);
    IconState qsIcon = new IconState(mCurrentState.connected,
            mWifiTracker.isCaptivePortal ? R.drawable.ic_qs_wifi_disconnected
                    : getQsCurrentIconId(), contentDescription);
    // callback为 CallbackHandler对象
    callback.setWifiIndicators(mCurrentState.enabled, statusIcon, qsIcon,
            ssidPresent && mCurrentState.activityIn, ssidPresent && mCurrentState.activityOut,
            wifiDesc, mCurrentState.isTransient, mCurrentState.statusLabel);
}

可以看到，这里回调了StatusBarSignalPolicy#setWifiIndicators() 方法：
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBarSignalPolicy.java

@Override
public void setWifiIndicators(boolean enabled, IconState statusIcon, IconState qsIcon,
        boolean activityIn, boolean activityOut, String description, boolean isTransient,
        String statusLabel) {
    boolean visible = statusIcon.visible && !mBlockWifi;
    boolean in = activityIn && mActivityEnabled && visible;
    boolean out = activityOut && mActivityEnabled && visible;
    WifiIconState newState = mWifiIconState.copy();
    newState.visible = visible;
    newState.resId = statusIcon.icon;
    newState.activityIn = in;
    newState.activityOut = out;
    newState.slot = mSlotWifi;
    newState.airplaneSpacerVisible = mIsAirplaneMode;
    newState.contentDescription = statusIcon.contentDescription;
    MobileIconState first = getFirstMobileState();
    newState.signalSpacerVisible = first != null && first.typeId != 0;
    updateWifiIconWithState(newState);
    mWifiIconState = newState;
}
private void updateWifiIconWithState(WifiIconState state) {
    if (state.visible && state.resId > 0) {
        mIconController.setSignalIcon(mSlotWifi, state);
        mIconController.setIconVisibility(mSlotWifi, true);
    } else {
        mIconController.setIconVisibility(mSlotWifi, false);
    }
}

通过StatusBarIconController接口设置图标的套路都是一样的:

• 获取图标名字
• 监听事件
• 通过StatusBarIconControllerImpl相应的方法设置图标。
  接下来再看StatusBarIconControllerImpl#setSignalIcon()：
  frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBarIconControllerImpl.java

/**
 * Signal icons need to be handled differently, because they can be
 * composite views
 */
@Override
public void setSignalIcon(String slot, WifiIconState state) {
    int index = getSlotIndex(slot);
    if (state == null) {
        removeIcon(index, 0);
        return;
    }
    StatusBarIconHolder holder = getIcon(index, 0);
    if (holder == null) {
        holder = StatusBarIconHolder.fromWifiIconState(state);
        setIcon(index, holder);
    } else {
        holder.setWifiState(state);
        handleSet(index, holder);
    }
}

首先设置WiFi的状态信息，遍历mIconGroups分别执行StatusBarIconController接口中静态类IconManager中的onIconAdded()和onSetIconHolder()的回调。
IconManager用于将信息从StatusBarIconController转换为ViewGroup中的ImageViews(com.android.systemui.statusbar.AlphaOptimizedImageView)。
接着看IconManager中的onIconAdded()和onSetIconHolder()方法：这两个方法一个用于添加、一个用于更新。
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBarIconController.java

public interface StatusBarIconController {
    ....
    public static class DarkIconManager extends IconManager {
        ....
        public DarkIconManager(LinearLayout linearLayout) {
            // 将布局传入IconManager
            super(linearLayout);
            mIconHPadding = mContext.getResources().getDimensionPixelSize(
                    R.dimen.status_bar_icon_padding);
            mDarkIconDispatcher = Dependency.get(DarkIconDispatcher.class);
        }
        ....
        @Override
        protected void onIconAdded(int index, String slot, boolean blocked,
                                   StatusBarIconHolder holder) {
            // 调用到父类的addHolder方法
            StatusIconDisplayable view = addHolder(index, slot, blocked, holder);
            ....
        }
    }
    
    public static class IconManager implements DemoMode {
        ....
        protected final ViewGroup mGroup;
        protected final Context mContext;
        public IconManager(ViewGroup group) {
            mGroup = group;
            mContext = group.getContext();
            mIconSize = mContext.getResources().getDimensionPixelSize(
                    R.dimen.status_bar_height);
            ....
        }
        ....
        protected StatusIconDisplayable addHolder(int index, String slot, boolean blocked,
                                                  StatusBarIconHolder holder) {
            switch (holder.getType()) {
                case TYPE_ICON:
                    return addIcon(index, slot, blocked, holder.getIcon());
                case TYPE_WIFI:
                    return addSignalIcon(index, slot, holder.getWifiState());
                case TYPE_MOBILE:
                    return addMobileIcon(index, slot, holder.getMobileState());
            }
            return null;
        }
        @VisibleForTesting
        protected StatusBarWifiView addSignalIcon(int index, String slot, WifiIconState state) {
            // 创建一个StatusBarWifiView 
            StatusBarWifiView view = onCreateStatusBarWifiView(slot);
            view.applyWifiState(state);
            // 将view 添加进ViewGroup
            mGroup.addView(view, index, onCreateLayoutParams());
            if (mIsInDemoMode) {
                mDemoStatusIcons.addDemoWifiView(state);
            }
            return view;
        }
        private StatusBarWifiView onCreateStatusBarWifiView(String slot) {
            StatusBarWifiView view = StatusBarWifiView.fromContext(mContext, slot);
            return view;
        }
    
        ....
        public void onSetIconHolder(int viewIndex, StatusBarIconHolder holder) {
            switch (holder.getType()) {
                case TYPE_ICON:
                    onSetIcon(viewIndex, holder.getIcon());
                    return;
                case TYPE_WIFI:
                    onSetSignalIcon(viewIndex, holder.getWifiState());
                    return;
                case TYPE_MOBILE:
                    onSetMobileIcon(viewIndex, holder.getMobileState());
                default:
                    break;
            }
        }
        public void onSetSignalIcon(int viewIndex, WifiIconState state) {
            StatusBarWifiView wifiView = (StatusBarWifiView) mGroup.getChildAt(viewIndex);
            if (wifiView != null) {
                wifiView.applyWifiState(state);
            }
            if (mIsInDemoMode) {
                mDemoStatusIcons.updateWifiState(state);
            }
        }
        ....
    }
    
}

这里根据不同的StatusBarIconHolder类型，设置不同的网络Icon，上面列出了 Wifi 图标相关的方法。
SystemUI状态栏图标根据源码可大体分为三种：

1. StatusBarIconView
2. StatusBarWifiView
3. StatusBarMobileView

这里主要以Wifi 相关图标（StatusBarWifiView）进行分析，添加Icon时首先会创建一个
StatusBarWifiView，然后调用StatusBarWifiView的applyWifiState更新其显示状态，最后将其加入到CollapsedStatusBarFragment中放置Icon的ViewGroup中，这样就完成了添加过程；
再来看看 CollapsedStatusBarFragment：
SystemUI/src/com/android/systemui/statusbar/phone/CollapsedStatusBarFragment.java

public class CollapsedStatusBarFragment extends Fragment implements CommandQueue.Callbacks {
    ....
    private DarkIconManager mDarkIconManager;
    ....
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
            Bundle savedInstanceState) {
        return inflater.inflate(R.layout.status_bar, container, false);
    }
    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        ....
        // 这里可以看出status_bar布局中的statusIcons就是我们展示各种Icon的区域
        mDarkIconManager = new DarkIconManager(view.findViewById(R.id.statusIcons));
        mDarkIconManager.setShouldLog(true);
        Dependency.get(StatusBarIconController.class).addIconGroup(mDarkIconManager);
        ....   
    }
}

补充：
notifyListenersIfNecessary()在其父类SignalController中定义，
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/policy/SignalController.java

    private final CallbackHandler mCallbackHandler;
    public void notifyListenersIfNecessary() {
        if (isDirty()) {
            saveLastState();
            notifyListeners();
        }
    }
    // 在这里注意了，在这里的的参数是 CallbackHandler 的对象
    public final void notifyListeners() {
        notifyListeners(mCallbackHandler);
    }
// callback 则是 CallbackHandler 的对象。
  public abstract void notifyListeners(SignalCallback callback);

CallbackHandler维护了所有需要监听的SignalCallback接口对象,我们的StatusBarSignalPolicy就实现了该接口。
StatusBarSignalPolicy主要执行网络图标的刷新动作，其实现了NetworkControllerImpl.SignalCallback接口,然后注册到NetworkController,其具体实现类NetworkControllerImpl会根据WIFI,SIM等状态广播来进一步派发给具体的Controller,例如WifiSignalController,每个Controller只与CallbackHandler交互,然后CallbackHandler继续转交给维护的SignalCallback接口的具体实现类,例如StatusBarSignalPolicy