解析Android 8.1平台SystemUI 导航栏加载流程

protected void makeStatusBarView() {
  final Context context = mContext;
  updateDisplaySize(); // populates mDisplayMetrics
  updateResources();
  updateTheme();
  ...
  ...
   try {
    boolean showNav = mWindowManagerService.hasNavigationBar();
    if (DEBUG) Log.v(TAG, "hasNavigationBar=" + showNav);
    if (showNav) {
      createNavigationBar();//创建导航栏
    }
  } catch (RemoteException ex) {
  }
}

protected void createNavigationBar() {
  mNavigationBarView = NavigationBarFragment.create(mContext, (tag, fragment) -  {
    mNavigationBar = (NavigationBarFragment) fragment;
    if (mLightBarController != null) {
      mNavigationBar.setLightBarController(mLightBarController);
    }
    mNavigationBar.setCurrentSysuiVisibility(mSystemUiVisibility);
  });
}

public static View create(Context context, FragmentListener listener) {
  WindowManager.LayoutParams lp = new WindowManager.LayoutParams(
      LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT,
      WindowManager.LayoutParams.TYPE_NAVIGATION_BAR,
      WindowManager.LayoutParams.FLAG_TOUCHABLE_WHEN_WAKING
          | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
          | WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL
          | WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH
          | WindowManager.LayoutParams.FLAG_SPLIT_TOUCH
          | WindowManager.LayoutParams.FLAG_SLIPPERY,
      PixelFormat.TRANSLUCENT);
  lp.token = new Binder();
  lp.setTitle("NavigationBar");
  lp.windowAnimations = 0;
  View navigationBarView = LayoutInflater.from(context).inflate(
      R.layout.navigation_bar_window, null);
  if (DEBUG) Log.v(TAG, "addNavigationBar: about to add " + navigationBarView);
  if (navigationBarView == null) return null;
  context.getSystemService(WindowManager.class).addView(navigationBarView, lp);
  FragmentHostManager fragmentHost = FragmentHostManager.get(navigationBarView);
  NavigationBarFragment fragment = new NavigationBarFragment();
  fragmentHost.getFragmentManager().beginTransaction()
      .replace(R.id.navigation_bar_frame, fragment, TAG) //注意！fragment里onCreateView加载的布局是add到这个Window属性的view里的。
      .commit();
  fragmentHost.addTagListener(TAG, listener);
  return navigationBarView;
 }
}

<com.android.systemui.statusbar.phone.NavigationBarFrame
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:systemui="http://schemas.android.com/apk/res-auto"
  android:id="@+id/navigation_bar_frame"
  android:layout_height="match_parent"
  android:layout_width="match_parent"  

</com.android.systemui.statusbar.phone.NavigationBarFrame 

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
    Bundle savedInstanceState) {
  return inflater.inflate(R.layout.navigation_bar, container, false);
}

<com.android.systemui.statusbar.phone.NavigationBarView
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:systemui="http://schemas.android.com/apk/res-auto"
  android:layout_height="match_parent"
  android:layout_width="match_parent"
android:background="@drawable/system_bar_background" 
<com.android.systemui.statusbar.phone.NavigationBarInflaterView
    android:id="@+id/navigation_inflater"
    android:layout_width="match_parent"
    android:layout_height="match_parent" / 

</com.android.systemui.statusbar.phone.NavigationBarView 

public NavigationBarInflaterView(Context context, AttributeSet attrs) {
  super(context, attrs);
  createInflaters();//根据屏幕旋转角度创建子view（单个back home or recent）的父布局
  Display display = ((WindowManager)
      context.getSystemService(Context.WINDOW_SERVICE)).getDefaultDisplay();
  Mode displayMode = display.getMode();
  isRot0Landscape = displayMode.getPhysicalWidth()   displayMode.getPhysicalHeight();
}
private void inflateChildren() {
  removeAllViews();
  mRot0 = (FrameLayout) mLayoutInflater.inflate(R.layout.navigation_layout, this, false);
  mRot0.setId(R.id.rot0);
  addView(mRot0);
  mRot90 = (FrameLayout) mLayoutInflater.inflate(R.layout.navigation_layout_rot90, this, false);
  mRot90.setId(R.id.rot90);
  addView(mRot90);
  updateAlternativeOrder();
}

@Override
protected void onFinishInflate() {
  super.onFinishInflate();
  inflateChildren();//进去看无关紧要 忽略
  clearViews();//进去看无关紧要 忽略
  inflateLayout(getDefaultLayout());//关键方法：加载了 back.home.recent三个按钮的layout
}

protected void inflateLayout(String newLayout) {
  mCurrentLayout = newLayout;
  if (newLayout == null) {
    newLayout = getDefaultLayout();
  }
  String[] sets = newLayout.split(GRAVITY_SEPARATOR, 3);//根据“；”号分割成长度为3的数组
  String[] start = sets[0].split(BUTTON_SEPARATOR);//根据“，”号分割，包含 left[.5W]和back[1WC]
  String[] center = sets[1].split(BUTTON_SEPARATOR);//包含home
  String[] end = sets[2].split(BUTTON_SEPARATOR);//包含recent[1WC]和right[.5W]
  // Inflate these in start to end order or accessibility traversal will be messed up.
  inflateButtons(start, mRot0.findViewById(R.id.ends_group), isRot0Landscape, true);
  inflateButtons(start, mRot90.findViewById(R.id.ends_group), !isRot0Landscape, true);
  inflateButtons(center, mRot0.findViewById(R.id.center_group), isRot0Landscape, false);
  inflateButtons(center, mRot90.findViewById(R.id.center_group), !isRot0Landscape, false);
  addGravitySpacer(mRot0.findViewById(R.id.ends_group));
  addGravitySpacer(mRot90.findViewById(R.id.ends_group));
  inflateButtons(end, mRot0.findViewById(R.id.ends_group), isRot0Landscape, false);
  inflateButtons(end, mRot90.findViewById(R.id.ends_group), !isRot0Landscape, false);
}
  protected String getDefaultLayout() {
  return mContext.getString(R.string.config_navBarLayout);
}

 <!-- Nav bar button default ordering/layout -- 
<string name="config_navBarLayout" translatable="false" left[.5W],back[1WC];home;recent[1WC],right[.5W]</string 

private void inflateButtons(String[] buttons, ViewGroup parent, boolean landscape,
    boolean start) {
  for (int i = 0; i < buttons.length; i++) {
    inflateButton(buttons[i], parent, landscape, start);
  }
}
@Nullable
protected View inflateButton(String buttonSpec, ViewGroup parent, boolean landscape,
    boolean start) {
  LayoutInflater inflater = landscape ? mLandscapeInflater : mLayoutInflater;
  View v = createView(buttonSpec, parent, inflater);//创建view
  if (v == null) return null;
  v = applySize(v, buttonSpec, landscape, start);
  parent.addView(v);//addView到父布局
  addToDispatchers(v);
  View lastView = landscape ? mLastLandscape : mLastPortrait;
  View accessibilityView = v;
  if (v instanceof ReverseFrameLayout) {
    accessibilityView = ((ReverseFrameLayout) v).getChildAt(0);
  }
  if (lastView != null) {
    accessibilityView.setAccessibilityTraversalAfter(lastView.getId());
  }
  if (landscape) {
    mLastLandscape = accessibilityView;
  } else {
    mLastPortrait = accessibilityView;
  }
  return v;
}

private View createView(String buttonSpec, ViewGroup parent, LayoutInflater inflater) {
  View v = null;
  ...
  ...
  if (HOME.equals(button)) {
    v = inflater.inflate(R.layout.home, parent, false);
  } else if (BACK.equals(button)) {
    v = inflater.inflate(R.layout.back, parent, false);
  } else if (RECENT.equals(button)) {
    v = inflater.inflate(R.layout.recent_apps, parent, false);
  } else if (MENU_IME.equals(button)) {
    v = inflater.inflate(R.layout.menu_ime, parent, false);
  } else if (NAVSPACE.equals(button)) {
    v = inflater.inflate(R.layout.nav_key_space, parent, false);
  } else if (CLIPBOARD.equals(button)) {
    v = inflater.inflate(R.layout.clipboard, parent, false);
  } 
  ...
  ...
  return v;
}
//SystemUIreslayouthome.xml 
//这里布局里没有src显示home的icon,肯定是在代码里设置了
//这里也是自定义view:KeyButtonView
<com.android.systemui.statusbar.policy.KeyButtonView
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:systemui="http://schemas.android.com/apk/res-auto"
android:id="@+id/home"
android:layout_width="@dimen/navigation_key_width"//引用了dimens.xml里的navigation_key_width
android:layout_height="match_parent"
android:layout_weight="0"
systemui:keyCode="3"//systemui自定义的属性
android:scaleType="fitCenter"
android:contentDescription="@string/accessibility_home"
android:paddingTop="@dimen/home_padding"
android:paddingBottom="@dimen/home_padding"
android:paddingStart="@dimen/navigation_key_padding"
android:paddingEnd="@dimen/navigation_key_padding"/ 

public KeyButtonView(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs);
  TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.KeyButtonView,
      defStyle, 0);
  mCode = a.getInteger(R.styleable.KeyButtonView_keyCode, 0);
  mSupportsLongpress = a.getBoolean(R.styleable.KeyButtonView_keyRepeat, true);
  mPlaySounds = a.getBoolean(R.styleable.KeyButtonView_playSound, true);
  TypedValue value = new TypedValue();
  if (a.getValue(R.styleable.KeyButtonView_android_contentDescription, value)) {
    mContentDescriptionRes = value.resourceId;
  }
  a.recycle();
  setClickable(true);
  mTouchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
  mAudioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
  mRipple = new KeyButtonRipple(context, this);
  setBackground(mRipple);
}
...
...
public boolean onTouchEvent(MotionEvent ev) {
  ...
  switch (action) {
    case MotionEvent.ACTION_DOWN:
      mDownTime = SystemClock.uptimeMillis();
      mLongClicked = false;
      setPressed(true);
      if (mCode != 0) {
        sendEvent(KeyEvent.ACTION_DOWN, 0, mDownTime);//关键方法
      } else {
        // Provide the same haptic feedback that the system offers for virtual keys.
        performHapticFeedback(HapticFeedbackConstants.VIRTUAL_KEY);
      }
      playSoundEffect(SoundEffectConstants.CLICK);
      removeCallbacks(mCheckLongPress);
      postDelayed(mCheckLongPress, ViewConfiguration.getLongPressTimeout());
      break;
    ...
    ...
  }
  return true;
}
void sendEvent(int action, int flags, long when) {
  mMetricsLogger.write(new LogMaker(MetricsEvent.ACTION_NAV_BUTTON_EVENT)
      .setType(MetricsEvent.TYPE_ACTION)
      .setSubtype(mCode)
      .addTaggedData(MetricsEvent.FIELD_NAV_ACTION, action)
      .addTaggedData(MetricsEvent.FIELD_FLAGS, flags));
  final int repeatCount = (flags & KeyEvent.FLAG_LONG_PRESS) != 0 ? 1 : 0;
  //这里根据mCode new了一个KeyEvent事件，通过injectInputEvent使事件生效。
  final KeyEvent ev = new KeyEvent(mDownTime, when, action, mCode, repeatCount,
      0, KeyCharacterMap.VIRTUAL_KEYBOARD, 0,
      flags | KeyEvent.FLAG_FROM_SYSTEM | KeyEvent.FLAG_VIRTUAL_HARD_KEY,
      InputDevice.SOURCE_KEYBOARD);
  InputManager.getInstance().injectInputEvent(ev,
      InputManager.INJECT_INPUT_EVENT_MODE_ASYNC);
}

public NavigationBarView(Context context, AttributeSet attrs) {
  super(context, attrs);
  mDisplay = ((WindowManager) context.getSystemService(
      Context.WINDOW_SERVICE)).getDefaultDisplay();
  ...
  ...
  updateIcons(context, Configuration.EMPTY, mConfiguration);//关键方法
  mBarTransitions = new NavigationBarTransitions(this);
  //mButtonDispatchers 是维护这些home back recent图标view的管理类，会传递到他的child,NavigationBarInflaterView类中
  mButtonDispatchers.put(R.id.back, new ButtonDispatcher(R.id.back));
  mButtonDispatchers.put(R.id.home, new ButtonDispatcher(R.id.home));
  mButtonDispatchers.put(R.id.recent_apps, new ButtonDispatcher(R.id.recent_apps));
  mButtonDispatchers.put(R.id.menu, new ButtonDispatcher(R.id.menu));
  mButtonDispatchers.put(R.id.ime_switcher, new ButtonDispatcher(R.id.ime_switcher));
  mButtonDispatchers.put(R.id.accessibility_button,new ButtonDispatcher(R.id.accessibility_button));
}
 private void updateIcons(Context ctx, Configuration oldConfig, Configuration newConfig) {
    ...
    iconLight = mNavBarPlugin.getHomeImage(
                  ctx.getDrawable(R.drawable.ic_sysbar_home));
    iconDark = mNavBarPlugin.getHomeImage(
                  ctx.getDrawable(R.drawable.ic_sysbar_home_dark));
    //mHomeDefaultIcon = getDrawable(ctx,
    //    R.drawable.ic_sysbar_home, R.drawable.ic_sysbar_home_dark);
    mHomeDefaultIcon = getDrawable(iconLight,iconDark);
    //亮色的icon资源
    iconLight = mNavBarPlugin.getRecentImage(
                  ctx.getDrawable(R.drawable.ic_sysbar_recent));
    //暗色的icon资源
    iconDark = mNavBarPlugin.getRecentImage(
                  ctx.getDrawable(R.drawable.ic_sysbar_recent_dark));
    //mRecentIcon = getDrawable(ctx,
    //    R.drawable.ic_sysbar_recent, R.drawable.ic_sysbar_recent_dark);
    mRecentIcon = getDrawable(iconLight,iconDark);
    mMenuIcon = getDrawable(ctx, R.drawable.ic_sysbar_menu,
                  R.drawable.ic_sysbar_menu_dark);
    ...
    ...

}

private void updateRecentsIcon() {
  getRecentsButton().setImageDrawable(mDockedStackExists ? mDockedIcon : mRecentIcon);
  mBarTransitions.reapplyDarkIntensity();
}

@Override
protected void onConfigurationChanged(Configuration newConfig) {
  super.onConfigurationChanged(newConfig);
  boolean uiCarModeChanged = updateCarMode(newConfig);
  updateTaskSwitchHelper();
  updateIcons(getContext(), mConfiguration, newConfig);
  updateRecentsIcon();
  if (uiCarModeChanged || mConfiguration.densityDpi != newConfig.densityDpi
      || mConfiguration.getLayoutDirection() != newConfig.getLayoutDirection()) {
    // If car mode or density changes, we need to reset the icons.
    setNavigationIconHints(mNavigationIconHints, true);
  }
  mConfiguration.updateFrom(newConfig);
}
public void setNavigationIconHints(int hints, boolean force) {
  ...
  ...
  mNavigationIconHints = hints;
  // We have to replace or restore the back and home button icons when exiting or entering
  // carmode, respectively. Recents are not available in CarMode in nav bar so change
  // to recent icon is not required.
  KeyButtonDrawable backIcon = (backAlt)
      ? getBackIconWithAlt(mUseCarModeUi, mVertical)
      : getBackIcon(mUseCarModeUi, mVertical);
  getBackButton().setImageDrawable(backIcon);
  updateRecentsIcon();
  ...
  ...
}

public void reorient() {
  updateCurrentView();
  ...
  setNavigationIconHints(mNavigationIconHints, true);
  getHomeButton().setVertical(mVertical);
}