前言

NavigationBar 和 StatusBar 都属于 SystemBar，也叫做 decor，就是说给 App 装饰的意思。一般的 window 的布局是在 PhoneWindowManager 的 layoutWindowLw() 方法中，而 SystemBar 是在 beginLayoutLw() 方法中布局。

当前最上层的 Activity 可以修改 SystemBar 的 visibility，可以调用 View#setSystemUiVisibility() 方法，系统也有一些针对 SystemBar visibility 的策略。最终的 visibility 保存在 PhoneWindowManager 中的 mLastSystemUiFlags 变量中。

一、简单认识Android9中的DisplayFrames

mDisplayId 是跟物理屏幕相关的，DEFAULT_DISPLAY 的值是 0，mDisplayWidth 是物理屏幕宽度，mDisplayHeight 是物理屏幕高度。

public class DisplayFrames {
    public final int mDisplayId;

    /**
     * The current size of the screen; really; extends into the overscan area of the screen and
     * doesn't account for any system elements like the status bar.
     */
    //当前的屏幕大小，包括过扫描区域。过扫描区域在输出到 TV 时会用到，
    // 对于移动设备来说 mOverscan 大小就是物理设备的大小 (0,0)-(dw,dh)。
    public final Rect mOverscan = new Rect();

    /**
     * The current visible size of the screen; really; (ir)regardless of whether the status bar can
     * be hidden but not extending into the overscan area.
     */
    //当前可见的屏幕大小，其实就是 (0,0)-(dw,dh)。
    public final Rect mUnrestricted = new Rect();

    /** Like mOverscan*, but allowed to move into the overscan region where appropriate. */
    //是应用可显示的区域，包含 StatusBar 的区域，不包含 NavigationBar 区域。
    public final Rect mRestrictedOverscan = new Rect();
    /**
     * The current size of the screen; these may be different than (0,0)-(dw,dh) if the status bar
     * can't be hidden; in that case it effectively carves out that area of the display from all
     * other windows.
     */
    //一般情况下 mRestrictedOverscan 与 mRestricted 相同
    public final Rect mRestricted = new Rect();

    /**
     * During layout, the current screen borders accounting for any currently visible system UI
     * elements.
     */
    //是布局过程中，当前画面的边界，包含 Translucent（半透明）区域。一般情况下 NavigationBar 区域不是 Translucent，而 StatusBar 是 Translucent。
    public final Rect mSystem = new Rect();

    /** For applications requesting stable content insets, these are them. */
    //是应用窗口的显示区域，不包含 StatusBar 和 NavigationBar。
    public final Rect mStable = new Rect();

    /**
     * For applications requesting stable content insets but have also set the fullscreen window
     * flag, these are the stable dimensions without the status bar.
     */
    //是当Activity设置Fullscreen flag 时候的窗口显示区域，这时 StatusBar 会隐藏。
    public final Rect mStableFullscreen = new Rect();

    /**
     * During layout, the current screen borders with all outer decoration (status bar, input method
     * dock) accounted for.
     */
    // 是布局的时候除去外部装饰的窗口（例如 StatusBar 和输入法窗口）。
    public final Rect mCurrent = new Rect();

    /**
     * During layout, the frame in which content should be displayed to the user, accounting for all
     * screen decoration except for any space they deem as available for other content. This is
     * usually the same as mCurrent*, but may be larger if the screen decor has supplied content
     * insets.
     */
    // 是当前应该给用户显示的窗口，通常与 mCurrent 相同。当装饰窗口提供内容插入的时候，有可能比 mCurrent 更大。
    public final Rect mContent = new Rect();

    /**
     * During layout, the frame in which voice content should be displayed to the user, accounting
     * for all screen decoration except for any space they deem as available for other content.
     */
    //mVoiceContent 通常与 mContent 相同。
    public final Rect mVoiceContent = new Rect();

    /** During layout, the current screen borders along which input method windows are placed. */
    //mDock 是输入法布局时的边界。
    public final Rect mDock = new Rect();

    /** The display cutout used for layout (after rotation) */
    //用于刘海屏布局的剪刀工具，Android 9.0 新加入的。
    @NonNull public WmDisplayCutout mDisplayCutout = WmDisplayCutout.NO_CUTOUT;

    /** The cutout as supplied by display info */
    //用于刘海屏布局的剪刀工具，Android 9.0 新加入的。
    @NonNull public WmDisplayCutout mDisplayInfoCutout = WmDisplayCutout.NO_CUTOUT;

    /**
     * During layout, the frame that is display-cutout safe, i.e. that does not intersect with it.
     */
    //是在刘海屏上可以安全显示的区域，即这个区域与刘海区域没有交集。
    public final Rect mDisplayCutoutSafe = new Rect();

二、系统对SystemBar的布局

1、Android9系统主要是在PhoneWindowManager的beginLayoutLw() 方法中对系统的SystemBar进行布局的。

frameworks/base/services/core/java/com/android/server/policy/PhoneWindowManager.java

public class PhoneWindowManager implements WindowManagerPolicy {
   ...代码省略...
    public void beginLayoutLw(DisplayFrames displayFrames, int uiMode) {
        displayFrames.onBeginLayout();
        mSystemGestures.screenWidth = displayFrames.mUnrestricted.width();
        mSystemGestures.screenHeight = displayFrames.mUnrestricted.height();
        mDockLayer = 0x10000000;
        mStatusBarLayer = -1;
        final Rect pf = mTmpParentFrame;
        final Rect df = mTmpDisplayFrame;
        final Rect of = mTmpOverscanFrame;
        final Rect vf = mTmpVisibleFrame;
        final Rect dcf = mTmpDecorFrame;
        vf.set(displayFrames.mDock);
        of.set(displayFrames.mDock);
        df.set(displayFrames.mDock);
        pf.set(displayFrames.mDock);
        dcf.setEmpty();  // Decor frame N/A for system bars.
        if (displayFrames.mDisplayId == DEFAULT_DISPLAY) {
            // For purposes of putting out fake window up to steal focus, we will
            // drive nav being hidden only by whether it is requested.
            //获取窗口systemui的标记类型
            final int sysui = mLastSystemUiFlags;
            //navigationBar是否可见
            boolean navVisible = (sysui & View.SYSTEM_UI_FLAG_HIDE_NAVIGATION) == 0;
            //navigationBar是否是半透明的
            boolean navTranslucent = (sysui & (View.NAVIGATION_BAR_TRANSLUCENT | View.NAVIGATION_BAR_TRANSPARENT)) != 0;
            boolean immersive = (sysui & View.SYSTEM_UI_FLAG_IMMERSIVE) != 0;
            boolean immersiveSticky = (sysui & View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY) != 0;
            //navigationBar是否允许隐藏
            boolean navAllowedHidden = immersive || immersiveSticky;
            navTranslucent &= !immersiveSticky;  // transient trumps translucent
            boolean isKeyguardShowing = isStatusBarKeyguard() && !mKeyguardOccluded;
            if (!isKeyguardShowing) {
                navTranslucent &= areTranslucentBarsAllowed();
            }
            boolean statusBarExpandedNotKeyguard = !isKeyguardShowing && mStatusBar != null
                    && mStatusBar.getAttrs().height == MATCH_PARENT
                    && mStatusBar.getAttrs().width == MATCH_PARENT;

            if (navVisible || navAllowedHidden) {
                if (mInputConsumer != null) {
                    mHandler.sendMessage(
mHandler.obtainMessage(MSG_DISPOSE_INPUT_CONSUMER, mInputConsumer));
                    mInputConsumer = null;
                }
            } else if (mInputConsumer == null && mStatusBar != null && canHideNavigationBar()) {
                mInputConsumer = mWindowManagerFuncs.createInputConsumer(mHandler.getLooper(),
                        INPUT_CONSUMER_NAVIGATION,
                        (channel, looper) -> new HideNavInputEventReceiver(channel, looper));
InputManager.getInstance().setPointerIconType(PointerIcon.TYPE_NULL);
            }

            navVisible |= !canHideNavigationBar();
            //调用layoutNavigationBar对NavigationBar进行布局
            boolean updateSysUiVisibility = layoutNavigationBar(displayFrames, uiMode, dcf,
                    navVisible, navTranslucent, navAllowedHidden, statusBarExpandedNotKeyguard);
            if (DEBUG_LAYOUT) Slog.i(TAG, "mDock rect:" + displayFrames.mDock);
            //调用layoutStatusBar对StatusBar进行布局
            updateSysUiVisibility |= layoutStatusBar(
                    displayFrames, pf, df, of, vf, dcf, sysui, isKeyguardShowing);
            if (updateSysUiVisibility) {
                updateSystemUiVisibilityLw();
            }
        }
        layoutScreenDecorWindows(displayFrames, pf, df, dcf);
        if (displayFrames.mDisplayCutoutSafe.top > displayFrames.mUnrestricted.top) {
            displayFrames.mDisplayCutoutSafe.top = Math.max(displayFrames.mDisplayCutoutSafe.top,
                    displayFrames.mStable.top);
        }
    }
   ...代码省略...
}

beginLayoutLw方法中首先会判断NavigationBar是否可见以及是否半透明，然后会
调用layoutNavigationBar对NavigationBar进行布局，随后还会调用layoutStatusBar对StatusBar进行布局。

三、布局导航栏的关键方法layoutNavigationBar

1、下面是layoutNavigationBar的相关代码，layoutNavigationBar方法首先会调用 navigationBarPosition()方法返回 NavigationBar 的位置，根据返回的位置的不同所执行的布局方式也会不同。这里以布局在下边为例，会计算 NavigationBar 的 top，然后更新 mTmpNavigationFrame和 DisplayFrames 的相关属性。其中比较重要的是更新DisplayFrames.mDock变量，然后用这个变量设置 mCurrent、mVoiceContent、mContent` 等变量。

    private boolean layoutNavigationBar(DisplayFrames displayFrames, int uiMode, Rect dcf,
            boolean navVisible, boolean navTranslucent, boolean navAllowedHidden,
            boolean statusBarExpandedNotKeyguard) {
        if (mNavigationBar == null) {
            return false;
        }
        boolean transientNavBarShowing = mNavigationBarController.isTransientShowing();
        //根据屏幕旋转角度，我们需要为导航栏设置相对应的合适位置和大小
        final int rotation = displayFrames.mRotation;
        final int displayHeight = displayFrames.mDisplayHeight;
        final int displayWidth = displayFrames.mDisplayWidth;
        final Rect dockFrame = displayFrames.mDock;
        //获取导航栏的位置
        mNavigationBarPosition = navigationBarPosition(displayWidth, displayHeight, rotation);

        //cutoutSafeUnrestricted是安全的窗口（Android9针对刘海平新增的），当没有Overscan的时候与mUnrestricted相同
        //即cutoutSafeUnrestricted.bottom的值与DisplayFrames.mDisplayHeight值相同。
        final Rect cutoutSafeUnrestricted = mTmpRect;
        cutoutSafeUnrestricted.set(displayFrames.mUnrestricted);
        cutoutSafeUnrestricted.intersectUnchecked(displayFrames.mDisplayCutoutSafe);

        //导航栏在底部
        if (mNavigationBarPosition == NAV_BAR_BOTTOM) {
            //计算导航栏的top，
            final int top = cutoutSafeUnrestricted.bottom - getNavigationBarHeight(rotation, uiMode);
            //mTmpNavigationFrame就是NavigationBar所对应的窗口区域。
            mTmpNavigationFrame.set(0, top, displayWidth, displayFrames.mUnrestricted.bottom);
            //mStable对应应用窗口的显示区域，mFullscreen对应应用窗口全屏的显示区域，这里的设置使得应用窗口正常状态和全屏的时候都在导航栏的上方
            displayFrames.mStable.bottom = displayFrames.mStableFullscreen.bottom = top;
            if (transientNavBarShowing) {
                mNavigationBarController.setBarShowingLw(true);
            } else if (navVisible) {
                //如果NavigationBar可见的话，更新dockFrame、mRestricted、mRestrictedOverscan的bottom值
                mNavigationBarController.setBarShowingLw(true);
                dockFrame.bottom = displayFrames.mRestricted.bottom = displayFrames.mRestrictedOverscan.bottom = top;
            } else {
                // We currently want to hide the navigation UI - unless we expanded the status bar.
                mNavigationBarController.setBarShowingLw(statusBarExpandedNotKeyguard);
            }
            if (navVisible && !navTranslucent && !navAllowedHidden
                    && !mNavigationBar.isAnimatingLw()
                    && !mNavigationBarController.wasRecentlyTranslucent()) {
                // If the opaque nav bar is currently requested to be visible and not in the process
                // of animating on or off, then we can tell the app that it is covered by it.
                displayFrames.mSystem.bottom = top;
            }
        } else if (mNavigationBarPosition == NAV_BAR_RIGHT) {
            ...代码省略...
        } else if (mNavigationBarPosition == NAV_BAR_LEFT) {
            ...代码省略...
        }

        //使用dockFrame的参数去更新mCurrent，mVoiceContent，mContent
        displayFrames.mCurrent.set(dockFrame);
        displayFrames.mVoiceContent.set(dockFrame);
        displayFrames.mContent.set(dockFrame);
        mStatusBarLayer = mNavigationBar.getSurfaceLayer();
        //计算NavigationBar的contentFrame大小
        mNavigationBar.computeFrameLw(mTmpNavigationFrame, mTmpNavigationFrame,
                mTmpNavigationFrame, displayFrames.mDisplayCutoutSafe, mTmpNavigationFrame, dcf,
                mTmpNavigationFrame, displayFrames.mDisplayCutoutSafe,
                displayFrames.mDisplayCutout, false /* parentFrameWasClippedByDisplayCutout */);
        //将NavigationBar的contentFrame绑定到mNavigationBarController。
        mNavigationBarController.setContentFrame(mNavigationBar.getContentFrameLw());
        if (DEBUG_LAYOUT) Slog.i(TAG, "mNavigationBar frame: " + mTmpNavigationFrame);
        return mNavigationBarController.checkHiddenLw();
    }

NavigationBar 的 top 是 cutoutSafeUnrestricted.bottom 减去 NavigationBar 高度的结果。cutoutSafeUnrestricted 是安全的窗口（Android 9 针对刘海平新增的），当没有 Overscan 的时候与 mUnrestricted 相同，即 cutoutSafeUnrestricted.bottom 的值与 DisplayFrames.mDisplayHeight 值相同。计算 NavigationBar 的 top 后设置 mTmpNavigationFrame。mTmpNavigationFrame 就是 NavigationBar 的窗口区域。
之后更新 mStable 和 mFullscreen 的 bottom 值为 top。mStable 就是应用窗口的显示区域，这就是 NavigationBar 占用应用显示区域的原因。
如果 NavigationBar 可见的话更新 dockFrame、mRestricted、mRestrictedOverscan 的 bottom 值。然后用 dockFrame 去更新 mCurrent、mVoiceContent、mContent。
最后调用 computeFrameLw() 方法计算 NavigationBar 的 contentFrame 大小，然后绑定到 mNavigationBarController。

四、布局状态栏的关键方法layoutStatusBar

StatusBar 的布局在 PhoneWindowManager 的 beginLayoutLw() 方法中，调用 layoutStatusBar() 方法。

frameworks/base/services/core/java/com/android/server/policy/PhoneWindowManager.java

public class PhoneWindowManager implements WindowManagerPolicy {
    @Override
    public void beginLayoutLw(DisplayFrames displayFrames, int uiMode) {
        boolean updateSysUiVisibility |= layoutNavigationBar(displayFrames, uiMode, dcf,
                    navVisible, navTranslucent, navAllowedHidden, statusBarExpandedNotKeyguard);
        updateSysUiVisibility |= layoutStatusBar(
                    displayFrames, pf, df, of, vf, dcf, sysui, isKeyguardShowing);
        if (updateSysUiVisibility) {
            updateSystemUiVisibilityLw();
        }
    }

    private boolean layoutStatusBar(DisplayFrames displayFrames, Rect pf, Rect df, Rect of, Rect vf,
            Rect dcf, int sysui, boolean isKeyguardShowing) {
        // decide where the status bar goes ahead of time
        if (mStatusBar == null) {
            return false;
        }
        // apply any navigation bar insets
        of.set(displayFrames.mUnrestricted);
        df.set(displayFrames.mUnrestricted);
        pf.set(displayFrames.mUnrestricted);
        vf.set(displayFrames.mStable);

        mStatusBarLayer = mStatusBar.getSurfaceLayer();

        // Let the status bar determine its size.
        mStatusBar.computeFrameLw(pf /* parentFrame */, df /* displayFrame */,
                vf /* overlayFrame */, vf /* contentFrame */, vf /* visibleFrame */,
                dcf /* decorFrame */, vf /* stableFrame */, vf /* outsetFrame */,
                displayFrames.mDisplayCutout, false /* parentFrameWasClippedByDisplayCutout */);

        // For layout, the status bar is always at the top with our fixed height.
        displayFrames.mStable.top = displayFrames.mUnrestricted.top
                + mStatusBarHeightForRotation[displayFrames.mRotation];
        // Make sure the status bar covers the entire cutout height
        displayFrames.mStable.top = Math.max(displayFrames.mStable.top,
                displayFrames.mDisplayCutoutSafe.top);

        // Tell the bar controller where the collapsed status bar content is
        mTmpRect.set(mStatusBar.getContentFrameLw());
        mTmpRect.intersect(displayFrames.mDisplayCutoutSafe);
        mTmpRect.top = mStatusBar.getContentFrameLw().top;  // Ignore top display cutout inset
        mTmpRect.bottom = displayFrames.mStable.top;  // Use collapsed status bar size
        mStatusBarController.setContentFrame(mTmpRect);

        boolean statusBarTransient = (sysui & View.STATUS_BAR_TRANSIENT) != 0;
        boolean statusBarTranslucent = (sysui
                & (View.STATUS_BAR_TRANSLUCENT | View.STATUS_BAR_TRANSPARENT)) != 0;
        if (!isKeyguardShowing) {
            statusBarTranslucent &= areTranslucentBarsAllowed();
        }

        // If the status bar is hidden, we don't want to cause windows behind it to scroll.
        if (mStatusBar.isVisibleLw() && !statusBarTransient) {
            // Status bar may go away, so the screen area it occupies is available to apps but just
            // covering them when the status bar is visible.
            final Rect dockFrame = displayFrames.mDock;
            dockFrame.top = displayFrames.mStable.top;
            displayFrames.mContent.set(dockFrame);
            displayFrames.mVoiceContent.set(dockFrame);
            displayFrames.mCurrent.set(dockFrame);

            if (DEBUG_LAYOUT) Slog.v(TAG, "Status bar: " + String.format(
                    "dock=%s content=%s cur=%s", dockFrame.toString(),
                    displayFrames.mContent.toString(), displayFrames.mCurrent.toString()));

            if (!mStatusBar.isAnimatingLw() && !statusBarTranslucent
                    && !mStatusBarController.wasRecentlyTranslucent()) {
                // If the opaque status bar is currently requested to be visible, and not in the
                // process of animating on or off, then we can tell the app that it is covered by it.
                displayFrames.mSystem.top = displayFrames.mStable.top;
            }
        }
        return mStatusBarController.checkHiddenLw();
}}

首先设定 of /* overscanFrame /、df / displayFrame /、pf / parentFrame /、vf / visibleFrame */，其中 of、df、pf 设定为 displayFrames.mUnrestricted，即屏幕大小。vf 设定为 displayFrames.mStable，mStable 的大小在调用 layoutNavigationBar() 方法后变成了除去 NavigationBar 的窗口。然后调用 mStatusBar.computeFrameLw() 方法计算 StatusBar 的 mContentFrame 大小。mContentFrame 在 IME 不存在时与 mDecorFrame 相同，IME 存在时是 mDecorFrame 除去 IME 窗口的大小。

 // apply any navigation bar insets
    of.set(displayFrames.mUnrestricted);
    df.set(displayFrames.mUnrestricted);
    pf.set(displayFrames.mUnrestricted);
    vf.set(displayFrames.mStable);
    
    mStatusBarLayer = mStatusBar.getSurfaceLayer();
    
    // Let the status bar determine its size.
    mStatusBar.computeFrameLw(pf /* parentFrame */, df /* displayFrame */,
            vf /* overlayFrame */, vf /* contentFrame */, vf /* visibleFrame */,
            dcf /* decorFrame */, vf /* stableFrame */, vf /* outsetFrame */,
            displayFrames.mDisplayCutout, false /* parentFrameWasClippedByDisplayCutout */);

下一步修改 displayFrames.mStable，即应用的窗口。因为 StatusBar 默认显示在顶部的，所以修改 mStable.top 值。

    // For layout, the status bar is always at the top with our fixed height.
    displayFrames.mStable.top = displayFrames.mUnrestricted.top
            + mStatusBarHeightForRotation[displayFrames.mRotation];
    // Make sure the status bar covers the entire cutout height
    displayFrames.mStable.top = Math.max(displayFrames.mStable.top,
            displayFrames.mDisplayCutoutSafe.top);

计算 StatusBar 的可显示区域，绑定到 mStatusBarController。

    // Tell the bar controller where the collapsed status bar content is
    mTmpRect.set(mStatusBar.getContentFrameLw());
    mTmpRect.intersect(displayFrames.mDisplayCutoutSafe);
    mTmpRect.top = mStatusBar.getContentFrameLw().top;  // Ignore top display cutout inset
    mTmpRect.bottom = displayFrames.mStable.top;  // Use collapsed status bar size
    mStatusBarController.setContentFrame(mTmpRect);

判断 StatusBar 是否是短暂显示的（Transient）或是半透明的（Translucent）。其中 sysui 就是 mLastSystemUiFlags。

    boolean statusBarTransient = (sysui & View.STATUS_BAR_TRANSIENT) != 0;
    boolean statusBarTranslucent = (sysui
            & (View.STATUS_BAR_TRANSLUCENT | View.STATUS_BAR_TRANSPARENT)) != 0;
    if (!isKeyguardShowing) {
        statusBarTranslucent &= areTranslucentBarsAllowed();
    }

如果 StatusBar 是可见的，且不是暂时显示的，则修改 displayFrames 的 mDock、mContent、mVoiceContent、mCurrent，其实就是除去 StatusBar 的窗口大小。还有，如果 StatusBar 是不透明的，则修改 mSystem.top 值为 mStable.top 值，这时 Activity 使用 @android:style/Theme.NoTitleBar.Fullscreen 也不会隐藏 StatusBar。

    // If the status bar is hidden, we don't want to cause windows behind it to scroll.
    if (mStatusBar.isVisibleLw() && !statusBarTransient) {
        // Status bar may go away, so the screen area it occupies is available to apps but just
        // covering them when the status bar is visible.
        final Rect dockFrame = displayFrames.mDock;
        dockFrame.top = displayFrames.mStable.top;
        displayFrames.mContent.set(dockFrame);
        displayFrames.mVoiceContent.set(dockFrame);
        displayFrames.mCurrent.set(dockFrame);
    
        if (DEBUG_LAYOUT) Slog.v(TAG, "Status bar: " + String.format(
                "dock=%s content=%s cur=%s", dockFrame.toString(),
                displayFrames.mContent.toString(), displayFrames.mCurrent.toString()));
    
        if (!mStatusBar.isAnimatingLw() && !statusBarTranslucent
                && !mStatusBarController.wasRecentlyTranslucent()) {
            // If the opaque status bar is currently requested to be visible, and not in the
            // process of animating on or off, then we can tell the app that it is covered by it.
            displayFrames.mSystem.top = displayFrames.mStable.top;
        }
    }

参考文章：https://xianzhu21.space/developer/navigationbar-statusbar-frame-and-layout/