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DisplayFrames
在分析NaivgationBar和StatusBar对应的窗口布局前，需要我们先来简单认识Android中的DisplayFrames对象。
frameworks/base/services/core/java/com/android/server/wm/DisplayFrames.java
public class DisplayFrames {
//物理屏幕相关的设备id
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();
private final Rect mDisplayInfoOverscan = new Rect();
private final Rect mRotatedDisplayInfoOverscan = new Rect();
public int mDisplayWidth;//物理屏幕宽度
public int mDisplayHeight;//物理屏幕高度
public int mRotation;//屏幕旋转角度
public DisplayFrames(int displayId, DisplayInfo info, WmDisplayCutout displayCutout) {
mDisplayId = displayId;
onDisplayInfoUpdated(info, displayCutout);
}
public void onDisplayInfoUpdated(DisplayInfo info, WmDisplayCutout displayCutout) {
mDisplayWidth = info.logicalWidth;
mDisplayHeight = info.logicalHeight;
mRotation = info.rotation;
mDisplayInfoOverscan.set(
info.overscanLeft, info.overscanTop, info.overscanRight, info.overscanBottom);
mDisplayInfoCutout = displayCutout != null ? displayCutout : WmDisplayCutout.NO_CUTOUT;
}
public void onBeginLayout() {
switch (mRotation) {
case ROTATION_90:
mRotatedDisplayInfoOverscan.left = mDisplayInfoOverscan.top;
mRotatedDisplayInfoOverscan.top = mDisplayInfoOverscan.right;
mRotatedDisplayInfoOverscan.right = mDisplayInfoOverscan.bottom;
mRotatedDisplayInfoOverscan.bottom = mDisplayInfoOverscan.left;
break;
case ROTATION_180:
mRotatedDisplayInfoOverscan.left = mDisplayInfoOverscan.right;
mRotatedDisplayInfoOverscan.top = mDisplayInfoOverscan.bottom;
mRotatedDisplayInfoOverscan.right = mDisplayInfoOverscan.left;
mRotatedDisplayInfoOverscan.bottom = mDisplayInfoOverscan.top;
break;
case ROTATION_270:
mRotatedDisplayInfoOverscan.left = mDisplayInfoOverscan.bottom;
mRotatedDisplayInfoOverscan.top = mDisplayInfoOverscan.left;
mRotatedDisplayInfoOverscan.right = mDisplayInfoOverscan.top;
mRotatedDisplayInfoOverscan.bottom = mDisplayInfoOverscan.right;
break;
default:
mRotatedDisplayInfoOverscan.set(mDisplayInfoOverscan);
break;
}
mRestrictedOverscan.set(0, 0, mDisplayWidth, mDisplayHeight);
mOverscan.set(mRestrictedOverscan);
mSystem.set(mRestrictedOverscan);
mUnrestricted.set(mRotatedDisplayInfoOverscan);
mUnrestricted.right = mDisplayWidth - mUnrestricted.right;
mUnrestricted.bottom = mDisplayHeight - mUnrestricted.bottom;
mRestricted.set(mUnrestricted);
mDock.set(mUnrestricted);
mContent.set(mUnrestricted);
mVoiceContent.set(mUnrestricted);
mStable.set(mUnrestricted);
mStableFullscreen.set(mUnrestricted);
mCurrent.set(mUnrestricted);
mDisplayCutout = mDisplayInfoCutout;
mDisplayCutoutSafe.set(Integer.MIN_VALUE, Integer.MIN_VALUE,
Integer.MAX_VALUE, Integer.MAX_VALUE);
if (!mDisplayCutout.getDisplayCutout().isEmpty()) {
final DisplayCutout c = mDisplayCutout.getDisplayCutout();
if (c.getSafeInsetLeft() > 0) {
mDisplayCutoutSafe.left = mRestrictedOverscan.left + c.getSafeInsetLeft();
}
if (c.getSafeInsetTop() > 0) {
mDisplayCutoutSafe.top = mRestrictedOverscan.top + c.getSafeInsetTop();
}
if (c.getSafeInsetRight() > 0) {
mDisplayCutoutSafe.right = mRestrictedOverscan.right - c.getSafeInsetRight();
}
if (c.getSafeInsetBottom() > 0) {
mDisplayCutoutSafe.bottom = mRestrictedOverscan.bottom - c.getSafeInsetBottom();
}
}
}
}
二、构建SystemBar对应的视图窗口区域坐标对象
1、系统主要是在PhoneWindowManager的beginLayoutLw() 方法中构建SystemBar对应的视图窗口区域坐标对象的。
frameworks/base/services/core/java/com/android/server/policy/PhoneWindowManager.java
public class PhoneWindowManager implements WindowManagerPolicy {
@Override
public void beginLayoutLw(DisplayFrames displayFrames, int uiMode) {
displayFrames.onBeginLayout();
// TODO(multi-display): This doesn't seem right...Maybe only apply to default display?
mSystemGestures.screenWidth = displayFrames.mUnrestricted.width();
mSystemGestures.screenHeight = displayFrames.mUnrestricted.height();
mDockLayer = 0x10000000;
mStatusBarLayer = -1;
// start with the current dock rect, which will be (0,0,displayWidth,displayHeight)
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;
// When the navigation bar isn't visible, we put up a fake input window to catch all
// touch events. This way we can detect when the user presses anywhere to bring back the
// nav bar and ensure the application doesn't see the event.
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));
// As long as mInputConsumer is active, hover events are not dispatched to the app
// and the pointer icon is likely to become stale. Hide it to avoid confusion.
InputManager.getInstance().setPointerIconType(PointerIcon.TYPE_NULL);
}
// For purposes of positioning and showing the nav bar, if we have decided that it can't
// be hidden (because of the screen aspect ratio), then take that into account.
navVisible |= !canHideNavigationBar();
//调用layoutNavigationBar构建导航栏
boolean updateSysUiVisibility = layoutNavigationBar(displayFrames, uiMode, dcf,
navVisible, navTranslucent, navAllowedHidden, statusBarExpandedNotKeyguard);
if (DEBUG_LAYOUT) Slog.i(TAG, "mDock rect:" + displayFrames.mDock);
//调用layoutStatusBar构建状态栏
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) {
// Make sure that the zone we're avoiding for the cutout is at least as tall as the
// status bar; otherwise fullscreen apps will end up cutting halfway into the status
// bar.
displayFrames.mDisplayCutoutSafe.top = Math.max(displayFrames.mDisplayCutoutSafe.top,
displayFrames.mStable.top);
}
}
}
beginLayoutLw方法中首先会判断NavigationBar是否可见以及是否半透明，然后调用layoutNavigationBar方法和layoutStatusBar方法构建NavigationBar和StatusBar所对应的视图窗口区域坐标。
三、导航栏视图窗口区域坐标对象的构建
构建导航栏视图窗口区域坐标的layoutNavigationBar方法r如下所示。
public class PhoneWindowManager implements WindowManagerPolicy {
//导航栏视图对应的窗口状态
WindowState mNavigationBar = null;
//导航栏视图对应的窗口区域坐标
static final Rect mTmpNavigationFrame = new Rect();
//导航栏视图控制器
private final BarController mNavigationBarController = new BarController("NavigationBar",
View.NAVIGATION_BAR_TRANSIENT,
View.NAVIGATION_BAR_UNHIDE,
View.NAVIGATION_BAR_TRANSLUCENT,
StatusBarManager.WINDOW_NAVIGATION_BAR,
FLAG_TRANSLUCENT_NAVIGATION,
View.NAVIGATION_BAR_TRANSPARENT);
//构建导航栏视图对应的窗口区域坐标
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，我们需要为导航栏视图设置相对应的位置和大小
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) {
//计算导航栏的左上角的Y坐标
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，mStableFullscreen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视图所对应的窗口坐标对应contentFrame的大小
mNavigationBar.computeFrameLw(mTmpNavigationFrame, mTmpNavigationFrame,
mTmpNavigationFrame, displayFrames.mDisplayCutoutSafe, mTmpNavigationFrame, dcf,
mTmpNavigationFrame, displayFrames.mDisplayCutoutSafe,
displayFrames.mDisplayCutout, false /* parentFrameWasClippedByDisplayCutout */);
//将NavigationBar的contentFrame给到mNavigationBarController。
mNavigationBarController.setContentFrame(mNavigationBar.getContentFrameLw());
if (DEBUG_LAYOUT) Slog.i(TAG, "mNavigationBar frame: " + mTmpNavigationFrame);
return mNavigationBarController.checkHiddenLw();
}
//获取导航栏的位置
private int navigationBarPosition(int displayWidth, int displayHeight, int displayRotation) {
if (mNavigationBarCanMove && displayWidth > displayHeight) {
if (displayRotation == Surface.ROTATION_270) {
return NAV_BAR_LEFT;
} else {
return NAV_BAR_RIGHT;
}
}
return NAV_BAR_BOTTOM;
}
}
对以上代码做个简单总结：
1）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。
2）我们这里以布局在下方为例，计算导航栏NavigationBar的左上角Y坐标top，top的值等于cutoutSafeUnrestricted.bottom减去NavigationBar高度。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。
3）在得到导航栏NavigationBar的左上角Y坐标top之后，会将导航栏相关的坐标给到mTmpNavigationFrame。mTmpNavigationFrame就是导航栏NavigationBar视图所对应的窗口区域。之后会更新应用程序安全显示区域mStable和应用程序全屏安全显示区域mStableFullscreen的最下方Y坐标bottom值为top。这就是为什么NavigationBar视图占用应用显示区域的原因。
4）如果 NavigationBar 可见的话则更新 dockFrame、mRestricted、mRestrictedOverscan 的 bottom 值。然后用 dockFrame 去更新 mCurrent、mVoiceContent、mContent。最后调用WindowState的computeFrameLw() 方法计算NavigationBar视图所对应的窗口坐标contentFrame的大小，并将结果给到mNavigationBarController对象。
四、状态栏视图窗口区域坐标对象的构建
1、构建状态栏视图窗口区域坐标的layoutStatusBar方法r如下所示。
public class PhoneWindowManager implements WindowManagerPolicy {
//状态栏视图所对应的窗口状态
WindowState mStatusBar = null;
private static final Rect mTmpRect = new Rect();
private final StatusBarController mStatusBarController = new StatusBarController();
//构建状态栏视图对应的窗口区域坐标
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();
//计算StatusBar的mContentFrame的大小
mStatusBar.computeFrameLw(pf /* parentFrame */, df /* displayFrame */,
vf /* overlayFrame */, vf /* contentFrame */, vf /* visibleFrame */,
dcf /* decorFrame */, vf /* stableFrame */, vf /* outsetFrame */,
displayFrames.mDisplayCutout, false /* parentFrameWasClippedByDisplayCutout */);
//因为StatusBar默认显示在顶部的，所以修改应用程序安全显示区域mStable.top的值。
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，并设置给mStatusBarController。
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();
}
if (mStatusBar.isVisibleLw() && !statusBarTransient) {
//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。
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()) {
//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。
displayFrames.mSystem.top = displayFrames.mStable.top;
}
}
return mStatusBarController.checkHiddenLw();
}}
1）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，前面有提到mStable 的大小在调用 layoutNavigationBar() 方法后变成了除去NavigationBar的窗口，即应用程序安全显示区域。
2）然后调用 mStatusBar.computeFrameLw() 方法计算 StatusBar 的 mContentFrame 大小。mContentFrame 在 IME (输入法)不存在时与 mDecorFrame 相同，IME 存在时是 mDecorFrame 除去 IME 窗口的大小。
3）然后修改 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。计算 StatusBar 的可显示区域，并将计算结果设置到mStatusBarController中。
4）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。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。