在将Android window Layer层级关系之前，我们先来直观的看看是什么样的：
adb shell dumpsys SurfaceFlinger
可以看到类似这样的输出：
Display 0 HWC layers:
-------------------------------------------------------------------------------
Layer name
Z | Comp Type | Disp Frame (LTRB) | Source Crop (LTRB)
-------------------------------------------------------------------------------
xxx xxx#0
231000 | Client | 0 1538 565 1578 | 0.0 0.0 565.0 426.0
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
xxx xxx#0
241000 | Client | 0 1180 565 1744 | 0.0 0.0 565.0 564.0
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
xxx xxx#0
241005 | Client | 0 1568 130 1578 | 0.0 0.0 130.0 220.0
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
xxx xxx#0
241010 | Client | 860 1568 565 1578 | 0.0 0.0 130.0 220.0
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
xxx xxx control#0
241015 | Client | 0 1180 130 1744 | 0.0 0.0 130.0 564.0
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
xxx xxx #0
241020 | Client | 860 1180 565 1744 | 0.0 0.0 130.0 564.0
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
这个输出包含了，Layer层级，Layer 画面的width, height 信息，要理解Layer 的层级关系，我们先要了解三个东西：WindowType, Z-Order, Layer
1.WindowType
在 WindowManager.java 中定义了Window的类型，这个类型引用 也是应用开发过程中直接使用的类型，主要分成三大类:
Application windows（应用窗口）: 1~99
Sub-windows(子窗口): 1000~1999
System windows（系统窗口）: 2000~2999
注意：
网上有很多说法，系统窗口 > 子窗口 > 应用窗口，从数值上来说是正确的，但是如果说 Window Layer的层级关系，这个说法就是错误的，举个很简单的例子，壁纸层 属于系统窗口管理，但是Window Layer的层级关系 却是最底端；WindowType 的值划分是为了更好的管理Window的类别、权限、依赖关系，和实际的Window Layer层级并没有关系。
2. Z-Order
可以去查一下这个词汇的含义，这个词汇一般出现在产品，UX设计过程中，而这个值的定义才是产品意义上的界面层级关系；Android WindowManagerPolicy中也给出了WindowType 和 Z-Order的对应关系：
/**
* Returns the layer assignment for the window type. Allows you to control how different
* kinds of windows are ordered on-screen.
*
* @param type The type of window being assigned.
* @param canAddInternalSystemWindow If the owner window associated with the type we are
* evaluating can add internal system windows. I.e they have
* {@link permission#INTERNAL_SYSTEM_WINDOW}. If true, alert window
* types {@link android.view.WindowManager.LayoutParams#isSystemAlertWindowType(int)}
* can be assigned layers greater than the layer for
* {@link android.view.WindowManager.LayoutParams#TYPE_APPLICATION_OVERLAY} Else, their
* layers would be lesser.
* @return int An arbitrary integer used to order windows, with lower numbers below higher ones.
*/
default int getWindowLayerFromTypeLw(int type, boolean canAddInternalSystemWindow) {
if (type >= FIRST_APPLICATION_WINDOW && type <= LAST_APPLICATION_WINDOW) {
return APPLICATION_LAYER;
}
switch (type) {
case TYPE_WALLPAPER:
// wallpaper is at the bottom, though the window manager may move it.
return 1;
case TYPE_PRESENTATION:
case TYPE_PRIVATE_PRESENTATION:
return APPLICATION_LAYER;
case TYPE_DOCK_DIVIDER:
return APPLICATION_LAYER;
case TYPE_QS_DIALOG:
return APPLICATION_LAYER;
case TYPE_PHONE:
return 3;
case TYPE_SEARCH_BAR:
case TYPE_VOICE_INTERACTION_STARTING:
return 4;
case TYPE_VOICE_INTERACTION:
// voice interaction layer is almost immediately above apps.
return 5;
case TYPE_INPUT_CONSUMER:
return 6;
case TYPE_SYSTEM_DIALOG:
return 7;
case TYPE_TOAST:
// toasts and the plugged-in battery thing
return 8;
case TYPE_PRIORITY_PHONE:
// SIM errors and unlock. Not sure if this really should be in a high layer.
return 9;
case TYPE_SYSTEM_ALERT:
// like the ANR / app crashed dialogs
return canAddInternalSystemWindow ? 11 : 10;
case TYPE_APPLICATION_OVERLAY:
return 12;
case TYPE_DREAM:
// used for Dreams (screensavers with TYPE_DREAM windows)
return 13;
case TYPE_INPUT_METHOD:
// on-screen keyboards and other such input method user interfaces go here.
return 14;
case TYPE_INPUT_METHOD_DIALOG:
// on-screen keyboards and other such input method user interfaces go here.
return 15;
case TYPE_STATUS_BAR_SUB_PANEL:
return 17;
case TYPE_STATUS_BAR:
return 18;
case TYPE_STATUS_BAR_PANEL:
return 19;
case TYPE_KEYGUARD_DIALOG:
return 20;
case TYPE_VOLUME_OVERLAY:
// the on-screen volume indicator and controller shown when the user
// changes the device volume
return 21;
case TYPE_SYSTEM_OVERLAY:
// the on-screen volume indicator and controller shown when the user
// changes the device volume
return canAddInternalSystemWindow ? 22 : 11;
case TYPE_NAVIGATION_BAR:
// the navigation bar, if available, shows atop most things
return 23;
case TYPE_NAVIGATION_BAR_PANEL:
// some panels (e.g. search) need to show on top of the navigation bar
return 24;
case TYPE_SCREENSHOT:
// screenshot selection layer shouldn't go above system error, but it should cover
// navigation bars at the very least.
return 25;
case TYPE_SYSTEM_ERROR:
// system-level error dialogs
return canAddInternalSystemWindow ? 26 : 10;
case TYPE_MAGNIFICATION_OVERLAY:
// used to highlight the magnified portion of a display
return 27;
case TYPE_DISPLAY_OVERLAY:
// used to simulate secondary display devices
return 28;
case TYPE_DRAG:
// the drag layer: input for drag-and-drop is associated with this window,
// which sits above all other focusable windows
return 29;
case TYPE_ACCESSIBILITY_OVERLAY:
// overlay put by accessibility services to intercept user interaction
return 30;
case TYPE_SECURE_SYSTEM_OVERLAY:
return 31;
case TYPE_BOOT_PROGRESS:
return 32;
case TYPE_POINTER:
// the (mouse) pointer layer
return 33;
default:
Slog.e("WindowManager", "Unknown window type: " + type);
return APPLICATION_LAYER;
}
}
可以看出Android8.0 Z-Order 定义了33 层, 相信看到这个兑换关系会解决你很多疑惑，到了这一层还是跟我们一开始看到的Layer 关系对不上。还有一层Z-Order 如何跟Surface Layer 进行对应？
3. Layer
Android 画面内容输出，是通过SurfaceFlinger 进行画面的合成、输出的，直接关联的就是Layer
Framework中计算Layer 的公式为：Layer = Z-Order * 10000 + 1000 :
getWindowLayerFromTypeLw() * WindowManagerService.TYPE_LAYER_MULTIPLIER + WindowManagerService.TYPE_LAYER_OFFSET
相同Layer 会根据添加的顺序，加上offset 值。