Android WindowManagerService架构分析

WindowManagerService(以下简称WMS) 是Android的核心服务。WMS管理所有应用程序窗口(Window)的Create、Display、Update、Destory。
因为Android系统中只有一个WMS（运行在SystemServer进程），可以称其为全局的WMS。其主要的任务有两个：
全局的窗口管理
应用程序的显示(在屏幕上看到应用）在WMS的协助下有序、有层次的输出给底层服务，最终显示到物理屏幕上。
全局的事件管理派发
WMS为Android输入系统（InputManagerService）提供窗口相关信息，让输入事件（比如touch、homekey等等）可派发给适合的应用（窗口）。
触摸屏：主流Android设备都使用了出触控屏，支持手势触控、多指触控。
鼠标：android系统加入鼠标，通过光标触发相应动作。
硬按键：Home、back、menu等等功能按键。

WMS的客户端WindowManager

WindowManager是WMS提供给使用者的API。Manager的命名方式遵循了Android通过的Service/Client框架的命名方法，即
Service端：XXXService
客户端API：XXXManager

WindowManager封装了WMS提供的AIDL对象，主要包括：

• IWindowManager.aidl：官方注释为**“System private interface to the window manager.”**，定义了WMS服务提供的能力接口。

//frameworks/base/core/java/android/view/IWindowManager.aidl

/*
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

package android.view;

import com.android.internal.os.IResultReceiver;
import com.android.internal.policy.IKeyguardDismissCallback;
import com.android.internal.policy.IShortcutService;

import android.app.IAssistDataReceiver;
import android.content.res.CompatibilityInfo;
import android.content.res.Configuration;
import android.graphics.Bitmap;
import android.graphics.GraphicBuffer;
import android.graphics.Insets;
import android.graphics.Point;
import android.graphics.Rect;
import android.graphics.Region;
import android.os.Bundle;
import android.os.IRemoteCallback;
import android.os.ParcelFileDescriptor;
import android.view.DisplayCutout;
import android.view.IApplicationToken;
import android.view.IAppTransitionAnimationSpecsFuture;
import android.view.ICrossWindowBlurEnabledListener;
import android.view.IDisplayWindowInsetsController;
import android.view.IDisplayWindowListener;
import android.view.IDisplayFoldListener;
import android.view.IDisplayWindowRotationController;
import android.view.IOnKeyguardExitResult;
import android.view.IPinnedTaskListener;
import android.view.IScrollCaptureResponseListener;
import android.view.RemoteAnimationAdapter;
import android.view.IRotationWatcher;
import android.view.ISystemGestureExclusionListener;
import android.view.IWallpaperVisibilityListener;
import android.view.IWindow;
import android.view.IWindowSession;
import android.view.IWindowSessionCallback;
import android.view.KeyEvent;
import android.view.InputEvent;
import android.view.InsetsState;
import android.view.MagnificationSpec;
import android.view.MotionEvent;
import android.view.InputChannel;
import android.view.InputDevice;
import android.view.IInputFilter;
import android.view.AppTransitionAnimationSpec;
import android.view.WindowContentFrameStats;
import android.view.WindowManager;
import android.view.SurfaceControl;
import android.view.displayhash.DisplayHash;
import android.view.displayhash.VerifiedDisplayHash;

/**
 * System private interface to the window manager.
 *
 * {@hide}
 */
interface IWindowManager
{
// 省略
}

• IWindowSession.aidl：官方注释为“System private per-application interface to the window manager.”，同样定义WMS服务提供的能力接口。

//frameworks/base/core/java/android/view/IWindowSession.aidl
/* //device/java/android/android/view/IWindowSession.aidl
**
** Copyright 2006, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

package android.view;

import android.content.ClipData;
import android.graphics.Point;
import android.graphics.Rect;
import android.graphics.Region;
import android.os.Bundle;
import android.os.RemoteCallback;
import android.util.MergedConfiguration;
import android.view.DisplayCutout;
import android.view.InputChannel;
import android.view.IWindow;
import android.view.IWindowId;
import android.view.MotionEvent;
import android.view.WindowManager;
import android.view.InsetsSourceControl;
import android.view.InsetsState;
import android.view.Surface;
import android.view.SurfaceControl;
import android.view.SurfaceControl.Transaction;
import android.window.ClientWindowFrames;

import java.util.List;

/**
* System private per-application interface to the window manager.
*
* {@hide}
*/
interface IWindowSession {
// 省略
}

WindowManager常用的方法有三个addView、removeView、updateViewLayout，分别对应添加窗口、移除窗口、更新窗口布局功能。

// 获取WindowManager对象
WindowManager mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
// 构建布局
WindowManager.LayoutParams wmParams = new WindowManager.LayoutParams();
wmParams.xxx = xxx;
// 添加窗口到wms
mWindowManager.addView(xxxView，wmParams);
// 更新窗口布局
mWindowManager.updateViewLayout(xxxView, xxxWmParams)
// 从wms中移除窗口
mWindowmanager.remove(xxxView, wmParams);

Android支持多Display（多块物理屏或虚拟屏），在多Display情况下可以指定WindowManager绑定的Display，从而在指定的屏幕上显示内容。默认情况下，WindowManager绑定到DefaultDisplay。

// 获取DisplayManager对象
DisplayManager mDisplayManager;
mDisplayManager = (DisplayManager) context.getSystemService(Context.DISPLAY_SERVICE);

// 获取指定DisplayID的Display，可以通过DisplayManager的getDisplays接口取得ID。
// 取得Display对象
Display display = mDisplayManager.getDisplay(displayId);
// 通过特定的Display创建Context
Context displayContext = mContext.createDisplayContext(display);
// 获取特定Display的WindowManager对象
WindowManager displayWindowManager = (WindowManager) displayContext.getSystemService(Context.WINDOW_SERVICE);

WMS提供的主要方法源码分析

这里针对WMS提供的主要方法，根据Android12源码进行分析。

获取WindowManager对象

在应用中可通过如下方法获取WindowManager对象。

// 获取WindowManager对象
WindowManager mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);

调用context的getSystemService方法，其实现在ContextImpl.java中。

//frameworks/base/core/java/android/app/ContextImpl.java
@Override
public Object getSystemService(String name) {
	if (vmIncorrectContextUseEnabled()) {
		// Check incorrect Context usage.
		// 省略
	}
	return SystemServiceRegistry.getSystemService(this, name);
}

调用SystemServiceRegistry对象的getSystemService方法。name为window(Context.WINDOW_SERVICE对应的值）

//frameworks/base/core/java/android/app/SystemServiceRegistry.java
public static Object getSystemService(ContextImpl ctx, String name) {
	if (name == null) {
		return null;
	}
	
	// 从SYSTEM_SERVICE_FETCHERS(map)中取的Key为"Window"的value
	final ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
	if (fetcher == null) {
		if (sEnableServiceNotFoundWtf) {
			Slog.wtf(TAG, "Unknown manager requested: " + name);
		}
		return null;
	}
	// 调用getService方法
	// CachedServiceFetcher<T>对应的getService
	final Object ret = fetcher.getService(ctx);
	if (sEnableServiceNotFoundWtf && ret == null) {
		// 省略
		return null;
	}
	return ret;
}


/**
 * Override this class when the system service constructor needs a
 * ContextImpl and should be cached and retained by that context.
 */
static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
	private final int mCacheIndex;

	CachedServiceFetcher() {
		// Note this class must be instantiated only by the static initializer of the
		// outer class (SystemServiceRegistry), which already does the synchronization,
		// so bare access to sServiceCacheSize is okay here.
		mCacheIndex = sServiceCacheSize++;
	}

	@Override
	@SuppressWarnings("unchecked")
	public final T getService(ContextImpl ctx) {
		final Object[] cache = ctx.mServiceCache;
		final int[] gates = ctx.mServiceInitializationStateArray;
		boolean interrupted = false;

		T ret = null;

		for (;;) {
			boolean doInitialize = false;
			synchronized (cache) {
				// Return it if we already have a cached instance.
				// 如果有缓存，从缓存中取出来
				T service = (T) cache[mCacheIndex];
				if (service != null) {
					ret = service;
					// 跳出循环，直接返回
					break; // exit the for (;;)
				}

				// If we get here, there's no cached instance.

				// Grr... if gate is STATE_READY, then this means we initialized the service
				// once but someone cleared it.
				// We start over from STATE_UNINITIALIZED.
				// Similarly, if the previous attempt returned null, we'll retry again.
				if (gates[mCacheIndex] == ContextImpl.STATE_READY
						|| gates[mCacheIndex] == ContextImpl.STATE_NOT_FOUND) {
					gates[mCacheIndex] = ContextImpl.STATE_UNINITIALIZED;
				}

				// It's possible for multiple threads to get here at the same time, so
				// use the "gate" to make sure only the first thread will call createService().

				// At this point, the gate must be either UNINITIALIZED or INITIALIZING.
				if (gates[mCacheIndex] == ContextImpl.STATE_UNINITIALIZED) {
					doInitialize = true;
					gates[mCacheIndex] = ContextImpl.STATE_INITIALIZING;
				}
			}

			if (doInitialize) {
				// Only the first thread gets here.

				T service = null;
				@ServiceInitializationState int newState = ContextImpl.STATE_NOT_FOUND;
				try {
					// This thread is the first one to get here. Instantiate the service
					// *without* the cache lock held.
					// 创建新的对象。对于Context.WINDOW_SERVICE，创建的是WindowManagerImpl
					// 在SystemServiceRegistry初始化时，注册了各个服务对应的代理对象，感兴趣的可自行阅读源码。
					service = createService(ctx);
					newState = ContextImpl.STATE_READY;

				} catch (ServiceNotFoundException e) {
					onServiceNotFound(e);

				} finally {
					synchronized (cache) {
						cache[mCacheIndex] = service;
						gates[mCacheIndex] = newState;
						cache.notifyAll();
					}
				}
				ret = service;
				break; // exit the for (;;)
			}
			// 省略
		}
		if (interrupted) {
			Thread.currentThread().interrupt();
		}
		return ret;
	}

	public abstract T createService(ContextImpl ctx) throws ServiceNotFoundException;
}

上面创建了WindowManagerImpl对象，这个对象实现了 WindowManager接口类，是WMS提供的客户端代理真正实现类。

//frameworks/base/core/java/android/view/WindowManagerImpl.java
public final class WindowManagerImpl implements WindowManager {
    @UnsupportedAppUsage
    private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
    @UiContext
    @VisibleForTesting
    public final Context mContext;
    private final Window mParentWindow;

    /**
     * If {@link LayoutParams#token} is {@code null} and no parent window is specified, the value
     * of {@link LayoutParams#token} will be overridden to {@code mDefaultToken}.
     */
    private IBinder mDefaultToken;

    /**
     * This token will be set to {@link LayoutParams#mWindowContextToken} and used to receive
     * configuration changes from the server side.
     */
    @Nullable
    private final IBinder mWindowContextToken;

    public WindowManagerImpl(Context context) {
        this(context, null /* parentWindow */, null /* clientToken */);
    }

    private WindowManagerImpl(Context context, Window parentWindow,
            @Nullable IBinder windowContextToken) {
        mContext = context;
        mParentWindow = parentWindow;
        mWindowContextToken = windowContextToken;
    }
}

WindowManagerImpl构造时，会调用WindowManagerGlobal单例类的getInstance方法。WindowManagerGlobal持有IWindowManager对象。所以对应一个进程来讲，默认情况下只需要一个IWindowManager对象。

//frameworks/base/core/java/android/view/WindowManagerGlobal.java
 private static IWindowManager sWindowManagerService;

// 应用启动加载Activity时就会调用这个初始化。
@UnsupportedAppUsage
public static void initialize() {
	getWindowManagerService();
}

@UnsupportedAppUsage
public static WindowManagerGlobal getInstance() {
	synchronized (WindowManagerGlobal.class) {
		if (sDefaultWindowManager == null) {
			sDefaultWindowManager = new WindowManagerGlobal();
		}
		return sDefaultWindowManager;
	}
}

@UnsupportedAppUsage
public static IWindowManager getWindowManagerService() {
	synchronized (WindowManagerGlobal.class) {
		if (sWindowManagerService == null) {
			sWindowManagerService = IWindowManager.Stub.asInterface(
					ServiceManager.getService("window"));
			try {
				if (sWindowManagerService != null) {
					ValueAnimator.setDurationScale(
							sWindowManagerService.getCurrentAnimatorScale());
					sUseBLASTAdapter = sWindowManagerService.useBLAST();
				}
			} catch (RemoteException e) {
				throw e.rethrowFromSystemServer();
			}
		}
		return sWindowManagerService;
	}
}

综上，getSystemService(Context.WINDOW_SERVICE)返回的实际上是WindowManagerImpl。WindowManagerImpl通过WindowManagerGlobal这个单例类获取IWindowManager。

WindowManager AddView

通过addView添加窗口到屏幕上，例如：

// 添加窗口到wms
mWindowManager.addView(xxxView，wmParams);

调用WindowManagerImpl的addView方法

@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyTokens(params);
    mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,
            mContext.getUserId());
}

直接调用WindowManagerGlobal的addView方法，在这个方法主要是创建了ViewRootImpl，更新了mViews和mRoots等变量。然后调用了ViewRootImpl的setView方法。

public void addView(View view, ViewGroup.LayoutParams params,
		Display display, Window parentWindow, int userId) {
	// 省略
	ViewRootImpl root;
	View panelParentView = null;

	synchronized (mLock) {
		// 创建ViewRoot
		// 一个Window对应一个ViewRoot
		root = new ViewRootImpl(view.getContext(), display);

		view.setLayoutParams(wparams);
		// mDyingViews：进程中所有要销毁的View
		// mViews：进程中所有View
		// mRoots：进程中所有ViewRootImpl
		mViews.add(view);
		mRoots.add(root);
		mParams.add(wparams);

		// do this last because it fires off messages to start doing things
		try {
			root.setView(view, wparams, panelParentView, userId);
		} catch (RuntimeException e) {
			// BadTokenException or InvalidDisplayException, clean up.
			if (index >= 0) {
				removeViewLocked(index, true);
			}
			throw e;
		}
	}
}

ViewRootImpl的setView方法中，通过IWindowSession调用了WMS的addToDisplayAsUser方法，向WMS添加Window。WMS收到请求后，后创建WindowState与客户端的Window 一对一对应。

/**
 * We have one child
 */
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
		int userId) {
	synchronized (this) {
		if (mView == null) {
			mView = view;
			// 省略
			try {
				// 调用IWindowSession，向WMS添加Window
				res = mWindowSession.addToDisplayAsUser(mWindow, mWindowAttributes,
						getHostVisibility(), mDisplay.getDisplayId(), userId,
						mInsetsController.getRequestedVisibility(), inputChannel, mTempInsets,
						mTempControls);
			} catch (RemoteException e) {
				mAdded = false;
				mView = null;
				mAttachInfo.mRootView = null;
				mFallbackEventHandler.setView(null);
				unscheduleTraversals();
				setAccessibilityFocus(null, null);
				throw new RuntimeException("Adding window failed", e);
			} finally {
				if (restore) {
					attrs.restore();
				}
			}

	}
}

WMS架构

Window

• Window是一个窗口，很多图形系统都会有Window这个概念。比如CEGUI（天龙八部使用的UI系统）将Window作为最小的渲染单位，每个画面都是系列Window组成的二叉树。对于WMS来说，Window是最终呈现给用户的一块显示容器，是一系列View组成的一个画面。
• 实现上Window是一个抽象类，PhoneWindow是它的实现类。PhoneWindow对View进行管理。
• 一个Activity对应一个PhoneWindow，Activity启动时会创建与自身一一对应的PhoneWindow。
• Window是View的容器，View是Window的表现内容。

WindowManager

• WMS的接口类，继承ViewManager。用于给客户端管理窗口，它的实现类是WindowManagerImpl

InputManagerService

• 通过EventHub方式，从系统的输入Device节点中读取Input事件。通过WMS的协助派发给适当的应用。

SurfaceFlinger

• 分配应用程序需要的图形缓冲区，对系统整个图像窗口做Composition，最终图形窗口更新显示到Display。

WMS的主要功能：

• Surface管理：wms会为所有窗口分配Surface（通过surfaceFlinger创建Surface）。客户端向WMS添加窗口（addView）的过程，实质上是WMS为其分配一块Surface的过程。一系列Surface通过WMS进行管理，有序排布（z-order）。所以，View是表象、Window载体、Surface是本质。
• 窗口属性管理：显示层次、size、posotion等等属性管理，这些属性经过WMS的管理后最终反馈到SurfaceFlinger中。
• 窗口动画：进场、退场动画。
• 输入中转：WMS是窗口的管理者，IMS通过EventHub输入的系统Input事件，会经由WMS转发给恰当的应用（窗口）

WMS的启动

WMS在SystemServer的startOtherServices阶段启动。

WMS的构造方法

WMS添加窗口

通过调用WMS的addWindow添加窗口，WMS在添加窗口过程中，主要完成了以下内容

• 登记Window：创建WindowState，WindowState与客户端的ViewRoot/View一一对应，通过WindowState对象，WMS可以通知到客户端的ViewRoot。创建完成后，将WindowState加入到WindowMap中进行管理。
• 设置DisplayContent：一个屏幕对应一个DisplayContent，将窗口与对应的DisplayContent进行绑定。
• 申请Surface画布：WMS向SurfaceFligner申请一块Window画布（在SurfaceFlinger中是一个Layer），Surface画布对应着一块内存（fb buffer），Surface画面申请成功后View上的内容才可能显示到屏幕上。

如果View没有通过WindowManager.addView添加到WMS之前，View的onDraw是不会被调用的。View上绘制的内容与WMS无关，应用端可以会用接口直接告知SurfaceFlinger进行重绘。针对描画来讲，WMS只负责窗口的管理。

WMS与SurfaceFlinger的关系

如何调试WMS

可以通过以下几种方法调试WMS

• dumpsys windows: WMS提供了dump方式，可以通过dumpsys查看wms内部状态。对比WMS的实现源码，以进行相关问题调查。

//frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java
private void doDump(FileDescriptor fd, PrintWriter pw, String[] args, boolean useProto){
}

• 使用dumpsys window -h查看支持的命令
• WMS的LogTag：TAG_WITH_CLASS_NAME可以让WMS以统一的Tag”WindowManager”输出。其配置在WindowManagerDebugConfig.java文件中。

// frameworks/base/services/core/java/com/android/server/wm/WindowManagerDebugConfig.java
/**
 * Common class for the various debug {@link android.util.Log} output configuration in the window
 * manager package.
 */
public class WindowManagerDebugConfig {
    // All output logs in the window manager package use the {@link #TAG_WM} string for tagging
    // their log output. This makes it easy to identify the origin of the log message when sifting
    // through a large amount of log output from multiple sources. However, it also makes trying
    // to figure-out the origin of a log message while debugging the window manager a little
    // painful. By setting this constant to true, log messages from the window manager package
    // will be tagged with their class names instead fot the generic tag.
    static final boolean TAG_WITH_CLASS_NAME = false;

    // Default log tag for the window manager package.
    static final String TAG_WM = "WindowManager";
}

关于WMS的扩展探讨

大多数系统上WMS都是核心模块之一，拥有良好人机交互是系统流行的基础。WMS虽然与描画系统有关联，但其应属于AppFramework范畴。
考虑WMS，其通用性设计应该包括几个方面：

• 北向：提供稳定的API接口，提供窗口管理、层次管理、动画管理、输入管理等功能。应用通过北向接口，可以申请一块用于上屏的画布。
• 南向：封装并适配底层描画系统。