if (context.getResources().getBoolean(R.bool.config_enableWallpaperService)) {
                t.traceBegin("StartWallpaperManagerService");
                mSystemServiceManager.startService(WALLPAPER_SERVICE_CLASS);
                t.traceEnd();
            } else {
                Slog.i(TAG, "Wallpaper service disabled by config");
            }

private void setWallpaperComponent(ComponentName name, int userId) {
        ...
        /* 首先調(diào)用該方法的時(shí)候回去校驗(yàn)權(quán)限，該權(quán)限定義在frameworks/base/core/res/AndroidManifest.xml，
            <permission android:name="android.permission.SET_WALLPAPER_COMPONENT"
        android:protectionLevel="signature|privileged" />
        查看protectionLevel，只有是特權(quán)應(yīng)用或者系統(tǒng)簽名的應(yīng)用才能獲取到這個(gè)系統(tǒng)權(quán)限，所以普通的應(yīng)用是沒有辦法進(jìn)行壁紙?jiān)O(shè)置的
        */
        checkPermission(android.Manifest.permission.SET_WALLPAPER_COMPONENT);

        int which = FLAG_SYSTEM;
        boolean shouldNotifyColors = false;
        WallpaperData wallpaper;

        synchronized (mLock) {
            Slog.v(TAG, "setWallpaperComponent name=" + name);
            /*
              此處會先通過當(dāng)前的用戶ID獲取到與該用戶相關(guān)的壁紙信息，WallpaperManagerService支持多用戶機(jī)制，用戶的信息在mWallpaperMap中存儲，每一個(gè)用戶對應(yīng)一個(gè)WallpaperData，WallpaperData存儲壁紙相關(guān)信息
			*/
            wallpaper = mWallpaperMap.get(userId);
            if (wallpaper == null) {
                throw new IllegalStateException("Wallpaper not yet initialized for user " + userId);
            }
      		 ...
      		    // 在這里真正會去拉起對應(yīng)的WallPaperService
                if (bindWallpaperComponentLocked(name, false, true, wallpaper, null)) {
              ...
    }

    private boolean bindWallpaperComponentLocked(ComponentName componentName, boolean force,
            boolean fromUser, WallpaperData wallpaper, IRemoteCallback reply) {
      	...
            int serviceUserId = wallpaper.userId;
            ServiceInfo si = mIPackageManager.getServiceInfo(componentName,
                    PackageManager.GET_META_DATA | PackageManager.GET_PERMISSIONS, serviceUserId);
            if (si == null) {
                // The wallpaper component we're trying to use doesn't exist
                Slog.w(TAG, "Attempted wallpaper " + componentName + " is unavailable");
                return false;
            }
            /*
            第一個(gè)校驗(yàn)：  
			啟動的時(shí)候首先會校驗(yàn)這個(gè)壁紙服務(wù)是否聲明權(quán)限為BIND_WALLPAPER權(quán)限，
              該權(quán)限的定義同樣也在fwk/base/core/res/manifest.xml
                  <permission android:name="android.permission.BIND_WALLPAPER"
        android:protectionLevel="signature|privileged" />
			  該權(quán)限也是系統(tǒng)級別的，防止三方應(yīng)用切換壁紙，
			*/
            if (!android.Manifest.permission.BIND_WALLPAPER.equals(si.permission)) {
                String msg = "Selected service does not have "
                        + android.Manifest.permission.BIND_WALLPAPER
                        + ": " + componentName;
                if (fromUser) {
                    throw new SecurityException(msg);
                }
                Slog.w(TAG, msg);
                return false;
            }

            WallpaperInfo wi = null;

            Intent intent = new Intent(WallpaperService.SERVICE_INTERFACE);
            if (componentName != null && !componentName.equals(mImageWallpaper)) {
                // Make sure the selected service is actually a wallpaper service.
                			/*
			   第二個(gè)校驗(yàn)：
			   這個(gè)檢查來校驗(yàn)服務(wù)是否聲明了android.service.wallpaper.WallpaperService這個(gè)action。如果這個(gè)服務(wù)沒有聲明這個(gè)action的話那么，ris中就不會含有這個(gè)component信息，
			*/
                List<ResolveInfo> ris =
                        mIPackageManager.queryIntentServices(intent,
                                intent.resolveTypeIfNeeded(mContext.getContentResolver()),
                                PackageManager.GET_META_DATA, serviceUserId).getList();
                for (int i=0; i<ris.size(); i++) {
                    ServiceInfo rsi = ris.get(i).serviceInfo;
                    if (rsi.name.equals(si.name) &&
                            rsi.packageName.equals(si.packageName)) {
                        try {
                        /*
                          第三個(gè)檢查：
                          獲取名為android.service.wallpaper中的meta-data信息，該meta-data信息中提供了縮略圖，開發(fā)者，簡單的描述等。會將這些信息轉(zhuǎn)換成WallpaperInfo
                        */
                            wi = new WallpaperInfo(mContext, ris.get(i));
                        } catch (XmlPullParserException e) {
                            if (fromUser) {
                                throw new IllegalArgumentException(e);
                            }
                            Slog.w(TAG, e);
                            return false;
                        } catch (IOException e) {
                            if (fromUser) {
                                throw new IllegalArgumentException(e);
                            }
                            Slog.w(TAG, e);
                            return false;
                        }
                        break;
                    }
                }
                if (wi == null) {
                    String msg = "Selected service is not a wallpaper: "
                            + componentName;
                    if (fromUser) {
                        throw new SecurityException(msg);
                    }
                    Slog.w(TAG, msg);
                    return false;
                }
            }

			// 當(dāng)壁紙服務(wù)支持在ambient模式下進(jìn)行繪制的時(shí)候，需要檢查是否有AMBIENT_WALLPAPER權(quán)限，
            if (wi != null && wi.supportsAmbientMode()) {
                final int hasPrivilege = mIPackageManager.checkPermission(
                        android.Manifest.permission.AMBIENT_WALLPAPER, wi.getPackageName(),
                        serviceUserId);
                if (hasPrivilege != PackageManager.PERMISSION_GRANTED) {
                    String msg = "Selected service does not have "
                            + android.Manifest.permission.AMBIENT_WALLPAPER
                            + ": " + componentName;
                    if (fromUser) {
                        throw new SecurityException(msg);
                    }
                    Slog.w(TAG, msg);
                    return false;
                }
            }
            // 檢驗(yàn)完畢，這里才會開始bind 壁紙服務(wù)，如果校驗(yàn)失敗的話，會返回false
			...
    }

    private boolean bindWallpaperComponentLocked(ComponentName componentName, boolean force,
            boolean fromUser, WallpaperData wallpaper, IRemoteCallback reply) {
	// 校驗(yàn)服務(wù)是否符合要求結(jié)束后，開始著手啟動服務(wù)
	...
	//1. 創(chuàng)建一個(gè)WallpaperConnection，該對象可以監(jiān)聽和WallpaperService之間的連接狀態(tài)，同時(shí)歸對象繼承了IWallpaperConnection.Stub，這樣該對象有擁有了跨進(jìn)程通信的能力，當(dāng)服務(wù)綁定成功后，onServiceConnected()方法調(diào)用中，WallpaperConnection實(shí)力會被發(fā)送到WallpaperService,該實(shí)例可以用于WallpaperService想WallpaperManagerService進(jìn)行通信的橋梁。
	intent.setComponent(componentName);
    intent.putExtra(Intent.EXTRA_CLIENT_LABEL,
                    com.android.internal.R.string.wallpaper_binding_label);
    ...
    /* 2. 這里啟動指定的壁紙服務(wù)，服務(wù)啟動后，壁紙還沒有辦法進(jìn)行顯示，還需要WallpaperConnection.onServiceConnected中進(jìn)行相應(yīng)的處理*/
    if (!mContext.bindServiceAsUser(intent, newConn,Context.BIND_AUTO_CREATE | Context.BIND_SHOWING_UI| Context.BIND_FOREGROUND_SERVICE_WHILE_AWAKE | 
    Context.BIND_INCLUDE_CAPABILITIES,new UserHandle(serviceUserId))) {
}

	/*3. 新的壁紙服務(wù)啟動之后，就開始銷毀舊服務(wù)*/
	if (wallpaper.userId == mCurrentUserId && mLastWallpaper != null && !wallpaper.equals(mFallbackWallpaper)) {
         detachWallpaperLocked(mLastWallpaper);
    }
    
    /* 4.將新的壁紙服務(wù)信息進(jìn)行保存*/
	wallpaper.wallpaperComponent = componentName;
    wallpaper.connection = newConn;
    ...

  class IWallpaperServiceWrapper extends IWallpaperService.Stub {
        private final WallpaperService mTarget;
        private IWallpaperEngineWrapper mEngineWrapper;

        public IWallpaperServiceWrapper(WallpaperService context) {
            mTarget = context;
        }
        
        @Override
        public void attach(IWallpaperConnection conn, IBinder windowToken,
        int windowType, boolean isPreview, int reqWidth, int reqHeight, Rect padding,int displayId) {
		...
		}

        @Override
        public void detach() {
        ...
        }
    }

        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            synchronized (mLock) {
                if (mWallpaper.connection == this) {
                    mService = IWallpaperService.Stub.asInterface(service);
                    attachServiceLocked(this, mWallpaper);
                    // XXX should probably do saveSettingsLocked() later
                    // when we have an engine, but I'm not sure about
                    // locking there and anyway we always need to be able to
                    // recover if there is something wrong.
                    if (!mWallpaper.equals(mFallbackWallpaper)) {
                    	// 保存當(dāng)前的壁紙信息到文件系統(tǒng)中，這樣重啟的時(shí)候就可以加載之前用戶設(shè)置過的壁紙
                        saveSettingsLocked(mWallpaper.userId);
                    }
                    FgThread.getHandler().removeCallbacks(mResetRunnable);
                    mContext.getMainThreadHandler().removeCallbacks(mTryToRebindRunnable);
                }
            }
        }

            void connectLocked(WallpaperConnection connection, WallpaperData wallpaper) {
                ...
                mWindowManagerInternal.addWindowToken(mToken, TYPE_WALLPAPER, mDisplayId);
                final DisplayData wpdData = getDisplayDataOrCreate(mDisplayId);
                try {
                    connection.mService.attach(connection, mToken, TYPE_WALLPAPER, false,
                            wpdData.mWidth, wpdData.mHeight,
                            wpdData.mPadding, mDisplayId);
                }
                ...
            }

    class IWallpaperServiceWrapper extends IWallpaperService.Stub {
        ...
        @Override
        public void attach(IWallpaperConnection conn, IBinder windowToken,
                int windowType, boolean isPreview, int reqWidth, int reqHeight, Rect padding,
                int displayId) {
            mEngineWrapper = new IWallpaperEngineWrapper(mTarget, conn, windowToken,
                    windowType, isPreview, reqWidth, reqHeight, padding, displayId);
        }
        ...
     }

IWallpaperEngineWrapper.java
        IWallpaperEngineWrapper(WallpaperService context,
                IWallpaperConnection conn, IBinder windowToken,
                int windowType, boolean isPreview, int reqWidth, int reqHeight, Rect padding,
                int displayId) {
            mCaller = new HandlerCaller(context, context.getMainLooper(), this, true);
        	...
            Message msg = mCaller.obtainMessage(DO_ATTACH);
            mCaller.sendMessage(msg);
        }
...
		@Override
        public void executeMessage(Message message) {
            switch (message.what) {
                case DO_ATTACH: {
                    try {
                    // 將IWallpaperEngineWapper對象傳遞給WallpaperConnection進(jìn)行保存，通過這個(gè)引用，WallpaperManagerService也可以通過它與engine進(jìn)行通信
                        mConnection.attachEngine(this, mDisplayId);
                    } catch (RemoteException e) {
                        Log.w(TAG, "Wallpaper host disappeared", e);
                        return;
                    }
                    // 創(chuàng)建一個(gè)引擎，該方法為抽象方法，需要子類根據(jù)自身實(shí)現(xiàn)具體的引擎
                    Engine engine = onCreateEngine();
                    mEngine = engine;
                    mActiveEngines.add(engine);
					// 該方法中會完成窗口的創(chuàng)建，surface創(chuàng)建等工作。
                    engine.attach(this);
                    return;
                }

        void attach(IWallpaperEngineWrapper wrapper) {
            ...
            mIWallpaperEngine = wrapper;
            mCaller = wrapper.mCaller;
            mConnection = wrapper.mConnection;
            mWindowToken = wrapper.mWindowToken;
            mSurfaceHolder.setSizeFromLayout();
            mInitializing = true;

			// 這個(gè)session用于和WMS進(jìn)行通信
            mSession = WindowManagerGlobal.getWindowSession();

			// mWindow是一個(gè)IWindow對象，用于接收從WMS發(fā)送過來的消息
            mWindow.setSession(mSession);

            mLayout.packageName = getPackageName();
            mIWallpaperEngine.mDisplayManager.registerDisplayListener(mDisplayListener,
                    mCaller.getHandler());
            mDisplay = mIWallpaperEngine.mDisplay;
            mDisplayContext = createDisplayContext(mDisplay);
            mDisplayState = mDisplay.getState();

            if (DEBUG) Log.v(TAG, "onCreate(): " + this);
            // 子類可以重寫該接口，在該接口中可以修改mSurfaceHolder相關(guān)的屬性，這個(gè)時(shí)候
            // 窗口尚未創(chuàng)建。設(shè)置的相關(guān)屬性將在updateSurface中創(chuàng)建窗口時(shí)使用
            onCreate(mSurfaceHolder);

            mInitializing = false;

            mReportedVisible = false;
			
			// updateSurface會進(jìn)行窗口以及Surface的創(chuàng)建。
            updateSurface(false, false, false);
        }