第 26 篇把单摄预览会话拆开CameraInput 负责打开设备PreviewOutput 负责画面PhotoOutput 负责拍照PhotoSession 负责把它们配置到同一个会话里。学习目标建立 CameraInput、PreviewOutput、PhotoOutput、PhotoSession 的层次关系。理解单摄回退为什么是双摄项目必须有的稳定路径。能读懂 beginConfig、addInput、addOutput、commitConfig、start 的顺序。知道并发能力为空时如何从双摄逻辑回到单摄逻辑。一、单摄不是备用按钮而是稳定主链路双镜记忆相机的最终目标是前后摄同时记录但真实设备上并发能力不一定存在。训练营项目因此把单摄链路做成完整能力而不是临时 fallback。只要有一个可用 CameraDevice页面就能通过单摄预览、单摄拍照和图库保存继续工作。这篇重点拆 ensureSinglePreview它把设备输入、预览输出、拍照输出和会话配置串起来。理解这条链路后再看双摄会话就只是多一组 input/output 和更严格的能力约束。图 1 单摄预览与图库结果形成闭环二、先看状态字段每种资源都有自己的位置Index.ets 里相机相关字段看起来很多但它们按层次分得很清楚。CameraDevice 表示设备选择CameraInput 表示已经打开的输入PreviewOutput 表示预览输出PhotoOutput 表示拍照输出PhotoSession 表示已经配置并启动的会话。把这些字段拆开保存是为了释放时能逐层关闭也是为了 UI 能知道当前是单摄 live、后摄 live还是前后摄都没连上。不要把所有东西塞进一个 currentCamera 对象否则失败和释放时很难判断谁已经创建、谁还没有。图 2 Index.ets 中按资源层次保存相机字段private backSurfaceId: string ; private frontSurfaceId: string ; private cameraManager?: camera.CameraManager; private backCameraDevice?: camera.CameraDevice; private frontCameraDevice?: camera.CameraDevice; private preferredBackSingleCameraDevice?: camera.CameraDevice; private singleCameraDevice?: camera.CameraDevice; private backLensOptions: ArrayCameraLensOption []; private cameraFlashSupportedModes: Arraycamera.FlashMode []; private concurrentInfos: Arraycamera.CameraConcurrentInfo []; private backCameraInput?: camera.CameraInput; private frontCameraInput?: camera.CameraInput; private singleCameraInput?: camera.CameraInput; private backPreviewOutput?: camera.PreviewOutput; private frontPreviewOutput?: camera.PreviewOutput; private singlePreviewOutput?: camera.PreviewOutput; private backPhotoSession?: camera.PhotoSession; private frontPhotoSession?: camera.PhotoSession; private singlePhotoSession?: camera.PhotoSession; private backPhotoOutput?: camera.PhotoOutput; private frontPhotoOutput?: camera.PhotoOutput; private singlePhotoOutput?: camera.PhotoOutput; private pendingCaptureId: string ;三、单摄会话的正确顺序ensureSinglePreview 的顺序非常典型先判断 activeTab、权限、单摄能力和 surfaceId再从 capability 中取 previewProfiles 与 photoProfiles然后 createCameraInput 并 open接着创建 PreviewOutput 和 PhotoOutput最后 createSession、beginConfig、addInput、addOutput、commitConfig、start。这里最容易犯的错是顺序混乱。例如 input 还没 open 就 addInput或者 previewProfiles 为空还硬取第一个。项目用多层 return 把这些无效状态挡在外面使真正进入 beginConfig 的时候资源已经满足会话配置要求。图 3 单摄 PhotoSession 从配置到启动的完整代码if (this.activeTab ! camera || !this.cameraPermissionReady || !this.singleCameraSupported) { return; } if (!this.cameraManager || !this.singleCameraDevice || this.backSurfaceId.length 0) { return; } const capability this.getSinglePhotoCapability(this.singleCameraDevice); if (!capability) { this.cameraStatusText ; return; } if (capability.previewProfiles.length 0 || capability.photoProfiles.length 0) { this.cameraStatusText ; return; } const activeRole this.singleCameraRole; this.cameraSessionPreparing true; this.singlePreviewLive false; this.backPreviewLive false; this.frontPreviewLive false; this.logCaptureTrace(ensure-single-preview-start, role${activeRole}); this.cameraStatusText ${this.getCameraRoleLabel(activeRole)}预览连接中...; try { this.singleCameraInput this.cameraManager.createCameraInput(this.singleCameraDevice); await this.singleCameraInput.open(); this.singlePreviewOutput this.cameraManager.createPreviewOutput( capability.previewProfiles[0], this.backSurfaceId ); this.singlePreviewOutput.on(frameStart, () { this.handleSinglePreviewFrameStart(activeRole); }); this.singlePhotoOutput this.cameraManager.createPhotoOutput(this.pickBestPhotoProfile(capability.photoProfiles)); this.bindPhotoOutput(activeRole, this.singlePhotoOutput, single); this.singlePhotoSession this.cameraManager.createSession(camera.SceneMode.NORMAL_PHOTO) as camera.PhotoSession; this.singlePhotoSession.beginConfig(); this.singlePhotoSession.addInput(this.singleCameraInput); this.singlePhotoSession.addOutput(this.singlePreviewOutput); this.singlePhotoSession.addOutput(this.singlePhotoOutput); await this.singlePhotoSession.commitConfig(); await this.singlePhotoSession.start(); this.cameraSessionActive true; this.syncZoomStateFromSession(); this.refreshCameraFlashState();四、双摄能力为空时如何回退单摄链路的价值来自前面的能力探测。项目先尝试用 getCameraDevice 找官方默认后摄和前摄再调用 getCameraConcurrentInfos。如果 concurrentInfos 为空说明设备不提供前后摄同时预览的组合。这种情况下不应该继续强开双摄而是保存 fallbackSingleDevice让 singleCameraSupported 为 true后面由 ensureCameraPreview 进入单摄预览。这和“捕获异常后重试”完全不同。异常重试不知道为什么失败能力回退知道当前设备能做什么。训练营文章后续讲双摄、镜头和闪光灯时都会把这个能力判断当作前置条件。图 4 官方并发能力探测与单摄回退代码private safeGetCameraDevice( cameraManager: camera.CameraManager, position: camera.CameraPosition, cameraType: camera.CameraType ): camera.CameraDevice | undefined { try { return cameraManager.getCameraDevice(position, cameraType); } catch (error) { console.error(Failed to get camera device: ${JSON.stringify(error)}); return undefined; } } private safeGetCameraConcurrentInfos( cameraManager: camera.CameraManager, devices: Arraycamera.CameraDevice ): Arraycamera.CameraConcurrentInfo { try { return cameraManager.getCameraConcurrentInfos(devices); } catch (error) { console.error(Failed to probe concurrent camera infos: ${JSON.stringify(error)}); return []; } } private findCameraDeviceByPosition( cameras: Arraycamera.CameraDevice, position: camera.CameraPosition ): camera.CameraDevice | undefined { const defaultDevice cameras.find((device: camera.CameraDevice) device.cameraPosition position device.cameraType camera.CameraType.CAMERA_TYPE_DEFAULT ); if (defaultDevice) { return defaultDevice; } return cameras.find((device: camera.CameraDevice) device.cameraPosition position); } private getCameraDevicesByPosition( cameras: Arraycamera.CameraDevice, position: camera.CameraPosition ): Arraycamera.CameraDevice { const preferredDevices cameras.filter((device: camera.CameraDevice) device.cameraPosition position device.cameraType camera.CameraType.CAMERA_TYPE_DEFAULT ); const fallbackDevices cameras.filter((device: camera.CameraDevice) device.cameraPosition position device.cameraType ! camera.CameraType.CAMERA_TYPE_DEFAULT ); return preferredDevices.concat(fallbackDevices); } private findConcurrentCameraPair( cameraManager: camera.CameraManager, cameras: Arraycamera.CameraDevice ): ConcurrentCameraPair { const backDevices this.getCameraDevicesByPosition(cameras, camera.CameraPosition.CAMERA_POSITION_BACK); const frontDevices this.getCameraDevicesByPosition(cameras, camera.CameraPosition.CAMERA_POSITION_FRONT); for (const frontDevice of frontDevices) { for (const backDevice of backDevices) { const concurrentInfos this.safeGetCameraConcurrentInfos(cameraManager, [frontDevice, backDevice]); if (concurrentInfos.length 0) { const concurrentPair: ConcurrentCameraPair { backDevice: backDevice, frontDevice: frontDevice, concurrentInfos: concurrentInfos }; return concurrentPair; } } } const fallbackPair: ConcurrentCameraPair { backDevice: backDevices[0], frontDevice: frontDevices[0], concurrentInfos: [] }; return fallbackPair; } private getOfficialConcurrentCameraPair(cameraManager: camera.CameraManager): ConcurrentCameraPair { const backDevice this.safeGetCameraDevice( cameraManager, camera.CameraPosition.CAMERA_POSITION_BACK, camera.CameraType.CAMERA_TYPE_DEFAULT ); const frontDevice this.safeGetCameraDevice( cameraManager, camera.CameraPosition.CAMERA_POSITION_FRONT, camera.CameraType.CAMERA_TYPE_DEFAULT ); if (!backDevice || !frontDevice) { const unsupportedPair: ConcurrentCameraPair { backDevice: backDevice, frontDevice: frontDevice, concurrentInfos: [] }; return unsupportedPair; } const concurrentInfos this.safeGetCameraConcurrentInfos(cameraManager, [frontDevice, backDevice]); const officialPair: ConcurrentCameraPair { backDevice: backDevice, frontDevice: frontDevice, concurrentInfos: concurrentInfos };五、把单摄会话写成可释放、可重进、可扩展相机页常见的隐性 bug 是第一次能打开切到别的页再回来就失败。原因通常是旧 session、output 或 input 没释放干净。项目把单摄资源分别保存并在 teardownDualPreview 里统一释放所以单摄会话可以多次进入、多次退出。另外单摄模式仍然保留镜头选择、变焦、闪光灯等扩展点。因为这些控制项依赖的是当前 active PhotoSession而不是“双摄”这个概念。只要单摄会话层次清晰后面的控制能力可以直接接上。本篇检查清单单摄启动前检查权限、Surface、设备和 profile。CameraInput、PreviewOutput、PhotoOutput、PhotoSession 分别保存便于释放和状态判断。PhotoSession 配置顺序符合 beginConfig、addInput/output、commitConfig、start。并发能力为空时走单摄回退而不是反复重试双摄。正文配图包含运行结果、字段结构、会话代码和并发能力探测源码。今日练习在 ensureSinglePreview 中记录 activeRole确认前后摄切换时日志正确。临时模拟 concurrentInfos 为空验证页面仍能进入单摄预览。拍一张单摄照片后进入图库页确认记录能出现在列表中。