目录

预览流黑屏但无报错信息该怎么解决

可能原因

未正确获取相机权限的情况下，进行相机初始化和相机输入流获取等操作。
应用切换到后台再返回时，相机资源被回收，未重新获取权限并开启预览导致。
配置的预览流尺寸不被支持。

解决措施

确保在初始化相机和获取相机输入流之前，先正确获取相机权限。

首先在module.json5文件中申请需要的权限（地理位置权限可以根据需要申请，非必选）；

"requestPermissions": [
  {
    "name": "ohos.permission.CAMERA",
    "usedScene": {
      "abilities": [
        "FormAbility"
      ],
      "when": "always"
    },
    "reason": "$string:Camera_Permission_Request"
  },
  {
    "name": "ohos.permission.MICROPHONE",
    "usedScene": {
      "abilities": [
        "FormAbility"
      ],
      "when": "always"
    },
    "reason": "$string:Camera_Permission_Request"
  },
  // The geographical location permission can be applied for as required. This parameter is optional.
  {
    "name": "ohos.permission.MEDIA_LOCATION",
    "usedScene": {
    "abilities": [
        "FormAbility"
    ],
    "when": "always"
    },
    "reason": "$string:Camera_Permission_Request"
  }
],

接着在EntryAbility.ets中onWindowStageCreate中使用requestPermissionsFromUser方法获取权限。

onWindowStageCreate(windowStage: window.WindowStage): void {
  let atManager = abilityAccessCtrl.createAtManager();
  atManager.requestPermissionsFromUser(this.context,
    [
      'ohos.permission.CAMERA',
      'ohos.permission.MICROPHONE',
      'ohos.permission.MEDIA_LOCATION'
    ]
  ).then((data) => {
    console.info('data:' + JSON.stringify(data));
    console.info('data permissions:' + data.permissions);
    console.info('data authResults:' + data.authResults);
    // Main window is created, set main page for this ability
    hilog.info(0x0000, 'testTag', '%{public}s', 'Ability onWindowStageCreate');

    windowStage.loadContent('pages/CameraPreviewFlow', (err, data) => {
      if (err.code) {
        hilog.error(0x0000, 'testTag', 'Failed to load the content. Cause: %{public}s', JSON.stringify(err) ?? '');
        return;
      }
      hilog.info(0x0000, 'testTag', 'Succeeded in loading the content. Data: %{public}s', JSON.stringify(data) ?? '');
    });
  }).catch((err: BusinessError) => {
    console.info('data:' + JSON.stringify(err));
  });
}

当应用被切换到后台后，相机资源会被全部回收，所以为了避免出现前后台切换时预览流黑屏的问题，需在onPageShow中进行重新创建会话、配置会话、启动等操作，并在onPageHide中对相机资源进行释放。
```
async onPageShow() {
  let baseContext = this.getUIContext().getHostContext()! as common.BaseContext;
  await this.initCamera(baseContext, this.surfaceId);
}

async onPageHide() {
  await this.releaseCamera();
}
```

获取相机设备支持的输出流能力，确定支持的预览尺寸。

// Obtain the output stream capability supported by the camera device
let cameraOutputCap: camera.CameraOutputCapability =
  cameraManager.getSupportedOutputCapability(cameraArray[0], camera.SceneMode.NORMAL_PHOTO);
if (!cameraOutputCap) {
  console.error(TAG, "cameraManager.getSupportedOutputCapability error");
  return;
}
console.info(TAG, "outputCapability: " +
JSON.stringify(cameraOutputCap));  //The aspect ratio of the preview stream and the video output stream resolution should be consistent
let previewProfilesArray: Array<camera.Profile> = cameraOutputCap.previewProfiles;
let position: number = 0;
if (previewProfilesArray != null) {
  previewProfilesArray.forEach((value: camera.Profile,
    index: number) => { // View supported preview sizes
    console.info(TAG,
      `Supported preview sizes: [${value.size.width},${value.size.height},${value.size.width / value.size.height}]`);
    if (value.size.width === 2592 && value.size.height === 1200) {
      position = index;
    }
  })
} else {
  console.error(TAG, "createOutput photoProfilesArray == null || undefined");
}