# 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。
    1.  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"
          }
        ],
        ```
        
    2.  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");
    }
    ```