Preview (ArkTS)

Before developing a camera application, request permissions by following the instructions provided in Camera Development Preparations.

Preview is the image you see after you start the camera application but before you take photos or record videos.

How to Develop

Read Camera for the API reference.

1. Import the camera module, which provides camera-related attributes and methods.

   import { camera } from '@kit.CameraKit';
   import { BusinessError } from '@kit.BasicServicesKit';
   

2. Create a surface.

   The XComponent, the capabilities of which are provided by the UI, offers the surface for preview streams. For details about how to obtain the surface ID, see getXcomponentSurfaceId. For details about the component, see XComponent.

   NOTE

   The preview stream and video output stream must have the same aspect ratio of the resolution. For example, the aspect ratio of the surface of the XComponent is 1920:1080 (which is equal to 16:9), then the aspect ratio of the resolution of the preview stream must also be 16:9. This means that the resolution can be 640:360, 960:540, 1920:1080, or the like.

3. Use previewProfiles in the CameraOutputCapability class to obtain the preview output capabilities, in the format of an previewProfilesArray array, supported by the current device. Then call createPreviewOutput to create a PreviewOutput object, with the first parameter set to the first item in the previewProfilesArray array and the second parameter set to the surface ID obtained in step 2.

   function getPreviewOutput(cameraManager: camera.CameraManager, cameraOutputCapability: camera.CameraOutputCapability, surfaceId: string): camera.PreviewOutput | undefined {
     let previewProfilesArray: Array<camera.Profile> = cameraOutputCapability.previewProfiles;
     let previewOutput: camera.PreviewOutput | undefined = undefined;
     try {
       previewOutput = cameraManager.createPreviewOutput(previewProfilesArray[0], surfaceId);
     } catch (error) {
       let err = error as BusinessError;
       console.error("Failed to create the PreviewOutput instance. error code: " + err.code);
     }
     return previewOutput;
   }
   

4. Call Session.start to start outputting the preview stream. If the call fails, an error code is returned. For details, see CameraErrorCode.

   async function startPreviewOutput(cameraManager: camera.CameraManager, previewOutput: camera.PreviewOutput): Promise<void> {
     let cameraArray: Array<camera.CameraDevice> = [];
     cameraArray = cameraManager.getSupportedCameras();
     if (cameraArray.length == 0) {
       console.error('no camera.');
       return;
     }
     // Obtain the supported modes.
     let sceneModes: Array<camera.SceneMode> = cameraManager.getSupportedSceneModes(cameraArray[0]);
     let isSupportPhotoMode: boolean = sceneModes.indexOf(camera.SceneMode.NORMAL_PHOTO) >= 0;
     if (!isSupportPhotoMode) {
       console.error('photo mode not support');
       return;
     }
     let cameraInput: camera.CameraInput | undefined = undefined;
     cameraInput = cameraManager.createCameraInput(cameraArray[0]);
     if (cameraInput === undefined) {
       console.error('cameraInput is undefined');
       return;
     }
     // Open the camera.
     await cameraInput.open();
     let session: camera.PhotoSession = cameraManager.createSession(camera.SceneMode.NORMAL_PHOTO) as camera.PhotoSession;
     session.beginConfig();
     session.addInput(cameraInput);
     session.addOutput(previewOutput);
     await session.commitConfig();
     await session.start();
   }
   

Status Listening

During camera application development, you can listen for the preview output stream status, including preview stream start, preview stream end, and preview stream output errors.

• Register the 'frameStart' event to listen for preview start events. This event can be registered when a PreviewOutput object is created and is triggered when the bottom layer starts exposure for the first time. The preview stream starts as long as a result is returned.

  function onPreviewOutputFrameStart(previewOutput: camera.PreviewOutput): void {
    previewOutput.on('frameStart', (err: BusinessError) => {
      if (err !== undefined && err.code !== 0) {
        return;
      }
      console.info('Preview frame started');
    });
  }
  

• Register the 'frameEnd' event to listen for preview end events. This event can be registered when a PreviewOutput object is created and is triggered when the last frame of preview ends. The preview stream ends as long as a result is returned.

  function onPreviewOutputFrameEnd(previewOutput: camera.PreviewOutput): void {
    previewOutput.on('frameEnd', (err: BusinessError) => {
      if (err !== undefined && err.code !== 0) {
        return;
      }
      console.info('Preview frame ended');
    });
  }
  

• Register the 'error' event to listen for preview output errors. The callback function returns an error code when an API is incorrectly used. For details about the error code types, see CameraErrorCode.

  function onPreviewOutputError(previewOutput: camera.PreviewOutput): void {
    previewOutput.on('error', (previewOutputError: BusinessError) => {
      console.error(`Preview output error code: ${previewOutputError.code}`);
    });
  }