1# Preview (C/C++) 2<!--Kit: Camera Kit--> 3<!--Subsystem: Multimedia--> 4<!--Owner: @qano--> 5<!--SE: @leo_ysl--> 6<!--TSE: @xchaosioda--> 7 8Preview is the image you see after you start the camera application but before you take photos or record videos. 9 10## How to Develop 11 12Read [Camera](../../reference/apis-camera-kit/capi-oh-camera.md) for the API reference. 13 141. Import the NDK, which provides camera-related attributes and methods. 15 16 ```c++ 17 // Include the NDK header files. 18 #include "hilog/log.h" 19 #include "ohcamera/camera.h" 20 #include "ohcamera/camera_input.h" 21 #include "ohcamera/capture_session.h" 22 #include "ohcamera/photo_output.h" 23 #include "ohcamera/preview_output.h" 24 #include "ohcamera/video_output.h" 25 #include "ohcamera/camera_manager.h" 26 ``` 27 282. Link the dynamic library in the CMake script. 29 30 ```txt 31 target_link_libraries(entry PUBLIC 32 libace_napi.z.so 33 libohcamera.so 34 libhilog_ndk.z.so 35 ) 36 ``` 37 383. Obtain the surface ID. 39 40 The **XComponent**, the capabilities of which are provided by the UI, offers the surface ID for preview streams. For details, see [XComponent](../../reference/apis-arkui/arkui-ts/ts-basic-components-xcomponent.md). 41 424. Call [OH_CameraManager_GetSupportedCameraOutputCapability()](../../reference/apis-camera-kit/capi-camera-manager-h.md#oh_cameramanager_getsupportedcameraoutputcapability) to obtain the preview capability supported by the current device based on the surface ID. Then call [OH_CameraManager_CreatePreviewOutput()](../../reference/apis-camera-kit/capi-camera-manager-h.md#oh_cameramanager_createpreviewoutput) to create a PreviewOutput instance, with the parameters set to the **cameraManager** pointer, the first item in the **previewProfiles** array, the surface ID obtained in step 3, and the returned **previewOutput** pointer, respectively. 43 44 ```c++ 45 Camera_PreviewOutput* CreatePreviewOutput(char* previewSurfaceIdstr, Camera_Manager* cameraManager, 46 const Camera_Profile* previewProfile) 47 { 48 Camera_PreviewOutput* previewOutput = nullptr; 49 Camera_ErrorCode ret = OH_CameraManager_CreatePreviewOutput(cameraManager, previewProfile, 50 previewSurfaceIdstr, &previewOutput); 51 if (previewOutput == nullptr || ret != CAMERA_OK) { 52 OH_LOG_ERROR(LOG_APP, "OH_CameraManager_CreatePreviewOutput failed."); 53 } 54 return previewOutput; 55 } 56 ``` 57 585. Configure the session. Call **commitConfig()** to commit the session configuration, and then call [OH_CaptureSession_Start()](../../reference/apis-camera-kit/capi-capture-session-h.md#oh_capturesession_start) to start outputting the preview stream. If the call fails, an error code is returned. For details, see [Camera_ErrorCode](../../reference/apis-camera-kit/capi-camera-h.md#camera_errorcode). 59 60 ```c++ 61 Camera_ErrorCode SessionStart(Camera_CaptureSession* captureSession) 62 { 63 Camera_ErrorCode ret = OH_CaptureSession_Start(captureSession); 64 if (ret != CAMERA_OK) { 65 OH_LOG_ERROR(LOG_APP, "OH_CaptureSession_Start failed."); 66 } 67 return ret; 68 } 69 ``` 70 716. Call [OH_CaptureSession_Stop()](../../reference/apis-camera-kit/capi-capture-session-h.md#oh_capturesession_stop) to stop outputting the preview stream. If the call fails, an error code is returned. For details, see[Camera_ErrorCode](../../reference/apis-camera-kit/capi-camera-h.md#camera_errorcode). 72 73 ```c++ 74 Camera_ErrorCode SessionStop(Camera_CaptureSession* captureSession) 75 { 76 Camera_ErrorCode ret = OH_CaptureSession_Stop(captureSession); 77 if (ret != CAMERA_OK) { 78 OH_LOG_ERROR(LOG_APP, "OH_CaptureSession_Stop failed."); 79 } 80 return ret; 81 } 82 ``` 83 84## Status Listening 85 86During camera application development, you can listen for the preview output stream status, including preview stream start, preview stream end, and preview stream output errors. 87 88- 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. 89 90 ```c++ 91 void PreviewOutputOnFrameStart(Camera_PreviewOutput* previewOutput) 92 { 93 OH_LOG_INFO(LOG_APP, "PreviewOutputOnFrameStart"); 94 } 95 ``` 96 97- 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. 98 99 ```c++ 100 void PreviewOutputOnFrameEnd(Camera_PreviewOutput* previewOutput, int32_t frameCount) 101 { 102 OH_LOG_INFO(LOG_APP, "PreviewOutput frameCount = %{public}d", frameCount); 103 } 104 ``` 105 106- 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 [Camera_ErrorCode](../../reference/apis-camera-kit/capi-camera-h.md#camera_errorcode). 107 108 ```c++ 109 void PreviewOutputOnError(Camera_PreviewOutput* previewOutput, Camera_ErrorCode errorCode) 110 { 111 OH_LOG_ERROR(LOG_APP, "PreviewOutput errorCode = %{public}d", errorCode); 112 } 113 ``` 114 ```c++ 115 PreviewOutput_Callbacks* GetPreviewOutputListener(void) 116 { 117 static PreviewOutput_Callbacks previewOutputListener = { 118 .onFrameStart = PreviewOutputOnFrameStart, 119 .onFrameEnd = PreviewOutputOnFrameEnd, 120 .onError = PreviewOutputOnError 121 }; 122 return &previewOutputListener; 123 } 124 Camera_ErrorCode RegisterPreviewOutputCallback(Camera_PreviewOutput* previewOutput) 125 { 126 Camera_ErrorCode ret = OH_PreviewOutput_RegisterCallback(previewOutput, GetPreviewOutputListener()); 127 if (ret != CAMERA_OK) { 128 OH_LOG_ERROR(LOG_APP, "OH_PreviewOutput_RegisterCallback failed."); 129 } 130 return ret; 131 } 132 ``` 133