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事件和影格通知機制

前一版 Exterior View System (EVS) 中 IEvsCameraStream 介面定義單一回呼方法來提供所擷取的影片 (只會) 標示頁框。雖然這可以簡化 EVS 服務用戶端實作程序，讓客戶難以辨別任何串流事件才能妥善處理為改善 EVS 開發體驗，Android 開放原始碼計畫現在包含額外的回呼，用來提供串流事件。

package android.hardware.automotive.evs@1.1;

import @1.0::IEvsCameraStream;

/**
 * Implemented on client side to receive asynchronous video frame deliveries.
 */
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
    /**
     * Receives calls from the HAL each time a video frame is ready for inspection.
     * Buffer handles received by this method must be returned via calls to
     * IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
     * to IEvsCamera::stopVideoStream(), this callback may continue to happen for
     * some time as the pipeline drains. Each frame must still be returned.
     * When the last frame in the stream has been delivered, STREAM_STOPPED
     * event must be delivered. No further frame deliveries may happen
     * thereafter.
     *
     * @param buffer a buffer descriptor of a delivered image frame.
     */
    oneway deliverFrame_1_1(BufferDesc buffer);

    /**
     * Receives calls from the HAL each time an event happens.
     *
     * @param  event EVS event with possible event information.
     */
    oneway notify(EvsEvent event);
};

這個方法提供的 EvsEventDesc 由三個欄位組成：

事件類型。
用來識別事件來源的字串。
4 個 32 位元文字資料，用來包含可能的事件資訊。

/**
 * Structure that describes informative events occurred during EVS is streaming
 */
struct EvsEvent {
    /**
     * Type of an informative event
     */
    EvsEventType aType;
    /**
     * Device identifier
     */
    string deviceId;
    /**
     * Possible additional information
     */
    uint32_t[4] payload;
};

並避免在 EVS 之間，避免圖形緩衝區說明出現任何差異和其他 Android 圖形元件，BufferDesc 已重新定義為請使用從 android.hardware.graphics.common@1.2 介面匯入的 HardwareBuffer。 HardwareBuffer 包含 HardwareBufferDescription，也就是 Android NDK 的 HIDL 對應項目 AHardwareBuffer_Desc，包含緩衝區控制代碼。

/**
 * HIDL counterpart of AHardwareBuffer_Desc.
 *
 * An AHardwareBuffer_Desc object can be converted to and from a
 * HardwareBufferDescription object by memcpy().
 *
 * @sa +ndk libnativewindow#AHardwareBuffer_Desc.
 */
typedef uint32_t[10] HardwareBufferDescription;

/**
 * HIDL counterpart of AHardwareBuffer.
 *
 * AHardwareBuffer_createFromHandle() can be used to convert a HardwareBuffer
 * object to an AHardwareBuffer object.
 *
 * Conversely, AHardwareBuffer_getNativeHandle() can be used to extract a native
 * handle from an AHardwareBuffer object. Paired with AHardwareBuffer_Desc,
 * AHardwareBuffer_getNativeHandle() can be used to convert between
 * HardwareBuffer and AHardwareBuffer.
 *
 * @sa +ndk libnativewindow#AHardwareBuffer".
 */
struct HardwareBuffer {
    HardwareBufferDescription description;
    handle nativeHandle;
}

/**
 * Structure representing an image buffer through our APIs
 *
 * In addition to the handle to the graphics memory, need to retain
 * the properties of the buffer for easy reference and reconstruction of
 * an ANativeWindowBuffer object on the remote side of API calls.
 * Not least because OpenGL expect an ANativeWindowBuffer* for us as a
 * texture via eglCreateImageKHR().
 */
struct BufferDesc {
    /**
     * HIDL counterpart of AHardwareBuffer_Desc. Please see
     * hardware/interfaces/graphics/common/1.2/types.hal for more details.
     */
    HardwareBuffer buffer;
    /**
     * The size of a pixel in the units of bytes
     */
    uint32_t pixelSize;
    /**
     * Opaque value from driver
     */
    uint32_t bufferId;
    /**
     * Unique identifier of the physical camera device that produces this buffer.
     */
    string deviceId;
    /**
     * Time that this buffer is being filled
     */
    int64_t timestamp;
    /**
     * Frame metadata. This is opaque to EVS manager
     */
    vec<uint8_t> metadata
};

注意： HardwareBufferDescription 定義為由十個 32 位元單字組成的陣列建議以「AHardwareBuffer_Desc」類型進行投放並填入內容

EvsEventDesc 是 enum EvsEventType 的結構。其中列出數個串流事件和 32 位元字詞酬載，其中開發人員可以提供可能的額外資訊。舉例來說可以為串流錯誤事件加入錯誤代碼。

/**
 * Types of informative streaming events
 */
enum EvsEventType : uint32_t {
    /**
     * Video stream is started
     */
    STREAM_STARTED = 0,
    /**
     * Video stream is stopped
     */
    STREAM_STOPPED,
    /**
     * Video frame is dropped
     */
    FRAME_DROPPED,
    /**
     * Timeout happens
     */
    TIMEOUT,
    /**
     * Camera parameter is changed; payload contains a changed parameter ID and
     * its value
     */
    PARAMETER_CHANGED,
    /**
     * Master role has become available
     */
    MASTER_RELEASED,
};

影格放送

新的 BufferDesc 也包含 IEvsCameraStream 導入了用於接收影格和串流事件的全新回呼方法以及實際呼叫的程式碼

/**
 * Implemented on client side to receive asynchronous streaming event deliveries.
 */
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
   /**
    * Receives calls from the HAL each time video frames are ready for inspection.
    * Buffer handles received by this method must be returned via calls to
    * IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
    * to IEvsCamera::stopVideoStream(), this callback may continue to happen for
    * some time as the pipeline drains. Each frame must still be returned.
    * When the last frame in the stream has been delivered, STREAM_STOPPED
    * event must be delivered. No further frame deliveries may happen
    * thereafter.
    *
    * A camera device delivers the same number of frames as number of
    * backing physical camera devices; it means, a physical camera device
    * sends always a single frame and a logical camera device sends multiple
    * frames as many as the number of backing physical camera devices.
    *
    * @param buffer Buffer descriptors of delivered image frames.
    */
   oneway deliverFrame_1_1(vec<BufferDesc> buffer);

   /**
    * Receives calls from the HAL each time an event happens.
    *
    * @param  event EVS event with possible event information.
    */
   oneway notify(EvsEventDesc event);
};

新版框架回呼方法的設計可提供。因此，採用 EVS 相機的方式管理多個來源。

此外，前一個通訊協定會在串流結束時發出通知傳送空值框架，已淘汰，並由 STREAM_STOPPED 取代活動。

事件通知序列圖表

圖 1. 事件通知序列圖表

使用事件和影格通知機制

識別由用戶端實作的 IEvsCameraStream 版本

此服務可以識別傳入 IEvsCameraStream 介面的版本所執行命令：

using IEvsCameraStream_1_0 =
    ::android::hardware::automotive::evs::V1_0::IEvsCameraStream;
using IEvsCameraStream_1_1 =
    ::android::hardware::automotive::evs::V1_1::IEvsCameraStream;

Return<EvsResult> EvsV4lCamera::startVideoStream(
    const sp<IEvsCameraStream_1_0>& stream)  {

    IEvsCameraStream_1_0 aStream = stream;
    // Try to downcast. This succeeds if the client implements
    // IEvsCameraStream v1.1.
    IEvsCameraStream_1_1 aStream_1_1 =
        IEvsCameraStream_1_1::castFrom(aStream).withDefault(nullptr);
    if (aStream_1_1 == nullptr) {
        ALOGI("Start a stream for v1.0 client.");
    } else {
        ALOGI("Start a stream for v1.1 client.");
    }

    // Start a video stream
    ...
}

notify() 回呼

EvsEvent 會透過 notify() 回呼傳遞，接著，用戶端就能根據鑑別器識別其類型，如下所示：

Return<void> StreamHandler::notify(const EvsEvent& event) {
    ALOGD("Received an event id: %u", event.aType);
    // Handle each received event.
    switch(event.aType) {
        case EvsEventType::ERROR:
            // Do something to handle an error
            ...
            break;
        [More cases]
    }
    return Void();
}

使用 BufferDesc

AHardwareBuffer_Desc 是 Android NDK 的資料類型，代表可繫結的原生硬體緩衝區 EGL/OpenGL 和 Vulkan 基元。其中含有先前因此會在新的 BufferDesc 定義中取代 EVS BufferDesc。不過，定義為 HIDL 介面中的陣列，無法直接建立索引成員變數。您可以改為將陣列轉換為 AHardwareBuffer_Desc 類型，如下所示：

BufferDesc bufDesc = {};
AHardwareBuffer_Desc* pDesc =
    reinterpret_cast<AHardwareBuffer_Desc *>(&bufDesc.buffer.description);
pDesc->width  = mVideo.getWidth();
pDesc->height = mVideo.getHeight();
pDesc->layers = 1;
pDesc->format = mFormat;
pDesc->usage  = mUsage;
pDesc->stride = mStride;
bufDesc_1_1.buffer.nativeHandle = mBuffers[idx].handle;
bufDesc_1_1.bufferId = idx;