在外部視圖系統 (EVS) 的先前版本中， IEvsCameraStream介面定義了一個回呼方法來僅傳送捕獲的視訊訊框。雖然這種簡化的 EVS 服務客戶端實現，但也使客戶端難以識別任何串流事件並因此難以正確處理它們。為了增強 EVS 開發體驗，AOSP 現在包含一個額外的回呼來傳遞流事件。
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);
};
此方法提供由三個欄位組成的EvsEventDesc ：
- 事件的類型。
- 用於標識事件起源的字串。
- 4x 32 位元字資料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;
};
並且，為了避免 EVS 和其他 Android 圖形元件之間的圖形緩衝區描述出現任何差異， BufferDesc已被重新定義為使用從 android.hardware.graphics.common@1.2 介面匯入的HardwareBuffer 。 HardwareBuffer包含HardwareBufferDescription ，它是Android NDK 的AHardwareBuffer_Desc的 HIDL 對應項，並帶有緩衝區句柄。
/**
* 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 10 個 32 位元字組成的陣列。您可能希望將其轉換為AHardwareBuffer_Desc類型並填寫內容。
EvsEventDesc是enum EvsEventType的結構，其中列出了多個串流事件和 32 位元字負載，開發人員可以在其中放置可能的附加資訊。例如，開發人員可以為流錯誤事件放置錯誤代碼。
/**
* 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,
};
影格交付
透過新的BufferDesc ， IEvsCameraStream也引入了新的回呼方法來從服務實作接收訊框和流事件。
/**
* 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);
};
幀回調方法的較新版本旨在傳遞多個緩衝區描述符。因此，如果 EVS 相機實作管理多個來源，則可以透過單一呼叫轉發多個幀。
此外，先前用於通知流結束（發送空幀）的協定已被棄用並替換為STREAM_STOPPED事件。
圖 1.事件通知序列圖
使用事件和幀通知機制
識別客戶端實作的 IEvsCameraStream 版本
該服務可以透過嘗試向下轉型來識別客戶端實現的傳入 IEvsCameraStream 介面的版本：
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
...
}
通知（）回調
EvsEvent透過notify()回呼傳遞，然後客戶端可以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();
}
使用緩衝區描述
AHardwareBuffer_Desc是 Android NDK 的資料類型，用於表示可以綁定到 EGL/OpenGL 和 Vulkan 原語的本機硬體緩衝區。它包含先前 EVS BufferDesc 中的大部分緩衝區元數據，因此在新的 BufferDesc 定義中將其替換。但是，由於它在 HIDL 介面中定義為數組，因此無法直接索引成員變數。相反，您可以將陣列轉換為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;