我正在尝试构建一个基于 NDK 的 c++ 低延迟音频播放器，它将遇到多个音频的三个操作。
- 从资产播放。
- 从在线来源流式传输。
- 从本地设备存储播放。
从 Google 提供的一个双簧管示例中，我向NDKExtractor.cpp类添加了另一个函数，以提取基于 URL 的音频并将其呈现到音频设备，同时从源读取。
int32_t NDKExtractor::decode(char *file, uint8_t *targetData, AudioProperties targetProperties) {
LOGD("Using NDK decoder: %s",file);
// Extract the audio frames
AMediaExtractor *extractor = AMediaExtractor_new();
//using this method instead of AMediaExtractor_setDataSourceFd() as used for asset files in the rythem game example
media_status_t amresult = AMediaExtractor_setDataSource(extractor, file);
if (amresult != AMEDIA_OK) {
LOGE("Error setting extractor data source, err %d", amresult);
return 0;
}
// Specify our desired output format by creating it from our source
AMediaFormat *format = AMediaExtractor_getTrackFormat(extractor, 0);
int32_t sampleRate;
if (AMediaFormat_getInt32(format, AMEDIAFORMAT_KEY_SAMPLE_RATE, &sampleRate)) {
LOGD("Source sample rate %d", sampleRate);
if (sampleRate != targetProperties.sampleRate) {
LOGE("Input (%d) and output (%d) sample rates do not match. "
"NDK decoder does not support resampling.",
sampleRate,
targetProperties.sampleRate);
return 0;
}
} else {
LOGE("Failed to get sample rate");
return 0;
};
int32_t channelCount;
if (AMediaFormat_getInt32(format, AMEDIAFORMAT_KEY_CHANNEL_COUNT, &channelCount)) {
LOGD("Got channel count %d", channelCount);
if (channelCount != targetProperties.channelCount) {
LOGE("NDK decoder does not support different "
"input (%d) and output (%d) channel counts",
channelCount,
targetProperties.channelCount);
}
} else {
LOGE("Failed to get channel count");
return 0;
}
const char *formatStr = AMediaFormat_toString(format);
LOGD("Output format %s", formatStr);
const char *mimeType;
if (AMediaFormat_getString(format, AMEDIAFORMAT_KEY_MIME, &mimeType)) {
LOGD("Got mime type %s", mimeType);
} else {
LOGE("Failed to get mime type");
return 0;
}
// Obtain the correct decoder
AMediaCodec *codec = nullptr;
AMediaExtractor_selectTrack(extractor, 0);
codec = AMediaCodec_createDecoderByType(mimeType);
AMediaCodec_configure(codec, format, nullptr, nullptr, 0);
AMediaCodec_start(codec);
// DECODE
bool isExtracting = true;
bool isDecoding = true;
int32_t bytesWritten = 0;
while (isExtracting || isDecoding) {
if (isExtracting) {
// Obtain the index of the next available input buffer
ssize_t inputIndex = AMediaCodec_dequeueInputBuffer(codec, 2000);
//LOGV("Got input buffer %d", inputIndex);
// The input index acts as a status if its negative
if (inputIndex < 0) {
if (inputIndex == AMEDIACODEC_INFO_TRY_AGAIN_LATER) {
// LOGV("Codec.dequeueInputBuffer try again later");
} else {
LOGE("Codec.dequeueInputBuffer unknown error status");
}
} else {
// Obtain the actual buffer and read the encoded data into it
size_t inputSize;
uint8_t *inputBuffer = AMediaCodec_getInputBuffer(codec, inputIndex,
&inputSize);
//LOGV("Sample size is: %d", inputSize);
ssize_t sampleSize = AMediaExtractor_readSampleData(extractor, inputBuffer,
inputSize);
auto presentationTimeUs = AMediaExtractor_getSampleTime(extractor);
if (sampleSize > 0) {
// Enqueue the encoded data
AMediaCodec_queueInputBuffer(codec, inputIndex, 0, sampleSize,
presentationTimeUs,
0);
AMediaExtractor_advance(extractor);
} else {
LOGD("End of extractor data stream");
isExtracting = false;
// We need to tell the codec that we've reached the end of the stream
AMediaCodec_queueInputBuffer(codec, inputIndex, 0, 0,
presentationTimeUs,
AMEDIACODEC_BUFFER_FLAG_END_OF_STREAM);
}
}
}
if (isDecoding) {
// Dequeue the decoded data
AMediaCodecBufferInfo info;
ssize_t outputIndex = AMediaCodec_dequeueOutputBuffer(codec, &info, 0);
if (outputIndex >= 0) {
// Check whether this is set earlier
if (info.flags & AMEDIACODEC_BUFFER_FLAG_END_OF_STREAM) {
LOGD("Reached end of decoding stream");
isDecoding = false;
} else {
// Valid index, acquire buffer
size_t outputSize;
uint8_t *outputBuffer = AMediaCodec_getOutputBuffer(codec, outputIndex,
&outputSize);
/*LOGV("Got output buffer index %d, buffer size: %d, info size: %d writing to pcm index %d",
outputIndex,
outputSize,
info.size,
m_writeIndex);*/
// copy the data out of the buffer
memcpy(targetData + bytesWritten, outputBuffer, info.size);
bytesWritten += info.size;
AMediaCodec_releaseOutputBuffer(codec, outputIndex, false);
}
} else {
// The outputIndex doubles as a status return if its value is < 0
switch (outputIndex) {
case AMEDIACODEC_INFO_TRY_AGAIN_LATER:
LOGD("dequeueOutputBuffer: try again later");
break;
case AMEDIACODEC_INFO_OUTPUT_BUFFERS_CHANGED:
LOGD("dequeueOutputBuffer: output buffers changed");
break;
case AMEDIACODEC_INFO_OUTPUT_FORMAT_CHANGED:
LOGD("dequeueOutputBuffer: output outputFormat changed");
format = AMediaCodec_getOutputFormat(codec);
LOGD("outputFormat changed to: %s", AMediaFormat_toString(format));
break;
}
}
}
}
// Clean up
AMediaFormat_delete(format);
AMediaCodec_delete(codec);
AMediaExtractor_delete(extractor);
return bytesWritten;
}
现在我面临的问题是，此代码首先提取所有音频数据并将其保存到缓冲区中，然后该缓冲区成为我从同一示例中的 DataSource 类派生的 AFileDataSource 的一部分。在完成提取整个文件后，它通过调用 Oboe AudioStreamBuilder 的 onAudioReady() 来播放。我需要的是在它流式传输音频缓冲区块时播放。
可选查询：除了问题之外，即使我创建了一个前台服务来与 NDK 函数通信以执行此代码，它也会阻止 UI。对此有什么想法吗？