ImageReader相关函数分析

一、前提知识：

       App中的一个surface对应SufaceFlinger中的一个layout，surface需要申请GraphicBuffer来绘制UI内容，然后交给SurfaceFlinger去合成，最后显示；ImageReader中获得的image，也就是GraphicBuffer，用于绘制surface中的UI。

应用层经常会搭配如下两个函数来获取camera数据：

// 需要什么样的图像
mImageReader = ImageReader.newInstance(1920, 1080, ImageFormat.YUV_420_888, 2);
// 设置图像绘制完毕的监听
mImageReader.setOnImageAvailableListener(mmOnImageAvailableListener, mBackgroundHandler);

二、函数解析：

       首先看下ImageReader::newInstance的实现:

文件路径：Z:\Android9_beta\frameworks\base\media\java\android\media\ImageReader.java

public static ImageReader newInstance(int width, int height, int format, int maxImages) {

        return new ImageReader(width, height, format, maxImages,  

               BUFFER_USAGE_UNKNOWN);

}

接下来看看ImageReader的构造：

protected ImageReader(int width, int height, int format, int maxImages) {
        mWidth = width;
        mHeight = height;
        mFormat = format;
        mMaxImages = maxImages;
      // 检查width 、height 、mMaxImages ，且不允许申请NV21格式的buffer
        if (width < 1 || height < 1) {
            throw new IllegalArgumentException(
                "The image dimensions must be positive");
        }
        if (mMaxImages < 1) {
            throw new IllegalArgumentException(
               "Maximum outstanding image count must be at least 1");
        }
      // 不允许创建nv21格式数据
        if (format == ImageFormat.NV21) {
            throw new IllegalArgumentException(
                    "NV21 format is not supported");
        }
        mNumPlanes = ImageUtils.getNumPlanesForFormat(mFormat);
      // 下面两个是主要实现
        nativeInit(new WeakReference<>(this), width, height, format, maxImages, usage);
        mSurface = nativeGetSurface();
      // 标记申请的是image是合法的
        mIsReaderValid = true;
        mEstimatedNativeAllocBytes = ImageUtils.getEstimatedNativeAllocBytes(
                width, height, format, /*buffer count*/ 1);
        VMRuntime.getRuntime().registerNativeAllocation(mEstimatedNativeAllocBytes);
 }

这里面主要的就是执行nativeInit和nativeGetSurface两个函数，这两个函数的实现在native文件android_media_ImageReader.cpp(Image.java和ImageReader.java共同的native文件)中，路径为：Z:\Android9_beta\frameworks\base\media\jni\android_media_ImageReader.cpp

函数映射如下：

{"nativeInit",             "(Ljava/lang/Object;IIIIJ)V",  (void*)ImageReader_init },
{"nativeImageSetup",       "(Landroid/media/Image;)I",   (void*)ImageReader_imageSetup },

ImageReader_init：

static void ImageReader_init(JNIEnv* env, jobject thiz, jobject weakThiz, jint width, jint height,jint format, jint maxImages, jlong ndkUsage) {
    status_t res;
    int nativeFormat;
    android_dataspace nativeDataspace;
    ALOGV("%s: width:%d, height: %d, format: 0x%x, maxImages:%d",
         __FUNCTION__, width, height, format, maxImages);
   // 这里是将申请的格式转化成公共格式，然后获取hal层中跟该格式对应的Format
    PublicFormat publicFormat = static_cast<PublicFormat>(format);
    nativeFormat = android_view_Surface_mapPublicFormatToHalFormat(
        publicFormat);
    nativeDataspace = android_view_Surface_mapPublicFormatToHalDataspace(
        publicFormat);
    jclass clazz = env->GetObjectClass(thiz);
    if (clazz == NULL) {
        jniThrowRuntimeException(env, "Can't find android/graphics/ImageReader");
        return;
    }
    // 注意这里new出来的JNIImageReaderContext，这是管理buffer的核心。
    // 在JNIImageReaderContext的构造里，会创建一个大小为maxImages的buffer队列mBuffers，
    // 用于将ConsumerBase中获得的buffer放进此队列中。
    sp<JNIImageReaderContext> ctx(new JNIImageReaderContext(env, weakThiz, clazz, maxImages));
    sp<IGraphicBufferProducer> gbProducer;
    sp<IGraphicBufferConsumer> gbConsumer;
    // 创建生产者和消费者。
    BufferQueue::createBufferQueue(&gbProducer, &gbConsumer);
    // 创建消费者，BufferItemConsumer是ConsumerBase的子类，这里可以从 
    // BufferQueueConsumer中拿取已经填充好的buffer数据。
    sp<BufferItemConsumer> bufferConsumer;
    String8 consumerName = String8::format("ImageReader-%dx%df%xm%d-%d-%d",
            width, height, format, maxImages, getpid(),
            createProcessUniqueId());
    uint32_t consumerUsage = GRALLOC_USAGE_SW_READ_OFTEN;
    bool needUsageOverride = ndkUsage != CONSUMER_BUFFER_USAGE_UNKNOWN;
    uint64_t outProducerUsage = 0;
    uint64_t outConsumerUsage = 0;
    android_hardware_HardwareBuffer_convertToGrallocUsageBits(&outProducerUsage, &outConsumerUsage,
            ndkUsage, 0);
    if (isFormatOpaque(nativeFormat)) {
        // Use the SW_READ_NEVER usage to tell producer that this format is not for preview or video
        // encoding. The only possibility will be ZSL output.
        consumerUsage = GRALLOC_USAGE_SW_READ_NEVER;
        if (needUsageOverride) {
            consumerUsage = android_convertGralloc1To0Usage(0, outConsumerUsage);
        }
    } else if (needUsageOverride) {
        ALOGW(&#