class BufferItem : public Flattenable<BufferItem> {
    friend class Flattenable<BufferItem>;
    size_t getPodSize() const;
    size_t getFlattenedSize() const;
    size_t getFdCount() const;
    status_t flatten(void*& buffer, size_t& size, int*& fds, size_t& count) const;
    status_t unflatten(void const*& buffer, size_t& size, int const*& fds, size_t& count);

BufferItem 继承了 Flattenable，它可以跨进程传递。

    public:
    // The default value of mBuf, used to indicate this doesn't correspond to a slot.
    enum { INVALID_BUFFER_SLOT = -1 };
    BufferItem();
    ~BufferItem();
    BufferItem(const BufferItem&) = default;
    BufferItem& operator=(const BufferItem&) = default;

    static const char* scalingModeName(uint32_t scalingMode);

    // mGraphicBuffer points to the buffer allocated for this slot, or is NULL
    // if the buffer in this slot has been acquired in the past (see
    // BufferSlot.mAcquireCalled).
    sp<GraphicBuffer> mGraphicBuffer;

    // mFence is a fence that will signal when the buffer is idle.
    sp<Fence> mFence;

    // The std::shared_ptr<FenceTime> wrapper around mFence.
    std::shared_ptr<FenceTime> mFenceTime{FenceTime::NO_FENCE};

    // mCrop is the current crop rectangle for this buffer slot.
    Rect mCrop;

    // mTransform is the current transform flags for this buffer slot.
    // refer to NATIVE_WINDOW_TRANSFORM_* in <window.h>
    uint32_t mTransform;

    // mScalingMode is the current scaling mode for this buffer slot.
    // refer to NATIVE_WINDOW_SCALING_* in <window.h>
    uint32_t mScalingMode;

如果该 slot 之前已经 acquire 过了，即 mAcquireCalled 是 true，则 mGraphicBuffer 是空。因为消费者已经持有该指针，不需要再次传递。

mCrop 是 buffer 当前裁剪矩形。mTransform 是在 window.h 定义的枚举：

mScalingMode 也是在 window.h 定义的枚举：