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我正在尝试使用该libvlc_media_new_callbacks功能使用 libvlc 从内存中播放媒体。此函数需要指向四个回调函数的指针：Open、Read、Seek和Close。

Declare Function libvlc_media_new_callbacks Lib "libvlc.dll" (Instance As Ptr, OpenCB As Ptr, ReadCB As Ptr, SeekCB As Ptr, CloseCB As Ptr, Opaque As Ptr) As Ptr

Dim opaque As Ptr = SomeOpaqueValue()
Dim handle As Ptr = libvlc_media_new_callbacks(instance, AddressOf MediaOpen, AddressOf MediaRead, AddressOf MediaSeek, AddressOf MediaClose, opaque)

如果我不指定回调Read，则Seek、 和回调都可以正常工作，但这意味着 libvlc 不会知道媒体的长度。CloseOpen

当我指定一个Open回调时，它会使用正确的opaque参数调用，但其他回调会获得一个空指针。这使得无法知道回调应该在哪个流上工作。

我是否误解了 open 回调的目的？

这是我的回调函数：

Shared Function MediaOpen(Opaque As Ptr, Buffer As Ptr, ByRef BufferSize As UInt64) As UInt32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 1 ' invalid Opaque

  Buffer = Nil  ' Not sure what to do with this parameter
  BufferSize = r.Length
  Return 0
End Function

Shared Sub MediaClose(Opaque As Ptr)
  If Streams.HasKey(Opaque) Then Streams.Remove(Opaque)
  If Streams.Count = 0 Then Streams = Nil
End Sub

Shared Function MediaRead(Opaque As Ptr, Buffer As Ptr, BufferSize As Integer) As UInt32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 0 ' invalid Opaque

  Dim mb As MemoryBlock = Buffer
  Dim data As MemoryBlock = r.Read(BufferSize)
  mb.StringValue(0, data.Size) = data
  Return data.Size
End Function

Shared Function MediaSeek(Opaque As Ptr, Offset As UInt64) As Int32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 1 ' invalid Opaque
  If Offset > r.Length Then Return 1 ' invalid offset

  r.Position = Offset
  Return 0
End Function
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1 回答 1

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根据文档，您必须分配一个缓冲区并在 open 的 Buffer 参数中返回其地址（为此，您需要将其更改为 ByRef 参数！）。然后，您将获得在其他函数中传递的相同缓冲区 ptr。

理想情况下，你会设置

Buffer = r

在 MediaOpen 中。但是Xojo不会让你。相反，您需要维护一个字典，在其中存储r然后将其与您可以分配给 Buffer 参数的值相关联。

这是一个示例（未测试）：

Static Property OpenedBuffers as Dictionary
Static Property OpenedBufferID as Integer

Shared Function MediaOpen(Opaque As Ptr, ByRef HandleOut As Integer, ByRef BufferSize As UInt64) As UInt32
  Dim r As BinaryStream = Streams.Lookup(Opaque, Nil)
  If r = Nil Then Return 1 ' invalid Opaque
  if OpenedBuffers = nil then OpenedBuffers = new Dictionary
  OpenedBufferID = OpenedBufferID + 1
  OpenedBuffers.Value(OpenedBufferID) = r
  HandleOut = OpenedBufferID
  BufferSize = r.Length
  Return 0
End Function

Shared Function MediaRead(Handle As Integer, Buffer As Ptr, BufferSize As Integer) As UInt32
  Dim r As BinaryStream = OpenedBuffers.Value(Handle)
  ...

Shared Sub MediaClose(Handle As Integer)
  Dim r As BinaryStream = OpenedBuffers.Value(Handle)
  OpenedBuffers.Remove (Handle)
  If Streams.HasKey(r) Then Streams.Remove(r)
  ...

希望有帮助

于 2018-05-02T19:59:11.470 回答