在Swift中使用AudioBufferList

Question

我在Swift中有一个桥接函数,其中一个参数在C中AudioBufferList *.在Swift中,这会生成一个UnsafePointer<AudioBufferList>.我设法通过调用来引用指针audioData[0](有更好的方法吗？).但我正在努力接下来的两个层次:他们和他们/ 成员的.mBuffers阵列. AudioBuffervoid *UnsafePointer<()> .mData

在C中它就是这样

Float32 *audioData = (Float 32*)abl->mBuffers[0]->mData;
output = audioData[sampleNum]...

在Swift中,第一个奇怪的事情是它不会让我访问元素,mBuffers但是当我作为属性访问它时非常高兴.换句话说,这工作,甚至有正确的数据(对于mBuffers我认为的第一个成员)...

println(abl[0].mBuffers.mNumberChannels)  // But .mBuffers should be an []!

其次,让我打印出.mData下标,但价值总是如此()

println(abl[0].mBuffers.mData[10])  // Prints '()'

我已经尝试了各种铸造操作和访问多个索引但无济于事......任何想法？

以下是C和Swift定义AudioBufferList,AudioBuffer为方便起见......

// C
struct AudioBufferList
{
    UInt32      mNumberBuffers;
    AudioBuffer mBuffers[1]; // this is a variable length array of mNumberBuffers elements
    // ...and a bit more for c++
}


struct AudioBuffer
{
    UInt32  mNumberChannels;
    UInt32  mDataByteSize;
    void*   mData;
};

...

// SWIFT

struct AudioBufferList {
    var mNumberBuffers: UInt32
    var mBuffers: (AudioBuffer)
}

struct AudioBuffer {
    var mNumberChannels: UInt32
    var mDataByteSize: UInt32
    var mData: UnsafePointer<()>
}

Answer 1

我偶然发现了这个.奇怪的是,当它提出UnsafeMutableAudioBufferListPointer时,前面的类型实际上正在使用Swift.您可以使用UnsafeMutablePointer参数进行初始化.此类型是MutableCollectionType,并提供对包含的音频缓冲区的下标和生成器访问.

例如,您可以使用以下代码将ABL设置为静音

func renderCallback(ioData: UnsafeMutablePointer<AudioBufferList>) -> OSStatus {

    let abl = UnsafeMutableAudioBufferListPointer(ioData)

    for buffer in abl {

        memset(buffer.mData, 0, Int(buffer.mDataByteSize))
    }

    return noErr
}

Answer 2

编辑: Adam Ritenauer的答案可能是现在最好的答案.要对其进行扩展,您可以查看iOS 8.3 Core Audio更改中的新实用程序功能/类型.

UnsafeMutableAudioBufferListPointer 可用于读取/访问某些给定数据:

struct UnsafeMutableAudioBufferListPointer {
    init(_ p: UnsafeMutablePointer<AudioBufferList>)
    var count: Int
    subscript (index: Int) -> AudioBuffer { get nonmutating set }
}

你可以使用AudioBuffer和AudioBufferList上的扩展来分配你自己的:

extension AudioBufferList {
    static func sizeInBytes(maximumBuffers maximumBuffers: Int) -> Int
    static func allocate(maximumBuffers maximumBuffers: Int) -> UnsafeMutableAudioBufferListPointer
}

extension AudioBuffer {
    init<Element>(_ typedBuffer: UnsafeMutableBufferPointer<Element>, numberOfChannels: Int)
}

_老答案:

这有点棘手,因为AudioBufferList它实际上是一个可变大小的结构.这意味着它被声明为具有单个AudioBuffer,但实际上它具有该mNumberBuffers成员指定的数量.这个概念并不能很好地转化为Swift,这就是你看到的原因var mBuffers: (AudioBuffer).

因此,访问这些缓冲区及其数据的规范方法将使用UnsafeArray.下面的代码提供了一些想法,但UnsafePointer并UnsafeArray没有得到很好的记录,所以这可能是错误的.

// ***WARNING: UNTESTED CODE AHEAD***

let foo: UnsafePointer<AudioBufferList> // from elsewhere...

// This looks intuitive, but accessing `foo.memory` may be doing a copy.
let bufs = UnsafeArray<AudioBuffer>(start: &foo.memory.mBuffers, length: Int(foo.memory.mNumberBuffers))

// This is another alternative that should work...
let bufsStart = UnsafePointer<AudioBuffer>(UnsafePointer<UInt32>(foo) + 1) // Offset to mBuffers member
let bufs = UnsafeArray<AudioBuffer>(start: bufsStart, length: Int(foo.memory.mNumberBuffers))

// Hopefully this isn't doing a copy, but it shouldn't be too much of a problem anyway.
let buf: AudioBuffer = bufs[0] // or you could use a for loop over bufs, etc.

typealias MySample = Float32
let numSamples = Int(buf.mDataByteSize / UInt32(sizeof(MySample)))
let samples = UnsafeArray<MySample>(start: UnsafePointer<MySample>(buf.mData), length: numSamples)

// Now use the samples array...

这似乎在操场上有效,但我很难测试真实的音频数据.特别是,我不是100%确定使用start: &foo.memory.mBuffers将按预期工作.(它返回与原始指针不同的指针,尽管数据似乎在那里.)试一试并报告回来!

编辑:顺便说一下,调试一下,例如:

(lldb) p foo
(UnsafePointer<AudioBufferList>) $R1 = (value = Builtin.RawPointer = 0x0000000100700740)
(lldb) expr -lc -- ((int*)0x0000000100700740)[0]
(int) $2 = 42
(lldb) expr -lc -- ((int*)0x0000000100700740)[1]
(int) $3 = 43
...