快速访问可变长度数组

Question

Core Audio 有一个 C API，可以将一些数据复制到您提供的内存中。在一种情况下，我需要传入一个指向 AudioBufferList 的指针，它定义为:

struct AudioBufferList {
    var mNumberBuffers: UInt32
    var mBuffers: (AudioBuffer) // this is a variable length array of mNumberBuffers elements
}

UInt32 标识缓冲区的数量，紧随其后的是实际缓冲区。

我可以成功得到这个:

let bufferList = UnsafeMutablePointer<AudioBufferList>.alloc(Int(propsize));
AudioObjectGetPropertyData(self.audioDeviceID, &address, 0, nil, &propsize, bufferList);

我不认识 (AudioBuffer) 语法，但我认为它不重要 - 我认为括号被忽略，mBuffers 只是一个 AudioBuffer，我需要做指针数学来找到第二个。

我试过这个:

let buffer = UnsafeMutablePointer<AudioBuffer>(&bufferList.memory.mBuffers);
// and index via buffer += index;
// Cannot invoke 'init' with an argument of type 'inout (AudioBuffer)'

还试过:

let buffer = UnsafeMutablePointer<Array<AudioBuffer>>(&bufferList.memory.mBuffers);
// and index via buffer[index];
// error: Cannot invoke 'init' with an argument of type '@lvalue (AudioBuffer)'

更笼统的措辞:在 Swift 中，我如何将 UnsafeMutablePointer 带到一个结构并将其视为这些结构的数组？

Answer 1

您可以创建一个从给定地址开始并具有给定元素数量:

let buffers = UnsafeBufferPointer<AudioBuffer>(start: &bufferList.memory.mBuffers,
    count: Int(bufferList.memory.mNumberBuffers))

for buf in buffers {
    // ...
}

Swift 3(及更高版本)的更新:

let buffers = UnsafeBufferPointer<AudioBuffer>(start: &bufferList.pointee.mBuffers,
    count: Int(bufferList.pointee.mNumberBuffers))

for buf in buffers {
    // ...
}