Creating an Array from UnsafeBufferPointer in Objective-C

Question

Usually people are asking for things to transform from Objective-C to Swift. I have to ask the other way round today.

Given the following line in Swift:

let values:[Float] = Array(UnsafeBufferPointer(start: buffer.floatChannelData?[0], count:Int(buffer.frameLength)))

Compiles successfully and I get an array of float values. Since floatChannelData on AVAudioPCMBuffer is defined as float * const I was assuming that the returning array of values is copied from the address floatChannelData[0] points to.

How would I achieve the same behaviour in Objective-C?

Charles Srstka · Accepted Answer · 2017-08-28 16:28:23Z

2

Depends on whether you want a C array of floats, or an NSArray.

(disclaimer: written in Chrome)

For a C array, we should be able to just copy the original values:

float *values = malloc(buffer.frameLength * sizeof(float)); // remember to free eventually
memcpy(values, buffer.floatChannelData[0], buffer.frameLength * sizeof(float));

For an NSArray we will have to wrap the floats in NSNumber objects:

NSMutableArray *values = [NSMutableArray new];

for (AVAudioFrameCount i = 0; i < buffer.frameLength; i++) {
    [values addObject:@(buffer.floatChannelData[0][i])];
}

edited Aug 28, 2017 at 16:28

answered Aug 28, 2017 at 15:50

Charles Srstka

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3 Comments

MrBr Over a year ago

Thanks for helping out. Compiler still not too happy Illegal type 'float *' used in a boxed expression.

Charles Srstka Over a year ago

My bad, misread your OP and didn't notice that floatChannelData was a float ** rather than a float *. I'll edit the answer.

MrBr Over a year ago

That's the solution! Thanks! Nice to know about the AVAudioFrameCount typedef as well.

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