[swift-users] Initialization Catch-22?

[Kenny Leung]

Hi Howard.

Yes, this would make it compile, but it still would not solve the issues that make it wrong in my mind:

1. I would have to make it a var, but it should not be mutable.

2. It’s not optional. Without it, the PDAudioPlayer class could not function.

Of course, this is just being stubbornly pedantic about the code,  but the whole point of this project is an exercise in pedantry.

-Kenny


> On Sep 25, 2017, at 10:35 PM, Howard Lovatt <howard.lovatt at gmail.com> wrote:
> 
> You could try making mQueue a force unwrapped optional or initialize it to a failed queue before overwriting on last line of `init`. 
> 
> On Tue, 26 Sep 2017 at 11:39 am, Kenny Leung via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> Hi Muthu.
> 
> Thanks for the suggestion.
> 
> I don’t want to do that because failure to create the AudioQueue should mean failure to create the AudioPlayer itself.
> 
> -Kenny
> 
> 
>> On Sep 25, 2017, at 6:33 PM, somu subscribe <somu.subscribe at gmail.com <mailto:somu.subscribe at gmail.com>> wrote:
>> 
>> Hi Kenny,
>> 
>> You could use a lazy var and initialise it with a closure.
>> 
>> class PDAudioPlayer {
>> 
>>     //Would be created lazily when first invoked
>>     lazy var mQueue : AudioQueueRef = {
>> 
>>         //create an audioQueue and return that instance
>>         return audioQueue
>>     }()
>> }
>> 
>> Thanks and regards,
>> Muthu
>> 
>> 
>>> On 26 Sep 2017, at 9:12 AM, Kenny Leung via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>> Hi All.
>>> 
>>> I’m trying to implement the AudioQueue example from Apple in Swift, and keep it as Swifty as I can. I’ve run into a problem where I have a let ivar for the AudioQueue, but the only way to initialize that let ivar is to pass a block to the function that creates the AudioQueue. I get the error, "Variable 'self.mQueue' used before being initialized”. Is there any way to get around this catch? 
>>> 
>>> Thanks!
>>> 
>>> -Kenny
>>> 
>>> The code follows.
>>> 
>>> ———8<————8<————
>>> class PDAudioPlayer {
>>>     static let kNumberBuffers :Int = 3                              // 1
>>>     //var mDataFormat:AudioStreamBasicDescription                     // 2
>>>     let mQueue :AudioQueueRef                                      // 3
>>>     //var mBuffers :[AudioQueueBufferRef]                            // 4
>>>     //var mAudioFile :AudioFileID?                                    // 5
>>>     var bufferByteSize :UInt32?                                     // 6
>>>     var mCurrentPacket :Int64?                                      // 7
>>>     var mNumPacketsToRead :UInt32?                                  // 8
>>>     var mPacketDescs :UnsafePointer<AudioStreamPacketDescription>?  // 9
>>>     var mIsRunning: Bool = false                                    // 10
>>>     
>>>     let dispatchQueue :DispatchQueue
>>>     let source :PDFileAudioSource
>>>     var audioBuffers :[PDAudioBuffer]
>>>     
>>>     init?(source:PDFileAudioSource) {
>>>         self.source = source
>>>         self.dispatchQueue = DispatchQueue(label:"AudioPlayer", qos:.default, attributes:[], autoreleaseFrequency:.workItem, target:nil)
>>>         self.audioBuffers = [PDAudioBuffer]()
>>> 
>>>         var tempAudioQueue :AudioQueueRef?
>>>         var dataFormat = source.mDataFormat
>>>         let status = AudioQueueNewOutputWithDispatchQueue(&tempAudioQueue, &dataFormat, 0, self.dispatchQueue) { [weak self] (queue, buffer) in   // ERROR on this line
>>>             guard let this = self else {
>>>                 return // block
>>>             }
>>>             guard let audioBuffer = this.audioBufferForAudioQueueBuffer(aqBuffer:buffer) else {
>>>                 return // block
>>>             }
>>>             this.source.fillBuffer(audioBuffer)
>>>         }
>>>         if status != 0 {
>>>             return nil
>>>         }
>>>         guard let audioQueue = tempAudioQueue else {
>>>             return nil
>>>         }
>>>         self.mQueue = audioQueue
>>>     }
>>> 
>>>     private func audioBufferForAudioQueueBuffer(aqBuffer:AudioQueueBufferRef) -> PDAudioBuffer? {
>>>         for buffer in self.audioBuffers {
>>>             if buffer.audioQueueBuffer == aqBuffer {
>>>                 return buffer
>>>             }
>>>         }
>>>         return nil
>>>     }
>>> }
>>> 
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>> 
> 
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> -- 
> -- Howard.

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