updated initWithComponentDescription

VolumePluginAudioUnit.m

- (instancetype)initWithComponentDescription:(AudioComponentDescription)componentDescription options:(AudioComponentInstantiationOptions)options error:(NSError **)outError {
    self = [super initWithComponentDescription:componentDescription options:options error:outError];
    
    if (self == nil) { return nil; }
    
    // initialize Buffer
    _buffer = Buffer();
    _buffer.maxFrames = 0;
    _buffer.pcmBuffer = nullptr;
    _buffer.mutableAudioBufferList = nullptr;
    // Initialize a default format for the busses.
    AVAudioFormat *defaultFormat = [[AVAudioFormat alloc] initStandardFormatWithSampleRate:44100.0 channels:2];
    
    // Create parameter objects.
    AUParameter *param1 = [AUParameterTree createParameterWithIdentifier:@"param1" name:@"Parameter 1" address:myParam1 min:0 max:100 unit:kAudioUnitParameterUnit_Percent unitName:nil flags:0 valueStrings:nil dependentParameters:nil];
    
    // Initialize the parameter values.
    param1.value = 1.0;
    
    // Create the parameter tree.
    _parameterTree = [AUParameterTree createTreeWithChildren:@[ param1 ]];
    
    // Create the input and output busses (AUAudioUnitBus).
    _inputBus = [[AUAudioUnitBus alloc] initWithFormat:defaultFormat error:nil];
    _outputBus = [[AUAudioUnitBus alloc] initWithFormat:defaultFormat error:nil];
    
    // Create the input and output bus arrays (AUAudioUnitBusArray).
    _inputBusArray = [[AUAudioUnitBusArray alloc] initWithAudioUnit:self busType:AUAudioUnitBusTypeInput busses:@[_inputBus]];
    _outputBusArray = [[AUAudioUnitBusArray alloc] initWithAudioUnit:self busType:AUAudioUnitBusTypeOutput busses:@[_outputBus]];
    
    // A function to provide string representations of parameter values.
    _parameterTree.implementorStringFromValueCallback = ^(AUParameter *param, const AUValue *__nullable valuePtr) {
        AUValue value = valuePtr == nil ? param.value : *valuePtr;
        
        switch (param.address) {
            case myParam1:
                return [NSString stringWithFormat:@"%.f", value];
            default:
                return @"?";
        }
    };
    
    __block Buffer *buffer = &_buffer;
    _parameterTree.implementorValueObserver = ^(AUParameter *param, AUValue value) {
        buffer->volume = value;
    };
    
    self.maximumFramesToRender = 512;
    
    return self;
}