iOS Audio

References:

• Core Audio Overview: Core Audio Essentials
• Audio Session Programming Guide: About Configuring Audio Behavior
• iOS Human Interface Guidelines: Designing for iOS 7

Steps outline:

1. Include the following minimal framework:
1. AVFoundationFramework
2. Import the following header files:
1. #import <AVFoundation/AVFoundation.h>
2. #import <CoreAudio/CoreAudioTypes.h>
3. (optional) to support delegates, for example responding to end of playback:
   @interface auaViewController : UIViewController
   <AVAudioRecorderDelegate, AVAudioPlayerDelegate>
   
3. Record
1. Open file
1. the following directory will work on a cell phone and the simulator:
   soundFileURL = [NSURL fileURLWithPath:[NSTemporaryDirectory() stringByAppendingString:fileName]];
2. The following, home directory, will only work with the simulator:
   soundFileURL = [NSURL fileURLWithPath:[NSHomeDirectory() stringByAppendingString:fileName]];
   
2. Set audio settings
3. Set up the recorder
   
4. Record and stop audio
4. Playback
1. Open file
2. Play the file

Example:

In the following example, the storyboard has two buttons, one to start and stop a recording (record), and the other to play it back (play).
The sound is recorded to a file, sound.aac.

Examining parts of the view controller file:

Data shared among methods:

• AVAudioRecorder * audioRecorder; - start and stop, must be external to method so reference not lost when record method ends
  
• NSURL * soundFileURL; - needed to retain file between record and playback
  
• AVAudioPlayer * audioPlayer; - needed for delegate responding to end of playback to work
  

Here are the relevant methods:

• setupRecorder
• file name - aac matches mpeg4aac specification in settings
  
• URL for sound file from file name, here stored in home directory
• settings - lots of choices, see the following reference page for some details:
• Core Audio Data Types Reference
• note that the data type is NSDictionary
  
• record
• uses a flag to start and stop recording
• save to file is automatic on recording stop
• optional elements in the code:
• NSLog to indicate operation
• changing the title of the button to indicate operation
• could do things like change color of button
  
• play
• AFTER recording done, create new audio player based on file - using the NSURL variable
  
• setting the delegate can only be done AFTER the new audio player is allocated and initialized
  
• simple playback, plays the entire recording from start to finish once
  
• audioPlayerDidFinishPlaying
• delegate method, here just notes end of playback to log console
• debugging - useful because if this method is not called, something in the configuration is wrong.
  

The view controller file:

//
//  auaViewController.m
//  Audio One
//
//  Created by Nick Duchon on 12/10/13.
//  Copyright (c) 2013 Nick Duchon. All rights reserved.
//

#import "auaViewController.h"

@interface auaViewController () {
    AVAudioRecorder * audioRecorder;
    NSURL * soundFileURL;
    AVAudioPlayer * audioPlayer;
}
@end

@implementation auaViewController

bool recording = FALSE;

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    [self setupRecorder];
} // end method viewDidLoad

- (void) setupRecorder {
    NSString * fileName = @"sound.aac";
    soundFileURL = [NSURL fileURLWithPath:[NSTemporaryDirectory() stringByAppendingString:fileName]];
    NSDictionary * soundSetting = @{AVSampleRateKey:@44100.0f,
                                    AVFormatIDKey:@(kAudioFormatMPEG4AAC),
                                    AVNumberOfChannelsKey:@2,
                                    AVEncoderAudioQualityKey:@(AVAudioQualityHigh)
                                    }; // end sound settings
    audioRecorder = [[AVAudioRecorder alloc]
                     initWithURL:soundFileURL
                     settings:soundSetting
                     error:Nil];
} // end method setupRecorder

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (IBAction)record:(id)sender {
    if (recording) {
        [audioRecorder stop];
        NSLog(@"Stopped");
        [self.recordButton setTitle:@"Start Recording" forState:UIControlStateNormal];
        recording = FALSE;
    } else {
        [audioRecorder record];
        NSLog(@"Recording");
        [self.recordButton setTitle:@"End Recording" forState:UIControlStateNormal];
        recording = true;
    } // end if, case not recording
} // end method record

- (IBAction)play:(id)sender {
    audioPlayer = [[AVAudioPlayer alloc]
                   initWithContentsOfURL:soundFileURL
                   error:nil];
    audioPlayer.delegate = self;
    [audioPlayer play];
    NSLog(@"Playing");
} // end method play

- (void) audioPlayerDidFinishPlaying:(AVAudioPlayer *)player successfully:(BOOL)flag {
    NSLog(@"Done Playing");
}

@end