Example for audio recording

ViewController.m

//
//  ViewController.m
//  AudioRecordTest
//
//  Created by -------- on 16/01/2019.
//  Copyright © 2019 -----. All rights reserved.
//

#import "ViewController.h"
#import <AVFoundation/AVFoundation.h>

@interface ViewController ()

@property (nonatomic) AVAudioEngine *audioEngine;
@property (nonatomic) AVAudioFile *outputFile;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(audioInterruptionHandler:) name:AVAudioSessionInterruptionNotification object:nil];
}


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

- (IBAction)startRecording:(id)sender {
    AVAudioSession *session = [AVAudioSession sharedInstance];
    NSError *error;
    if (![session setCategory:AVAudioSessionCategoryRecord error:&error])  {
        NSLog(@"Failed to set session category: %@", error);
    }
    
    NSURL *outputURL = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask][0] URLByAppendingPathComponent:@"output-4.m4a"];
    
    __block BOOL outputFileInited = NO;
    
    self.audioEngine = [[AVAudioEngine alloc] init];
    
    AVAudioInputNode *inputNode = self.audioEngine.inputNode;
    
    [inputNode installTapOnBus:0 bufferSize:512 format:nil block:^(AVAudioPCMBuffer *buffer, AVAudioTime * when) {
        NSError *error;
        
        if (self.outputFile == nil && !outputFileInited) {
            NSDictionary *settings = @{
                                       AVFormatIDKey: @(kAudioFormatMPEG4AAC),
                                       AVNumberOfChannelsKey: @(buffer.format.channelCount),
                                       AVSampleRateKey: @(buffer.format.sampleRate)
                                       };
            
            self.outputFile = [[AVAudioFile alloc] initForWriting:outputURL settings:settings error:&error];
            
            if (!self.outputFile) {
                NSLog(@"output file error: %@", error);
                abort();
            }
            
            outputFileInited = YES;
        }
        
        if (self.outputFile && ![self.outputFile writeFromBuffer:buffer error:&error]) {
            NSLog(@"AVAudioFile write error: %@", error);
        }
    }];
    
    if (![self.audioEngine startAndReturnError:&error]) {
        NSLog(@"engine start error: %@", error);
    }
    else {
        NSLog(@"Successfully start audioEngine");
    }
}

- (IBAction)stopRecording:(id)sender {
    // To stop recording, nil the outputFile at some point in the future.
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"Finished");
        self.outputFile = nil;
    });
}


// https://developer.apple.com/library/archive/documentation/Audio/Conceptual/AudioSessionProgrammingGuide/HandlingAudioInterruptions/HandlingAudioInterruptions.html
- (void)audioInterruptionHandler:(NSNotification *)notification {
    NSDictionary *info = notification.userInfo;
    AVAudioSessionInterruptionType type = [info[AVAudioSessionInterruptionTypeKey] unsignedIntegerValue];
    
    switch (type) {
        case AVAudioSessionInterruptionTypeBegan:
            NSLog(@"Begin interruption");
            break;
        case AVAudioSessionInterruptionTypeEnded:
            NSLog(@"End interruption");
            // or ignore shouldResume if you're really keen to resume recording
            AVAudioSessionInterruptionOptions endOptions = [info[AVAudioSessionInterruptionOptionKey] unsignedIntegerValue];
            if (AVAudioSessionInterruptionOptionShouldResume == endOptions) {
                NSError *error;
                if (![self.audioEngine startAndReturnError:&error]) {
                    NSLog(@"Error restarting engine: %@", error);
                }
                else {
                    NSLog(@"Successfully restarting engine: %@", error);
                }
            }
            break;
    }
}

@end