说明：
一般情况下，我们采用UIImagePickerController来调用系统提供的相机来拍照，非常好用。但是有时UIImagePickerController控件无法满足我们的需求，例如我们需要更加复杂的相机界面，这时候我们就要自定义相机了。
本文用的是AVFoundation框架做的一个自定义相机demo，包含了基本的对焦以及焦距调整功能。因为自己要用到，所以写了这样一个Demo，方便有同样需求的人查阅。直接上代码：
<pre>#import "ViewController.h"

import <AVFoundation/AVFoundation.h>

import <CoreGraphics/CoreGraphics.h>

import <QuartzCore/QuartzCore.h>

define kWidth ([UIScreen mainScreen].bounds.size.width)

define kHeight ([UIScreen mainScreen].bounds.size.height)

@interface ViewController ()

@property(nonatomic,strong)AVCaptureSession *session;
@property(nonatomic,strong)AVCaptureDeviceInput *input;
@property(nonatomic,strong)AVCaptureStillImageOutput *output;
@property(nonatomic,strong)AVCaptureVideoPreviewLayer *previewLayer;
//拍照
@property(nonatomic,strong)UIButton *shutterBtn;
//对焦
@property(nonatomic,strong)UIView *focalReticule;
//焦距Button
@property(nonatomic,strong)UIButton *focalBtn;

@end

//焦距
static float kCameraScale=1.0;

@implementation ViewController

• (void)viewDidLoad
  {
  [super viewDidLoad];
  [self setUpSession];
  [self initCameraLayer];
  [self ceateUI];
  [self createFunctionalUI];
  }
  //开始任务
  -(void)viewDidAppear:(BOOL)animated
  {
  [super viewDidAppear:animated];
  [_session startRunning];
  }
  //停止任务
  -(void)viewWillDisappear:(BOOL)animated
  {
  [super viewWillDisappear:animated];
  [_session stopRunning];
  }
  //初始化抓取任务
  -(void)setUpSession
  {
  _session=[[AVCaptureSession alloc]init];
  AVCaptureDevice *device=nil;
  NSArray *devices=[AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
  for(AVCaptureDevice *tmp in devices)
  {
  if(tmp.position==AVCaptureDevicePositionBack)
  device=tmp;
  }
  _input=[[AVCaptureDeviceInput alloc]initWithDevice:device error:nil];
  _output=[[AVCaptureStillImageOutput alloc]init];
  _output.outputSettings=@{AVVideoCodecKey:AVVideoCodecJPEG};
  if([_session canAddInput:_input])
  [_session addInput:_input];
  if([_session canAddOutput:_output])
  [_session addOutput:_output];
  }
  //初始化相机预览层
  -(void)initCameraLayer
  {
  _previewLayer=[[AVCaptureVideoPreviewLayer alloc]initWithSession:_session];
  //self.view.layer.masksToBounds=YES;
  [self.previewLayer setFrame:self.view.bounds];
  [self.previewLayer setVideoGravity:AVLayerVideoGravityResizeAspect];
  [self.view.layer addSublayer:_previewLayer];
  }
  //搭建UI
  -(void)ceateUI
  {
  //拍照按钮
  _shutterBtn=[[UIButton alloc]initWithFrame:CGRectMake(kWidth-50, kHeight-60, 30, 40)];
  [_shutterBtn setImage:[UIImage imageNamed:@"camera"] forState:UIControlStateNormal];
  [_shutterBtn setImage:[UIImage imageNamed:@"camera_h"] forState:UIControlStateHighlighted];
  [self.view addSubview:_shutterBtn];
  [_shutterBtn addTarget:self action:@selector(shutter) forControlEvents:UIControlEventTouchUpInside];
  _shutterBtn.backgroundColor=[UIColor colorWithWhite:0.8 alpha:0.8];
  _shutterBtn.layer.cornerRadius=10;
  _shutterBtn.imageEdgeInsets=UIEdgeInsetsMake(10, 5, 10, 5);
  _shutterBtn.layer.borderColor=[UIColor whiteColor].CGColor;
  _shutterBtn.layer.borderWidth=1;

  //对焦十字
  _focalReticule=[[UIView alloc]initWithFrame:CGRectMake(0, 0, 60, 60)];
  _focalReticule.backgroundColor=[UIColor clearColor];
  //十字
  UIView *line1=[[UIView alloc]initWithFrame:CGRectMake(0, 29.5, 60, 1)];
  line1.backgroundColor=[UIColor whiteColor];
  [_focalReticule addSubview:line1];

  UIView *line2=[[UIView alloc]initWithFrame:CGRectMake(29.5, 0, 1, 60)];
  line2.backgroundColor=[UIColor whiteColor];
  [_focalReticule addSubview:line2];
  [self.view addSubview:_focalReticule];
  //默认隐藏
  _focalReticule.hidden=YES;

  //点击屏幕对焦的手势
  UITapGestureRecognizer *foucusTap=[[UITapGestureRecognizer alloc]initWithTarget:self action:@selector(foucus:)];
  [self.view addGestureRecognizer:foucusTap];
  }
  //功能性UI
  -(void)createFunctionalUI
  {
  //1x 2x 3x 4x焦距按钮
  _focalBtn=[[UIButton alloc]initWithFrame:CGRectMake(20, kHeight-110, 40, 30)];
  [_focalBtn setTitle:@"1X" forState:UIControlStateNormal];
  [_focalBtn setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
  _focalBtn.backgroundColor=[UIColor colorWithWhite:0.8 alpha:0.8];
  _focalBtn.layer.cornerRadius=10;
  _focalBtn.layer.borderColor=[UIColor whiteColor].CGColor;
  _focalBtn.layer.borderWidth=1.0;
  _focalBtn.transform=CGAffineTransformMakeRotation(M_PI_2);
  [_focalBtn addTarget:self action:@selector(adjustFocalDistance:) forControlEvents:UIControlEventTouchUpInside];
  [self.view addSubview:_focalBtn];
  CGRect frame=_focalBtn.frame;
  frame.origin.x=_shutterBtn.frame.origin.x;
  _focalBtn.frame=frame;

  //闪光灯开启关闭按钮
  UIButton *flashBtn=[[UIButton alloc]initWithFrame:CGRectMake(CGRectGetMinX(_shutterBtn.frame), 20, 40, 30)];
  flashBtn.transform=CGAffineTransformMakeRotation(M_PI_2);
  [flashBtn setTitle:@"⚡️关" forState:UIControlStateNormal];
  [flashBtn setTitle:@"⚡️开" forState:UIControlStateSelected];
  [flashBtn setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
  flashBtn.titleLabel.font=[UIFont systemFontOfSize:12];
  flashBtn.backgroundColor=[UIColor colorWithWhite:0.8 alpha:0.8];
  flashBtn.layer.cornerRadius=10;
  flashBtn.layer.borderColor=[UIColor whiteColor].CGColor;
  flashBtn.layer.borderWidth=1;
  [self.view addSubview:flashBtn];
  [flashBtn addTarget:self action:@selector(flasAction:) forControlEvents:UIControlEventTouchUpInside];

  //切换前后摄像头
  UIButton *shiftBtn=[[UIButton alloc]initWithFrame:CGRectMake(CGRectGetMinX(_shutterBtn.frame), 80, 40, 30)];
  shiftBtn.transform=CGAffineTransformMakeRotation(M_PI_2);
  [shiftBtn setImage:[UIImage imageNamed:@"shift"] forState:UIControlStateNormal];
  shiftBtn.imageEdgeInsets=UIEdgeInsetsMake(5, 10, 5, 10);
  [shiftBtn setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
  shiftBtn.titleLabel.font=[UIFont systemFontOfSize:12];
  shiftBtn.backgroundColor=[UIColor colorWithWhite:0.8 alpha:0.8];
  shiftBtn.layer.cornerRadius=10;
  shiftBtn.layer.borderColor=[UIColor whiteColor].CGColor;
  shiftBtn.layer.borderWidth=1;
  [self.view addSubview:shiftBtn];
  [shiftBtn addTarget:self action:@selector(shiftCamera:) forControlEvents:UIControlEventTouchUpInside];
  }
  //切换前后相机
  -(void)shiftCamera:(UIButton *)sender
  {
  sender.selected=!sender.isSelected;
  //切换至前置摄像头
  if(sender.isSelected)
  {
  AVCaptureDevice *device=nil;
  NSArray *devices=[AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
  for(AVCaptureDevice *tmp in devices)
  {
  if(tmp.position==AVCaptureDevicePositionFront)
  device=tmp;
  }
  [_session beginConfiguration];
  [_session removeInput:_input];
  _input=nil;
  _input=[[AVCaptureDeviceInput alloc]initWithDevice:device error:nil];
  if([_session canAddInput:_input])
  [_session addInput:_input];
  [_session commitConfiguration];
  }
  //切换至后置摄像头
  else
  {
  AVCaptureDevice *device=nil;
  NSArray *devices=[AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
  for(AVCaptureDevice *tmp in devices)
  {
  if(tmp.position==AVCaptureDevicePositionBack)
  device=tmp;
  }
  [_session beginConfiguration];
  [_session removeInput:_input];
  _input=nil;
  _input=[[AVCaptureDeviceInput alloc]initWithDevice:device error:nil];
  if([_session canAddInput:_input])
  [_session addInput:_input];
  [_session commitConfiguration];
  }
  }
  //闪光灯按钮的操作
  -(void)flasAction:(UIButton *)sender
  {
  sender.selected=!sender.isSelected;
  AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
  if ([device hasTorch] && [device hasFlash])
  {
  [device lockForConfiguration:nil];
  //闪光灯开
  if (sender.isSelected)
  {
  [device setFlashMode:AVCaptureFlashModeOn];
  }
  //闪光灯关
  else
  {
  [device setFlashMode:AVCaptureFlashModeOff];
  }
  //闪光灯自动，这里就不写了，可以自己尝试
  //[device setFlashMode:AVCaptureFlashModeAuto];
  [device unlockForConfiguration];
  }
  }
  //调整焦距
  -(void)adjustFocalDistance:(UIButton *)sender
  {
  kCameraScale+=1.0;
  if(kCameraScale>4.0)
  kCameraScale=1.0;
  //改变焦距
  AVCaptureConnection *connect=[_output connectionWithMediaType:AVMediaTypeVideo];
  [CATransaction begin];
  [CATransaction setAnimationDuration:0.2];
  [_focalBtn setTitle:[NSString stringWithFormat:@"%dX",(int)kCameraScale] forState:UIControlStateNormal];
  [_previewLayer setAffineTransform:CGAffineTransformMakeScale(kCameraScale, kCameraScale)];
  connect.videoScaleAndCropFactor=kCameraScale;
  [CATransaction commit];
  }
  //对焦
  -(void)foucus:(UITapGestureRecognizer *)sender
  {
  if(_input.device.position==AVCaptureDevicePositionFront)
  return;
  if(sender.state==UIGestureRecognizerStateRecognized)
  {
  CGPoint location=[sender locationInView:self.view];
  //对焦
  __weak typeof(self) weakSelf=self;
  [self focusOnPoint:location completionHandler:^{
  weakSelf.focalReticule.center=location;
  weakSelf.focalReticule.alpha=0.0;
  weakSelf.focalReticule.hidden=NO;
  [UIView animateWithDuration:0.3 animations:^{
  weakSelf.focalReticule.alpha=1.0;
  }completion:^(BOOL finished) {
  [UIView animateWithDuration:0.3 animations:^{
  weakSelf.focalReticule.alpha=0.0;
  }];
  }];
  }];
  }
  }
  //对某一点对焦
  -(void)focusOnPoint:(CGPoint)point completionHandler:(void(^)())completionHandler
  {
  AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];;
  CGPoint pointOfInterest = CGPointZero;
  CGSize frameSize = self.view.bounds.size;
  pointOfInterest = CGPointMake(point.y / frameSize.height, 1.f - (point.x / frameSize.width));

  if ([device isFocusPointOfInterestSupported] && [device isFocusModeSupported:AVCaptureFocusModeAutoFocus])
  {

    NSError *error;
    if ([device lockForConfiguration:&error])
    {
        
        if ([device isWhiteBalanceModeSupported:AVCaptureWhiteBalanceModeAutoWhiteBalance])
        {
            [device setWhiteBalanceMode:AVCaptureWhiteBalanceModeAutoWhiteBalance];
        }
        
        if ([device isFocusModeSupported:AVCaptureFocusModeContinuousAutoFocus])
        {
            [device setFocusMode:AVCaptureFocusModeAutoFocus];
            [device setFocusPointOfInterest:pointOfInterest];
        }
        
        if([device isExposurePointOfInterestSupported] && [device isExposureModeSupported:AVCaptureExposureModeContinuousAutoExposure])
        {
            [device setExposurePointOfInterest:pointOfInterest];
            [device setExposureMode:AVCaptureExposureModeContinuousAutoExposure];
        }
        
        [device unlockForConfiguration];
        if(completionHandler)
            completionHandler();
    }
  

  }
  else
  {
  if(completionHandler)
  completionHandler();
  }
  }
  //拍照
  -(void)shutter
  {
  AVCaptureConnection *connect=[_output connectionWithMediaType:AVMediaTypeVideo];
  if(!connect)
  {
  NSLog(@"拍照失败");
  return;
  }
  __weak typeof(self) weakSelf=self;
  [_output captureStillImageAsynchronouslyFromConnection:connect completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) {
  if(imageDataSampleBuffer==NULL)
  return;
  NSData *imageData=[AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
  UIImage *image=[UIImage imageWithData:imageData];
  [weakSelf shutterSuccessAlert];
  [weakSelf showCapturedImage:image];
  }];
  }
  //显示拍摄到的照片
  -(void)showCapturedImage:(UIImage *)image
  {
  UIImageView *imv=[[UIImageView alloc]initWithFrame:self.view.bounds];
  imv.image=image;
  if(_input.device.position==AVCaptureDevicePositionFront)
  imv.transform=CGAffineTransformMakeRotation(M_PI);
  [self.view addSubview:imv];
  imv.userInteractionEnabled=YES;
  CGFloat xpos=20;
  CGFloat ypos=100;
  if(_input.device.position==AVCaptureDevicePositionFront)
  {
  xpos=kWidth-80;
  ypos=kHeight-100;
  }
  UIButton cancelBtn=[[UIButton alloc]initWithFrame:CGRectMake(xpos, ypos, 60, 40)];
  [cancelBtn setTitle:@"取消" forState:UIControlStateNormal];
  [cancelBtn setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
  cancelBtn.backgroundColor=[UIColor colorWithWhite:0.8 alpha:0.8];
  cancelBtn.layer.cornerRadius=5;
  cancelBtn.layer.borderColor=[UIColor whiteColor].CGColor;
  cancelBtn.layer.borderWidth=1.0;
  [cancelBtn addTarget:self action:@selector(cancel:) forControlEvents:UIControlEventTouchUpInside];
  CGFloat raduis=M_PI_2;
  if(_input.device.position==AVCaptureDevicePositionFront)
  raduis=M_PI_23;
  cancelBtn.transform=CGAffineTransformMakeRotation(raduis);
  [imv addSubview:cancelBtn];
  UIButton *saveBtn=[[UIButton alloc]initWithFrame:CGRectMake(xpos, kHeight-ypos, 60, 40)];
  [saveBtn setTitle:@"保存" forState:UIControlStateNormal];
  [saveBtn setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
  saveBtn.backgroundColor=[UIColor colorWithWhite:0.8 alpha:0.8];
  saveBtn.layer.cornerRadius=5;
  saveBtn.layer.borderColor=[UIColor whiteColor].CGColor;
  saveBtn.layer.borderWidth=1.0;
  [saveBtn addTarget:self action:@selector(save:) forControlEvents:UIControlEventTouchUpInside];
  saveBtn.transform=CGAffineTransformMakeRotation(raduis);
  [imv addSubview:saveBtn];

}
//取消
-(void)cancel:(UIButton *)sender
{
[sender.superview removeFromSuperview];
}
//保存
-(void)save:(UIButton *)sender
{
UIImageView *imv=(UIImageView *)sender.superview;
//保存到相册
UIImageWriteToSavedPhotosAlbum(imv.image, nil, nil, nil);
[imv removeFromSuperview];
}
//播放拍照音效
-(void)shutterSuccessAlert
{
//播放音效
SystemSoundID soundID;
AudioServicesCreateSystemSoundID((__bridge CFURLRef)[NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"sound.mp3" ofType:nil]],&soundID);
//播放短音频
AudioServicesPlaySystemSound(soundID);
//增加震动效果
//AudioServicesPlaySystemSound(kSystemSoundID_Vibrate);
}
//隐藏状态栏
-(BOOL)prefersStatusBarHidden
{
return YES;
}

@end</pre>
<pre>
附上：GitHub代码地址，希望能多多支持！</pre>