我有一张带照片的应用程序。当用户按下按钮拍照时，我将名为self.hasUserTakenAPhoto的全局NSString变量设置为YES。使用后置摄像头时，这种方式可以100％完美地工作。但是，只有50％的时间使用前置摄像头，我不知道为什么。
下面是重要的代码片段以及他们所做的快速描述。
这是我的viewDidLoad：
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
self.topHalfView.frame = CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.height/2);
self.takingPhotoView.frame = CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.height);
self.afterPhotoView.frame = CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.height);
self.bottomHalfView.frame = CGRectMake(0, 240, self.view.bounds.size.width, self.view.bounds.size.height/2);
PFFile *imageFile = [self.message objectForKey:@"file"];
NSURL *imageFileURL = [[NSURL alloc]initWithString:imageFile.url];
imageFile = nil;
self.imageData = [NSData dataWithContentsOfURL:imageFileURL];
imageFileURL = nil;
self.topHalfView.image = [UIImage imageWithData:self.imageData];
//START CREATING THE SESSION
self.session =[[AVCaptureSession alloc]init];
[self.session setSessionPreset:AVCaptureSessionPresetPhoto];
self.inputDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
NSError *error;
self.deviceInput = [AVCaptureDeviceInput deviceInputWithDevice:self.inputDevice error:&error];
if([self.session canAddInput:self.deviceInput])
[self.session addInput:self.deviceInput];
_previewLayer = [[AVCaptureVideoPreviewLayer alloc]initWithSession:_session];
self.rootLayer = [[self view]layer];
[self.rootLayer setMasksToBounds:YES];
[_previewLayer setFrame:CGRectMake(0, 240, self.rootLayer.bounds.size.width, self.rootLayer.bounds.size.height/2)];
[_previewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
[self.rootLayer insertSublayer:_previewLayer atIndex:0];
self.videoOutput = [[AVCaptureVideoDataOutput alloc] init];
self.videoOutput.videoSettings = @{ (NSString *)kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_32BGRA) };
[self.session addOutput:self.videoOutput];
dispatch_queue_t queue = dispatch_queue_create("MyQueue", NULL);
[self.videoOutput setSampleBufferDelegate:self queue:queue];
[_session startRunning];
}
viewDidLoad的重要部分从我离开//START CREATING THE SESSION
我基本上创建会话然后开始运行它。我已将此视图控制器设置为AVCaptureVideoDataOutputSampleBufferDelegate，因此只要会话开始运行，就会开始调用以下方法。
- (void)captureOutput:(AVCaptureOutput *)captureOutput
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
fromConnection:(AVCaptureConnection *)connection
{
//Sample buffer data is being sent, but don't actually use it until self.hasUserTakenAPhoto has been set to YES.
NSLog(@"Has the user taken a photo?: %@", self.hasUserTakenAPhoto);
if([self.hasUserTakenAPhoto isEqualToString:@"YES"]) {
//Now that self.hasUserTakenAPhoto is equal to YES, grab the current sample buffer and use it for the value of self.image aka the captured photo.
self.image = [self imageFromSampleBuffer:sampleBuffer];
}
}
此代码每秒都会从摄像头接收视频输出，但在self.hasUserTakenAPhoto等于YES之前，我实际上并没有对其进行任何操作。一旦字符串值为YES，我就会使用相机中的当前sampleBuffer并将其放在名为self.image
所以，这是self.hasUserTakenAPhoto实际设置为YES的时候。
下面是我的IBAction代码，当用户按下按钮捕获照片时调用该代码。当这段代码运行时会发生很多事情，但真正重要的是第一句话：self.hasUserTakenAPhoto = @"YES";
-(IBAction)stillImageCapture {
self.hasUserTakenAPhoto = @"YES";
[self.session stopRunning];
if(self.inputDevice.position == 2) {
self.image = [self selfieCorrection:self.image];
} else {
self.image = [self rotate:UIImageOrientationRight];
}
CGFloat widthToHeightRatio = _previewLayer.bounds.size.width / _previewLayer.bounds.size.height;
CGRect cropRect;
// Set the crop rect's smaller dimension to match the image's smaller dimension, and
// scale its other dimension according to the width:height ratio.
if (self.image.size.width < self.image.size.height) {
cropRect.size.width = self.image.size.width;
cropRect.size.height = cropRect.size.width / widthToHeightRatio;
} else {
cropRect.size.width = self.image.size.height * widthToHeightRatio;
cropRect.size.height = self.image.size.height;
}
// Center the rect in the longer dimension
if (cropRect.size.width < cropRect.size.height) {
cropRect.origin.x = 0;
cropRect.origin.y = (self.image.size.height - cropRect.size.height)/2.0;
NSLog(@"Y Math: %f", (self.image.size.height - cropRect.size.height));
} else {
cropRect.origin.x = (self.image.size.width - cropRect.size.width)/2.0;
cropRect.origin.y = 0;
float cropValueDoubled = self.image.size.height - cropRect.size.height;
float final = cropValueDoubled/2;
finalXValueForCrop = final;
}
CGRect cropRectFinal = CGRectMake(cropRect.origin.x, finalXValueForCrop, cropRect.size.width, cropRect.size.height);
CGImageRef imageRef = CGImageCreateWithImageInRect([self.image CGImage], cropRectFinal);
UIImage *image2 = [[UIImage alloc]initWithCGImage:imageRef];
self.image = image2;
CGImageRelease(imageRef);
self.bottomHalfView.image = self.image;
if ([self.hasUserTakenAPhoto isEqual:@"YES"]) {
[self.takingPhotoView setHidden:YES];
self.image = [self screenshot];
[_afterPhotoView setHidden:NO];
}
}
所以基本上viewDidLoad方法运行并且会话开始，会话将相机看到的所有内容发送到captureOutput方法，然后一旦用户按下“拍照”按钮我们将self.hasUserTakenAPhoto的字符串值设置为YES，会话停止，并且由于self.hasUserTakenAPhoto现在等于YES，captureOutput方法将最后一个相机缓冲区放入我可以使用self.image对象。
我无法弄清楚这一点，因为就像我说的那样，使用后置摄像头时它有100％的工作时间。但是，使用前置摄像头时，它只能在50％的时间内工作。
我已经将问题缩小到self.hasUserTakenAPhoto在使用面对相机时没有足够快地更新到YES的事实，我知道因为如果你查看我的第二个代码我发布了它有NSLog(@"Has the user taken a photo?: %@", self.hasUserTakenAPhoto);的陈述。
如果此操作正常，并且用户刚刚按下按钮以捕获也停止会话的照片，则NSLog(@"Has the user taken a photo?: %@", self.hasUserTakenAPhoto);运行的最后一次将以正确的值YES打印。
但是，当它无法正常工作且更新速度不够快时，它最后一次运行时仍然会打印到值为null的日志。
为什么self.hasUserTakenAPhoto在使用前置摄像头时50％的时间内没有足够快速更新？即使我们无法弄清楚这一点，也没关系。我只需要帮助，然后提出替代解决方案。
感谢您的帮助。
答案 0 :(得分：0)
我认为这是一个调度问题。在方法的返回点
– captureOutput:didOutputSampleBuffer:fromConnection:
– captureOutput:didDropSampleBuffer:fromConnection:
添加CFRunLoopRun()