然后切换到ViewController.m文件，在startStopAction:方法中添加下面代码以启用和暂停计时器：
- (IBAction)startStopAction:(id)sender
{
if (!self.isReading) {
...
[self.view bringSubviewToFront:self.scanView]; // display scanView
self.scanView.timer.fireDate = [NSDate distantPast]; //start timer
...
}
else {
[self stopScanning];
self.scanView.timer.fireDate = [NSDate distantFuture]; //stop timer
...
}
...
}
最后，再在viewWillAppear:的重写方法中添加下面代码：
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
self.scanView.timer.fireDate = [NSDate distantFuture];
}
可以运行看下：
2.3.4 从图片解析二维码
从iOS 8开始，可以使用Core Image框架中的CIDetector解析图片中的二维码。在这个应用中，我们通过点击Album按钮，从相册选取二维码来解析。
在写代码之前，需要在Info.plist文件中添加NSPhotoLibraryAddUsageDescription键及相应描述以访问相册：
然后在ViewController.m文件中添加UIImagePickerControllerDelegate和UINavigationControllerDelegate协议：
复制代码
@interface ViewController ()<AVCaptureMetadataOutputObjectsDelegate, UIImagePickerControllerDelegate, UINavigationControllerDelegate>
在实现部分找到readingFromAlbum:方法，添加下面代码以访问相册中的图片：
- (IBAction)readingFromAlbum:(id)sender
{
UIImagePickerController *picker = [[UIImagePickerController alloc] init];
picker.delegate = self;
picker.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
picker.allowsEditing = YES;
[self presentViewController:picker animated:YES completion:nil];
}
然后实现UIImagePickerControllerDelegate的imagePickerController:didFinishPickingMediaWithInfo:方法以解析选取的二维码图片：
- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary<NSString *,id> *)info
{
[picker dismissViewControllerAnimated:YES completion:nil];
UIImage *selectedImage = [info objectForKey:UIImagePickerControllerEditedImage];
CIImage *ciImage = [[CIImage alloc] initWithImage:selectedImage];
CIDetector *detector = [CIDetector detectorOfType:CIDetectorTypeQRCode context:nil options:@{CIDetectorAccuracy:CIDetectorAccuracyLow}];
NSArray *features = [detector featuresInImage:ciImage];
if (features.count > 0) {
CIQRCodeFeature *feature = features.firstObject;
NSString *message = feature.messageString;
// display message
[self displayMessage:message];
// play sound
if (self.audioPlayer) {
[self.audioPlayer play];
}
}
}
