QLThumbnailGenerationRequest无法从iPhone相机拍摄图生成缩略图求助

问题分析 #

你遇到的QLThumbnailErrorDomain Code=102错误对应QLThumbnailErrorUnsupportedDocumentType，说明缩略图生成器无法识别你保存的文件类型。核心原因是通过UIImageJPEGRepresentation转换并保存的图片丢失了原始图片的元数据，导致QL无法正确判定它为有效JPEG文件。

解决方案 #

方案一：直接使用相机返回的原始文件URL（推荐） #

相机拍摄的图片会自动保存到系统临时目录，无需自行转换保存，直接从UIImagePickerController的info字典中获取原始文件URL即可：

// 从info字典获取相机拍摄的原始图片URL
NSURL *originalImageURL = info[UIImagePickerControllerImageURL];

CGSize thumbnailSize = CGSizeMake(720, 720);
QLThumbnailGenerator *generator = [[QLThumbnailGenerator alloc] init];
QLThumbnailGenerationRequest *request = [[QLThumbnailGenerationRequest alloc] initWithFileAtURL:originalImageURL 
                                                                                           size:thumbnailSize 
                                                                                          scale:[UIScreen mainScreen].scale 
                                                                             representationTypes:QLThumbnailGenerationRequestRepresentationTypeThumbnail];

[generator generateBestRepresentationForRequest:request completionHandler:^(QLThumbnailRepresentation * _Nullable thumbnail, NSError * _Nullable error) {
    if (error) {
        NSLog(@"ERROR = %@", error.localizedDescription);
    } else {
        NSLog(@"SUCCESS");
        // 获取生成的缩略图
        UIImage *thumbnailImage = thumbnail.image;
    }
}];

方案二：自行保存UIImage时添加正确的UTI标识 #

如果必须将UIImage转换为Data保存，需要在创建QLThumbnailGenerationRequest时手动指定文件的UTI（统一类型标识符），确保QL能识别文件类型：

UIImage *source = info[UIImagePickerControllerOriginalImage];
NSData *imageData = UIImageJPEGRepresentation(source, 1.0);

NSString *directoryPath = NSTemporaryDirectory();
NSString *filePath = [directoryPath stringByAppendingPathComponent:@"CapturedImageFromCamera.jpg"];

NSError *error = nil;
BOOL success = [imageData writeToFile:filePath options:NSDataWritingAtomic error:&error];

if (success) {
    CGSize thumbnailSize = CGSizeMake(720, 720);
    QLThumbnailGenerator *generator = [[QLThumbnailGenerator alloc] init];
    
    // 手动指定文件UTI为JPEG类型
    QLThumbnailGenerationRequest *request = [[QLThumbnailGenerationRequest alloc] initWithFileAtURL:[NSURL fileURLWithPath:filePath] 
                                                                                               size:thumbnailSize 
                                                                                              scale:[UIScreen mainScreen].scale 
                                                                                         UTI:(NSString *)kUTTypeJPEG 
                                                                             representationTypes:QLThumbnailGenerationRequestRepresentationTypeThumbnail];

    [generator generateBestRepresentationForRequest:request completionHandler:^(QLThumbnailRepresentation * _Nullable thumbnail, NSError * _Nullable error) {
        if (error) {
            NSLog(@"ERROR = %@", error.localizedDescription);
        } else {
            NSLog(@"SUCCESS");
        }
    }];
}

注意：使用kUTTypeJPEG需要导入MobileCoreServices框架：

#import <MobileCoreServices/MobileCoreServices.h>

额外验证步骤 #

你可以在保存文件后，尝试用以下代码验证文件是否有效：

UIImage *testImage = [UIImage imageWithContentsOfFile:filePath];
if (!testImage) {
    NSLog(@"保存的图片文件损坏");
}

内容的提问来源于stack exchange，提问作者Adeel Ur Rehman