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在我的应用程序中，我需要使用 Grand Central Dispatch (CGD) 在后台执行大量操作（调整一组图像的大小）。

我有EngineViewController一个应用程序的 rootViewController 类：

AppDelegate.m

EngineViewController * engineVc = [[EngineViewController alloc]init];
self.window.rootViewController = engineVC;

在我的viewDidAppear方法中，engineVC我有以下代码：

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //this method resize a set of image and save them in Document Folder of the app
        //this method needs about 40 seconds to perform all operations
        [[JSONParser sharedJSONParser]writeToDiskResizedImage]; 
        NSLog(@"Finished");

    });

}

方法[[JSONParser sharedJSONParser]writeToDiskResizedImage]返回后，NSLog在控制台中打印消息和应用程序崩溃。我所有的项目都是 ARC，JSONParser上课除外。在控制台中没有错误消息；我只有这个：

在此处输入图像描述

执行的代码[[JSONParser sharedJSONParser]writeToDiskResizedImage]如下：

CGFloat screenScale = [UIScreen mainScreen].scale;

NSString * imagePath = [[NSBundle mainBundle]pathForResource:imageName ofType:nil];
//UIImage * image = [UIImage imageWithContentsOfFile:imagePath];

UIImage * image = [[UIImage alloc]initWithContentsOfFile:imagePath];

CGSize newImageSize = [image resizeImageSettingWidth:(kArticleImageWidth * screenScale)];

UIGraphicsBeginImageContext(newImageSize);
[image drawInRect:CGRectMake(0,0,newImageSize.width,newImageSize.height)];
UIImage* newImage =UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

if (image) {
    [image release];
}

NSData *jpegData = UIImageJPEGRepresentation(newImage, 0.9f);
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsPath = [paths objectAtIndex:0]; //Get the docs directory

NSString * newImageName;
if ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] &&
    ([UIScreen mainScreen].scale == 2.0)) {

    newImageName = [NSString stringWithFormat:@"%@-article@2x.jpg", [imageName stringByDeletingPathExtension]];

}
else {

    newImageName = [NSString stringWithFormat:@"%@-article.jpg", [imageName stringByDeletingPathExtension]];
}

NSLog(@"newImageName %@", newImageName);

NSString *filePath = [documentsPath stringByAppendingPathComponent:newImageName]; //Add the file name

[jpegData writeToFile:filePath atomically:YES]; //Write the file

if (newImage) {
    [newImage release];
}

if (jpegData) {
    [jpegData release];

}
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