关于屏幕截图时会报内存不足 的问题

捕捉视图的屏幕截图，保存图片在文件夹中：

CGSize size = self.view.bounds.size;
CGRect cropRect;
CGRect screenBounds = [[UIScreen mainScreen] bounds];
if([self isPad])
{
    cropRect = CGRectMake(145, 110, 476, 476);
}
else
{
    if (screenBounds.size.height ==568)
    {
        cropRect = CGRectMake(40, 69, 240, 240);
    }
    else
    {
        cropRect = CGRectMake(40, 62, 240, 240);
    }
}
/* Get the entire on screen map as Image */
UIGraphicsBeginImageContext(size);
[self.view.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage * mapImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
/* Crop the desired region */
CGImageRef imageRef = CGImageCreateWithImageInRect(mapImage.CGImage, cropRect);
UIImage * cropImage = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
/* Save the cropped image
 UIImageWriteToSavedPhotosAlbum(cropImage, nil, nil, nil);*/
//save to document folder
NSData * imageData = UIImageJPEGRepresentation(cropImage, 1.0);
NSArray* paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString* documentsDirectory = [paths objectAtIndex:0];
    imagename=[NSString stringWithFormat:@"Fff.jpg"];
NSString* fullPathToFile = [documentsDirectory stringByAppendingPathComponent:imagename];
////NSLog(@"full path %@",fullPathToFile);
[imageData writeToFile:fullPathToFile atomically:NO];

在截图15到20次后，就会出现内存空间不足的警告，然后应用就会崩溃。

不知道有没有更好的方法不引起这种内存问题？