Question

I need to read images from NSDocumentDirectory to multiple uiimageview async so it won’t block the UI.
I know i can use perform selector in background to load a uiimage, but then how can i associate it with the dynamic uiimageview ?

Answer 1

One convenient way is to use blocks, something like:

[self loadFullImageAt:imagePath completion:^(UIIMage * image){
    self.imageView.image = image;
}];

Where you would load the image as data (since UIImage otherwise loads the image data deferred – when you first access it). It’s also a good idea to decompress the image while still in the background thread, so the main thread doesn’t have to do it when we first use the image.

- (void)loadFullImageAt:(NSString *)imageFilePath completion:(MBLoaderCompletion)completion {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        NSData *imageData = [NSData dataWithContentsOfFile:imageFilePath];
        UIImage *image = nil;
        if (imageData) {
            image = [[[UIImage alloc] initWithData:imageData] decodedImage];
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            completion(image);
        });
    });
}

The callback is defined as:

typedef void (^MBLoaderCompletion)(UIImage *image);

Here’s an UIImage category that implements the decompression code:

UIIMage+Decode.h

#import <UIKit/UIKit.h>


@interface UIImage (Decode)

- (UIImage *)decodedImage;

@end

UIIMage+Decode.m

#import "UIImage+Decode.h"


@implementation UIImage (Decode)

- (UIImage *)decodedImage {
    CGImageRef imageRef = self.CGImage;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 CGImageGetWidth(imageRef),
                                                 CGImageGetHeight(imageRef),
                                                 8,
                                                 // Just always return width * 4 will be enough
                                                 CGImageGetWidth(imageRef) * 4,
                                                 // System only supports RGB, set explicitly
                                                 colorSpace,
                                                 // Makes system don't need to do extra conversion when displayed.
                                                 kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
    CGColorSpaceRelease(colorSpace);
    if (!context) return nil;

    CGRect rect = (CGRect){CGPointZero,{CGImageGetWidth(imageRef), CGImageGetHeight(imageRef)}};
    CGContextDrawImage(context, rect, imageRef);
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(context);
    CGContextRelease(context);

    UIImage *decompressedImage = [[UIImage alloc] initWithCGImage:decompressedImageRef scale:self.scale orientation:self.imageOrientation];
    CGImageRelease(decompressedImageRef);
    return decompressedImage;
}

@end

The sample code provided here assumes that we’re using ARC.