将图像异步加载到 UIImage 中

Question

I'm developing an iOS 4 application with iOS 5.0 SDK and XCode 4.2.

I have to show some post blogs into a UITableView. When I have retreived all web service data, I use this method to create an UITableViewCell:

- (BlogTableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString* cellIdentifier = @"BlogCell";

    BlogTableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

    if (cell == nil)
    {
        NSArray* topLevelObjects =  [[NSBundle mainBundle] loadNibNamed:@"BlogTableViewCell" owner:nil options:nil];

        for(id currentObject in topLevelObjects)
        {
            if ([currentObject isKindOfClass:[BlogTableViewCell class]])
            {
                cell = (BlogTableViewCell *)currentObject;
                break;
            }
        }
    }

    BlogEntry* entry = [blogEntries objectAtIndex:indexPath.row];

    cell.title.text = entry.title;
    cell.text.text = entry.text;
    cell.photo.image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:entry.photo]]];

    return cell;
}

But this line:

cell.photo.image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:entry.photo]]];

it is so slow (entry.photo has a http url).

Is there any way to load that image asynchronously? I think it is difficult because tableView:cellForRowAtIndexPath is called very often.

Answer 1

I wrote a custom class to do just this, using blocks and GCD:

WebImageOperations.h

#import <Foundation/Foundation.h>

@interface WebImageOperations : NSObject {
}

// This takes in a string and imagedata object and returns imagedata processed on a background thread
+ (void)processImageDataWithURLString:(NSString *)urlString andBlock:(void (^)(NSData *imageData))processImage;
@end

WebImageOperations.m

#import "WebImageOperations.h"
#import <QuartzCore/QuartzCore.h>

@implementation WebImageOperations


+ (void)processImageDataWithURLString:(NSString *)urlString andBlock:(void (^)(NSData *imageData))processImage
{
    NSURL *url = [NSURL URLWithString:urlString];

    dispatch_queue_t callerQueue = dispatch_get_current_queue();
    dispatch_queue_t downloadQueue = dispatch_queue_create("com.myapp.processsmagequeue", NULL);
    dispatch_async(downloadQueue, ^{
        NSData * imageData = [NSData dataWithContentsOfURL:url];

        dispatch_async(callerQueue, ^{
            processImage(imageData);
        });
    });
    dispatch_release(downloadQueue);
}

@end

And in your ViewController

    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

    // Pass along the URL to the image (or change it if you are loading there locally)
    [WebImageOperations processImageDataWithURLString:entry.photo andBlock:^(NSData *imageData) {
    if (self.view.window) {
        UIImage *image = [UIImage imageWithData:imageData];

        cell.photo.image = image;
    }

    }];
}

It is very fast and will load the images without affecting the UI or scrolling speed of the TableView.

*** Note - This example assumes ARC is being used. If not, you will need to manage your own releases on objects)

Answer 2

In iOS 6 and later dispatch_get_current_queue gives deprecation warnings.

Here is an alternative that is a synthesis of the @ElJay answer above and the article by @khanlou here .

Create a category on UIImage:

UIImage+Helpers.h

@interface UIImage (Helpers)

+ (void) loadFromURL: (NSURL*) url callback:(void (^)(UIImage *image))callback;

@end

UIImage+Helpers.m

#import "UIImage+Helpers.h"

@implementation UIImage (Helpers)

+ (void) loadFromURL: (NSURL*) url callback:(void (^)(UIImage *image))callback {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
    dispatch_async(queue, ^{
        NSData * imageData = [NSData dataWithContentsOfURL:url];
        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *image = [UIImage imageWithData:imageData];
            callback(image);
        });
    });
}

@end

Answer 3

Take a look at SDWebImage:

https://github.com/rs/SDWebImage

It's a fantastic set of classes that handle everything for you.

Tim

Answer 4

Swift:

extension UIImage {

    // Loads image asynchronously
    class func loadFromURL(url: NSURL, callback: (UIImage)->()) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {

            let imageData = NSData(contentsOfURL: url)
            if let data = imageData {
                dispatch_async(dispatch_get_main_queue(), {
                    if let image = UIImage(data: data) {
                        callback(image)
                    }
                })
            }
        })
    }
}

Usage:

// First of all remove the old image (required for images in cells)
imageView.image = nil 

// Load image and apply to the view
UIImage.loadFromURL("http://...", callback: { (image: UIImage) -> () in
     self.imageView.image = image
})

Answer 5

Considering Failure Case.

- (void) loadFromURL: (NSURL*) url callback:(void (^)(UIImage *image))callback {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
    dispatch_async(queue, ^{
        NSError * error = nil;
        NSData * imageData = [NSData dataWithContentsOfURL:url options:0 error:&error];
        if (error)
            callback(nil);

        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *image = [UIImage imageWithData:imageData];
            callback(image);
        });
    });
}

Answer 6

Swift 4 | Async loading of image

Make a new class named ImageLoader.swift

import UIKit

class ImageLoader {

var cache = NSCache<AnyObject, AnyObject>()

class var sharedInstance : ImageLoader {
    struct Static {
        static let instance : ImageLoader = ImageLoader()
    }
    return Static.instance
}

func imageForUrl(urlString: String, completionHandler:@escaping (_ image: UIImage?, _ url: String) -> ()) {
        let data: NSData? = self.cache.object(forKey: urlString as AnyObject) as? NSData

        if let imageData = data {
            let image = UIImage(data: imageData as Data)
            DispatchQueue.main.async {
                completionHandler(image, urlString)
            }
            return
        }

    let downloadTask: URLSessionDataTask = URLSession.shared.dataTask(with: URL.init(string: urlString)!) { (data, response, error) in
        if error == nil {
            if data != nil {
                let image = UIImage.init(data: data!)
                self.cache.setObject(data! as AnyObject, forKey: urlString as AnyObject)
                DispatchQueue.main.async {
                    completionHandler(image, urlString)
                }
            }
        } else {
            completionHandler(nil, urlString)
        }
    }
    downloadTask.resume()
    }
}

To Use in your ViewController class:

ImageLoader.sharedInstance.imageForUrl(urlString: "https://www.logodesignlove.com/images/classic/apple-logo-rob-janoff-01.jpg", completionHandler: { (image, url) in
                if image != nil {
                    self.imageView.image = image
                }
            })

Answer 7

Yes it's relatively easy. The idea is something like:

1. Create a mutable array to hold your images
2. Populate the array with placeholders if you like (or NSNull objects if you don't)
3. Create a method to fetch your images asynchronously in the background
4. When an image has arrived, swap your placeholder with the real image and do a [self.tableView reloadData]

I have tested this approach many times and gives great results. If you need any help / examples with the async part (I recommend to use gcd for that) let me know.

Answer 8

you can easily do it perfectly if you use the sample code provided by Apple for this purpose: the sample code : Lazy Image

Just look at the delegate rowforcell and add icondownloader files to you project.

The only change you have to do is to change apprecord object with your object.

Answer 9

While SDWebImage and other 3rd party maybe great solutions, if you are not keen on using 3rd party APIs, you can develop your own solution too.

Refer to this tutorial about lazy loading which also talks about how should you model your data inside table view.

Answer 10

AsyncImage can synchronously load and display an image. is officially introduced after iOS 15

cell.photo = AsyncImage(url: URL(string: entry.photo))
    .frame(width: 200, height: 200)

It also supports:

• specify a custom placeholder using init(url:scale:content:placeholder:) .
• gain more control over the loading process, use the init(url:scale:transaction:content:)

See more in doc

Answer 11

Use below code snippet to loading image into imageview

func imageDownloading() {

DispatchQueue.global().async {

    let url = URL(string: "http://verona-api.municipiumstaging.it/system/images/image/image/22/app_1920_1280_4.jpg")!

    do {

        let data = try Data(contentsOf: url)

        DispatchQueue.main.async {

        self.imageView.image = UIImage(data: data)

        }

    } catch {
        print(error.localizedDescription)
    }
}
}

Answer 12

You'll probably have to subclass your UIImageView. I've recently made a simple project to explain this particular task - background asynchronous image loading - take a look at my project at GitHub. Specifically, look at KDImageView class.