Ios NSOperation类运行多个操作

所以我问了几个关于UICollectionView的问题。了解它的工作原理，我尝试实现延迟加载，将15个图像加载到视图控制器上。我发现了很多例子，…第一个和第三个例子只处理一个操作，第二个例子我认为根本不使用操作，只使用线程。我的问题是，是否可以使用NSOperation类并使用/重用操作？我读到您不能重新运行操作，但我认为您可以在再次初始化操作后重新运行。这是我的密码：

视图控制器：

UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc]init];
layout.sectionInset = UIEdgeInsetsMake(10, 20, 10, 20);
[layout setItemSize:CGSizeMake(75, 75)];
self.images = [[UICollectionView alloc]initWithFrame:CGRectMake(0, 230, self.view.frame.size.width, 200) collectionViewLayout:layout];
self.images.delegate = self;
self.images.dataSource = self;
[self.images registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellIdentifier"];
[self.view addSubview:self.images];
self.operation = [[NSOperationQueue alloc]init];
[self.operation addOperationWithBlock:^{
    NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"getgallery.php?user=%@", userId] relativeToURL:[NSURL URLWithString:@"http://www.mywebsite.com/"]];
    NSData *data = [NSData dataWithContentsOfURL:url];
//datasource for all images
    self.imagesGalleryPaths = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers error:nil];
    [[NSOperationQueue mainQueue]addOperationWithBlock:^{
        [self.images reloadData];
//reload collection view to place placeholders
    }];
}];

- (void)viewDidAppear:(BOOL)animated{
//get the visible cells right away
visibleCellPaths = [NSArray new];
visibleCellPaths = self.images.indexPathsForVisibleItems;
self.processedImages = [[NSMutableDictionary alloc]initWithCapacity:visibleCellPaths.count];
}
#pragma mark - collection view
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView{
return 1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
int i;
if (self.imagesGalleryPaths.count == 0)
    i = 25;
else
    i = self.imagesGalleryPaths.count;
return i;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
UICollectionViewCell *cell = [UICollectionViewCell new];
cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cellIdentifier" forIndexPath:indexPath];
cell.layer.borderWidth = 1;
cell.layer.borderColor = [[UIColor whiteColor]CGColor];
UIImageView *image = [[UIImageView alloc]init];
if (self.imagesGalleryPaths.count != 0) {
    if ([visibleCellPaths containsObject:indexPath]) {
        [self setUpDownloads:visibleCellPaths];
    }
    image.image = [UIImage imageNamed:@"default.png"];
    image.frame = CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height);
    [cell.contentView addSubview:image];
}
return cell;
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
visibleCellPaths = [NSArray new];
visibleCellPaths = self.images.indexPathsForVisibleItems;
[self setUpDownloads:visibleCellPaths];
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
visibleCellPaths = [NSArray new];
visibleCellPaths = self.images.indexPathsForVisibleItems;
[self setUpDownloads:visibleCellPaths];
}

- (void)setUpDownloads:(NSArray *)visiblePaths{
//I want to pass the visiblePaths to the NSOperation class if visible cells changed
GalleryOps *gallery = [[GalleryOps alloc]init];
//I will use a dictionary to keep track of which indexPaths are being downloaded...
//...so there are no duplicate downloads
}

厨房操作

@implementation GalleryOps

- (void)main{
    //would I initialize all the operations here and perform them?
}

---

显示空的GalleryOps类几乎毫无意义，因为我不知道如何使用多个操作初始化它。我知道我必须重写main方法，一旦我从URL获取了图像数据，我就需要更新UI，为此我需要一个委托方法……另一件事我还不知道如何做，但有很多例子可以说明这一点。我最大的问题是如何将可见单元格传递给这个类并运行多个操作？当新的可见单元格出现时，我将运行一个检查，查看哪些要取消，哪些要保留。有什么建议吗？提前谢谢

您可以使用队列和操作本身来管理多个操作。如果我没有看错您的问题，那么您有一个操作（从JSON获取图像URL列表）需要生成子操作。您可以通过让父操作将子操作添加到队列中，或者通过使用依赖操作（子操作将父操作作为依赖项）来实现这一点

对于您试图做的事情，您不需要将NSOperation子类化，NSBlockOperation应该满足您的需要。子类化NSOperation比看起来更复杂，因为它依赖于KVO（很容易出错）

但对于你问题的细节：

我的问题是，是否可以使用NSOperation类并使用/重用操作？我读到您不能重新运行操作，但我认为您可以在再次初始化操作后重新运行

如果您再次初始化它们，它们就是新对象（或者至少应该是）。NSO操作无法重新运行，因为它们具有内部状态——我上面提到的棘手的KVO位。一旦它们转到“finished”，该实例就不能返回到干净状态

操作应该是相当轻量级的对象，重用它们不应该有任何显著的价值，并且存在大量的潜在问题。创建新的操作应该是一条路

---

Apple示例代码“”可能会为您提供一些提示，说明如何完成您正在尝试的操作。

基于

的图像延迟加载操作的组成元素可能包括：

创建用于下载操作的专用
NSOperationQueue
。通常，这是配置为4或5的
maxConcurrentOperationCount
，这样您可以享受并发性，但不会超过并发网络操作的最大数量

如果您不使用此
maxConcurrentOperationCount
，且网络连接速度较慢（如蜂窝网络），则可能会导致网络请求超时

具有支持集合视图或表视图的模型对象（例如数组）。这通常只有图像的一些标识符（例如URL），而不是图像本身

实现缓存机制来存储下载的图像，以防止需要重新下载已下载的图像。有些实现只进行基于内存的缓存（通过
NSCache
）。其他（例如）将执行两层缓存，内存（
NSCache
，以获得最佳性能）和辅助永久性存储缓存（这样，当内存压力迫使您清除
NSCache
，您仍然在设备上保存了一个格式副本，因此您不必从网络中重新检索它）。其他人（例如）依靠
NSURLCache
将来自服务器的响应缓存到持久存储中

编写用于下载单个图像的
NSOperation
子类。您希望使此操作可取消。这意味着两种不同的设计考虑


首先，关于操作本身，您可能希望执行并发操作（请参阅《并发编程指南》中的部分）

第二，关于网络请求，您需要一个可取消的网络请求。如果使用
NSURLConnection
，这意味着使用基于委托的格式副本（不是任何方便的方法）。如果使用
NSURLConnection
，诀窍是必须在持续的运行循环中安排它。有许多技巧可以实现这一点，但最简单的是在主运行循环中调度它（使用
scheduleinrunlop:forMode:
）（尽管有更优雅的方法），即使您将从
NSOperationQueue
中的操作运行它。就我个人而言，我为此目的启动了一个新的专用线程（像AFNetworking一样），但主运行循环更简单，适合这种过程

如果使用
NSURLSession
，这个过程可能会稍微容易一些，因为您可以不使用
dataTaskWithRequest
的完成块格式副本，而不必进入基于代理的实现（如果您不想）。但这只是iOS 7+版本（如果您需要处理身份验证质询请求之类的奇特操作，那么最终还是会采用基于代理的方法）

结合这两个先前的点，定制的
NSOperation
子类将检测何时取消操作，然后取消网络请求并完成操作


顺便说一句，操作实例永远不会被重用。为正在下载的每个映像创建一个新的操作实例UICollectionViewCell *cell = [UICollectionViewCell new];
cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cellIdentifier" forIndexPath:indexPath];
cell.layer.borderWidth = 1;
cell.layer.borderColor = [[UIColor whiteColor]CGColor];
UIImageView *image = [[UIImageView alloc]init];
if (self.imagesGalleryPaths.count != 0) {
    image.image = [UIImage imageNamed:@"profile_default.png"];
    image.frame = CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height);
    [cell.contentView addSubview:image];
}
return cell;

if (self.imagesGalleryPaths.count != 0) {
    [[NSOperationQueue mainQueue]addOperationWithBlock:^{
        [self.images reloadData];
        [self startOperationForVisibleCells];
    }];
}

[self.operation addOperationWithBlock:^{
    int i=0;
    while (i < visibleCellPaths.count) {
        NSIndexPath *indexPath = [visibleCellPaths objectAtIndex:i];
        if (![self.imageGalleryData.allKeys containsObject:indexPath]) {
            NSURL *url = [@"myurl"];
            NSData *responseData = [NSData dataWithContentsOfURL:url];
            if (responseData != nil) {
                [self.imageGalleryData setObject:responseData forKey:indexPath];
                [[NSOperationQueue mainQueue]addOperationWithBlock:^{
                    UICollectionViewCell *cell = [self.images cellForItemAtIndexPath:indexPath];
                    UIImageView *image = [UIImageView new];
                    image.image = [UIImage imageWithData:responseData];
                    image.frame = CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height);
                    [cell.contentView addSubview:image];
                }];
            }
        }
        i++;
    }
}];

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
visibleCellPaths = [NSArray new];
visibleCellPaths = self.images.indexPathsForVisibleItems;
for (int i=0; i<visibleCellPaths.count; i++) {
    NSIndexPath *indexPath = [visibleCellPaths objectAtIndex:i];
    if ([self.imageGalleryData.allKeys containsObject:indexPath]) {
        UICollectionViewCell *cell = [self.images cellForItemAtIndexPath:indexPath];
        UIImageView *image = [UIImageView new];
        image.image = [UIImage imageWithData:[self.imageGalleryData objectForKey:indexPath]];
        image.frame = CGRectMake(0, 0, cell.frame.size.width, cell.frame.size.height);
        [cell.contentView addSubview:image];
    }else{
        [self startOperationForVisibleCells];
    }
}
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
//same functions as previous
}

static NSString * const kCellIdentifier = @"CustomCellIdentifier";

- (void)viewDidLoad
{
    [super viewDidLoad];

    UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc]init];
    layout.sectionInset = UIEdgeInsetsMake(10, 20, 10, 20);
    [layout setItemSize:CGSizeMake(75, 75)];

    // renamed `images` collection view to `collectionView` to follow common conventions

    self.collectionView = [[UICollectionView alloc]initWithFrame:CGRectMake(0, 230, self.view.frame.size.width, 200) collectionViewLayout:layout];
    self.collectionView.delegate = self;
    self.collectionView.dataSource = self;

    // you didn't show where you instantiated this in your examples, but I'll do it here

    self.imageGalleryData = [NSMutableDictionary dictionary];

    // register a custom class, not `UICollectionViewCell`

    [self.collectionView registerClass:[CustomCell class] forCellWithReuseIdentifier:kCellIdentifier];
    [self.view addSubview:self.collectionView];

    // (a) change queue variable name;
    // (b) set maxConcurrentOperationCount to prevent timeouts

    self.queue = [[NSOperationQueue alloc]init];
    self.queue.maxConcurrentOperationCount = 5;

    [self.queue addOperationWithBlock:^{
        NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"getgallery.php?user=%@", userId] relativeToURL:[NSURL URLWithString:@"http://www.mywebsite.com/"]];
        NSData *data = [NSData dataWithContentsOfURL:url];
        //datasource for all images
        self.imagesGalleryPaths = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers error:nil];
        [[NSOperationQueue mainQueue]addOperationWithBlock:^{
            [self.collectionView reloadData];
        }];
    }];
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return [self.imagesGalleryPaths count];          // just use whatever is the right value here, don't make this unnecessarily smaller
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CustomCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:kCellIdentifier forIndexPath:indexPath];

    NSString *key = self.imagesGalleryPaths[indexPath.row];                 // I don't know whether this was simply array, or some nested structure, so tweak this accordingly
    UIImage *image = self.imageGalleryData[key]; 

    if (image) {
        cell.imageView.image = image;                                       // if we have image already, just use it
    } else {
        cell.imageView.image = [UIImage imageNamed:@"profile_default.png"]; // otherwise set the placeholder ...

        [self.queue addOperationWithBlock:^{                                // ... and initiate the asynchronous retrieval
            NSURL *url = [NSURL URLWithString:...];                         // build your URL from the `key` as appropriate
            NSData *responseData = [NSData dataWithContentsOfURL:url];
            if (responseData != nil) {
                UIImage *downloadedImage = [UIImage imageWithData:responseData];
                if (downloadedImage) {
                    [[NSOperationQueue mainQueue]addOperationWithBlock:^{
                        self.imageGalleryData[key] = downloadedImage;
                        CustomCell *updateCell = (id)[collectionView cellForItemAtIndexPath:indexPath];
                        if (updateCell) {
                            updateCell.imageView.image = downloadedImage;
                        }
                    }];
                }
            }
        }];
    }

    return cell;
}

// don't forget to purge your gallery data if you run low in memory

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    [self.imageGalleryData removeAllObjects];
}

//  CustomCell.h

#import <UIKit/UIKit.h>

@interface CustomCell : UICollectionViewCell

@property (nonatomic, weak) UIImageView *imageView;

@end

//  CustomCell.m

#import "CustomCell.h"

@implementation CustomCell

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        self.layer.borderWidth = 1;
        self.layer.borderColor = [[UIColor whiteColor]CGColor];

        UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, frame.size.width, frame.size.height)];
        [self addSubview:imageView];
        _imageView = imageView;
    }
    return self;
}
    
@end