注意:沙盒緩存應在ZYXDownloadOperation中完成封裝不應該在代理中完成,后續(xù)完善

ZYXApp.h

#import <Foundation/Foundation.h>

@interface ZYXApp : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, copy) NSString *download;
@property (nonatomic, copy) NSString *icon;        
+ (instancetype)appWithDict:(NSDictionary *)dict;
@end

ZYXApp.m

#import "ZYXApp.h"

@implementation ZYXApp
+ (instancetype)appWithDict:(NSDictionary *)dict
{
    ZYXApp *app = [[self alloc] init];
    [app setValuesForKeysWithDictionary:dict]; // KVC
    return app;
}
@end

ZYXDownloadOperation.h

#import <Foundation/Foundation.h>

@class ZYXDownloadOperation;


@protocol ZYXDownloadOperationDelegate <NSObject>
@optional
- (void)downloadOperation:(ZYXDownloadOperation *)operation didFinishDownloadWithImage:(UIImage *)image;
@end


@interface ZYXDownloadOperation : NSOperation
@property (nonatomic, copy) NSString *urlString;
@property (nonatomic, strong) NSIndexPath *indexPath;
@property (nonatomic, weak) id<ZYXDownloadOperationDelegate> delegate; // 代理對象屬性使用weak
@end

ZYXDownloadOperation.m

#import "ZYXDownloadOperation.h"

@implementation ZYXDownloadOperation

/**
 *  自定義NSOperation的步驟很簡單
 *  重寫 - (void)main 方法，在里面實現(xiàn)想執(zhí)行的任務
 */
- (void)main{
#warning - 自己創(chuàng)建自動釋放池（因為如果是異步執(zhí)行，無法訪問主線程的自動釋放池）
    @autoreleasepool{
        NSURL *downloadUrl  = [NSURL URLWithString:self.urlString];
        NSData *data = [NSData dataWithContentsOfURL:downloadUrl]; // 這行會比較耗時
        UIImage *image      = [UIImage imageWithData:data];
        if ([self.delegate respondsToSelector:@selector(downloadOperation:didFinishDownloadWithImage:)]){
            // 線程間通信,NSOperation和GCD的混合使用,子線程獲取數(shù)據(jù)->主線程使用子線程獲取的數(shù)據(jù)
            dispatch_async(dispatch_get_main_queue(), ^{ // 回到主線程, 傳遞圖片數(shù)據(jù)給代理對象
                [self.delegate downloadOperation:self didFinishDownloadWithImage:image];
            });
        }
    }
}

@end

ViewController.m

#import "ViewController.h"

#import "ZYXApp.h"
#import "ZYXDownloadOperation.h"

@interface ViewController ()  <ZYXDownloadOperationDelegate>
@property (nonatomic, strong) NSArray *apps;
@property (nonatomic, strong) NSOperationQueue *queue;
/** key:url value:operation對象 */
@property (nonatomic, strong) NSMutableDictionary *operations;
/** key:url value:image對象*/
@property (nonatomic, strong) NSMutableDictionary *images;
@end

@implementation ViewController

#pragma mark - 懶加載
- (NSArray *)apps
{
    if (!_apps) {
        NSArray *dictArray = [NSArray arrayWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"apps.plist" ofType:nil]];
        
        NSMutableArray *appArray = [NSMutableArray array];
        for (NSDictionary *dict in dictArray) {
            ZYXApp *app = [ZYXApp appWithDict:dict];
            [appArray addObject:app];
        }
        _apps = appArray;
    }
    return _apps;
}

- (NSOperationQueue *)queue
{
    if (!_queue) {
        _queue = [[NSOperationQueue alloc] init];
        // 最大并發(fā)數(shù) == 3
        _queue.maxConcurrentOperationCount = 3;
    }
    return _queue;
}

- (NSMutableDictionary *)operations
{
    if (!_operations) {
        _operations = [NSMutableDictionary dictionary];
    }
    return _operations;
}

- (NSMutableDictionary *)images
{
    if (!_images) {
        _images = [NSMutableDictionary dictionary];
    }
    return _images;
}

#pragma mark - download task

#pragma mark - UITableViewDatasource 數(shù)據(jù)源方法
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{
    return self.apps.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    static NSString *ID = @"app";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:ID];
    if (!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                      reuseIdentifier:ID];
    }
    
    ZYXApp *app = self.apps[indexPath.row];
    cell.textLabel.text = app.name;
    cell.detailTextLabel.text = app.download;
    
    // 顯示圖片
    // 保證一個url對應一個ZYXDownloadOperation
    // 保證一個url對應UIImage對象
    
    UIImage *image = self.images[app.icon];
    if (image) { // 緩存中有圖片
        cell.imageView.image = image;
    }
    else { // 緩存中沒有圖片, 看沙盒緩存是否有
        // 獲得Library/Caches文件夾
        NSString *cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
        // 獲得文件名
        NSString *filename = [app.icon lastPathComponent];
        // 計算出文件的全路徑
        NSString *file = [cachesPath stringByAppendingPathComponent:filename];
        // 加載沙盒的文件數(shù)據(jù)
        NSData *data = [NSData dataWithContentsOfFile:file];
        
        if (data) { // 直接利用沙盒中圖片
            UIImage *image = [UIImage imageWithData:data];
            cell.imageView.image = image;
            // 存到字典中
            self.images[app.icon] = image;
        }
        else { // 下載圖片
            cell.imageView.image = [UIImage imageNamed:@"57437179_42489b0"];

            ZYXDownloadOperation *operation = self.operations[app.icon];
            if (operation) { // 正在下載
                // ... 暫時不需要做其他事
                
            } else { // 沒有正在下載
                // 創(chuàng)建操作
                operation = [[ZYXDownloadOperation alloc] init];
                operation.urlString = app.icon;
                operation.delegate = self;
                operation.indexPath = indexPath;
                [self.queue addOperation:operation]; // 異步下載
                self.operations[app.icon] = operation;
            }
        }
    }
    
    // SDWebImage : 專門用來下載圖片
    return cell;
}

#pragma mark - ZYXDownloadOperationDelegate 

- (void)downloadOperation:(ZYXDownloadOperation *)operation didFinishDownloadWithImage:(UIImage *)image{
    // 數(shù)據(jù)加載失敗
    if (image == nil) {
        [self.operations removeObjectForKey:operation.urlString];
        return;
    }
    
    // 1.移除執(zhí)行完畢的操作
    [self.operations removeObjectForKey:operation.urlString];
    
    if (image) {
        // 2.將圖片放到緩存中(images)
        self.images[operation.urlString] = image;
        
        // 3.刷新表格
        [self.tableView reloadRowsAtIndexPaths:@[operation.indexPath]
                              withRowAnimation:UITableViewRowAnimationNone];

        // 4.將圖片寫入沙盒
        NSString *cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
        NSIndexPath *indexPath = operation.indexPath;
        ZYXApp *app = self.apps[indexPath.row];
        NSString *filename = [app.icon lastPathComponent];
        NSString *file = [cachesPath stringByAppendingPathComponent:filename];
        
        NSData *data = UIImagePNGRepresentation(image);
        [data writeToFile:file atomically:YES];
    }
}

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{
    // 開始拖拽
    [self.queue setSuspended:YES];
}

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView
                     withVelocity:(CGPoint)velocity
              targetContentOffset:(inout CGPoint *)targetContentOffset{
    // 開始隊列
    [self.queue setSuspended:NO];
}

@end

沙盒緩存.png