网络图片下载的实例

有一个plist描述图片信息：

<array>
    <dict>
        <key>name</key>
        <string>植物大战僵尸</string>
        <key>icon</key>
        <string>http://p16.qhimg.com/dr/48_48_/t0125e8d438ae9d2fbb.png</string>
        <key>download</key>
        <string>10311万</string>
    </dict>
    <dict>
        <key>name</key>
        <string>捕鱼达人2</string>
        <key>icon</key>
        <string>http://p19.qhimg.com/dr/48_48_/t0101e2931181bb540d.png</string>
        <key>download</key>
        <string>9982万</string>
    </dict>
...
</array>

创建一个字典模型：

AppItem.h

#import <Foundation/Foundation.h>

@interface AppItem : NSObject

@property (strong,nonatomic) NSString *name;
@property (strong,nonatomic) NSString *icon;
@property (strong,nonatomic) NSString *download;

+ (instancetype)itemWithDict:(NSDictionary*)dict;

@end

AppItem.m

#import "AppItem.h"

@implementation AppItem

+ (instancetype)itemWithDict:(NSDictionary*)dict
{
    AppItem *item = [[AppItem alloc]init];

    [item setValuesForKeysWithDictionary:dict];

    return item;
}

@end

ViewController.h

#import "ViewController.h"
#import "AppItem.h"

@interface ViewController ()

@property (strong,nonatomic)NSArray<AppItem*> *appItems;

@property (strong,nonatomic)NSMutableDictionary *imagesDict;

@property (strong,nonatomic)NSMutableDictionary *operationDict;

@property (strong,nonatomic)NSOperationQueue *queue;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
}

- (NSArray<AppItem *> *)appItems
{
    if(_appItems == nil)
    {
        NSArray *dicts = [NSArray arrayWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"apps.plist" ofType:nil]];

        NSMutableArray *items = [NSMutableArray array];

        for(NSDictionary *dict in dicts)
        {
            AppItem *appItem = [AppItem itemWithDict:dict];
            [items addObject:appItem];
        }

        _appItems = items;
    }
    return _appItems;
}

- (NSMutableDictionary *)imagesDict
{
    if(_imagesDict == nil)
    {
        _imagesDict = [NSMutableDictionary dictionary];
    }
    return _imagesDict;
}

- (NSMutableDictionary *)operationDict
{
    if(_operationDict == nil)
    {
        _operationDict = [NSMutableDictionary dictionary];
    }
    return _operationDict;
}

- (NSOperationQueue *)queue
{
    if(_queue == nil)
    {
        _queue = [[NSOperationQueue alloc]init];
    }
    return _queue;
}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

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

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellId = @"appCell";

    UITableViewCell *cell = [self.tableView dequeueReusableCellWithIdentifier:cellId];

    __block UIImage *iconImage = [self.imagesDict objectForKey:self.appItems[indexPath.row].name];
    if(iconImage == nil)
    {
        NSString *cachePath = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES)[0];
        NSString *fileName = [self.appItems[indexPath.row].icon lastPathComponent];
        NSString *fileFullPath = [cachePath stringByAppendingPathComponent:fileName];

        NSData *cacheData = [NSData dataWithContentsOfFile:fileFullPath];
        iconImage = [UIImage imageWithData:cacheData];

        if(iconImage == nil)
        {
            iconImage = [UIImage imageNamed:@"defaultImage"];

            __block NSBlockOperation *downloadOp = [self.operationDict objectForKey:self.appItems[indexPath.row].name];
            if(downloadOp == nil)
            {
                cell.imageView.image = [UIImage imageNamed:@"defaultImage"];
                downloadOp = [NSBlockOperation blockOperationWithBlock:^{
                    NSLog(@"%ld-----图片下载-----%@",indexPath.row,[NSThread currentThread]);

                    NSURL *url = [NSURL URLWithString:self.appItems[indexPath.row].icon];
                    NSData *downloadData = [NSData dataWithContentsOfURL:url];
                    UIImage *image = [UIImage imageWithData:downloadData];

                    if(image == nil)
                    {
                        NSLog(@"%ld-----图片错误，没能被下载-----%@",indexPath.row,[NSThread currentThread]);
                        [self.operationDict removeObjectForKey:self.appItems[indexPath.row].name];
                        return;
                    }

                    [NSThread sleepForTimeInterval:1.0];

                    [[NSOperationQueue mainQueue] addOperationWithBlock:^{

                        [self.tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];

                    }];

                    [self.imagesDict setObject:image forKey:self.appItems[indexPath.row].name];

                    [downloadData writeToFile:fileFullPath atomically:YES];

                    [self.operationDict removeObjectForKey:self.appItems[indexPath.row].name];
                }];

                [self.operationDict setObject:downloadOp forKey:self.appItems[indexPath.row].name];

                [self.queue addOperation:downloadOp];
            }
            else
            {
                NSLog(@"%ld-----图片已经在下载，不会重复下载-----%@",indexPath.row,[NSThread currentThread]);
            }
        }
        else
        {
            //cell.imageView.image = iconImage;

            NSLog(@"%ld-----图片在本地找到",indexPath.row);
        }
    }
    else
    {
        //cell.imageView.image = iconImage;

        NSLog(@"%ld-----图片已经存入缓存",indexPath.row);
    }

    cell.imageView.image = iconImage;
    cell.textLabel.text = self.appItems[indexPath.row].name;
    cell.detailTextLabel.text = self.appItems[indexPath.row].download;

    return cell;
}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    return 80;
}

- (void)didReceiveMemoryWarning
{
    [self.imagesDict removeAllObjects];

    [self.queue cancelAllOperations];
}

@end