自定义NSOperation的话，只是需要将要下载图片的操作下载它的main方法里面，考虑到，图片下载完毕，需要回传到控制器里，这里可以采用block，也可以采用代理的方式实现，我采用的是代理的方式实现的。
重点应该是如何避免同一个url的图片被重复下载？！事实上，可以有这样两个字典，key值是图片的url，value的话，一个字典可以是当前的operation对象，表示，这个url对应的图片正在下载，如果传入图片的url，value不为空的话，说明图片正在下载，那么就不需要再重复下载。另一个字典，可以是存放下载好的图片，也就是常说的缓存，如果一个url取出来的value不为nil，那么这张图片就存在缓存中，不需要再次下载。当然，别忘了设置占位图片~~
这是model的代码：
#import <Foundation/Foundation.h>
@interface ZYApp : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, copy) NSString *icon;
@property (nonatomic, copy) NSString *download;
+ (id)appWithDict:(NSDictionary *)dict;
- (id)initWithDict:(NSDictionary *)dict;
@end
#import "ZYApp.h"
@implementation ZYApp
- (id)initWithDict:(NSDictionary *)dict
{
if (self = [super init]){
[self setValuesForKeysWithDictionary:dict];
// NSLog(@"%@----%@---%@",self.icon,self.name,self.download);
}
return self;
}
+ (id)appWithDict:(NSDictionary *)dict
{
return [[self alloc] initWithDict:dict];
}
@end
这是自定义的Operation代码：
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
@class ZYDownLoadImageOperation;
@protocol ZYDownLoadImageOperationDelegate <NSObject>
@optional
- (void)DownLoadImageOperation:(ZYDownLoadImageOperation *)operation didFinishDownLoadImage:(UIImage *)image;
@end
@interface ZYDownLoadImageOperation : NSOperation
@property (nonatomic, weak) id<ZYDownLoadImageOperationDelegate> delegate;
@property (nonatomic, copy) NSString *url;
@property (nonatomic, strong) NSIndexPath *indexPath;
@end
#import "ZYDownLoadImageOperation.h"
@implementation ZYDownLoadImageOperation
- (void)main //重写main方法即可
{
@autoreleasepool { //在子线程中，并不会自动添加自动释放池，所以，手动添加，免得MARC的情况下，出现内存泄露的问题
NSURL *DownLoadUrl = [NSURL URLWithString:self.url];
if (self.isCancelled) return; //如果下载操作被取消，那么就无需下面操作了
NSData *data = [NSData dataWithContentsOfURL:DownLoadUrl];
if (self.isCancelled) return;
UIImage *image = [UIImage imageWithData:data];
if (self.isCancelled) return;
if ([self.delegate respondsToSelector:@selector(DownLoadImageOperation:didFinishDownLoadImage:)]) {
dispatch_async(dispatch_get_main_queue(), ^{ //回到主线程，更新UI
[self.delegate DownLoadImageOperation:self didFinishDownLoadImage:image];
});
}
}
}
@end
这是控制器代码：
#import <UIKit/UIKit.h>
@interface ViewController : UIViewController
@end
#import "ViewController.h"
#import "ZYApp.h"
#import "ZYDownLoadImageOperation.h"
@interface ViewController () <UITableViewDataSource, UITableViewDelegate, ZYDownLoadImageOperationDelegate, UIScrollViewDelegate>
@property (weak, nonatomic) IBOutlet UITableView *tableView;
@property (nonatomic, strong) NSArray *apps;
@property (nonatomic, strong) NSOperationQueue *queue;
// key:图片的url values: 相对应的operation对象 （判断该operation下载操作是否正在执行，当同一个url地址的图片正在下载，那么不需要再次下载，以免重复下载，当下载操作执行完，需要移除）
@property (nonatomic, strong) NSMutableDictionary *operations;
// key:图片的url values: 相对应的图片 （缓存，当下载操作完成，需要将所下载的图片放到缓存中，以免同一个url地址的图片重复下载）
@property (nonatomic, strong) NSMutableDictionary *images;
@end
@implementation ViewController
#define ZYCellIdentifier @"ZYCellIdentifier"
- (NSArray *)apps
{
if (_apps == nil) {
NSArray *dictArray = [NSArray arrayWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"apps.plist" ofType:nil]];
NSMutableArray *tmpArray = [NSMutableArray array];
for (NSDictionary *dict in dictArray) {
ZYApp *app = [ZYApp appWithDict:dict];
[tmpArray addObject:app];
}
_apps = tmpArray;
}
return _apps;
}
- (NSOperationQueue *)queue
{
if (_queue == nil) {
_queue = [[NSOperationQueue alloc] init];
_queue.maxConcurrentOperationCount = 3;
}
return _queue;
}
- (NSMutableDictionary *)operations
{
if (_operations == nil) {
_operations = [NSMutableDictionary dictionary];
}
return _operations;
}
- (NSMutableDictionary *)images
{
if (_images == nil) {
_images = [NSMutableDictionary dictionary];
}
return _images;
}
- (void)viewDidLoad {
[super viewDidLoad];
[self setupView];
}
- (void)setupView
{
self.tableView.delegate = self;
self.tableView.dataSource = self;
}
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return self.apps.count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:ZYCellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:ZYCellIdentifier];
}
ZYApp *app = self.apps[indexPath.row];
cell.textLabel.text = app.name;
cell.detailTextLabel.text = app.download;
UIImage *image = self.images[app.icon];
if (image) {
cell.imageView.image = image;
}
else{
cell.imageView.image = [UIImage imageNamed:@"TestMam"];
ZYDownLoadImageOperation *operation = self.operations[app.icon];
if (operation) { //正在下载（可以在里面取消下载）
}
else{ //没有在下载
operation = [[ZYDownLoadImageOperation alloc] init];
operation.delegate = self;
operation.url = app.icon;
operation.indexPath = indexPath;
[self.queue addOperation:operation]; //异步下载
self.operations[app.icon] = operation; //加入字典，表示正在执行此次操作
}
}
return cell;
}
#pragma mark -- ZYDownLoadImageOperationDelegate
- (void)DownLoadImageOperation:(ZYDownLoadImageOperation *)operation didFinishDownLoadImage:(UIImage *)image
{
[self.operations removeObjectForKey:operation.url]; //下载操作完成，所以把它清掉，表示没有正在下载
if (image){
self.images[operation.url] = image; //下载完毕，放入缓存，免得重复下载
[self.tableView reloadRowsAtIndexPaths:@[operation.indexPath] withRowAnimation:UITableViewRowAnimationNone];
}
}
#pragma mark --- UIScrollViewDelegate
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
[self.queue setSuspended:YES]; //在拖拽的时候，停止队列下载
}
- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset
{
[self.queue setSuspended:NO]; //在停止拖拽的时候，开始队列下载
}
@end
这是此次mode代码github地址：https://github.com/wzpziyi1/CustomOperation