研究過NSOperation後,想通過實戰更好的理解NSOperation,適用於對下載圖片不頻繁的項目,免得爲了一個小需求而導入比較重的框架。Demo(直通車)主要利用自定義NSOperation子類,同時借鑑了
AFNetworking、SDWebImage、YYKit
的部分思想來實現具有緩存支持的異步圖像下載器。
架構思想
整個架構共分爲三層:Manager(單例,主要來管理總體的控制)、Operation(進行隊列的控制)和UIImageView(對外)層。通過UIImageView的分類來通知Manager,Manager中註冊了清理操作通知,Manager接收到信號後再把信號下發到對應的Operation中,Operation進行隊列的操作。
UIImageView的分類主要對外提供接口,內部通過URL進行判斷當前操作是否需要下載,如果需要下載則通知Manager來進行下載,下載完成後設置image。UIImageView分類的部分代碼:
- (void)v_setImageWithUrlString:(NSString *)urlString title:(NSString *)title {
if (!urlString) {
VLog(@"下載地址爲空");
return;
}
if ([self.v_urlString isEqualToString:urlString]) {
VLog(@"%@ 兩次下載地址一樣的 沒必要重複下載",title);
return;
}
if (self.v_urlString && self.v_urlString.length > 0 && ![self.v_urlString isEqualToString:urlString]) {
VLog(@"取消之前的下載操作:%@---%@ \n%@---%@",self.v_title,title,self.v_urlString,urlString);
[[VWebImageManager sharedManager] cancelDownloadImageWithUrlString:self.v_urlString];
}
//記錄新操作開始下載
self.v_urlString = urlString;
self.v_title = title;
// 複用
self.image = nil;
[[VWebImageManager sharedManager] downloadImageWithUrlString:urlString completeHandle:^(UIImage *downloadImage,NSString *urlString) {
//下載完成 要置空
if ([urlString isEqualToString:self.v_urlString]) {
self.v_urlString = @"";
self.v_title = @"";
self.image = downloadImage;
}
} title:title];
}
- (void)v_setImageWithUrlString:(NSString *)urlString {
[self v_setImageWithUrlString:urlString title:@""];
}
Manager接收到需要下載(或者取消下載)的信號後,首先從內存獲取圖片,有則利用block直接返回,否則繼續從沙盒獲取圖片(內存的讀取速度比沙盒快,所以先讀內存),找到了則返回並把圖片存入緩存。如果還是沒有找到則進行判斷是否需要下載,如果當前URL已經在下載中,則不需要再次進行下載,否則通知Operation來進行創建下載操作。Manager部分代碼:
- (void)downloadImageWithUrlString:(NSString *)urlString completeHandle:(VCompleteHandle)completeHandle title:(NSString *)title {
//內存獲取圖片
UIImage *cacheImage = self.imageCacheDict[urlString];
if (cacheImage) {
VLog(@"從內存緩存獲取數據 %@",title);
completeHandle(cacheImage,urlString);
return;
}
//沙盒獲取圖片
NSString *cachePath = [urlString getDowloadImagePath];
cacheImage = [UIImage imageWithContentsOfFile:cachePath];
if (cacheImage) {
VLog(@"從沙盒緩存獲取數據 %@",title);
//沙盒圖片 存入緩存
[self.imageCacheDict setObject:cacheImage forKey:urlString];
completeHandle(cacheImage,urlString);
return ;
}
//對當前下載圖片判斷,是否需要創建操作
if (self.operationDict[urlString]) {
VLog(@"正在下載的回調Block %@的%@",title,completeHandle);
NSMutableArray *mArray = self.handleDict[urlString];
if (mArray == nil) {
mArray = [NSMutableArray arrayWithCapacity:1];
}
[mArray addObject:completeHandle];
[self.handleDict setObject:mArray forKey:urlString];
return;
}
// 創建操作 下載 --- 自定義
VWebImageDownloadOperation *downOp = [[VWebImageDownloadOperation alloc] initWithDownloadImageUrl:urlString completeHandle:^(NSData * _Nonnull imageData, NSString * _Nonnull v_urlString) {
UIImage *downloadImage = [UIImage imageWithData:imageData];
if (downloadImage) {
[self.imageCacheDict setObject:downloadImage forKey:urlString];
[self.operationDict removeObjectForKey:urlString];
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
completeHandle(downloadImage, v_urlString);
// 去取回調
if (self.handleDict[v_urlString]) {
NSMutableArray *mArray = self.handleDict[v_urlString];
for (VCompleteHandle completeHandle in mArray) {
completeHandle(downloadImage, v_urlString);
}
[self.handleDict removeObjectForKey:urlString];
}
}];
}
} title:title];
// 操作加入隊列
[self.queue addOperation:downOp];
// 操作緩存
[self.operationDict setObject:downOp forKey:urlString];
}
自定義NSOperation子類需要重寫Start等函數,這裏參考了SDWebImage的思想進行手動KVO觀察當前的狀態,防止自動KVO的影響。比如這裏可以根據相應的url進行取消當前任務,而此時GCD就無法滿足需求。自定義NSOperation子類部分代碼:
- (instancetype)initWithDownloadImageUrl:(NSString *)urlString completeHandle:(VDownCompleteHandle)completeHandle title:(NSString *)title{
if (self = [super init]) {
_urlString = urlString;
_executing = NO;
_finished = NO;
_cancelled = NO;
_lock = [NSRecursiveLock new];
_completeHandle = completeHandle;
_title = title;
}
return self;
}
使用
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
VCustomCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseID forIndexPath:indexPath];
VModel *model = self.dataArray[indexPath.row];
cell.titleLabel.text = model.title;
cell.moneyLabel.text = model.money;
[cell.imageView v_setImageWithUrlString:model.imageUrl];
// debug
// [cell.imageView v_setImageWithUrlString:model.imageUrl title:model.title];
return cell;
}
Demo地址:https://github.com/JBWangWork/VWebImage
該文章爲記錄本人的學習路程,也希望能夠幫助大家,知識共享,共同成長,共同進步!!!