我们用AFNetworking小试牛刀，写一个简单的下载器来演示功能。
前言
为什么AFNetworking能够成为顶级框架？我们究竟该如何领悟它的精髓所在？这都是很难的问题。安全，高效，流畅，这3个特性缺一不可。假如我们要封装一个通用的网络框架，提供一个文件下载器是很有必要的。按照 管理编程原则 ，这个下载管理器应该管理所有的下载任务和依据。
这是一个简单的下载器，只为了功能演示
下载器提供的功能
- 根据一个url下载文件 我们下载一个文件，最重要的就是url，因此我们应该把这个url作为下载的唯一标识。
- 提供下载进度 为了增加用户体验，往往在下载文件的同时，展示一个下载进度告诉用户当前的下载情况，有的人喜欢使用bytesWriten/totalBytesWriten/totalExpectedBytesWriten，但AFNetworking中使用的都是NSProgress。因此，我们也采用NSProgress表示进度。
- 下载完成的回调 通知下载完成的方式有通知/代理/Block，我们采用的是Block。
- 下载失败的回调 同上
- 根据url获取下载对象 我们把下载的对象包装成了MCDownloadReceipt，我能够在MCDownloadReceipt对象中获取到我们需要的所有内容。
- 回复/暂停/取消 下载任务 这些功能，我们使用协议来实现。
- 下载限制和顺序 我们能够自定义同时下载文件的个数，默认为4个。能够自定义等待队列中的任务是先进先出还是后进先出。
设计思路
写一个下载器，一定需要一个对象来描述下载的文件。在这个下载器中，我们使用MCDownloadReceipt。既然是一个信息的载体，那么从设计角度来说，我们应该使用它来存储跟文件相关的内容，不应该让他完成其他更多的事情，比如说开始，暂停等等。
MCDownloadReceipt使用归档进行本地化存储。
核心下载使用NSURLSession实现，下边我们会介绍详情。
MCDownloadReceipt
MCDownloadReceipt的主要功能是用于记录下载信息。即使下载未完成，也能在MCDownloadReceipt的filePath路径下找个这个文件。
我们先来看看暴露出来的头文件信息：
-
NSString *url作为MCDownloadReceipt的唯一标识。 -
NSString *filePathMCDownloadReceipt的文件索引。 -
NSString *filenameMCDownloadReceipt的文件名，命名规则为：把url进行MD5编码后作为文件名，url中如果有后缀，就拼接后缀。 -
MCDownloadState stateMCDownloadReceipt的状态- MCDownloadStateNone,
- MCDownloadStateWillResume,
- MCDownloadStateDownloading,
- MCDownloadStateSuspened,
- MCDownloadStateCompleted,
- MCDownloadStateFailed
-
long long totalBytesWritten总共写入的数据的大小 -
totalBytesExpectedToWrite文件总大小 -
NSOutputStream *stream用于把数据写入到路径中
1.获取filename
通常我们把文件的下载URL进行MD5编码后在拼接上后缀名来作为本地文件的名称。
把一个字符串转为MD5字符串：
static NSString * getMD5String(NSString *str) {
if (str == nil) return nil;
const char *cstring = str.UTF8String;
unsigned char bytes[CC_MD5_DIGEST_LENGTH];
CC_MD5(cstring, (CC_LONG)strlen(cstring), bytes);
NSMutableString *md5String = [NSMutableString string];
for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i++) {
[md5String appendFormat:@"%02x", bytes[i]];
}
return md5String;
}
拼接名称：
- (NSString *)filename {
if (_filename == nil) {
NSString *pathExtension = self.url.pathExtension;
if (pathExtension.length) {
_filename = [NSString stringWithFormat:@"%@.%@", getMD5String(self.url), pathExtension];
} else {
_filename = getMD5String(self.url);
}
}
return _filename;
}
2.获取filePath
首先我们要获取一个缓存的路径：
NSString * const MCDownloadCacheFolderName = @"MCDownloadCache";
static NSString * cacheFolder() {
NSFileManager *filemgr = [NSFileManager defaultManager];
static NSString *cacheFolder;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
if (!cacheFolder) {
NSString *cacheDir = NSHomeDirectory();
cacheFolder = [cacheDir stringByAppendingPathComponent:MCDownloadCacheFolderName];
}
NSError *error = nil;
if(![filemgr createDirectoryAtPath:cacheFolder withIntermediateDirectories:YES attributes:nil error:&error]) {
NSLog(@"Failed to create cache directory at %@", cacheFolder);
cacheFolder = nil;
}
});
return cacheFolder;
}
拼接路径和文件名：
- (NSString *)filePath {
NSString *path = [cacheFolder() stringByAppendingPathComponent:self.filename];
if (![path isEqualToString:_filePath] ) {
if (_filePath && ![[NSFileManager defaultManager] fileExistsAtPath:_filePath]) {
NSString *dir = [_filePath stringByDeletingLastPathComponent];
[[NSFileManager defaultManager] createDirectoryAtPath:dir withIntermediateDirectories:YES attributes:nil error:nil];
}
_filePath = path;
}
return _filePath;
}
3.获取文件的大小
获取某个路径下文件的大小：
static unsigned long long fileSizeForPath(NSString *path) {
signed long long fileSize = 0;
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:path]) {
NSError *error = nil;
NSDictionary *fileDict = [fileManager attributesOfItemAtPath:path error:&error];
if (!error && fileDict) {
fileSize = [fileDict fileSize];
}
}
return fileSize;
}
获取本对象的文件大小：
- (long long)totalBytesWritten {
return fileSizeForPath(self.filePath);
}
4.初始化stream
- (NSOutputStream *)stream
{
if (_stream == nil) {
_stream = [NSOutputStream outputStreamToFileAtPath:self.filePath append:YES];
}
return _stream;
}
5.设置progress
- (NSProgress *)progress {
if (_progress == nil) {
_progress = [[NSProgress alloc] initWithParent:nil userInfo:nil];
}
_progress.totalUnitCount = self.totalBytesExpectedToWrite;
_progress.completedUnitCount = self.totalBytesWritten;
return _progress;
}
6.初始化和归档
- (instancetype)initWithURL:(NSString *)url {
if (self = [self init]) {
self.url = url;
self.totalBytesExpectedToWrite = 1;
}
return self;
}
#pragma mark - NSCoding
- (void)encodeWithCoder:(NSCoder *)aCoder
{
[aCoder encodeObject:self.url forKey:NSStringFromSelector(@selector(url))];
[aCoder encodeObject:self.filePath forKey:NSStringFromSelector(@selector(filePath))];
[aCoder encodeObject:@(self.state) forKey:NSStringFromSelector(@selector(state))];
[aCoder encodeObject:self.filename forKey:NSStringFromSelector(@selector(filename))];
[aCoder encodeObject:@(self.totalBytesWritten) forKey:NSStringFromSelector(@selector(totalBytesWritten))];
[aCoder encodeObject:@(self.totalBytesExpectedToWrite) forKey:NSStringFromSelector(@selector(totalBytesExpectedToWrite))];
}
- (id)initWithCoder:(NSCoder *)aDecoder
{
self = [super init];
if (self) {
self.url = [aDecoder decodeObjectForKey:NSStringFromSelector(@selector(url))];
self.filePath = [aDecoder decodeObjectForKey:NSStringFromSelector(@selector(filePath))];
self.state = [[aDecoder decodeObjectOfClass:[NSNumber class] forKey:NSStringFromSelector(@selector(state))] unsignedIntegerValue];
self.filename = [aDecoder decodeObjectForKey:NSStringFromSelector(@selector(filename))];
self.totalBytesWritten = [[aDecoder decodeObjectOfClass:[NSNumber class] forKey:NSStringFromSelector(@selector(totalBytesWritten))] unsignedIntegerValue];
self.totalBytesExpectedToWrite = [[aDecoder decodeObjectOfClass:[NSNumber class] forKey:NSStringFromSelector(@selector(totalBytesExpectedToWrite))] unsignedIntegerValue];
}
return self;
}
MCDownloadControlDelegate
是这样的，如果我们要给某个对象扩展一类的功能或者方法，那么我们最好使用协议。在AFNetworking的AFURLResponseSerialization和AFURLRequestSerialization就是最好的例子。
@protocol MCDownloadControlDelegate <NSObject>
- (void)resumeWithURL:(NSString * _Nonnull)url;
- (void)resumeWithDownloadReceipt:(MCDownloadReceipt * _Nonnull)receipt;
- (void)suspendWithURL:(NSString * _Nonnull)url;
- (void)suspendWithDownloadReceipt:(MCDownloadReceipt * _Nonnull)receipt;
- (void)removeWithURL:(NSString * _Nonnull)url;
- (void)removeWithDownloadReceipt:(MCDownloadReceipt * _Nonnull)receipt;
@end
实现部分：
#pragma mark - MCDownloadControlDelegate
- (void)resumeWithURL:(NSString *)url {
if (url == nil) return;
MCDownloadReceipt *receipt = [self downloadReceiptForURL:url];
[self resumeWithDownloadReceipt:receipt];
}
- (void)resumeWithDownloadReceipt:(MCDownloadReceipt *)receipt {
if ([self isActiveRequestCountBelowMaximumLimit]) {
[self startTask:self.tasks[receipt.url]];
}else {
receipt.state = MCDownloadStateWillResume;
[self saveReceipts:self.allDownloadReceipts];
[self enqueueTask:self.tasks[receipt.url]];
}
}
- (void)suspendAll {
for (NSURLSessionDownloadTask *task in self.queuedTasks) {
[task suspend];
MCDownloadReceipt *receipt = [self downloadReceiptForURL:task.taskDescription];
receipt.state = MCDownloadStateSuspened;
}
[self saveReceipts:self.allDownloadReceipts];
}
-(void)suspendWithURL:(NSString *)url {
if (url == nil) return;
MCDownloadReceipt *receipt = [self downloadReceiptForURL:url];
[self suspendWithDownloadReceipt:receipt];
}
- (void)suspendWithDownloadReceipt:(MCDownloadReceipt *)receipt {
[self updateReceiptWithURL:receipt.url state:MCDownloadStateSuspened];
NSURLSessionDataTask *task = self.tasks[receipt.url];
if (task) {
[task suspend];
}
}
- (void)removeWithURL:(NSString *)url {
if (url == nil) return;
MCDownloadReceipt *receipt = [self downloadReceiptForURL:url];
[self removeWithDownloadReceipt:receipt];
}
- (void)removeWithDownloadReceipt:(MCDownloadReceipt *)receipt {
NSURLSessionDataTask *task = self.tasks[receipt.url];
if (task) {
[task cancel];
}
[self.queuedTasks removeObject:task];
[self safelyRemoveTaskWithURLIdentifier:receipt.url];
[self.allDownloadReceipts removeObject:receipt];
[self saveReceipts:self.allDownloadReceipts];
NSFileManager *fileManager = [NSFileManager defaultManager];
[fileManager removeItemAtPath:receipt.filePath error:nil];
}
MCDownloadManager
初始化MCDownloadManager跟AFNetworking中AFImageDownloader的初始化很像，做一些网络配置。参数配置。我们规定下载任务的创建都放在一个专有的同步队列中完成。我们还要监听applicationWillTerminate 和 applicationDidReceiveMemoryWarning这两个通知，并在通知方法中，暂停多有的下载任务。
初始化示例代码：
+ (NSURLSessionConfiguration *)defaultURLSessionConfiguration {
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
configuration.HTTPShouldSetCookies = YES;
configuration.HTTPShouldUsePipelining = NO;
configuration.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
configuration.allowsCellularAccess = YES;
configuration.timeoutIntervalForRequest = 60.0;
return configuration;
}
- (instancetype)init {
NSURLSessionConfiguration *defaultConfiguration = [self.class defaultURLSessionConfiguration];
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
queue.maxConcurrentOperationCount = 1;
NSURLSession *session = [NSURLSession sessionWithConfiguration:defaultConfiguration delegate:self delegateQueue:queue];
return [self initWithSession:session
downloadPrioritization:MCDownloadPrioritizationFIFO
maximumActiveDownloads:4 ];
}
- (instancetype)initWithSession:(NSURLSession *)session downloadPrioritization:(MCDownloadPrioritization)downloadPrioritization maximumActiveDownloads:(NSInteger)maximumActiveDownloads {
if (self = [super init]) {
self.session = session;
self.downloadPrioritizaton = downloadPrioritization;
self.maximumActiveDownloads = maximumActiveDownloads;
self.queuedTasks = [[NSMutableArray alloc] init];
self.tasks = [[NSMutableDictionary alloc] init];
self.activeRequestCount = 0;
NSString *name = [NSString stringWithFormat:@"com.mc.downloadManager.synchronizationqueue-%@", [[NSUUID UUID] UUIDString]];
self.synchronizationQueue = dispatch_queue_create([name cStringUsingEncoding:NSASCIIStringEncoding], DISPATCH_QUEUE_SERIAL);
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillTerminate:) name:UIApplicationWillTerminateNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidReceiveMemoryWarning:) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
}
return self;
}
+ (instancetype)defaultInstance {
static MCDownloadManager *sharedInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedInstance = [[self alloc] init];
});
return sharedInstance;
}
初始化完成后，我们需要在MCDownloadManager中拿到所有的下载的数据，以及能够保存这些数据到本地。
示例代码：
- (NSMutableArray *)allDownloadReceipts {
if (_allDownloadReceipts == nil) {
NSArray *receipts = [NSKeyedUnarchiver unarchiveObjectWithFile:LocalReceiptsPath()];
_allDownloadReceipts = receipts != nil ? receipts.mutableCopy : [NSMutableArray array];
}
return _allDownloadReceipts;
}
- (void)saveReceipts:(NSArray <MCDownloadReceipt *>*)receipts {
[NSKeyedArchiver archiveRootObject:receipts toFile:LocalReceiptsPath()];
}
下载的核心方法：
- (MCDownloadReceipt *)downloadFileWithURL:(NSString *)url
progress:(void (^)(NSProgress * _Nonnull,MCDownloadReceipt *receipt))downloadProgressBlock
destination:(NSURL * (^)(NSURL * _Nonnull, NSURLResponse * _Nonnull))destination
success:(nullable void (^)(NSURLRequest * _Nullable, NSHTTPURLResponse * _Nullable, NSURL * _Nonnull))success
failure:(nullable void (^)(NSURLRequest * _Nullable, NSHTTPURLResponse * _Nullable, NSError * _Nonnull))failure {
__block MCDownloadReceipt *receipt = [self downloadReceiptForURL:url];
dispatch_sync(self.synchronizationQueue, ^{
NSString *URLIdentifier = url;
if (URLIdentifier == nil) {
if (failure) {
NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorBadURL userInfo:nil];
dispatch_async(dispatch_get_main_queue(), ^{
failure(nil, nil, error);
});
}
return;
}
receipt.successBlock = success;
receipt.failureBlock = failure;
receipt.progressBlock = downloadProgressBlock;
if (receipt.state == MCDownloadStateCompleted) {
dispatch_async(dispatch_get_main_queue(), ^{
if (receipt.successBlock) {
receipt.successBlock(nil,nil,[NSURL URLWithString:receipt.url]);
}
});
return ;
}
if (receipt.state == MCDownloadStateDownloading) {
dispatch_async(dispatch_get_main_queue(), ^{
if (receipt.progressBlock) {
receipt.progressBlock(receipt.progress,receipt);
}
});
return ;
}
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:receipt.url]];
NSString *range = [NSString stringWithFormat:@"bytes=%zd-", receipt.totalBytesWritten];
[request setValue:range forHTTPHeaderField:@"Range"];
NSURLSessionDataTask *task = [self.session dataTaskWithRequest:request];
task.taskDescription = receipt.url;
self.tasks[receipt.url] = task;
[self.queuedTasks addObject:task];
[self resumeWithURL:receipt.url];
});
return receipt;
}
--
- (NSURLSessionDownloadTask*)safelyRemoveTaskWithURLIdentifier:(NSString *)URLIdentifier {
__block NSURLSessionDownloadTask *task = nil;
dispatch_sync(self.synchronizationQueue, ^{
task = [self removeTaskWithURLIdentifier:URLIdentifier];
});
return task;
}
//This method should only be called from safely within the synchronizationQueue
- (NSURLSessionDownloadTask *)removeTaskWithURLIdentifier:(NSString *)URLIdentifier {
NSURLSessionDownloadTask *task = self.tasks[URLIdentifier];
[self.tasks removeObjectForKey:URLIdentifier];
return task;
}
- (void)safelyDecrementActiveTaskCount {
dispatch_sync(self.synchronizationQueue, ^{
if (self.activeRequestCount > 0) {
self.activeRequestCount -= 1;
}
});
}
- (void)safelyStartNextTaskIfNecessary {
dispatch_sync(self.synchronizationQueue, ^{
if ([self isActiveRequestCountBelowMaximumLimit]) {
while (self.queuedTasks.count > 0) {
NSURLSessionDownloadTask *task = [self dequeueTask];
MCDownloadReceipt *receipt = [self downloadReceiptForURL:task.taskDescription];
if (task.state == NSURLSessionTaskStateSuspended && receipt.state == MCDownloadStateWillResume) {
[self startTask:task];
break;
}
}
}
});
}
- (void)startTask:(NSURLSessionDownloadTask *)task {
[task resume];
++self.activeRequestCount;
[self updateReceiptWithURL:task.taskDescription state:MCDownloadStateDownloading];
}
- (void)enqueueTask:(NSURLSessionDownloadTask *)task {
switch (self.downloadPrioritizaton) {
case MCDownloadPrioritizationFIFO: //
[self.queuedTasks addObject:task];
break;
case MCDownloadPrioritizationLIFO: //
[self.queuedTasks insertObject:task atIndex:0];
break;
}
}
- (NSURLSessionDownloadTask *)dequeueTask {
NSURLSessionDownloadTask *task = nil;
task = [self.queuedTasks firstObject];
[self.queuedTasks removeObject:task];
return task;
}
- (BOOL)isActiveRequestCountBelowMaximumLimit {
return self.activeRequestCount < self.maximumActiveDownloads;
}
根据URL获取receipt对象：
- (MCDownloadReceipt *)downloadReceiptForURL:(NSString *)url {
if (url == nil) return nil;
for (MCDownloadReceipt *receipt in self.allDownloadReceipts) {
if ([receipt.url isEqualToString:url]) {
return receipt;
}
}
MCDownloadReceipt *receipt = [[MCDownloadReceipt alloc] initWithURL:url];
receipt.state = MCDownloadStateNone;
receipt.totalBytesExpectedToWrite = 1;
[self.allDownloadReceipts addObject:receipt];
[self saveReceipts:self.allDownloadReceipts];
return receipt;
}
NSURLSessionDataDelegate：
在接到响应后,保存totalBytesExpectedToWrite和state
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSHTTPURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler
{
MCDownloadReceipt *receipt = [self downloadReceiptForURL:dataTask.taskDescription];
receipt.totalBytesExpectedToWrite = dataTask.countOfBytesExpectedToReceive;
receipt.state = MCDownloadStateDownloading;
@synchronized (self) {
[self saveReceipts:self.allDownloadReceipts];
}
[receipt.stream open];
completionHandler(NSURLSessionResponseAllow);
}
在接收到数据后，写入文件并且调用progressBlock
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data
{
MCDownloadReceipt *receipt = [self downloadReceiptForURL:dataTask.taskDescription];
[receipt.stream write:data.bytes maxLength:data.length];
receipt.progress.totalUnitCount = receipt.totalBytesExpectedToWrite;
receipt.progress.completedUnitCount = receipt.totalBytesWritten;
dispatch_async(dispatch_get_main_queue(), ^{
if (receipt.progressBlock) {
receipt.progressBlock(receipt.progress,receipt);
}
});
}
下载完成后
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
MCDownloadReceipt *receipt = [self downloadReceiptForURL:task.taskDescription];
[receipt.stream close];
receipt.stream = nil;
if (error) {
receipt.state = MCDownloadStateFailed;
dispatch_async(dispatch_get_main_queue(), ^{
if (receipt.failureBlock) {
receipt.failureBlock(task.originalRequest,(NSHTTPURLResponse *)task.response,error);
}
});
}else {
receipt.state = MCDownloadStateCompleted;
dispatch_async(dispatch_get_main_queue(), ^{
if (receipt.successBlock) {
receipt.successBlock(task.originalRequest,(NSHTTPURLResponse *)task.response,task.originalRequest.URL);
}
});
}
@synchronized (self) {
[self saveReceipts:self.allDownloadReceipts];
}
[self safelyDecrementActiveTaskCount];
[self safelyStartNextTaskIfNecessary];
}
总结
这个下载器就介绍到这里了，可以在https://github.com/agelessman/MCDownloadManager.git下载demo。如发现任何问题或改进意见，可以留言，我会尽力完成。
