Object Storage Service:Resumable download (OSS SDK for iOS)

During the download, the app displays a progress dialog that shows the current download percentage, a progress bar, and buttons to pause or cancel the download.

The following sample code demonstrates how to perform a resumable download in iOS:

#import "DownloadService.h"
#import "OSSTestMacros.h"
@implementation DownloadRequest
@end
@implementation Checkpoint
- (instancetype)copyWithZone:(NSZone *)zone {
    Checkpoint *other = [[[self class] allocWithZone:zone] init];
    other.etag = self.etag;
    other.totalExpectedLength = self.totalExpectedLength;
    return other;
}
@end
@interface DownloadService()<NSURLSessionTaskDelegate, NSURLSessionDataDelegate>
@property (nonatomic, strong) NSURLSession *session;         // Network session.
@property (nonatomic, strong) NSURLSessionDataTask *dataTask;   // Data request task.
@property (nonatomic, copy) DownloadFailureBlock failure;    // Called on request failure.
@property (nonatomic, copy) DownloadSuccessBlock success;    // Called on request success.
@property (nonatomic, copy) DownloadProgressBlock progress;  // Tracks download progress.
@property (nonatomic, copy) Checkpoint *checkpoint;        // The checkpoint.
@property (nonatomic, copy) NSString *requestURLString;    // The URL of the resource to download.
@property (nonatomic, copy) NSString *headURLString;       // The URL for the HEAD request.
@property (nonatomic, copy) NSString *targetPath;     // The local path to store the file.
@property (nonatomic, assign) unsigned long long totalReceivedContentLength; // The size of the content received so far.
@property (nonatomic, strong) dispatch_semaphore_t semaphore;
@end
@implementation DownloadService
- (instancetype)init
{
    self = [super init];
    if (self) {
        NSURLSessionConfiguration *conf = [NSURLSessionConfiguration defaultSessionConfiguration];
        conf.timeoutIntervalForRequest = 15;
        NSOperationQueue *processQueue = [NSOperationQueue new];
        _session = [NSURLSession sessionWithConfiguration:conf delegate:self delegateQueue:processQueue];
        _semaphore = dispatch_semaphore_create(0);
        _checkpoint = [[Checkpoint alloc] init];
    }
    return self;
}
// DownloadRequest is the core of the download logic.
+ (instancetype)downloadServiceWithRequest:(DownloadRequest *)request {
    DownloadService *service = [[DownloadService alloc] init];
    if (service) {
        service.failure = request.failure;
        service.success = request.success;
        service.requestURLString = request.sourceURLString;
        service.headURLString = request.headURLString;
        service.targetPath = request.downloadFilePath;
        service.progress = request.downloadProgress;
        if (request.checkpoint) {
            service.checkpoint = request.checkpoint;
        }
    }
    return service;
}
/**
 * Gets file information by using the HEAD method and compares the file's ETag with the one in the local checkpoint.
 */
- (BOOL)getFileInfo {
    __block BOOL resumable = NO;
    NSURL *url = [NSURL URLWithString:self.headURLString];
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:url];
    [request setHTTPMethod:@"HEAD"];
    // Process object information. For example, the ETag is used for pre-checks in resumable downloads, and content-length is used to calculate download progress.
    NSURLSessionDataTask *task = [[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        if (error) {
            NSLog(@"Failed to get file metadata. Error: %@", error);
        } else {
            NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
            NSString *etag = [httpResponse.allHeaderFields objectForKey:@"Etag"];
            if ([self.checkpoint.etag isEqualToString:etag]) {
                resumable = YES;
            } else {
                resumable = NO;
            }
        }
        dispatch_semaphore_signal(self.semaphore);
    }];
    [task resume];
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    return resumable;
}
/**
 * Gets the size of the local file.
 */
- (unsigned long long)fileSizeAtPath:(NSString *)filePath {
    unsigned long long fileSize = 0;
    NSFileManager *dfm = [NSFileManager defaultManager];
    if ([dfm fileExistsAtPath:filePath]) {
        NSError *error = nil;
        NSDictionary *attributes = [dfm attributesOfItemAtPath:filePath error:&error];
        if (!error && attributes) {
            fileSize = attributes.fileSize;
        } else if (error) {
            NSLog(@"error: %@", error);
        }
    }
    return fileSize;
}
- (void)resume {
    NSURL *url = [NSURL URLWithString:self.requestURLString];
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:url];
    [request setHTTPMethod:@"GET"];
    BOOL resumable = [self getFileInfo];    // A `NO` return value indicates that the conditions for a resumable download are not met.
    if (resumable) {
        self.totalReceivedContentLength = [self fileSizeAtPath:self.targetPath];
        NSString *requestRange = [NSString stringWithFormat:@"bytes=%llu-", self.totalReceivedContentLength];
        [request setValue:requestRange forHTTPHeaderField:@"Range"];
    } else {
        self.totalReceivedContentLength = 0;
    }
    if (self.totalReceivedContentLength == 0) {
        [[NSFileManager defaultManager] createFileAtPath:self.targetPath contents:nil attributes:nil];
    }
    self.dataTask = [self.session dataTaskWithRequest:request];
    [self.dataTask resume];
}
- (void)pause {
    [self.dataTask cancel];
    self.dataTask = nil;
}
- (void)cancel {
    [self.dataTask cancel];
    self.dataTask = nil;
    [self removeFileAtPath: self.targetPath];
}
- (void)removeFileAtPath:(NSString *)filePath {
    NSError *error = nil;
    [[NSFileManager defaultManager] removeItemAtPath:self.targetPath error:&error];
    if (error) {
        NSLog(@"remove file with error : %@", error);
    }
}
#pragma mark - NSURLSessionDataDelegate
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
// Determines if the download is complete and reports the result to the upper-layer service.
didCompleteWithError:(nullable NSError *)error {
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)task.response;
    if ([httpResponse isKindOfClass:[NSHTTPURLResponse class]]) {
        if (httpResponse.statusCode == 200) {
            self.checkpoint.etag = [[httpResponse allHeaderFields] objectForKey:@"Etag"];
            self.checkpoint.totalExpectedLength = httpResponse.expectedContentLength;
        } else if (httpResponse.statusCode == 206) {
            self.checkpoint.etag = [[httpResponse allHeaderFields] objectForKey:@"Etag"];
            self.checkpoint.totalExpectedLength = self.totalReceivedContentLength + httpResponse.expectedContentLength;
        }
    }
    if (error) {
        if (self.failure) {
            NSMutableDictionary *userInfo = [NSMutableDictionary dictionaryWithDictionary:error.userInfo];
            [userInfo oss_setObject:self.checkpoint forKey:@"checkpoint"];
            NSError *tError = [NSError errorWithDomain:error.domain code:error.code userInfo:userInfo];
            self.failure(tError);
        }
    } else if (self.success) {
        self.success(@{@"status": @"success"});
    }
}
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler
{
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)dataTask.response;
    if ([httpResponse isKindOfClass:[NSHTTPURLResponse class]]) {
        if (httpResponse.statusCode == 200) {
            self.checkpoint.totalExpectedLength = httpResponse.expectedContentLength;
        } else if (httpResponse.statusCode == 206) {
            self.checkpoint.totalExpectedLength = self.totalReceivedContentLength +  httpResponse.expectedContentLength;
        }
    }
    completionHandler(NSURLSessionResponseAllow);
}
// Appends the received data to the file and updates the download progress.
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:self.targetPath];
    [fileHandle seekToEndOfFile];
    [fileHandle writeData:data];
    [fileHandle closeFile];
    self.totalReceivedContentLength += data.length;
    if (self.progress) {
        self.progress(data.length, self.totalReceivedContentLength, self.checkpoint.totalExpectedLength);
    }
}
@end