Resumable download

• The Range field is added to the HTTP 1.1 header to specify the scope within which to obtain data. The formats of the Range field value are as follows:
  • Range: bytes=100-: starts from the 101st Byte until the download is complete.
  • Range: bytes=100-200: starts the download from the 101st Byte and stops the download at the 201st Byte. The Range field value is counted from 0 by default. This method is typically used for multipart transmission of large objects, such as videos.
  • Range: bytes =-100: The start position is not specified. The server downloads the last 100 Bytes, instead of the 100 Bytes starting from 0.
  • Range: bytes=0-100, 200-300: downloads content within multiple ranges at the same time. This is untypical.
• During resumable download, the server requires to use the If-Match value in the header to verify whether the object on the server changes. The If-Match field value corresponds to the ETag value.
• When the client initiates a request, the Range and If-Match field values are included in the request. The OSS server verifies the ETag value that corresponds to the If-Match field value. If the received ETag value does not match the ETag value saved in OSS, OSS returns the 412 precondition status code.
• The OSS server currently supports the following fields for GetObject: Range, If-Match, If-None-Match, If-Modified-Since, If-Unmodified-Since. Therefore, you can use resumable download to download resources from OSS on your mobile device.

#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;         //The network session.
@property (nonatomic, strong) NSURLSessionDataTask *dataTask;   //The data request task.
@property (nonatomic, copy) DownloadFailureBlock failure;    //The request error.
@property (nonatomic, copy) DownloadSuccessBlock success;    //The request succeeds.
@property (nonatomic, copy) DownloadProgressBlock progress;  //The download progress.
@property (nonatomic, copy) Checkpoint *checkpoint;        //The checkpoint file.
@property (nonatomic, copy) NSString *requestURLString;    //The object resource URL used in a download request.
@property (nonatomic, copy) NSString *headURLString;       //The object resource URL used in a HEAD request.
@property (nonatomic, copy) NSString *targetPath;     //The path to the object.
@property (nonatomic, assign) unsigned long long totalReceivedContentLength; //The size of the downloaded content.
@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;
}

+ (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;
}

/**
 * Use HeadObject to obtain Object Meta. Compare the ETag value saved in OSS with that saved in the local checkpoint file. The result is returned.
 */
- (BOOL)getFileInfo {
    __block BOOL resumable = NO;
    NSURL *url = [NSURL URLWithString:self.headURLString];
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:url];
    [request setHTTPMethod:@"HEAD"];
    
    NSURLSessionDataTask *task = [[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        if (error) {
            cNSLog(@"Failed to obtain Object Meta. 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;
}

/**
 * Obtain 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];    // If the resumable field value is NO, resumable upload is unfeasible. Otherwise, continue the resumable upload logic.
    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
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);
}

- (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

• DownloadService: the core of the download logic.
• URLSession:dataTask:didReceiveData: The received network data is appended to the object. The download progress record is updated.
• URLSession:task:didCompleteWithError: determines whether the download task is complete, and then returns the result to the upper layer business.
• URLSession:dataTask:didReceiveResponse:completionHandler: processes object-relevant information, such as the ETag value for the precheck of resumable download and the content-length field value to calculate the download progress.

#import <Foundation/Foundation.h>

typedef void(^DownloadProgressBlock)(int64_t bytesReceived, int64_t totalBytesReceived, int64_t totalBytesExpectToReceived);
typedef void(^DownloadFailureBlock)(NSError *error);
typedef void(^DownloadSuccessBlock)(NSDictionary *result);

@interface Checkpoint : NSObject<NSCopying>

@property (nonatomic, copy) NSString *etag;     // The ETag value of the resource.
@property (nonatomic, assign) unsigned long long totalExpectedLength;    //The total size of the object.

@end

@interface DownloadRequest : NSObject

@property (nonatomic, copy) NSString *sourceURLString;      // The URL for the download.

@property (nonatomic, copy) NSString *headURLString;        // The URL used to obtain the original information of the object.

@property (nonatomic, copy) NSString *downloadFilePath;     // The local path to the object.

@property (nonatomic, copy) DownloadProgressBlock downloadProgress; // The download progress.

@property (nonatomic, copy) DownloadFailureBlock failure;   // The callback that is sent after the download fails.

@property (nonatomic, copy) DownloadSuccessBlock success;   // The callback that is sent after the download succeeds.

@property (nonatomic, copy) Checkpoint *checkpoint;         // The checkpoint file that stores the ETag value of the object.

@end


@interface DownloadService : NSObject

+ (instancetype)downloadServiceWithRequest:(DownloadRequest *)request;

/**
 * Start the download.
 */
- (void)resume;

/**
 * Pause the download.
 */
- (void)pause;

/**
 * Cancel the download.
 */
- (void)cancel;

@end

- (void)initDownloadURLs {
    OSSPlainTextAKSKPairCredentialProvider *pCredential = [[OSSPlainTextAKSKPairCredentialProvider alloc] initWithPlainTextAccessKey:OSS_ACCESSKEY_ID secretKey:OSS_SECRETKEY_ID];
    _mClient = [[OSSClient alloc] initWithEndpoint:OSS_ENDPOINT credentialProvider:pCredential];
    
    // Generate the signed URL for the GET request.
    OSSTask *downloadURLTask = [_mClient presignConstrainURLWithBucketName:@"aliyun-dhc-shanghai" withObjectKey:OSS_DOWNLOAD_FILE_NAME withExpirationInterval:1800];
    [downloadURLTask waitUntilFinished];
    _downloadURLString = downloadURLTask.result;
    
    // Generate the signed URL for the HEAD request.
    OSSTask *headURLTask = [_mClient presignConstrainURLWithBucketName:@"aliyun-dhc-shanghai" withObjectKey:OSS_DOWNLOAD_FILE_NAME httpMethod:@"HEAD" withExpirationInterval:1800 withParameters:nil];
    [headURLTask waitUntilFinished];
    
    _headURLString = headURLTask.result;
}

- (IBAction)resumeDownloadClicked:(id)sender {
    _downloadRequest = [DownloadRequest new];
    _downloadRequest.sourceURLString = _downloadURLString;       // Set the resource URL.
    _downloadRequest.headURLString = _headURLString;
    NSString *documentPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject;
    _downloadRequest.downloadFilePath = [documentPath stringByAppendingPathComponent:OSS_DOWNLOAD_FILE_NAME];   //Set the local path to the object you want to download.
    
    __weak typeof(self) wSelf = self;
    _downloadRequest.downloadProgress = ^(int64_t bytesReceived, int64_t totalBytesReceived, int64_t totalBytesExpectToReceived) {
        // totalBytesReceived is the number of Bytes already cached by the client. totalBytesExpectToReceived is the total number of Bytes that need to be downloaded.
        dispatch_async(dispatch_get_main_queue(), ^{
            __strong typeof(self) sSelf = wSelf;
            CGFloat fProgress = totalBytesReceived * 1.f / totalBytesExpectToReceived;
            sSelf.progressLab.text = [NSString stringWithFormat:@"%. 2f%%", fProgress * 100];
            sSelf.progressBar.progress = fProgress;
        });
    };
    _downloadRequest.failure = ^(NSError *error) {
        __strong typeof(self) sSelf = wSelf;
        sSelf.checkpoint = error.userInfo[@"checkpoint"];
    };
    _downloadRequest.success = ^(NSDictionary *result) {
        NSLog(@"The download succeeds.");
    };
    _downloadRequest.checkpoint = self.checkpoint;
    
    NSString *titleText = [[_downloadButton titleLabel] text];
    if ([titleText isEqualToString:@"download"]) {
        [_downloadButton setTitle:@"pause" forState: UIControlStateNormal];
        _downloadService = [DownloadService downloadServiceWithRequest:_downloadRequest];
        [_downloadService resume];
    } else {
        [_downloadButton setTitle:@"download" forState: UIControlStateNormal];
        [_downloadService pause];
    }
}

- (IBAction)cancelDownloadClicked:(id)sender {
    [_downloadButton setTitle:@"download" forState: UIControlStateNormal];
    [_downloadService cancel];
}