如果在網速一定的情境下，大文件（目前指的是100M以上的文件）的下載對用戶來說是一段不短的時間，用戶體驗不是很好。
分塊下載文件實現原理
如果要實現文件的分段下載，我們首先需要知道要下載的文件的大小，這里需要向服務器發送HTTP請求，請求方法為HEAD，這樣服務器只會給客戶端返回response的包頭信息，不會發送數據信息，然后我們通過包頭信息中的Content-Length字段可以得到要下載的文件的總長度。
- (void)getFileTotalLengthWithURL:(NSString *)url
completion:(void(^)(NSInteger length))completion{
NSURL *URL = [NSURL URLWithString:url];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
request.HTTPMethod = @"HEAD";
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
if ([response isKindOfClass:[NSHTTPURLResponse class]]) {
NSHTTPURLResponse *tmpResponse = (NSHTTPURLResponse *)response;
NSLog(@"allHeaderFields:%@", tmpResponse.allHeaderFields);
}
NSInteger fileTotalLength = response.expectedContentLength;
completion(fileTotalLength);
}];
[dataTask resume];
}
獲取到要下載的文件的總長度之后，在本地沙盒中創建一個同樣大小的文件
- (void)multiDownloadWithFileLength:(NSInteger)fileLength url:(NSURL *)url{
_wholeFileLength = fileLength;
NSString *filePath = [self filePathWithFileName:url.lastPathComponent];
NSFileManager *fm = [NSFileManager defaultManager];
if ([fm fileExistsAtPath:filePath]) {
[fm removeItemAtPath:filePath error:nil];
}
[[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil];
_filePath = filePath;
_fileHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];
[_fileHandle truncateFileAtOffset:fileLength];
NSBlockOperation *addOperationOP = [NSBlockOperation blockOperationWithBlock:^{
while (_completedLength < fileLength) {
long long startSize = _completedLength;
long long endSize = startSize+blockSize;
if (endSize > fileLength) {
endSize = fileLength - 1;
_completedLength = fileLength;
} else {
_completedLength += blockSize;
}
//一個operation對應一個downloadTask
NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
NSString *range=[NSString stringWithFormat:@"bytes=%lld-%lld", startSize, endSize];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
[request setValue:range forHTTPHeaderField:@"Range"];
NSLog(@"requestHeader:%@", request.allHTTPHeaderFields);
NSURLSessionDownloadTask *task = [self.session downloadTaskWithRequest:request];
[task resume];
}];
[_queue addOperation:operation];
}
}];
[_queue addOperation:addOperationOP];
}
每個請求塊的大小
#define blockSize 1024*1024
新建一個下載隊列，循環發送請求，在請求頭中設置Range字段
NSOperationQueue *queue = [NSOperationQueue currentQueue];
NSBlockOperation *addOperationOP = [NSBlockOperation blockOperationWithBlock:^{
while (_completedLength < fileLength) {
long long startSize = _completedLength;
long long endSize = startSize+blockSize;
if (endSize > fileLength) {
endSize = fileLength - 1;
_completedLength = fileLength;
} else {
_completedLength += blockSize;
}
//一個operation對應一個downloadTask
NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
NSString *range=[NSString stringWithFormat:@"bytes=%lld-%lld", startSize, endSize];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
[request setValue:range forHTTPHeaderField:@"Range"];
NSLog(@"requestHeader:%@", request.allHTTPHeaderFields);
NSURLSessionDownloadTask *task = [self.session downloadTaskWithRequest:request];
[task resume];
}];
[queue addOperation:operation];
}
}];
[queue addOperation:addOperationOP];
在代理方法中將獲取到的數據寫到已經創建好的空文件的對應的位置中
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location{
DLQData *tmpReceivedData = [[DLQData alloc] init];
NSInteger startSize = 0;
NSInteger endSize = 0;
if ([downloadTask.response isKindOfClass:[NSHTTPURLResponse class]]) {
NSHTTPURLResponse *tmpResponse = (NSHTTPURLResponse *)downloadTask.response;
NSDictionary *dic = tmpResponse.allHeaderFields;
NSLog(@"diiiiiic: %@", dic[@"Content-Range"]);
NSString *fileRange = dic[@"Content-Range"];
fileRange = [fileRange stringByReplacingOccurrencesOfString:@"bytes" withString:@""];
fileRange = [fileRange stringByReplacingOccurrencesOfString:@" " withString:@""];
NSArray *aTmp1 = [fileRange componentsSeparatedByString:@"/"];
NSArray *aTmp2 = @[];
if (aTmp1.count) {
NSString *tmpStr = aTmp1[0];
aTmp2 = [tmpStr componentsSeparatedByString:@"-"];
if (aTmp1.count >= 2) {
NSString *startSizeStr = aTmp2[0];
NSString *endSizeStr = aTmp2[1];
startSize = startSizeStr.integerValue;
endSize = endSizeStr.integerValue;
tmpReceivedData.data = [NSData dataWithContentsOfURL:location];
tmpReceivedData.startSize = startSize;
tmpReceivedData.endSize = endSize;
[_fileHandle seekToFileOffset:tmpReceivedData.startSize];
[_fileHandle writeData:tmpReceivedData.data];
[_fileData appendData:tmpReceivedData.data];
double progress = _fileData.length/_wholeFileLength;
progress = progress >= 1 ? 1 : progress;
if (progress == 1) {
NSLog(@"分段下載完成");
NSLog(@"downloadProgress:%f", progress);
[_operationQueue cancelAllOperations];
_operationQueue = nil;
if ([self.delegate respondsToSelector:@selector(multiDownloadDidFinished:)] && [self.delegate respondsToSelector:@selector(multiDownloadProgress:)]) {
NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
[mainQueue addOperationWithBlock:^{
[self.delegate multiDownloadProgress:progress];
[self.delegate multiDownloadDidFinished:_filePath];
}];
}
}else{
if ([self.delegate respondsToSelector:@selector(multiDownloadProgress:)]) {
NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
[mainQueue addOperationWithBlock:^{
[self.delegate multiDownloadProgress:progress];
}];
}
}
}
}
}
}
參考文章
iOS開發網絡篇—多線程斷點下載
