iOS开发中断点下载功能很常见，网上也有很多框架，本文选择了原生的
NSURLSession和NSFileHandle，实现了多任务、大文件的断点下载，保证了较低的内存占用。
NSMutableData 如果文件很大会出现内存警告
NSURLConnection iOS9之后弃用，没有暂停的方法
NSURLSession 推荐
将下载记录保存到plist，重启应用时就能拿到下载记录，从而继续下载。
NSMutableData 如果文件很大会出现内存警告
NSOutputStream 推荐
NSFileHandle 推荐
实际使用发现NSOutputSteam和NSFileHandle差别不大，但要注意不再写入时需要调用close方法。
内存占用：
创建下载任务时我们采用了NSURLSessionDataTask，它是NSURLSessionTask的子类，其拥有只读属性taskIdentifier，若要将其作为任务的唯一标识符需要利用KVC。这里我们没有这么做，而是创建了NSURLSession的分类，为其添加了taskKey属性作为任务的为一标识符。创建任务时将传入的URL的MD5值作为key，并将其作查询下载任务、本地文件缓存、下载记录的为一索引。
创建下载管理者类MYDownloadManager，考虑到会在多个地方调用下载功能，所以将其设计为单例模式。管理者拥有多个下载任务，每个任务有其唯一的key，所以创建一个保存着多个任务的字典。
@interface MYDownloadManager ()<NSURLSessionDataDelegate>
@property (nonatomic, strong) NSMutableDictionary *downloadDict; // {Key : md5, Value : <MYDonwload *>}
@end
创建下载类
@interface MYDownload : NSObject
@property (nonatomic, copy) NSString *url; // 下载地址
@property (nonatomic, assign) long long downloadedLength; // 已下载大小
@property (nonatomic, assign) long long totalLength; // 总大小
@property (nonatomic, strong) NSURLSessionDataTask *task; // 任务
@property (nonatomic, strong) NSFileHandle *fileHandle; // 文件句柄
@property (nonatomic, copy) ProgressBlock progressBlock; // 下载进度回调
@property (nonatomic, copy) StateBlock stateBlock; // 下载状态回调
@end
下载记录Download.plist结构
我们要从上次结束的位置开始下载，所以需要设置请求头，下载指定范围的文件，设置规则如下：
表示头500个字节：Range: bytes=0-499 表示第二个500字节：Range: bytes=500-999 表示最后500个字节：Range: bytes=-500 表示500字节以后的范围：Range: bytes=500- 同时指定几个范围：Range: bytes=100-199,400-500
- (void)downloadWithUrl:(NSString *)url resume:(BOOL)resume progress:(ProgressBlock)progressBlock state:(StateBlock)stateBlock {
if (!url.length) {
return;
}
// 将url的md5值作为key
NSString *key = [url md5];
long long totalLength = [self getTotalLengthWithKey:key];
long long downloadedLength = [self getDownloadedLengthWithKey:key];
// 任务已完成
if (totalLength == downloadedLength && totalLength > 0) {
if (progressBlock) {
progressBlock(1.0, downloadedLength, totalLength);
}
if (stateBlock) {
stateBlock(MYDownloadStateComplete);
}
}
// 查询任务是否存在
MYDownload *download = [self.downloadDict valueForKey:key];
if (download) {
// 取出下载任务
if (resume) {
[download.task resume];
} else {
[download.task suspend];
if (download.stateBlock) {
download.stateBlock(MYDownloadStateSuspend);
}
}
} else {
// 创建下载任务
NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:nil];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url]];
NSString *rangeValue = [NSString stringWithFormat:@"bytes=%lld-", downloadedLength];
[request setValue:rangeValue forHTTPHeaderField:@"Range"];
NSURLSessionDataTask *task = [session dataTaskWithRequest:request];
task.taskKey = key;
task.url = url;
if (resume) {
[task resume];
}
// 创建并保存下载对象
download = [MYDownload new];
download.url = url;
download.task = task;
download.progressBlock = progressBlock;
download.stateBlock = stateBlock;
[self.downloadDict setValue:download forKey:key];
}
}
根据dataTask的taskKey可以得到当前下载对象，从服务器返回的response我们可以得到任务的总大小。开辟缓存文件，创建文件句柄准备写入文件，保存下载记录到plist。
// 收到服务器相应
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(nonnull NSURLResponse *)response completionHandler:(nonnull void (^)(NSURLSessionResponseDisposition))completionHandler {
NSString *key = dataTask.taskKey;
NSString *url = dataTask.url;
NSString *filePath = [self getDownloadedPathWithFileName:response.suggestedFilename];
MYDownload *download = [self.downloadDict valueForKey:key];
// 计算总大小并保存到plist
long long expectedLength = response.expectedContentLength;
long long downloadedLength = [self getDownloadedLengthWithKey:key];
long long totalLength = expectedLength + downloadedLength;
if (totalLength == 0) {
if (download.progressBlock) {
download.progressBlock(0.f, downloadedLength, totalLength);
}
if (download.stateBlock) {
download.stateBlock(MYDownloadStateError);
}
return;
}
NSDictionary *dict = @{@"TotalLength" : @(totalLength),
@"Url" : url,
@"FileName" : response.suggestedFilename
};
[self setPlistValue:dict forKey:key];
// 创建NSFileHandle
NSFileManager *fileManager = [NSFileManager defaultManager];
if (![fileManager fileExistsAtPath:filePath]) {
[fileManager createFileAtPath:filePath contents:nil attributes:nil];
}
NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];
// 设置下载对象
download.totalLength = totalLength;
download.downloadedLength = downloadedLength;
download.fileHandle = fileHandle;
completionHandler(NSURLSessionResponseAllow);
}
开始接收数据，利用NSFileHandle将文件写入沙盒，不会导致内存占用过高。
// 收到数据（多次调用）
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
NSString *key = dataTask.taskKey;
// 写数据
MYDownload *download = [self.downloadDict valueForKey:key];
if (download) {
[download.fileHandle seekToEndOfFile];
[download.fileHandle writeData:data];
download.downloadedLength += data.length;
CGFloat progress = (CGFloat) download.downloadedLength / download.totalLength;
if (download.progressBlock) {
download.progressBlock(progress, download.downloadedLength, download.totalLength);
}
if (download.stateBlock) {
download.stateBlock(MYDownloadStateDownloading);
}
}
}
任务完成、中止时关闭NSFileHandle，移除下载对象
// 任务完成、中止
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
NSString *key = task.taskKey;
// 关闭写数据流
MYDownload *download = [self.downloadDict valueForKey:key];
[download.fileHandle closeFile];
download.fileHandle = nil;
if (download.stateBlock) {
if (error) {
download.stateBlock(MYDownloadStateError);
} else {
download.stateBlock(MYDownloadStateComplete);
}
}
[self.downloadDict removeObjectForKey:key];
}
- Git地址： MultitaskSuspendDownload