使用NSProgress查找下载速度和剩余时间

Question

根据NSProgress，我看到-[NSProgress localizedAdditionalDescription]可以报告下载速度和剩余时间，例如：

1.61 GB 3.22 GB (2KB/秒)-剩余2分钟

但是，当我将NSProgress与NSURLSessionDownloadTask相关联时，我无法获得这些细节。这是我的密码：

Downloader.h

@interface Downloader : NSObject
@property NSProgress *overallProgress;
-(void)startDownload;
@end

Downloader.m

- (void)startDownload {

    self.overallProgress = [NSProgress progressWithTotalUnitCount:100];

    [self.overallProgress setKind:NSProgressKindFile];
    [self.overallProgress setUserInfoObject:NSProgressFileOperationKindKey forKey:NSProgressFileOperationKindDownloading];


    [self.overallProgress becomeCurrentWithPendingUnitCount:100];
    [self work1];
    [self.overallProgress resignCurrent];
}

- (void)work1 {

    NSProgress *firstTaskProgress = [NSProgress progressWithTotalUnitCount:1];
    [firstTaskProgress setKind:NSProgressKindFile];
    [firstTaskProgress setUserInfoObject:NSProgressFileOperationKindKey forKey:NSProgressFileOperationKindDownloading];

    NSURL *downloadURL = [NSURL URLWithString:@"http://ipv4.download.thinkbroadband.com/200MB.zip"];
    NSURL *destinationDirectory = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] firstObject];
    NSURL *destinationURL = [destinationDirectory URLByAppendingPathComponent:[downloadURL lastPathComponent]];

    NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
    NSURLSessionDownloadTask *fileDownloadTask =
    [session downloadTaskWithURL:downloadURL
               completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error){

                   [[NSFileManager defaultManager] removeItemAtURL:destinationURL error:NULL];

                   [[NSFileManager defaultManager] moveItemAtURL:location toURL:destinationURL error:nil];

                   [firstTaskProgress setCompletedUnitCount:1];
               }];

    [fileDownloadTask resume];
}

DownloadObserver.m

-(void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];

    downloader = [Downloader new];

    [downloader addObserver:self
                 forKeyPath:@"overallProgress.fractionCompleted"
                    options:NSKeyValueObservingOptionNew
                    context:NULL];

    [downloader startDownload];

}

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog(@"%@", [downloader.overallProgress localizedAdditionalDescription]);
}

这只会打印出来：

100中的0 100字节的零KB

如何让localizedAdditionalDescription打印下载速度和剩余时间？

Answer 1

首先，totalUnitCount应该对应文件的大小(以字节为单位)。在下载过程中，我们修改了completedUnitCount，它反映在localizedAdditionalDescription上。NSProgressKindFile是将这些值格式化为文件大小的提示。

要正确设置进度，我们需要使用NSURLSessionDownloadDelegate方法而不是块处理程序。在-URLSession: downloadTask: didWriteData: totalBytesWritten: totalBytesExpectedToWrite:中，我们有所有要启动(totalBytesExpectedToWrite)和更新(totalBytesWritten)进度的信息。

static void *myContext = &myContext;

- (void)download {
    NSURL *downloadURL = [NSURL URLWithString:@"http://ipv4.download.thinkbroadband.com/10MB.zip"];
    NSURLSession *session =
    [NSURLSession
     sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]
     delegate:self
     delegateQueue:[NSOperationQueue mainQueue]];

    [[session downloadTaskWithURL:downloadURL] resume];
}

#pragma mark - url session download delegate

- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    if (!_progress) {
        _progress = [NSProgress progressWithTotalUnitCount:totalBytesExpectedToWrite];
        _progress.kind = NSProgressKindFile;
        [_progress addObserver:self forKeyPath:@"fractionCompleted" options:0 context:myContext];
    }

    _progress.completedUnitCount = totalBytesWritten;
    //[_overallProgress setUserInfoObject:@1024 forKey:NSProgressEstimatedTimeRemainingKey];
}

- (void)URLSession:(NSURLSession *)session 
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
    [_progress removeObserver:self forKeyPath:@"fractionCompleted"];
    _progress = nil;
    NSLog(@"finished: %@", location);
}

-(void)observeValueForKeyPath:(NSString *)keyPath
                     ofObject:(id)object
                       change:(NSDictionary *)change
                      context:(void *)context
{
    if (context == myContext) {
        self.label.text = [_progress localizedAdditionalDescription];
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

事实上，NSProgress不做任何计算。它只是根据userInfo和其他类属性格式化本地化描述。请参阅文档中的常量部分。您可以使用这些值并比较本地化的描述输出。

如果要显示剩余的下载时间，可以使用NSProgressEstimatedTimeRemainingKey。坏消息是，您需要手动计算剩余时间。参考资料：如何估算下载剩余时间(准确)？

我尝试设置NSProgressThroughputKey，它指示以每秒字节为单位的数据处理速度。我原以为NSProgress会计算剩余时间，但没有发生。

还请参阅AFNetworking源：AFURLSessionManagerTaskDelegate使用NSProgress实例进行上载和下载任务。