delphi - 使用 TIdHttp 逐步下载文件

Question

我想使用 TIdHttp (Indy10) 实现一个简单的 http 下载器。我从网上找到了两种代码示例。不幸的是，他们中没有一个能让我 100% 满意。这是代码，我需要一些建议。

---

变体 1

var
  Buffer: TFileStream;
  HttpClient: TIdHttp;
begin
  Buffer := TFileStream.Create('somefile.exe', fmCreate or fmShareDenyWrite);
  try
    HttpClient := TIdHttp.Create(nil);
    try
      HttpClient.Get('http://somewhere.com/somefile.exe', Buffer); // wait until it is done
    finally
      HttpClient.Free;
    end;
  finally
    Buffer.Free;
  end;
end;

代码紧凑，非常容易理解。问题是它在下载开始时分配磁盘空间。另一个问题是我们无法直接在 GUI 中显示下载进度，除非代码在后台线程中执行(或者我们可以绑定(bind) HttpClient.OnWork 事件)。

---

变体 2:

const
  RECV_BUFFER_SIZE = 32768;
var
  HttpClient: TIdHttp;
  FileSize: Int64;
  Buffer: TMemoryStream;
begin
  HttpClient := TIdHttp.Create(nil);
  try
    HttpClient.Head('http://somewhere.com/somefile.exe');
    FileSize := HttpClient.Response.ContentLength;

    Buffer := TMemoryStream.Create;
    try
      while Buffer.Size < FileSize do
      begin
        HttpClient.Request.ContentRangeStart := Buffer.Size;
        if Buffer.Size + RECV_BUFFER_SIZE < FileSize then
          HttpClient.Request.ContentRangeEnd := Buffer.Size + RECV_BUFFER_SIZE - 1
        else
          HttpClient.Request.ContentRangeEnd := FileSize;

        HttpClient.Get(HttpClient.URL.URI, Buffer); // wait until it is done
        Buffer.SaveToFile('somefile.exe');
      end;
    finally
      Buffer.Free;
    end;
  finally
    HttpClient.Free;
  end;
end;

首先我们从服务器查询文件大小，然后我们分段下载文件内容。检索到的文件内容在完全接收后将保存到磁盘。潜在的问题是我们必须向服务器发送多个 GET 请求。我不确定某些服务器(例如 megaupload)是否会限制特定时间段内的请求数量。

---

我的期望

1. 下载器应该只向服务器发送一个 GET 请求。
2. 下载开始时不得分配磁盘空间。

如有任何提示，我们将不胜感激。

Answer 1

变体 #1 是最简单的，也是 Indy 的用途。

关于磁盘分配问题，可以从TFileStream派生一个新的类，覆盖它的SetSize()方法什么也不做。 TIdHTTP 仍会在适当的时候尝试预分配文件，但实际上不会分配任何磁盘空间。写入 TFileStream 将根据需要增大文件。

关于状态报告，TIdHTTP 有用于该目的的OnWork... 事件。 OnWorkBegin 的 AWorkCountMax 参数将是实际文件大小(如果已知)(响应未分块)，如果未知则为 0。 OnWork 事件的 AWorkCount 参数将是迄今为止已传输的累计字节数。如果文件大小已知，您可以通过简单地将 AWorkCount 除以 AWorkCountMax 并乘以 100 来显示总百分比，否则只显示 AWorkCount本身的值(value)。如果要显示传输速度，可以根据 AWorkCount 值的差值和多个 OnWork 事件之间的时间间隔来计算。

试试这个:

type
  TNoPresizeFileStream = class(TFileStream)
  procedure
    procedure SetSize(const NewSize: Int64); override;
  end;

procedure TNoPresizeFileStream.SetSize(const NewSize: Int64);
begin
end;

.

type
  TSomeClass = class(TSomething)
  ...
    TotalBytes: In64;
    LastWorkCount: Int64;
    LastTicks: LongWord;
    procedure Download;
    procedure HttpWorkBegin(ASender: TObject; AWorkMode: TWorkMode; AWorkCountMax: Int64);
    procedure HttpWork(ASender: TObject; AWorkMode: TWorkMode; AWorkCount: Int64);
    procedure HttpWorkEnd(ASender: TObject; AWorkMode: TWorkMode);
  ...
  end;

procedure TSomeClass.Download;
var
  Buffer: TNoPresizeFileStream;
  HttpClient: TIdHttp;
begin
  Buffer := TNoPresizeFileStream.Create('somefile.exe', fmCreate or fmShareDenyWrite);
  try
    HttpClient := TIdHttp.Create(nil);
    try
      HttpClient.OnWorkBegin := HttpWorkBegin;
      HttpClient.OnWork := HttpWork;
      HttpClient.OnWorkEnd := HttpWorkEnd;

      HttpClient.Get('http://somewhere.com/somefile.exe', Buffer); // wait until it is done
    finally
      HttpClient.Free;
    end;
  finally
    Buffer.Free;
  end;
end;

procedure TSomeClass.HttpWorkBegin(ASender: TObject; AWorkMode: TWorkMode; AWorkCountMax: Int64);
begin
  if AWorkMode <> wmRead then Exit;

  // initialize the status UI as needed...
  //
  // If TIdHTTP is running in the main thread, update your UI
  // components directly as needed and then call the Form's
  // Update() method to perform a repaint, or Application.ProcessMessages()
  // to process other UI operations, like button presses (for
  // cancelling the download, for instance).
  //
  // If TIdHTTP is running in a worker thread, use the TIdNotify
  // or TIdSync class to update the UI components as needed, and
  // let the OS dispatch repaints and other messages normally...

  TotalBytes := AWorkCountMax;
  LastWorkCount := 0;
  LastTicks := Ticks;
end;

procedure TSomeClass.HttpWork(ASender: TObject; AWorkMode: TWorkMode; AWorkCount: Int64);
var
  PercentDone: Integer;
  ElapsedMS: LongWord;
  BytesTransferred: Int64;
  BytesPerSec: Int64;
begin
  if AWorkMode <> wmRead then Exit;

  ElapsedMS := GetTickDiff(LastTicks, Ticks);
  if ElapsedMS = 0 then ElapsedMS := 1; // avoid EDivByZero error

  if TotalBytes > 0 then
    PercentDone := (Double(AWorkCount) / TotalBytes) * 100.0;
  else
    PercentDone := 0.0;

  BytesTransferred := AWorkCount - LastWorkCount;

  // using just BytesTransferred and ElapsedMS, you can calculate
  // all kinds of speed stats - b/kb/mb/gm per sec/min/hr/day ...
  BytesPerSec := (Double(BytesTransferred) * 1000) / ElapsedMS;

  // update the status UI as needed...

  LastWorkCount := AWorkCount;
  LastTicks := Ticks;
end;

procedure TSomeClass.HttpWorkEnd(ASender: TObject; AWorkMode: TWorkMode);
begin
  if AWorkMode <> wmRead then Exit;

  // finalize the status UI as needed...
end;