本文整理自网络，侵删。
环境delphi2007 + indy
网上有大量利用TIdHttp控件下载并支持断点续传的例子，但在delphi2007环境下运行都不成功。最近应为项目需要编写了一个测试程序，可以成功实现分块断点续传，并支持链接重定向后，自动获得真实的下载文件名。供大家参考。
（想修改成多线程下载也只是增加一些逻辑就可以了。）
procedure TForm3.RzButton1Click(Sender: TObject);
var
IdHttp: TIdHttp;
MemStream: TMemoryStream;
UrlStr,
DFileName: string;
FilePosition: int64;
FileSize: integer;
begin
//初始化进度条
RzProgressBar1.TotalParts := 0;
RzProgressBar1.PartsComplete := 0;
//获取下载地址
UrlStr := trim(RzEditUrl.Text);
IdHttp := tIdHttp.Create(nil);
IdHttp.ProtocolVersion := pv1_1 ;
//IdHttp.RecvBufferSize := 1024 ;
IdHttp.HandleRedirects := true;
try
//获取重定向后的下载地址和文件大小
IdHttp.Head(UrlStr);
FileSize := IdHttp.Response.ContentLength;
DFileName := idhttp.URL.URI;
//由下载地址转换为保存文件名
while pos('/',DFileName) > 0 do
begin
delete(DFileName, 1, pos('/',DFileName));
end;
while pos('/',DFileName) > 0 do
begin
delete(DFileName, 1, pos('/',DFileName));
end;
if DFileName = '' then
exit;
FilePosition := 0;
MemStream := TMemoryStream.Create;
if FileExists(ExtractFilePath(Application.ExeName) + DFileName) then
begin
try
MemStream.LoadFromFile(ExtractFilePath(Application.ExeName) + DFileName);
MemStream.Seek(0, soFromEnd);
FilePosition := MemStream.Size;
except
MemStream.Free;
RzMemo1.lines.Add(Format('Open file "%S" failed!', [DFileName]));
exit;
end;
end;
//初始化进度条
RzProgressBar1.TotalParts := FileSize;
RzProgressBar1.PartsComplete := FilePosition;
RzMemo1.Lines.Add('FileName:' + DFileName);
RzMemo1.Lines.Add('FileSize:' + IntToStr(RzProgressBar1.TotalParts));
while FilePosition < FileSize do
begin
//方法1：未验证
//IdHttp.Request.ContentRangeStart := FilePosition;
//IdHttp.Request.ContentRangeEnd := FilePosition + 1024;
//方法2：已验证
IdHttp.Request.Range := IntToStr(FilePosition) + '-' ;
if FilePosition + 10240 < FileSize then
IdHttp.Request.Range := IdHttp.Request.Range + IntToStr(FilePosition + 10239);
IdHttp.get(idhttp.URL.URI, MemStream);
//MemStream.LoadFromStream(IdHttp.Response.ContentStream);
MemStream.SaveToFile(ExtractFilePath(Application.ExeName) + DFileName);
FilePosition := MemStream.Size;
//刷新进度条
RzProgressBar1.PartsComplete := FilePosition;
Application.ProcessMessages;
//sleep(50); //为确保能够看到下载过程，添加此延时
end;
finally
IdHttp.Free;
MemStream.Free;
end;
end;
来源 http://blog.csdn.net/arbin_he/article/details/6325708
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