使用TIdHttp渐进式下载文件
使用TIdHttp渐进式下载文件
提问于 2012-11-30 15:53:12
我想用TIdHttp (Indy10)实现一个简单的http下载器。我在网上找到了两种代码示例。不幸的是，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;代码很紧凑，很容易理解。问题是它会在下载开始时分配磁盘空间。另一个问题是，我们不能在图形用户界面中直接显示下载进度，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，然后分块下载文件内容。检索到的文件内容将在完全接收后保存到磁盘。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)是否会在特定时间段内限制请求的数量。
My expectations
- 下载程序应该只向服务器发送一个GET-request。
- 下载开始时不能分配磁盘空间。
任何提示都是值得感谢的。
回答 3
Stack Overflow用户
回答已采纳
发布于 2012-12-01 09:43:37
变体#1是最简单的，也是如何使用Indy的。
关于磁盘分配问题，您可以从TFileStream派生一个新类，并覆盖它的SetSize()方法而不执行任何操作。TIdHTTP仍会在适当的时候尝试预分配文件，但不会实际分配任何磁盘空间。写入TFileStream将根据需要增大文件。
关于状态报告，TIdHTTP提供了用于此目的的OnWork...事件。如果已知(响应未分块)，OnWorkBegin的AWorkCountMax参数将为实际文件大小；如果未知，则为0。OnWork事件的AWorkCount参数将是到目前为止已传输的累积字节数。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，否则仅显示AWorkCount值本身。如果要显示传输速度，可以根据多个OnWork事件之间的AWorkCount值和时间间隔的差异来计算。
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;Stack Overflow用户
发布于 2012-11-30 16:39:42
下面的示例展示了如何使用components OnWork显示进度条：
Download a File from internet programatically with an Progress event using Delphi and Indy
您不必担心磁盘分配的问题。分配的磁盘空间实际上并未写入，因此不会损坏您的磁盘。值得庆幸的是，它被分配了，这样就不会有另一个进程占用磁盘空间并让您耗尽空间！
Stack Overflow用户
发布于 2013-12-24 23:21:13
不要忘记为变体2添加以下内容
: Else HttpClient.Request.ContentRangeEnd := FileSize;替换
if Buffer.Size + RECV_BUFFER_SIZE < FileSize then
HttpClient.Request.ContentRangeEnd := Buffer.Size + RECV_BUFFER_SIZE - 1;通过
if Buffer.Size + RECV_BUFFER_SIZE < FileSize then
HttpClient.Request.ContentRangeEnd := Buffer.Size + RECV_BUFFER_SIZE - 1;
Else HttpClient.Request.ContentRangeEnd := FileSize;页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/13641055
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