从网络上下载文件的两种方法

两种方法使用的Com函数都来源于UrlMon，记得先引用。

一、URLDownloadToFile

直接下载文件到磁盘，URLDownloadToFile声明如下：

functionURLDownloadToFile(
Caller: IUnknown;
URL: PWideChar; { 下载的路径 }
FileName: PWideChar; { 保存到的路径 }
Reserved: DWORD;
StatusCB: IBindStatusCallback
): HResult; stdcall;

function URLDownloadToFileA(Caller: IUnknown; URL: PAnsiChar; FileName: PAnsiChar; Reserved: DWORD; StatusCB: IBindStatusCallback): HResult; stdcall;
function URLDownloadToFileW(Caller: IUnknown; URL: PWideChar; FileName: PWideChar; Reserved: DWORD; StatusCB: IBindStatusCallback): HResult; stdcall;

调用示例：
if UrlDownloadToFile(nil, 'http://www.xxxx.com/xxx.xx', 'c:\1.xx', 0, nil) = 0 then
MessageBox(handle, '下载成功', '', 0)

else

MessageBox(handle, '下载失败', '', 0);

URLDownloadToFile下载文件前先在本地的缓存中查找文件，如果缓存有则不会再去网上抓最新的文件，希望总是获取新文件时可在URL后加一个随机数，或用DeleteUrlCacheEntry(URL)清理缓存后再下载

二、URLOpenStream

下载到 IStream 流，声明为：

functionURLOpenStream(
p1: IUnknown;

p2: PWideChar; { 下载的路径 }

p3: DWORD;

p4: IBindStatusCallback{接口, 下载后的通过它取得数据 }

): HResult; stdcall;

functionURLOpenStreamA(p1: IUnknown; p2: PAnsiChar; p3: DWORD; p4: IBindStatusCallback): HResult; stdcall;

functionURLOpenStreamW(p1: IUnknown; p2: PWideChar; p3: DWORD; p4: IBindStatusCallback): HResult; stdcall;

IBindStatusCallback 接口的实现

  TBindStatusCallback = class(TInterfaceList, IBindStatusCallback)
  public
    function OnStartBinding(dwReserved: DWORD; pib: IBinding): HResult; stdcall;
    function GetPriority(out nPriority): HResult; stdcall;
    function OnLowResource(reserved: DWORD): HResult; stdcall;
    function OnProgress(ulProgress, ulProgressMax, ulStatusCode: ULONG;
      szStatusText: LPCWSTR): HResult; stdcall;
    function OnStopBinding(hresult: HResult; szError: LPCWSTR): HResult; stdcall;
    function GetBindInfo(out grfBINDF: DWORD; var bindinfo: TBindInfo): HResult; stdcall;
    function OnDataAvailable(grfBSCF: DWORD; dwSize: DWORD; formatetc: PFormatEtc;
      stgmed: PStgMedium): HResult; stdcall;
    function OnObjectAvailable(const iid: TGUID; punk: IUnknown): HResult; stdcall;

end;

{ TBindStatusCallback }

function TBindStatusCallback.GetBindInfo(out grfBINDF: DWORD;
  var bindinfo: TBindInfo): HResult;
begin
  Result := S_OK;
end;

function TBindStatusCallback.GetPriority(out nPriority): HResult;
begin
  Result := S_OK;
end;

function TBindStatusCallback.OnDataAvailable(grfBSCF, dwSize: DWORD;
  formatetc: PFormatEtc; stgmed: PStgMedium): HResult;
var
  Stream: IStream;
  MS: TMemoryStream;
begin
  if dwSize > 0then
  begin
    Stream := IStream(stgmed.stm);
    MS := TMemoryStream.Create;
    MS.SetSize(dwSize);
    Stream.Read(MS.Memory, dwSize, nil);
    //从MS取数据...
    MS.Free;
    Result := S_OK;
  end else Result := E_ABORT;
end;

function TBindStatusCallback.OnLowResource(reserved: DWORD): HResult;
begin
  Result := S_OK;
end;

function TBindStatusCallback.OnObjectAvailable(const iid: TGUID;
  punk: IInterface): HResult;
begin
  Result := S_OK;
end;

function TBindStatusCallback.OnProgress(ulProgress, ulProgressMax,
  ulStatusCode: ULONG; szStatusText: LPCWSTR): HResult;
begin
  //可实现侦测进度，实现代码在此写
  Result := S_OK;
end;

function TBindStatusCallback.OnStartBinding(dwReserved: DWORD;
  pib: IBinding): HResult;
begin
  Result := S_OK;
end;

function TBindStatusCallback.OnStopBinding(hresult: HResult;
  szError: LPCWSTR): HResult;
begin
  Result := S_OK;
end;

调用代码：
var
 url: string;
 MyBindStatusCallback: IBindStatusCallback;

begin
  MyBindStatusCallback := TBindStatusCallback.Create;
  if Succeeded(URLOpenStream(nil, 'http://www.xxxx.com/xxx.xx', 0, MyBindStatusCallback)) then
    //下载成功!
  else
    //下载失败!;
end;