|
Howto
create a new thread and download asynchronously
In the
latest API-Guide version, the Internet Update function
performs its task asynchronously. The WinInet-functions
provide a way to download files asynchronously,
without having to create a second thread, but after
weeks of trying to find out how these functions
worked, I decided to create a new thread and perform
the Internet tasks synchronously in that new thread.
Multi-threading
without API is VERY difficult! Without API, you have
to create a server application and you need tons of
code to get it working.
With API, we need three lines of code to call a
function asynchronously. Here's how we did it:
Create a
new project and add a module to it.
Place this code in the module:
Declare Function
CreateThread Lib "kernel32" (lpThreadAttributes
As Any, ByVal dwStackSize As Long, ByVal
lpStartAddress As Long, lpParameter As Any, ByVal
dwCreationFlags As Long, lpThreadID As Long) As Long
Declare Function CloseHandle Lib "kernel32"
(ByVal hObject As Long) As Long
Declare Sub Sleep Lib "kernel32" (ByVal
dwMilliseconds As Long)
Sub AsyncTest()
Sleep 10000
End Sub
If we
would call the AsyncTest-function like a normal Visual
Basic function, the program would be completely locked
for 10 seconds. However, if we call this function
asynchronously, we won't even notice that the function
is "sleeping".
Dim hThread As
Long, hThreadID As Long
hThread = CreateThread(ByVal 0&, ByVal 0&,
AddressOf AsyncTest, ByVal 0&, ByVal 0&,
hThreadID)
CloseHandle hThread
The following part
is quite simple. Replace the sleep function with the
download code, and your asynchronous function is
complete.
We have an example on our site that shows you how to
use the WinInet-functions to download a file. This
example can be found here.
|
