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Starting the Download With the AsyncRead Method

You call the AsyncRead method when you need to download extensive information (perhaps a bitmap image) for a property or some other feature of your UserDocument. You call AsyncRead with three arguments:

  • Target. Despite its name, this argument is a string representing the path to the source of the downloaded information. Target will usually be an Internet URL or a path on an Intranet site.

  • AsyncType. This long argument can take one of three values specifying where you plan to store the information that you are going to download. This argument will help the download process determine exactly what download format to use. The possible values of the AsyncType parameter are:

    vbAsyncTypePicture (0). You plan to store the downloaded information in a Picture object such as the Picture property of a PictureBox control.

    vbAsyncTypeFile (1). You plan to store the downloaded information in a file.

    vbAsyncTypeByteArray (2). You plan to store the downloaded information in an array of type Byte.

  • PropertyName (optional). You can use this third argument to identify the download (this is especially useful if you might have more than one asynchronous download going on at once).

    You can then use the string that you assign here to PropertyName in the AsyncReadComplete event procedure or in the CancelAsyncRead method. Despite the term PropertyName, this argument does not automatically guarantee that the information will be assigned to a property. You must write code in the AsyncReadComplete event procedure to do that. Listing 14.8 initiates an asynchronous download from a site on a local network. You will put the downloaded information into a Picture object, and you will use the identifier "PRETTYPIX" to identify this download.


UserDocument.AsyncRead _
"file://g:/Intranet/Graphics/PrettyPix.gif", _
vbAsyncTypePicture, "PRETTYPIX"

After your code has initiated the asynchronous download, nothing happens until the download has finished and the AsyncReadComplete event fires—or until your user or your code decides to cancel the download, as discussed in the next session.


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