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<hr><p align="left"><small>发信人: strayli (stray), 信区: Delphi <br>
标 题: A Technical View of Borland MIDAS Part II (5) <br>
发信站: BBS 水木清华站 (Thu Nov 12 22:17:45 1998) WWW-POST <br>
<br>
The Briefcase Model <br>
<br>
<br>
The briefcase model depends on two methods of TClientDataSet called <br>
LoadFromFile and SaveToFile: <br>
<br>
CustomerClientDataSet.SaveToFile('Customer.dta'); <br>
<br>
<br>
<br>
CustomerClientDataSet.LoadFromFile('Customer.dta'); <br>
<br>
If you have two tables you want to save, then you should save and read them <br>
both to separate files: <br>
<br>
procedure TForm1.BriefcaseSave1Click(Sender: TObject); <br>
<br>
begin <br>
<br>
<br>
CustomerClientDataSet.SaveToFile(CustomerFile); <br>
<br>
OrdersClientDataSet.SaveToFile(OrdersFile); <br>
<br>
end; <br>
<br>
Clearly the act of saving and reading files from disk is trivial in the <br>
extreme. However, there are some additional points that you need to take into <br>
consideration to use the briefcase model properly. Most of these points will <br>
be covered in the next section on PacketRecords. <br>
<br>
PacketRecords <br>
PacketRecords is a very important property of TClientDataSet which you need <br>
to spend some time contemplating. In the next few paragraphs I will discuss <br>
it from several angles. In particular, I will make several references to its <br>
importance when using the briefcase model. <br>
<br>
To make the briefcase model work correctly it is probably best to make sure <br>
the files on the server are not arranged in a one to many relationship and <br>
that you have set the PacketRecords property on both the ClientDataSets to <br>
-1. Setting PacketRecords to 0 brings down the metadata, setting it to -1 <br>
brings down all the data, and setting it to some positive number n brings <br>
down n records per request. <br>
<br>
If you have already gotten the metadata for a dataset, then setting <br>
PacketRecords to -1, or to some positive number other than zero, will only <br>
retrieve data. However, if you have not gotten the metadata, then setting <br>
PacketRecords to -1 or to some positive number other than zero, will retrieve <br>
both the metadata and the records. <br>
<br>
If you want to use the briefcase model, then you usually want to bring down <br>
the whole dataset, which means you want to set PacketRecords to -1. (Of <br>
course, a query on a server could already filter a large portion of a table <br>
for you, but you would still want to set PacketRecords to -1 to retrieve all <br>
of the records from the query. ) <br>
<br>
If you just want to establish a one to many relationship, and don't care <br>
about the briefcase model, then you will probably want to set PacketRecords <br>
to zero on the detail table and to -1 on the master table. These settings <br>
retrieve all the records from the master table but only the metadata for the <br>
detail table. Then internally, Delphi will call TClientDataSet.AppendData to <br>
bring down just those detail records that are needed when viewing one <br>
particular master record. This is great for many situations, but it is <br>
probably not what you want if you are using the briefcase model. Instead, <br>
probably not what you want if you are using the briefcase model. Instead, <br>
when using the briefcase model, you will usually set PacketRecords to -1 and <br>
the master detail will still be done, but the whole detail dataset will be <br>
available on the server at all times. This is obviously impractical when <br>
working with very large datasets. <br>
<br>
Because this is such an important issue, I am going to show you how to write <br>
some code that allows you to fine tune this process. The following code <br>
represents a nonsensical case where you first retrieve the metadata and then <br>
retrieve the records. I say this is nonsensical because the metadata will be <br>
retrieved automatically the first time you access the data. However, assuming <br>
you had some reason to get the metadata first and then get all the records, <br>
you could write code that looks like this: <br>
<br>
with ClientDataSet1 do begin <br>
<br>
Close; <br>
<br>
PacketRecords := 0; <br>
<br>
Open; <br>
<br>
PacketRecords := -1; <br>
<br>
GetNextPacket; <br>
<br>
end; <br>
<br>
<br>
<br>
Or, if you wanted to be very fancy, you can study this code from Delphi guru <br>
Josh Dahlby: <br>
<br>
var <br>
<br>
RecsOut: Integer; <br>
<br>
V: OleVariant; <br>
<br>
begin <br>
<br>
CustomerClientDataSet.Close; <br>
<br>
V := CustomerClientDataSet.Provider.GetMetaData; <br>
<br>
<br>
CustomerClientDataSet.AppendData(V, False) <br>
<br>
V := CustomerClientDataSet.Provider.GetRecords(-1,RecsOut); <br>
<br>
CustomerClientDataSet.AppendData(V, True); <br>
<br>
end; <br>
<br>
<br>
<br>
In this case you first close the client dataset, then use the GetMetaData <br>
function to retrieve the metadata inside an OleVariant. At this time, there <br>
is not much you can do with the result of this function other than pass it <br>
directly to AppendData, or else start hacking away at it with a series of low <br>
level functions. However, in the future, it is reasonable to suppose that <br>
Borland may provide functions for easily accessing the information from the <br>
data packet that is stored in the OleVariant. <br>
<br>
AppendData takes two parameters. The first is the data retrieved from the <br>
server, the second is whether or not you hit EOF when retrieving the data. <br>
Remember that you don't have to use either GetRecords or AppendData, and that <br>
you should normally use GetNextPacket. Furthermore, the simplest way to <br>
perform this operation is simply to call Open or to set Active to True. I've <br>
shown you GetNextPacket and AppendData simply so you can have more control <br>
over the process if you happen to need it. <br>
<br>
<br>
-- <br>
※ 来源:·BBS 水木清华站 bbs.net.tsinghua.edu.cn·[FROM: 202.38.79.111] <br>
</small><hr>
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