Reading inline data using <Object> data property 
Author Message
 Reading inline data using <Object> data property

Does anybody know how to use the data property of the HTML <object> tag to
instantiate an ActiveX control? I ran across a reference to this in
Knowledge Base article Q201515 and in the Web Workshop. Unfortunately the
Web Workshop discussion relates to C code and I haven't found anything
explaining this topic for VB.


Sat, 07 Jun 2003 10:06:07 GMT  
 Reading inline data using <Object> data property
Hi Kelina,

If the control is created in C++, it must implement the IPersistPropertyBag
interface and use its Save method to persist the properties. If the control
is created in VB, we can use the PropertyBag to persist its properties.
Here is a article from MSDN:


Persisting a Component's Data

Most components have properties; in most cases you'll want to establish
default values for those properties in the Initialize event of the class.
Those default values are frozen when you compile the component, so how do
you allow a developer to change the default values to meet their own
special conditions? Classes have a special property, Persistable, that
allow you to store a component's values between instances.

Suppose that you had an ActiveX DLL that calculates loans, with an
InterestRate property used in the calculations. You could initialize the
InterestRate to some arbitrary value, but since interest rates periodically
go up or down, the InterestRate property would need to be modified each
time the component is run. With class persistence, you can store the
InterestRate value and modify it only when the interest rate changes. Each
time your component is run it can retrieve the InterestRate from storage,
so the component will always provide the latest rate.

While ActiveX controls have always been able to persist their data;
persistence for ActiveX components is slightly different. A control stores
property settings inside it's .cls file, but a component can't do that.
Instead, it uses a PropertyBag object that can be saved just about anywhere
C in a file, a database, a cell in a spreadsheet, or even in the registry.

Setting Up Class Persistence
In order to be persistable, a class must meet two conditions: it must be
public and creatable. If you think about, this makes sense C after all,
persistence wouldn't be useful in a private component. If a class meets
both conditions, the Persistable property appears in the Properties window.

By default, the Persistable property is set to 0 (NotPersistable). By
changing this value to 1 (Persistable), three new events are added to the
class: ReadProperties, WriteProperties, and InitProperties. As you might
guess, these events are use to read, write, and initialize the class's

Persisting a Property
You can mark a property as persistable by implementing the PropertyChanged
method in a Property Let or Property Set procedure, as in the following

Private mInterestRate As Single
Public Property Let InterestRate(newRate As Single)
   mInterestRate = newRate
   PropertyChanged "InterestRate"
End Sub

Calling the PropertyChanged method marks the InterestRate property as
dirty. The WriteProperties event will fire when the class is terminated if
any property in the class has called PropertyChanged.

The ReadProperties, WriteProperties and InitProperties Events
The WriteProperties event procedure is used when a class is terminating to
write the current property values to a private storage known as a
PropertyBag object. The following code is used to save a property to the
built-in PropertyBag:

Private Sub Class_WriteProperties(PropBag As PropertyBag)
   PropBag.WriteProperty "InterestRate", mInterestRate, conDefaultRate
End Sub

The Property Bag's WriteProperty method in the above code takes three
arguments: the name of the property to save ("InterestRate"), the value to
save (mInterestRate), and a default value (DefaultRate). If the new value
matches the constant conDefaultRate, the WriteProperty method doesn't have
to write out the value.

The ReadProperties event is fired when a class is initialized C but only
if the PropertyBag has something in it. If the PropertyBag is empty, the
InitProperties event will be fired instead. Code in the ReadProperties and
InitProperties events is used to set the initial property values:

Private Sub Class_ReadProperties(PropBag As PropertyBag)
   mInterestRate = PropBag.ReadProperty("InterestRate", conDefaultRate)
End Sub
Private Sub Class_InitProperties ()
   mInterestRate = conDefaultRate
End Sub

Note that the constant conDefaultRate is used in both procedures to provide
a default value. By using a constant to define the default values you
eliminate the risk of accidentally defining different default values in
different procedures.

When you first create an instance of the Loan class using the New keyword,
the InitProperties event will be fired; once the class has been persisted
and is then recreated, the ReadProperties event will be fired instead.

Using the PropertyBag Object to Persist an Object
In order to persist an ActiveX component, you need to create an instance of
a PropertyBag object. This may seem redundant ?a after all, the class
already has it's own PropertyBag. Why can't you just use that? Simple. When
the object goes away, so does its PropertyBag. It only exists in memory;
for persistence you need to store a copy of the object somewhere so you can
retrieve it later.

Think of a PropertyBag as a sack that you can fill up with stuff and stash
away somewhere for safe keeping. Where you stash it is entirely up to you.
The following form code demonstrates how you can persist an object to a
text file:

Private pb As PropertyBag      ' Declare a PropertyBag object.
Private LoanObject As Loan      ' Declare a Loan object.

Private Sub Form_Unload(Cancel As Integer)
   Dim varTemp as Variant

   ' Instantiate the PropertyBag object.
   Set pb = New PropertyBag
   ' Save the object to the PropertyBag using WriteProperty.
   pb.WriteProperty "MyLoanObject", LoanObject
   ' Assign the Contents of the PropertyBag to a Variant.
   varTemp = pb.Contents
   ' Save to a text file.
   Open "C:\Loandata.txt" For Binary As #1
   Put #1, , varTemp
   Close #1
End Sub

The Contents property of the PropertyBag object contains the Loan object
stored as an array of bytes. In order to save it to a text file, you first
must convert it to a data type that a text file understands ?a in this
case, a Variant.

Depersisting An Object
Once the object is contained inside a text file (or any other type of
storage), it can easily be transported to another location. Imagine that
our Loan object contains not only the InterestRate, but also property
values to represent all of the fields in a loan application. You could take
the Loandata.txt file and send it to the central office for approval. The
code for a form that would reuse the Loan object would look something like

Private pb As PropertyBag      ' Declare a PropertyBag object.
Private LoanObject As Loan      ' Declare a Loan object.

Private Sub Form_Load()
   Dim varTemp As Variant
   Dim byteArr() as Byte

   ' Instantiate the PropertyBag object.
   Set pb = New PropertyBag
   ' Read the file contents into a Variant.
   Open "C:\Loandata.txt" For Binary As #1
   Get #1, , varTemp
   Close #1
   ' Assign the Variant to a Byte array.
   ByteArr = varTemp
   ' Assign to the PropertyBag Contents property.
   Pb.Contents  = ByteArr
   ' Instantiate the object from the PropertyBag
   Set LoanObject = pb.ReadProperty("MyLoanObject")
End If

You may have noticed that the object had to be assigned three times: first
from the text file to a Variant, then from a Variant to a Byte array, then
to the Contents property. That's because the Contents property will only
accept a Byte array ?a if you tried to assign any other data type you would
get an error.

So what's going on here? Can you actually take an object created in one
place and reuse it in another, complete with its data? Well, not exactly.
The original object is long gone. What you are passing in a PropertyBag is
an exact copy of the object, not the object itself. This ability to "clone"
an object for reuse is a powerful concept, especially when it comes to
designing workflow applications.


Sun, 08 Jun 2003 18:00:31 GMT  
 [ 2 post ] 

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