As documented the TEXT_BY_POS message may be used to retrieve the contents of a nominated cell. What is not documented is the fact that you can use it to update the contents of a cell as well. It's simply a matter of adding an extra parameter containing the data you wish to set.

E.g.



OpenInsight trivia bonus: The EditTable CELLPOS property is a simple wrapper around the TEXT_BY_POS message.

Labels: EditTable, EditTable Cookbook, OpenInsight

While recently adding a new window into our internal admin system we ran into a subtle problem with the CHANGED event and the NOTIFYPOS property.

NOTIFYPOS is an EditTable property that is always updated to contain the coordinates of the last cell to raise an event, but this is not restricted to the CHANGED event: Any EditTable event that is cell-oriented will also update NOTIFYPOS when triggered, common examples being DBLCLK and POSCHANGED.

In our case the sequence of events ran like so:

1. The user edited data in a cell and hit the down arrow to move to the cell beneath.


2. The EditTable registered that the data had changed and set NOTIFYPOS to the edited cell position.


3. The EditTable raised a CHANGED event.


4. The EditTable registered that the cell position had changed and set NOTIFYPOS to the position of the 'new' cell.


5. The EditTable raised a POSCHANGED event.


6. The Basic+ event handler for the CHANGED event executed - but now NOTIFYPOS was pointing to the 'new' cell, not the edited one - and our code mangled the data!

Now you'd think that step (6) would actually have taken place straight after step (3) but unfortunately that's not the case due to the way that OpenInsight communicates with OpenEngine to run Basic+ event handlers.

Normal Basic+ event handlers are executed in an asynchronous fashion, i.e. they are not executed directly when the notification is received, but are placed into a queue and executed when the queue is processed by the application's "message pump" (For those of you familiar with the Windows API they are dispatched via the PostMessage function).

The solution to the problem was to ensure that our Basic+ event handler ran in a synchronous fashion instead - i.e. it should be executed as soon as OpenInsight is notified by the EditTable that the CHANGED event has taken place. That way we know that NOTIFYPOS will still contain the correct coordinates when our handler runs.

Doing this was simply a matter of qualifying the CHANGED event with the synchronous flag in the window CREATE event handler like so:



The new sequence of events now ran like this:

1. The user edited data in a cell and hit the down arrow to move to the cell beneath.


2. The EditTable registered that the data had changed and set NOTIFYPOS to the edited cell position.


3. The EditTable raised a CHANGED event.


4. The Basic+ event handler for the CHANGED event executed


5. The EditTable registered that the cell position had changed and set NOTIFYPOS to the position of the 'new' cell.


6. The EditTable raised a POSCHANGED event.

Labels: EditTable, EditTable Cookbook, OpenInsight

For any of you using our Basic+ Source code publisher we've now got an update to correct a Notes parsing issue when creating content for the Revelation Forum.

You can download version 1.5.1 from here in standard RDK format or here as an NSIS installer version.

Labels: Basic+, freeware, OpenInsight, ZZ_IDE_BP_PUBLISHER

Many of the applications we write need to display things like option dialog boxes near a specific control. In most cases this is quite easy to handle as we can easily obtain the SIZE property of a control and work out our positioning from that. A slightly more difficult task is to position something relative to an EditTable cell because OpenInsight doesn't expose this information as a property or a method.

We've seen many attempts to calculate cell coordinates in Basic+ - we've even done it a few times ourselves and it's quite a pain, having to take into account all the different styles of the edit table, the width of columns, which columns are hidden and so forth.

Well, there's a really easy way to do this, and that's by asking the EditTable itself what the coordinates are via the standard Windows API SendMessage function. We just need to know what message to send to the EditTable.


DTM_READCELLRECT

The message we need is called DTM_READCELLRECT, and it returns the coordinates of the cell identified via the ACCESSPOS property. All we need to do is pass it the address of a RECT structure to fill in, which we then translate into a dynamic array which we can use further.

Here's a simple function to demonstrate this:



(The more pedantic amongst you may notice that we didn't reset ACCESSPOS after we updated it. The reason for this is simple - every low-level function in the EditTable updates ACCESSPOS to the required coordinates before executing, and you should never assume ACCESSPOS is at the correct coordinates - always set it yourself before use!)

You can download a text version of edt_GetCellRect here

Labels: EditTable, EditTable Cookbook, OpenInsight

Whenever we have to deal with raw pointers in Basic+, as we did recently, we frequently end up using the GetPointer function. The documentation for GetPointer states that you should always use the LockVariable statement to "lock the string" before retrieving the pointer, but what does that actually mean?


In the beginning...

OpenInsight was originally designed as a 16-bit application running on a 16-bit platform (Windows 3.x), and that platform had a habit of moving chunks of memory around for performance reasons which meant that direct pointers to the memory could be made invalid unless certain steps were taken to guard against this.

So that pointers could be used safely in a Windows 3.x, program memory was tracked by handles instead, and when we wanted a pointer we "locked" the memory via it's handle and got a pointer to it which we could then use, because Windows promised not to pull the bytes out from under our feet.

This is essentially what the Basic+ LockVariable statement did. It marked the memory section used by the Basic+ variable as unmoveable, which meant that a subsequent GetPointer() statement got a valid pointer to it. Afterwards we used the UnlockVariable statement to release the memory section so Windows could move it as needed.


Did you say "did"?

Yep - Memory management changed with the advent of 32-bit Windows operating systems - each application (or "Process" to use the proper term) could access memory in a linear fashion via a virtual address space. This basically meant that the OS handled mapping all pointers to "real memory" internally - no need for any more handle and locking shenanigans.

This of course rendered the primary function of the LockVariable statement obsolete on 32-bit operating systems, but we can't just do away with it entirely because it can perform one other very important function: variable-type coercion.


Basic+ Variable Typing

As we mentioned in our previous post variables in Basic+ are typeless - i.e. they can change their type at runtime based on the context in which they are used. When we're dealing with a typed language like C/C++, we must ensure that any Basic+ variables we pass to a DLL function are actually held internally in the correct binary format.

This is where the other function of the LockVariable statement comes into play - it allows us to specify how we want the Basic+ variable to be typed.

E.g.



Likewise for a string we could do this:



Of course there are other ways of coercing a variable to a certain type:

1. Adding 0 to a numeric string will ensure it is held in numeric format


2. Concatenating an empty string to a numeric variable will ensure it's held as a string.

We must beware of using method (1) above to ensure a number however, because Basic+ holds numeric variables in one of two binary formats internally - one as an integer, the other is as a floating point number. The "adding 0" method will not let us specify which one to use - only the LockVariable statement can do that, so when we need to ensure a type we must use it in these circumstances.


So do we really need LockVariable for strings then?

Providing that we're sure of how Basic+ is holding the format of a variable we can actually get away without using the LockVariable statements in our programs, and call GetPointer directly.

However, we would recommend using it in your programs in case the memory model that OpenInsight uses changes at some point in the future. For example, if it changed to a fully garbage-collected environment akin to the .NET runtime, we might need to lock or "pin" the variable in memory. Basically it's just a good habit to adopt.


What about the UnlockVariable statement?

If we're going to use LockVariable then we must also ensure that we call UnlockVariable. Right now it does absolutely nothing whatsoever. Of course that may not always be the case, so we'd recommend you use this too.

Labels: DLL, DLL Prototyping, OpenInsight, Windows API

Welcome to part 3 of the "Strings and Things" series on OpenInsight DLL prototyping. So far we've covered the theory of how Windows defines functions that use ANSI and Unicode versions (in Part 1), and we've also looked at the standard way of calling those functions from Basic+ (in Part 2).

This time we're going to look at another way of passing string data to a Windows API function, but we'll show you how to handle the entire process yourself, and afterwards we'll look at why this is sometimes necessary.

(Note: This article assumes you are familiar with the basics of DLL prototyping in OpenInsight. If not please consult the OpenInsight on-line help for more details)


Passing Basic+ variables as string parameters - The Long Way Round

Previously we looked at a method whereby OpenInsight took care of all the low-level details for you when passing string parameters, but you do have the option to handle this process yourself via the following steps:

1. Prototype the DLL function so that it expects a raw pointer to be passed rather than a Basic+ variable.


2. Ensure that the variable is null-terminated - i.e. that it ends with a Char(0).


3. Encode the Basic+ variable as a Unicode or ANSI string depending on the function you are passing the data to, taking into account your application's UTF8 mode.


4. Get a pointer to the variable that you want to pass to the DLL function.


5. Invoke the function


6. Clean up any loose ends.

It appears like quite a lot of work but it's not as bad as it looks when you put it into practice.


Creating the LPVOID prototype

The first task you have to do is prototype the DLL function, and for now we'll use the same Unicode SetWindowText function that we used before so you can see the difference. Here's how it's documented:



Which we know is actually means this for the Unicode version:



So far so good but as you're handling the string passing yourself the prototype must be created like this instead:



Notice that for the string parameter we are now using the LPVOID prototype rather than the LPWSTR prototype as we did in Part 2. This means that when we call SetWindowTextW OpenInsight is expecting us to pass a pointer to the string data rather than the string data itself.

Now we have the prototyped function we can look at using it in a program, but as mentioned above we have to do some preparation to the Basic+ variables before they can be used.


Null termination

Strings used in C use a Char(0) as an end of string marker (i.e. it is "null-terminated"), so you must ensure that any strings you pass also follow this convention - simply appending a Char(0) to the end of the string will suffice here.



String Encoding

Before the string is passed to the DLL function you must make sure that it is encoded correctly. OpenInsight provides an easy way to do this via the str_ANSI and str_Unicode functions, which automatically take into account your application's UTF8 setting:



The str_ANSI and str_Unicode functions are very useful and save you a bit of coding, otherwise you would have to do something like this to get a Unicode string:



A note on variable typing

Variables in Basic+ are typeless - i.e. they can change their type at runtime based on the context in which they are used (They are actually very similar to the Variant type in Microsoft COM/OLE programming). While having typeless variables is very convenient from a standard Basic+ programming viewpoint, it's not so helpful when you're dealing with a typed language like C/C++, so you must ensure that any Basic+ variables you pass to a DLL function expecting strings are being held internally by OpenInsight as strings also.

There are several ways this can be done but the most common is to use the concatenation operator and append a null variable like so:



For our purposes in the SetWindowText example the act of appending a Char(0) to the variable we are passing performs any required coercion to a string (as would the Unicode encoding functions too actually).


Getting the pointer

All that remains to do now is obtain a pointer to the OI string so it can be passed to the DLL function, and for this there is the aptly-named GetPointer function:



(You'll notice the use of the LockVariable statement here as well - We're going to fully cover LockVariable later in separate post.)


Putting it altogether

And you're now in a position to finally call your DLL function. As an example look at how you would use all this with SetWindowText:



Clean-up

Any clean-up tasks you may need to perform are somewhat dependant on the actual function called, but the primary cleanup task is to call the UnlockVariable statement if you used the LockVariable statement as you can see in the example above. Another common occurrence is to convert a returned string to the correct OI string type as you can see from the GetWindowText example below:



So, tell me again why I need to do this?

Well, as well as being a good intellectual exercise and helping understand how OpenInsight works behind the scenes there's one important area where the ability to pass a pointer is really critical - and that's when you need to pass a NULL pointer (i.e. the numeric value '0') to a function.

For example many Windows API functions exhibit special behaviour when passed a NULL pointer such as returning the length of a buffer needed to contain a value (e.g. the GetShortPathName function), so it is quite important to be able to do this. However, you cannot to this with the LPWSTR and LPASTR prototypes we looked at in part 2, because they will always pass a pointer to something even if it's a null OpenInsight variable!

e.g. This will NOT work:



Neither will this:



The first example will end up passing a pointer to a Char(0) (i.e. an empty string), while the second example will pass a pointer to an ASCII '0' character.

The proper way to tackle this is:



The best of both worlds

Of course having to do all of this work just to pass a NULL pointer seems slightly unreasonable, so thanks to the magic of DLL function aliasing you can "have your cake and eat it". It's simply a matter of how you prototype the function - you prototype one version to use the LPWSTR/LPASTR prototype, and another to use LPVOID, just ensuring that you give them different names.

E.g. continuing with the GetShortPathName example here's how you would prototype the functions:



Conclusion

That concludes this small series of posts on string handling in DLL Prototyping. In the next post in the series we'll take a closer look at the LockVariable statement and why it is still relevant in 32-bit OpenInsight.

Labels: DLL, DLL Prototyping, OpenInsight, Windows API