I am think I am doing something pretty basic - but I am missing something. Within iOS, I want to the user to click anywhere on the screen and then a little disk (filled oval) appears at that position.
So I created a module, with a property “Clicked” (boolean). When the Pointer is down, this is changed to true, and when it the Pointer is up, this is false.
Within the Paint method for the canvas, I have something like
if modClick.Clicked
i = Xojo.Math.Round( ClickPoint.X ) //****
j = Xojo.Math.Round( ClickPoint.Y )
g.FillOval( i, j, 100, 100 )
Then in the OPEN method of the canvas I put
Dim ClickPoint As New Xojo.Core.Point( 0, 0)
However, when I run my program, the statement above //**** gets a nil object exception. And in fact, it gets this if I comment out that Dim statement too.
So apparently I am missing something basic and fundamental. Any ideas?
Clickpoint has a scope limited to the OPEN event. ie. once the Open Event is completed, ClickPoint is no longer valid
it needs to be defined at a higher scope… relative to the Canvas, not to a specific event of the canvas
I forgot to mention, that in the PointerDown action, I also store the point
modClick.ClickPoint = pos
So that I have global access to that clicked point
I did a search, and ClickPoint is only defined in the App open - although its listed as a property in a module. But I seem to recall that it has to be explicitly dimensioned, even when listed as a property.
Maybe the problem is with how I am using a point. This seems to fly :
The REFERENCE to the object must NOT be inside any method or event… (ie. the command the creates or DIM’s it)
otherwise the Scope is limited to that location
Is it possible that you have a ClickPoint property in your object, but in the open event you do something like Dim C as new Clickpoint(0,0)?
In that case you would have created a temporary variable only instead of initializing the objects property.
Another chance would be that the object you are referring to gets out of scope.
It would otherwise help to see more of your code. Currently it is very unclear what properties and variables exist and where.
Well how should this be done? I just want to capture the “mouse click” point which is delivered to the PointerUp action, so that when the PAINT event fires, it knows that point. The way I thought to do it was to create a global static point variable within a module, which would hold the point information. But then I need to DIM it somewhere - either in a method or an action - and it does not make sense to dimension it every time the PAINT event fires, nor should it be local to PointerUp. So this seems pretty basic.
Should I make a special Canvas with a point property or something?
No, if you have an object as a module property, you dont need to Dim it. Reserving memory space ( = Dim) is done by the existence of the property in the module implicitly.
You have to initialize it anyway. Thats why the code should be like
(ModuleName).ClickPoint = New Xojo.Core.Point(0,0)
Dimming in this case would mean you created a local temporary variable with the same name as the modules property like Dave and I suspected.
Thanks. But where does that code go (your dimension)? I tried exactly what you did in the App>Open, and in a module method called by the Canvas>Open. In both cases I got compiler errors that I was trying to declare a local variable in some other way.
In this case I would recommend to put the code like above into the Canvas Open event. This way you can be sure the property will be initialized before the first draw event.
You could also avoid the clicked property if you change your drawing code and include it in a
If (ModuleName).ClickPoint <> Nil then
// your drawing code here
End If
Then in PointerDown
(ModuleName).ClickPoint = New Xojo.Core.Point(x,y)
and in PointerUp
(ModuleName).ClickPoint = Nil
The reason you are having problems with the compiler could be the scope of ClickPoint in your module.
If it happens to be Global, it acts as a Global Property and is available anywhere in you project by just its name.
With a scope set to protected, you have to qualify its namespace fully (include the module name).
If it should be private, you cannot change the property from outside the module.
