[Interest] Can QML model specify a range including negative numbers? For example -180 to +180 ?

[André Somers]

Op 6-11-2015 om 14:52 schreef Edward Sutton:
>
>> On Nov 6, 2015, at 3:53 AM, André Somers <andre at familiesomers.nl 
>> <mailto:andre at familiesomers.nl>> wrote:
>>
>> Op 6-11-2015 om 10:10 schreef Curtis Mitch:
>>> You can use the columnForeground component [1] for this purpose:
>>> import QtQuick 2.3
>>> import QtQuick.Controls 1.2
>>> import QtQuick.Extras 1.2
>>> import QtQuick.Controls.Styles 1.2
>>> ApplicationWindow {
>>>     visible: true
>>>     width: 640
>>>     height: 480
>>>     title: qsTr("Hello World")
>>>     Tumbler {
>>>         id: tumbler
>>> anchors.centerIn: parent
>>> TumblerColumn {
>>> model: 90
>>>         }
>>> TumblerColumn {
>>> model: 60
>>>         }
>>> TumblerColumn {
>>> model: 60
>>>         }
>>> TumblerColumn {
>>> model: ["N", "S"]
>>>         }
>> I don't think I would keep the N/S (or E/W) option in a separate 
>> column. Why not integrate that into the first column?
>
> Can you please elaborate? Do you mean create a model with 180 items 
> ranging from "N 0" to "N 90” and "S 0" to "S 90”?
>
> Or 180 numbers and the delegate adds a “N” or “S” and subtracts 90 
> when needed?
The latter. That allows you to just specify model: 180 as your model. 
Then you let the delegate display a N or S and adjust the value to the 
right range (for instance by shifting the range from 0-180 to -90-90 and 
then displaying a negative value as S and positive value as N, removing 
the minus itself.) I guess you'll need to specify 182 though, as you 
probably want all of N 0, N 90, S 0 and S 90.
>
>> You can easily use a delegate to on the column to display negative 
>> values as S or W depending on latitude or longitude. To stick to a 
>> standard notation, you would have to put the N/S/E/W first in the 
>> column, like S 17|33|08.
>>
>> Using a delegate can also solve your model problem the whole thread 
>> started with. Just use a number again as the model, but only use the 
>> positive range from 0 to 360. Then in your delegate simply adjust the 
>> value from the model to the actual value you want to display.
>
> So a QML delegate can transform magic model numbers into real-world 
> user display data? For example a number 0 into “Apple”?
Sure.
>
> I am not sure I understand what delegates are operating on.  In the 
> case of a TumblerColumn it would be the current index?
>
> http://doc.qt.io/qt-5/qml-qtqml-models-delegatemodel.html
>
Delegates gets a styleData attached property, which in turn contains an 
index, column, value, current (bool) and displacement (distance from 
current). That is what you can use to do your rendering.

See
http://doc.qt.io/qt-5/qml-qtquick-controls-styles-tumblerstyle.html#delegate-prop

Something like

TumblerColumn {
   model: 182
   delegate: Text {
     property int degrees: styleData.value - 90
     text: degrees <= 0 ? "S " + -1 * degrees : "N " + degrees - 1
     opacity: 0.4 + Math.max(0, 1 - Math.abs(styleData.displacement)) * 
0.6 //from example
   }
}

should work I think. This should yield a column with items from S 90 to 
S 0 followed by N 0 to N 90. Did not try it though.

André


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