Select¶
Description¶
The Select object is a PropertyModifier object which reduces a source list to a single value using an arbitrary expression. The expression is evaluated for each element in the source list and the final result written to the target property. Select manages strong bindings between elements in the source list, the expression and the reduced value, i.e. whenever either an element in the source list or other variable parts of the evaluated expression change the target property is being updated instantly.
Select primarily is designed to use with DataObject lists. Select replaces traditional for
loops in a declarative manner which often allows simplifying the code and thus reduce its complexity.
› Inherits: | PropertyModifier |
---|
Properties¶
defaultValue¶
This property holds a value to write to the target in case select does not evaluate to true
for any source list element.
This property was introduced in InCore 2.0.
› Type: | Variant |
---|---|
› Signal: | defaultValueChanged() |
› Attributes: | Writable |
error¶
This property holds the most recently occurred error or Select.NoError if no error occurred. If the same error occurs multiple times this property does not change. Use the errorOccurred() signal to detect multiple occurrences of the same error.
› Type: | Error |
---|---|
› Signal: | errorChanged() |
› Attributes: | Readonly |
errorString¶
This property holds the current human readable error string corresponding to the current value in the error property. It may include additional information such as failure reasons or locations.
› Type: | String |
---|---|
› Signal: | errorStringChanged() |
› Attributes: | Readonly |
eval¶
This property holds an expression which is evaluated for the selected element in the source list. The respective source list element is available in a local item
property inside the expression. This allows transforming the selected item or select subproperties. If unset the item itself is written to the target.
› Type: | <QML expression> |
---|---|
› Signal: | evalChanged() |
› Attributes: | Writable, Optional |
select¶
This property holds an expression which is used to select a list item. The expression is evaluated for each item and needs to evaluate to true
for the particular item to select and write to the target.
› Type: | <QML expression> |
---|---|
› Signal: | selectChanged() |
› Attributes: | Writable |
Signals¶
Enumerations¶
Error¶
This enumeration describes all errors which can occur in Select objects. The most recently occurred error is stored in the error property.
Name | Value | Description |
---|---|---|
Select.NoError |
0 |
No error occurred or was detected. |
Select.InvalidSource |
1 |
Operation not supported for empty or non-list source. |
Select.ExpressionError |
2 |
Error while evaluating select or eval expression: <Unknown File>: . |
Select.TargetWriteError |
3 |
The result value could not be written to the target property, likely due to type incompatibilities. |
Example¶
import InCore.Foundation 2.5
Application {
List {
id: simpleValueList
items: [ 10, 20, 30 ]
}
property int simpleValue
Select on simpleValue {
source: simpleValueList
select: item > 20
}
onSimpleValueChanged: console.log("Selected simple value:", simpleValue)
MeasurementGroup {
id: measurements
Measurement { name: "A"; data: 1 }
Measurement { name: "B"; data: 2 }
Measurement { name: "C"; data: 3 }
Measurement { name: "D"; data: 4 }
Measurement { name: "E"; data: 5 }
}
property int measurementValue
Select on measurementValue {
source: measurements.objects
select: item.name === "C"
eval: item.data
}
onMeasurementValueChanged: console.log("Selected measurement value:", measurementValue)
property var threshold: 3
property bool anyMeasurementExceedsThreshold: false
Select on anyMeasurementExceedsThreshold {
defaultValue: false
source: measurements.objects
select: item.data > threshold
eval: true
}
onAnyMeasurementExceedsThresholdChanged: {
if(anyMeasurementExceedsThreshold)
console.log("There's at least one measurement above", threshold)
else
console.log("There's no measurement above", threshold)
}
Timer {
onTriggered: threshold++
}
}