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  4. Is it possible to recreate the ListView.delegate model Property behavior in pure QML?

Is it possible to recreate the ListView.delegate model Property behavior in pure QML?

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  • L Offline
    L Offline
    LowLevelM
    wrote on last edited by LowLevelM
    #1

    If im using the delegate property im able to overwrite the visual appearace of the list items
    inside of the delegate all the model.item properties are available + the index of the model-item

    im nearly able to reproduce the model forwarding but have no idea how i can also get the model.index property into my delegate, could it be that the ListView implementation is doing C++ magic here that i can't rebuild with pure QML?

    Sample-Source:

    i've got a simple example with a ListModel with an attribute name and myValue, a ListView showing the model and my own list view like implementation that try to show the same content, as you can see the index is undefined in my implementation, its clear to me that i have not introduced this property but i also have no idea how to extend model property with the index attribute

    I hope someone got an idea how to replicate (at best) the interface of ListView

    DelegateModelList.qml

    import QtQuick 2.5
    
    Item {
        id: root
    
        property alias model : repeater.model // needs to be an alias
        property Component delegate
    
        Column {
            Repeater {
                id: repeater
                Loader {
                    // introduces "model" into loaded delegate scope
                    // how to add index to model?
                    property var model: repeater.model.get(index)
                    sourceComponent: root.delegate
                }
            }
        }
    }
    

    DelegateModelListTest.qml

    import QtQuick 2.5
    import QtQuick.Window 2.5
    
    Window {
        width: 600
        height: 600
    
        ListModel {
            id: testModel
            ListElement { name: "Item 1"; myValue: 10 }
            ListElement { name: "Item 2"; myValue: 20 }
            ListElement { name: "Item 3"; myValue: 30 }
            ListElement { name: "Item 4"; myValue: 40 }
            ListElement { name: "Item 5"; myValue: 50 }
        }
    
        Row {
            ListView {
                id: listView
    
                width: 300
                height: 300
    
                model: testModel
    
                delegate: Rectangle {
                    width: parent.width
                    height: 30
                    border { width: 1 }
                    Text { text: model.name+"("+model.index+"), myValue: "+model.myValue }
                }
            }
    
            DelegateModelList {
                id: myList
    
                width: 300
                height: 300
    
                model: testModel
    
                delegate: Rectangle {
                    width: 300
                    height: 30
                    border { width: 1 }
                    Text { text: model.name+"("+model.index+"), myValue: "+model.myValue }
                }
            }
        }
    }
    

    0_1563465056245_Screenshot from 2019-07-18 17-49-03.png

    Thanks for any help

    1 Reply Last reply
    0
    • S Offline
      S Offline
      shaan7
      wrote on last edited by
      #2

      I was able to achieve it like this-

              Loader {
                      // introduces "model" into loaded delegate scope
                      // how to add index to model?
                      property var model: repeater.model.get(index)
                      sourceComponent: root.delegate
      
                      Binding {
                          target: model
                          property: "index"
                          value: index
                      }
                  }
      
      1 Reply Last reply
      1
      • L Offline
        L Offline
        LowLevelM
        wrote on last edited by
        #3

        Thanks for this ultra short solution

        1 Reply Last reply
        0
        • GrecKoG Offline
          GrecKoG Offline
          GrecKo
          Qt Champions 2018
          wrote on last edited by
          #4

          Why don't you just do property alias delegate: repeater.delegate?
          You won't have to do anything then.
          Note that while having supbar performances, your solution currently only works with ListModel (or custom models providing a get function). It won't work with a generic QAbstractListModel, or a js array.
          I also believe your item won't be updated if some role data gets updated in the ListModel.

          Also you might want to remove the wrapping Item and just use the Column as the root item. It will have useful implicitWidth and implicitHeight properties.

          Can you tell us a bit more about what you are trying to achieve and why ListView or Column +Repeater don't fill your needs?

          L 1 Reply Last reply
          1
          • GrecKoG GrecKo

            Why don't you just do property alias delegate: repeater.delegate?
            You won't have to do anything then.
            Note that while having supbar performances, your solution currently only works with ListModel (or custom models providing a get function). It won't work with a generic QAbstractListModel, or a js array.
            I also believe your item won't be updated if some role data gets updated in the ListModel.

            Also you might want to remove the wrapping Item and just use the Column as the root item. It will have useful implicitWidth and implicitHeight properties.

            Can you tell us a bit more about what you are trying to achieve and why ListView or Column +Repeater don't fill your needs?

            L Offline
            L Offline
            LowLevelM
            wrote on last edited by LowLevelM
            #5

            @GrecKo

            Why don't you just do property alias delegate: repeater.delegate?
            You won't have to do anything then.

            perfect - QML is still very magical to me, where does the model property come from when using the alias?

            DelegateModelList.qml

            import QtQuick 2.5
            
            Item {
                id: root
            
                property alias model : repeater.model // needs to be an alias
                property alias delegate: repeater.delegate // prevents using loader
            
                Column {
                    Repeater {
                        id: repeater
                    }
                }
            }
            

            Also you might want to remove the wrapping Item and just use the Column as the root item

            Where to put my alias properties then?

            Can you tell us a bit more about what you are trying to achieve and why ListView or Column +Repeater don't fill your needs?

            i asked that in another post, i've already build a Flickable based version of the list which perfectly suits my animation needs, but still would use ListView if the animation behavior can be "fixed", i use this type of list in serveral places of my software so it should be delegateable etc. - but as i said i would be more then happy if ListView is capable enough, that also the reason why i try to replicate the ListView Interface a litte, just to be able to replace all the usages of ListView with my new list (without the need to change the model usage)

            https://forum.qt.io/topic/105085/can-t-get-my-listview-scrollbehavior-correct-in-my-simple-example

            L 1 Reply Last reply
            0
            • L LowLevelM

              @GrecKo

              Why don't you just do property alias delegate: repeater.delegate?
              You won't have to do anything then.

              perfect - QML is still very magical to me, where does the model property come from when using the alias?

              DelegateModelList.qml

              import QtQuick 2.5
              
              Item {
                  id: root
              
                  property alias model : repeater.model // needs to be an alias
                  property alias delegate: repeater.delegate // prevents using loader
              
                  Column {
                      Repeater {
                          id: repeater
                      }
                  }
              }
              

              Also you might want to remove the wrapping Item and just use the Column as the root item

              Where to put my alias properties then?

              Can you tell us a bit more about what you are trying to achieve and why ListView or Column +Repeater don't fill your needs?

              i asked that in another post, i've already build a Flickable based version of the list which perfectly suits my animation needs, but still would use ListView if the animation behavior can be "fixed", i use this type of list in serveral places of my software so it should be delegateable etc. - but as i said i would be more then happy if ListView is capable enough, that also the reason why i try to replicate the ListView Interface a litte, just to be able to replace all the usages of ListView with my new list (without the need to change the model usage)

              https://forum.qt.io/topic/105085/can-t-get-my-listview-scrollbehavior-correct-in-my-simple-example

              L Offline
              L Offline
              LowLevelM
              wrote on last edited by LowLevelM
              #6

              But what is the solution if i want to add a spacing Rectangle to my repeated delegate
              can i still work with alias or is then the Loader needed? how is then the aliasing of the delegate/model property handled?

              this nearly(index binding does not work) works, but is not as nice as your example(with alias), but i can't get it working, still fighting with alias put through i think

              DelegateModelList.qml

              import QtQuick 2.5
              
              Item {
                  id: root
              
                  property alias model : repeater.model // needs to be an alias
                  property Component delegate // i would like to alias, but how?
                  property real spacing
              
                  Column {
                      Repeater {
                          id: repeater
              
                          Column {
                              Loader {
                                  property var model: repeater.model.get(index)
                                  sourceComponent: root.delegate
              
                                  // does not work anymore (and i think its as "working" but no good solution, the alias is much better)
                                  Binding {
                                      target: model
                                      property: "index"
                                      value: index
                                  }
                              }
              
                              // spacing
                              Rectangle { width: parent.width; height: root.spacing }
                          }
              
                      }
                  }
              }
              
              

              DelegateModelListTest.qml

              import QtQuick 2.5
              import QtQuick.Window 2.5
              
              Window {
                  width: 600
                  height: 600
              
                  ListModel {
                      id: testModel
                      ListElement { name: "Item 1"; myValue: 10 }
                      ListElement { name: "Item 2"; myValue: 20 }
                      ListElement { name: "Item 3"; myValue: 30 }
                      ListElement { name: "Item 4"; myValue: 40 }
                      ListElement { name: "Item 5"; myValue: 50 }
                  }
              
                  Row {
                      ListView {
                          id: listView
              
                          width: 300
                          height: 300
              
                          model: testModel
              
                          spacing: 11
              
                          delegate: Rectangle {
                              width: parent.width
                              height: 30
                              border { width: 1 }
                              Text { text: model.name+"("+model.index+"), myValue: "+model.myValue }
                          }
                      }
              
                      DelegateModelList {
                          id: myList
              
                          width: 300
                          height: 300
              
                          model: testModel
              
                          spacing: 11
              
                          delegate: Rectangle {
                              width: 300
                              height: 30
                              border { width: 1 }
                              Text { text: model.name+"("+model.index+"), myValue: "+model.myValue }
                          }
                      }
                  }
              }
              
              

              0_1563515708912_Screenshot from 2019-07-19 07-54-57.png

              i want to solve my animation problem AND want to understand how QML works inside

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              0
              • GrecKoG Offline
                GrecKoG Offline
                GrecKo
                Qt Champions 2018
                wrote on last edited by
                #7

                If you made the Column the root item, you could put aliases there. Your Item is useless and even removes features from your Column.
                Column also has a spacing property, so no need to add your own.

                perfect - QML is still very magical to me, where does the model property come from when using the alias?

                Those are set as context properties for each delegate when they are instantiated by the ListView

                L 1 Reply Last reply
                0
                • GrecKoG GrecKo

                  If you made the Column the root item, you could put aliases there. Your Item is useless and even removes features from your Column.
                  Column also has a spacing property, so no need to add your own.

                  perfect - QML is still very magical to me, where does the model property come from when using the alias?

                  Those are set as context properties for each delegate when they are instantiated by the ListView

                  L Offline
                  L Offline
                  LowLevelM
                  wrote on last edited by LowLevelM
                  #8

                  @GrecKo

                  Column also has a spacing property

                  a that works

                  DelegateModelList.qml

                  import QtQuick 2.5
                  
                  Column {
                      property alias model : repeater.model
                      property alias delegate : repeater.delegate
                  
                      Repeater {
                          id: repeater
                          delegate: item
                      }
                  }
                  
                  

                  but what if i still want to add for example a green rectangle to every item inside my DelegateModelList.qml, as long as i just alias everything around it seems to work - but when i try to add some other components i need to use the Loader and everything gets messy, how would you add (just for the sake of it) a rectangle spacing feature to this example without using the spacing of the column? the spacing should not be part of my delegate component, only in my list

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                  0
                  • GrecKoG Offline
                    GrecKoG Offline
                    GrecKo
                    Qt Champions 2018
                    wrote on last edited by GrecKo
                    #9

                    You could do it like so:

                    import QtQuick 2.5
                    
                    Column {
                        id: root
                        property alias model : repeater.model
                        property Component delegate
                    
                        Repeater {
                            id: repeater
                            delegate: Column {
                                id: column
                                property var _model: model
                                width: parent.width
                                Rectangle {
                                    width: parent.width
                                    height: 10
                                    color: "lightblue"
                                }
                                Loader {
                                    property var model: column._model
                                    width: parent.width
                                    sourceComponent: root.delegate
                                }
                            }
                        }
                    }
                    

                    What it does is that it explicitely set a model property (equals to the model context property of the delegate) in the Loader. Properties of a Loader are exposed as context properties in its loaded object.

                    L 1 Reply Last reply
                    1
                    • GrecKoG GrecKo

                      You could do it like so:

                      import QtQuick 2.5
                      
                      Column {
                          id: root
                          property alias model : repeater.model
                          property Component delegate
                      
                          Repeater {
                              id: repeater
                              delegate: Column {
                                  id: column
                                  property var _model: model
                                  width: parent.width
                                  Rectangle {
                                      width: parent.width
                                      height: 10
                                      color: "lightblue"
                                  }
                                  Loader {
                                      property var model: column._model
                                      width: parent.width
                                      sourceComponent: root.delegate
                                  }
                              }
                          }
                      }
                      

                      What it does is that it explicitely set a model property (equals to the model context property of the delegate) in the Loader. Properties of a Loader are exposed as context properties in its loaded object.

                      L Offline
                      L Offline
                      LowLevelM
                      wrote on last edited by LowLevelM
                      #10

                      @GrecKo
                      thanks alot, getting more into QML in 20minutes writing with you then 2 days of trial&error :)

                      also seen my current ListView question with the animation, any idea or is that really something that needs a special list?

                      1 Reply Last reply
                      0
                      • GrecKoG Offline
                        GrecKoG Offline
                        GrecKo
                        Qt Champions 2018
                        wrote on last edited by
                        #11

                        Your other question would require trial and error for me, I guess it can work but I don't know how.
                        Sorry, I can't really take the time to answer it.

                        L 1 Reply Last reply
                        0
                        • GrecKoG GrecKo

                          Your other question would require trial and error for me, I guess it can work but I don't know how.
                          Sorry, I can't really take the time to answer it.

                          L Offline
                          L Offline
                          LowLevelM
                          wrote on last edited by
                          #12

                          @GrecKo

                          Sorry, I can't really take the time to answer it.

                          absolutely no problem - you already helped enough - thank you

                          1 Reply Last reply
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