[SOLVED] ListView's flickable problems

[Milnadar]

Hi everyone.

I noticed that when (vertical) listView's items has different height, contentHeight value doesn't equal to height of all items. When changing ListView's height, contentHeight may be larger or smaller than it should be. Why is it so? How does ListView count it's contentHeight?
The same issue when items have different height, and i am swapping two items and trying to flick listView on item's height by changing contentY value;

[edit : typo in title fixed, Eddy]

[Alicemirror]

Hi, please can you post a piece of your code ? ListViews are objects that don't need to implement the flickable mechanism because they are already flickable objects by default.

[Milnadar]

I understand that they are flickable objects. The problem is that if items have different height, the listView's contentHeight is counted not correct;
Unfortunaly i am not able to post my code here, because it is to much of it, but i'll try to post an example.
ListDelegate.qml
@Compenent{
Item{
height: edit.height
TextEdit{
id: edit
text: someText //(role)
}
}
}@

The sometext value may be multiline, so each delegate may have different height

[Alicemirror]

Mmmh... I see. The problem I think maybe in the text. As a matter of fact I have expereienced that the height of your edit example is not always correct if the text is higher. You should check the height of the text.

But, just to be clear, I understand that you don't need to show your code, but this example is so simple. It is difficult to disucss about something that is not possible to see.

[Milnadar]

Thanks for your help. I think i found where my bug is. TextEdit height depends on paintedHeight value, and when list is scrolling it is "jumping" from one value to another. Is it because of that paintedHeight depends on visibility of the text on the screen?

[Alicemirror]

Partially true. The real text height depends on the font size and the number of characters. Then the program shows text with word wrap (I suppose yes in this case). This means that you can't know before how much lines your text is really height. Thus you should manage the height of every list element based on the painted height. Eventually you can decide in your model to put a control like elide the test that exceeds the previous decided height or if the painted height exceeds the height of your list model you should remove lines or words from your sentence before it is shown.

[Milnadar]

Thanks again for help.

[Milnadar]

Even if i set all delegates height to 20, and to one of them to 50, contentHeight value counts not correctly when scrolling the listView

[Alicemirror]

Thank you for this news. I had not experienced this condition but I am developing an application that can happen. So I manage it a workaround before.

[Milnadar]

I was thinking to set ContentHeight manually, by counting every item's text's length, and number of lines of text, but! ListView just ignores my value, and uses it's own. :)
Is it ok?

[Alicemirror]

I think yes. The reason will be that you try setting the value of the element that is built automatically depending on the ListModel, where you define the aspect of the elements that formerly are part of the generic abstract representation of any element of the list.

ListView just fill the model based on the ListElements (or any other element i.e. the XmlList). I think that you should try to set the sizes in the ListModel. Thus when you define the model you don't know yet what will be the size of that particular instance of the model when the ListView will fill it with the ListData. If in the model you create an area that changes depending on every element content I think that everything works fine.

A short advice in your case: if you set a text in a rectangle the object that you manage as the size reference is the rectangle, despite the text size. To avoid this problem you should do something like the following:

Create a rectangle set with the initial width and height you want.

Set the rectangle opacity to 0 if it should be hidden: you only need a fake object

Create a text object and bind the rectangle height to the full text height: this means that depending by how much text you have the rectangle will be always height to contain it.

Align the text object on top margin of the rectangle (one over the other)

Try it and let me know if it works. I you have some doubt I will try to create this kind of coupled object.

[Alicemirror]

See the following example. Sorry but I get it from a more complex piece of code so I think it can't work. BTW you can get the idea applying it in your code without difficult.
@
// Login form for registered users
Item {
id: login

width: parent.width;
// Height is set based on the height value of the text area.
height: background.textHeight

signal newuserClicked; signal loginClicked

[... some custom properties definition ...]

// Main background recatngle with information text
Rectangle {
    id: background
    width: parent.width
    property alias textHeight: textIntro.paintedHeight
    height: parent.height
    anchors.horizontalCenter: parent.horizontalCenter
    anchors.bottom: parent.bottom
    color: "#d1cece"; border.color: "#eeeeee";
    border.width: Geometry.getFormBorder();
    radius: Geometry.getRadius()
    // Introductory text over the login area.
    Text {
        id: textIntro
        width: background.width * 0.95; anchors.top: background.top
        anchors.topMargin: 5;
        anchors.horizontalCenter: background.horizontalCenter
        color: "#ffffff"
        wrapMode: TextEdit.Wrap; font.pointSize: Geometry.getFontPoints();
        text: login.loginIntro
    }
}

[... other stuff ...]
}
@

Hope it can help you.

[Milnadar]

Or it is me who make it wrong, or i don't know :)
As for rectangles and text inside of it, in my current code text is in rectangle already, and my delegate's height depends on text height and if Loader component is loaded, it's height added to delegate's height, and there is also separator line to separate two types of items in the view; I thought that something in my current code is making contentHeight value "jumping", so i just created small example project and test everything using it;
I also noticed that contentHeight value changed when item with higher height scrolls away or returns to screen; So i think that contentHeight depends on items visibility or smth.
p.s. tried to set item's height from model - didn't help
Here is example that i am using
ListDelegate.qml
@
Component{
Item{
height: layout.height
Rectangle{
id: layout
height: index == 3 ? 50 : 20
TextEdit{
id: edit
height: paintedHeight
font {pixelSize: 20}
text: title
}
}
}
}
@

ListModel.qml
@
ListModel{
id: model
ListElement{
title: "this is test text"
}
ListElement{
title: "World"
}
ListElement{
title: "Some more text"
}
ListElement{
title: "And some more :)"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
ListElement{
title: "Dummy text"
}
}
@

main.qml
@
import QtQuick 1.0

Rectangle {
width: 360
height: 360

ListView{
    id: view
    anchors {fill: parent;}
    spacing: 5
    delegate: ListDelegate{}
    model: ListModel{}
    Rectangle{
        anchors {fill: view.contentItem;}
        color: "transparent"
        border {color: "red"; width: 2;}
    }
    onContentYChanged: {console.log(contentHeight)}
}

}

@

[Milnadar]

Project from which you posted part of code, as i understand there are no problems with contentHeight?
Or they are just invisible? )

[Alicemirror]

This is the solution that I have adopted to solve the problem of content height.

Two strange things in your example: ListDelegate.qml is the ListItem ? It seems you have misunderstood the role of the three elements Item, Model and Delegate.

[Milnadar]

It's probably when i say item i mean the item in the list, and when i say delegate, i mean the delegate. I think i do understand the role of three elements, and just build my sentences wrong.
Could you please explain what you find strange in that example?
ListDelegate.qml it is a delegate;

[alex2202375]

I have the similar problem with ListView. The each model data provides one line of text, and the delegate shows the text with Text. And then I find two problems:

1. If I use ListView.positionViewAtIndex(index, ListView.Beginning) to position the view at certain line of text, the contentY is always incorrect.

2. If the text has large number of lines, after flicking more than 200 times, the ListView stops loading text from model. Then the listView becomes blank, no matter flick up or down...

[Alicemirror]

Hi,

later today I should manage the two problems of the variable text and a text list. I apply the example I posted before in a text-only list so we can check with an example.

@alex: is possible that it is a problem of memory? The documentaion reports that the best behavior in a list is obtained when you load few elements at a time and update the list at the end of the scrolling. Thus you should manage a large number of lines loading them by blocks. As a matter of fact it is not userful to have a list longer so much than the number of elements that can be shown at a time.

A general consideration to your issue is: a correct method to show variable height elements in a list exist because most of the lists i.e. the twitter feeds programs have a final button - usually thicker than the other - showing "load more ..."

As I have a working piece of code I will post here.

[alex2202375]

Alicemirror: I don't think this is a memory problem, because I use QAbstractListModel to provide the data, and following is the data function that I used:
@QVariant RFileModel::data(const QModelIndex & index, int /role/) const {
int row = index.row();
return data(row);
}

QVariant RFileModel::data(int row) const {
// return QVariant();
if (row < m_file_line_pos.count()) {
if (m_file->seek(m_file_line_pos[row])) {
QTextStream stream(m_file);
stream.setCodec(m_codec);
stream.setAutoDetectUnicode(true);
QString str = stream.readLine();
if (str.length() || !m_file->atEnd()) {
qDebug()<<"Read str row"<<row
<<"pos"<<m_file_line_pos[row]
<<"length"<<str.length()<<str;
return str+KNewLine;
}
else {
qDebug()<<"read file line failed, line number"<<row
<<"Position"<<m_file_line_pos[row]
<<m_file->errorString();
}
}
else {
qDebug()<<"could not seek to position/line"<<m_file_line_pos[row]<<row;
}
}
else {
qDebug()<<"row number is too large:row/max"<<row<<m_file_line_pos.count();
}
return QVariant();
}@

[Alicemirror]

Thank yo Alex, just a ping :) I'll answer to this shortly