Performance Penalty when using Text.implicitWidth in QML

Question

In the Documentation of Itemyou can find this:

Note: Using implicitWidth of Text or TextEdit and setting the width explicitly incurs a performance penalty as the text must be laid out twice.

So I should not write something like this:

Text {
    width: implicitWidth
    text: 'my text defines the width'
}

This means, if I don't set the width by some other means, I can not meaningful anchor to the Text.

I wonder if the same performance penalty applies to this structure:

Item {
    id: myAnchorableTextBoundingBox
    width: myText.implicitWidth
    height: myText.implicitHeight
    Text {
        id: myText
        text: 'my text defines the width'
    }
}

Or is this a probable workaround for this usecase?

It even allows me to elide a maximum width like this:

Rectangle {
    color: 'transparent'
    border.color: 'red'
    width: myText.width + 2
    height: myText.height + 2
}

Rectangle {
    id: myAnchorableTextBoundingBox
    y: 1
    x: 1
    border.color: 'black'
    width: myText.truncated ? myText.width : myText.implicitWidth
    height: myText.implicitHeight
    Text {
        id: myText
        text: 'my text defines the width until it elides, then the width is used as limit'
        elide: Text.ElideRight
        width: 200
    }
}

I would imediately build a Component out of this, if I can be sure, there won't be (too large) a penalty.

Tim Tim · Accepted Answer · 2016-12-19T20:40:44

This workaround should work just fine. As the documentation states, the performance penalty is only an issue when setting the width as well:

Using implicitWidth [...] and setting the width explicitly incurs a performance penalty

In this case, you are only reading the implicitWidth property, and not then setting the width of the Text element, so the warning does not apply.

Performance Penalty when using Text.implicitWidth in QML

1 Answers