QML - Tutorial 038. Using the clip property to crop child objects inside Item or Rectangle

clip, OpacityMask, ListView, QML

Content

Quite often, the same question arises on the forum regarding the problem of displaying content embedded in any QML objects.

The most common manifestation of this problem is when ListView content scrolls beyond its scope. In any case, this is one of those frequent cases when beginners encounter this problem when starting to learn QML. Well, or another option when faced with such a problem when using Rectangle objects.

In the following image below, you can see two application windows that illustrate this problem.

  • The left window shows what the layout looks like initially
  • The right window shows that if you start scrolling through ListView content, it will go under the Title text, although it should have been cropped.

Initial code example

This is what the original program code looks like.

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Text {
        id: topText
        text: qsTr("Title")

        anchors.horizontalCenter: parent.horizontalCenter
        anchors.top: parent.top
    }

    ListView {
        width: 180; height: 200

        model: contactModel
        delegate: Text {
            text: name + ": " + number
        }

        anchors.horizontalCenter: parent.horizontalCenter
        anchors.top: topText.bottom
    }

    ListModel {
        id: contactModel
        ListElement {
            name: "Bill Smith"
            number: "555 3264"
        }
        ListElement {
            name: "John Brown"
            number: "555 8426"
        }
        ListElement {
            name: "Sam Wise"
            number: "555 0473"
        }
    }
}

To solve this problem, just set the clip: true property of ListView .

ListView {
    clip: true

    /* код */
}

And now ListView content will not go beyond ListView

Using Rectangle Objects

Similarly, clip works for Rectangle objects. Here is the program code, as well as an example before and after including clip.

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Rectangle {
        x: 27
        y: 31
        width: 100
        height: 100
        color: "blue"
        clip: true

        Rectangle {
            x: -20
            y: -20
            width: 100
            height: 100

            color: "green"
        }
    }
}

Rounding

But what does not work when using clip is rounding, even if you adjust the radius of rounding at the parent Rectangle object.

For example, here we have such a code will not give the expected result

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Rectangle {
        x: 27
        y: 31
        width: 100
        height: 100
        color: "blue"
        radius: 30 // set rounding
        clip: true // enable cropping

        Rectangle {
            x: -20
            y: -20
            width: 100
            height: 100

            color: "green"
        }
    }
}

The result

This clip behavior is standard and has been done for performance reasons, since almost all QML graphic elements are inherited from the Item type. This means that additional processing of masks throughout the application is redundant.

But the expected result can be achieved with the following program code using the graft effect OpacityMask

import QtQuick 2.12
import QtQuick.Window 2.12
import QtGraphicalEffects 1.13

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Rectangle {
        id: rect
        x: 27
        y: 31
        width: 100
        height: 100
        color: "blue"

        layer.enabled: true
        layer.effect: OpacityMask {
            maskSource: Item {
                width: rect.width
                height: rect.height
                Rectangle {
                    anchors.centerIn: parent
                    width: rect.adapt ? rect.width : Math.min(rect.width, rect.height)
                    height: rect.adapt ? rect.height : width
                    radius: 30
                }
            }
        }

        Rectangle {
            x: -20
            y: -20
            width: 100
            height: 100

            color: "green"
        }
    }
}

And the result will be as follows

Conclusion

In conclusion, I’ll say that clip is a very useful property, which for performance reasons was set to false by default. At the same time, this property performs only simple cropping in the form of a rectangle, and something more complex needs to be cropped using a mask.

We recommend hosting TIMEWEB
We recommend hosting TIMEWEB
Stable hosting, on which the social network EVILEG is located. For projects on Django we recommend VDS hosting.
- company blog
Support the author Donate

Comments

Only authorized users can post comments.
Please, Log in or Sign up
Donate

Hello, Dear Users of EVILEG!!!

If the site helped you, then support the development of the site financially, please.

You can do it by following ways:

Thank you, Evgenii Legotckoi

A
March 29, 2020, 12:14 p.m.
Alexanderv66

C++ - Тест 003. Условия и циклы

  • Result:71points,
  • Rating points1
A
March 29, 2020, 12:05 p.m.
Alexanderv66

C ++ - Test 004. Pointers, Arrays and Loops

  • Result:80points,
  • Rating points4
DE
March 28, 2020, 3:13 p.m.
Denis Erokhin

C++ - Test 005. Structures and Classes

  • Result:100points,
  • Rating points10
Last comments
March 27, 2020, 2:40 p.m.
Evgenij Legotskoj

Добрый день. В конце пятой статьи скачать можете.
March 27, 2020, 2:28 p.m.
mkdir _

Здравствуйте, а можно, пожалуйста, ссылку на целые исходники, если есть?
March 27, 2020, 4:36 a.m.
Evgenij Legotskoj

Скорее всего также, как и для установки всех остальных переменых в CMake, через использование set
March 27, 2020, 2:47 a.m.
Andrej Jankovich

Ошибка с ярлыками в Windows исправлена в версиии 1.4.0.4
March 26, 2020, 1:37 p.m.
Andrej Jankovich

но появятся ярлыки, сейчас поправим
Now discuss on the forum
March 30, 2020, 4:16 a.m.
Evgenij Legotskoj

Попробуйте запустить через плейлист, а не через setMedia. Пример есть в этой статье
March 30, 2020, 3:25 a.m.
Evgenij Legotskoj

Добрый день. Почитайте документацию, в исходниках этой библиотеки есть исходнные классы логгеров. Вам наверняка достаточно только правильно вызвать пару функций, чтобы задать имя файла…
March 30, 2020, 3:17 a.m.
Evgenij Legotskoj

Нет, не нужно ничего умножать, если у вас включена поддержа High DPI в приложении а QML, то достаточно держать в памяти, что все значения задаются в DPI, и просто задаёте значения равные DPI сра…
March 27, 2020, 10 a.m.
Mihailll

Оказывается нужно сделать столбец уникальным ALTER TABLE public.drivers ADD UNIQUE (agregator1_id); Потом так работает INSERT INTO drivers( name_driver, surname, middle_name, agr…
s
March 27, 2020, 8:25 a.m.
shuric

Спасибо за ответы. Скорее всего оставлю на неопределенное время под macOS. Хоть будет полезно другим начинающим разработчикам.
EVILEG
About
Services
© EVILEG 2015-2019
Recommend hosting TIMEWEB