I want to make a component that can change a property of another component "by reference".
For example:
TextField {
id: lat
text: MyDataSource.latitude
}
LatLongButton {
latitude: lat.text
// or:
// latitude: MyDataSource.latitude
}
When the user clicks the button, for example, I want to update the value of lat.text. Now the problem is when I create LatLongButton as a component in a separate file:
LatLongButton {
// doesn't work, alias has to be set initially to something
// property alias latitude: ???
// overriding it in the main file is also not possible
property real latitude: 0
onClicked: {
// I want this to change some other property indirectly
// that is, one that I specify in the main file
latitude = 123
// but this here doesn't set the value through the binding -
// it seems to destroy the binding and set the value of this
// component directly
}
}
Is there any way to achive this? I think there must be, because the builtin widgets like TextField work like this. The user interacts, and then they can change a property across a binding.
Related
i got two Windows inside the same .qml file.
Window1 has a textinput1 and a button, and when I press the button, i'd like to send the string value from that textinput to the Window2 textInput2.
I'm new to Qt and QML, been reading a lot on signals, Loaders, properties and can't materialize this kind of transfer. Can anyone do a simple 10-line example of such transfer please?
Window {
id:window
TextInput {
id:text1
text: "This value is needed in the second Window!"
}
Button {
onClicked: window2.open()
}
Window {
id.window2
function open(){
visible = true
}
Text {
text: text1.text
}
}
}
If I do this it gives me ReferenceError: text1 is not defined, how can I reference the text1 from the first Window?
I would prefer to use signals in such case:
Window {
id: window1
title: "window 1"
visible: true
width: 600
height: 600
signal someSignal()
Button {
anchors.centerIn: parent
text: "Open window"
onClicked: window1.someSignal()
}
Window {
id: window2
title: "window 2"
width: 400
height: 400
// you can use this instead of Connections
//Component.onCompleted: {
// window1.someSignal.connect(show);
//}
}
Connections {
target: window1
onSomeSignal: {
window2.show();
}
}
}
I think this is more ... how do you say? ... more imperative -)
i got two Windows inside the same .qml file.
If you did then your code will work. Since it doesn't work, I will assume each window is defined in its own qml file, and you only instantiate the two windows in the same qml file.
If I do this it gives me ReferenceError: text1 is not defined, how can
I reference the text1 from the first Window?
You will have to be able to reference the window first, and it should provide an interface to reference the text.
Keep in mind that ideally ids should only be used to reference stuff in the same source. On rare occasions you could go further, and reference ids down the object tree, but only parents, and none of their out-of-line children, it will however work for in-line children that are given ids in that same source. Meaning that if window2 is created inside window then you will be able to reference window from inside window2. But if both windows are siblings in another object, the id won't resolve.
Obj1
Obj2
Obj4
Obj3
In this example object tree, Obj1 will resolve from any of the objects. However, Obj3 will not be able to resolve Obj2 if the id is given inside Obj2, but will resolve if the id for Obj2 is given inside Obj1. But there is no way to resolve Obj4 from Obj3. because the id doesn't act like a property, you can't do someId.someOtherId, that's only possible for properties. You cannot do somePropertyObject.someId neither. You can only begin with either an id or a property, and continue only with sub-properties.
When the id is not applicable, can expose objects or properties either as properties or property aliases. The first is useful when you want to expose a whole object, the second when you want to expose a particular property of an object:
Item {
property Item innerItem: inner // gives you access to the "entire" inner object
property alias innerWidth: inner.width // gives you access to a property of inner
Item {
id: inner
}
}
You can also have aliases to aliases.
This is a simplification of the situation I am dealing with in main.qml file:
Component {
id: component1
property string stringIneedToPass: "Hello"
Text { text: stringIneedToPass }
}
Component {
id: component2
Rectangle {
id: myRectangle
property string stringIneedToReceive = component1.stringIneedToPass; //this doesn't work
}
}
Obviously my situation is more complicated. But in the end I just need to understand how this kind of transfer should be done!
Thank you all!
First of all, a Component element cannot have properties. Components are either loaded from files, or defined declaratively, in the latter case they can contain only one single root element and an id.
Second - you cannot do assignment in the body of an element, only bindings.
Third - you cannot reference properties defined inside an element inside a component from the outside, as that object doesn't exist until the component is instantiated. Such objects can only be referenced from inside.
Other than that, it will work as expected, if you can reference it, you can bind or assign it to a property, depending on what you want.
So you can simply have the string property external:
property string stringIneedToPass: "Hello"
Component {
id: component1
Text {
text: stringIneedToPass
}
}
Component {
id: component2
Rectangle {
id: myRectangle
property string stringIneedToReceive: stringIneedToPass
}
}
I have a string id of an object I need to find in QML tree.
For example:
var idToFind = "myBtnId"
Can I do something like the following?
var objectThatINeed = myMainWindow.findObjectById(idToFind)
As far as I understand I can use objectName for this purpose (at least from C++). Can I still reuse existing ids somehow without introducing the names?
I want to use this object as a parent for some other dynamically created controls.
No, you have to use objectName or some other property.
The id Attribute:
Once an object instance is created, the value of its id attribute cannot be changed. While it may look like an ordinary property, the id attribute is not an ordinary property attribute, and special semantics apply to it; for example, it is not possible to access myTextInput.id in the above example.
If you know the IDs of all items you want beforehand, you can create an object map for them and use that to look them up. For example:
Item {
property var idMap: ({ foo:foo, bar:bar })
Rectangle { id:foo }
Item { id:bar }
function findItemById(id) {
return idMap[id];
}
Component.onCompleted: console.log(findItemById('foo'), findItemById('bar'))
// qml: QQuickRectangle(0x1479e40) QQuickItem(0x148fbf0)
}
QJSValue MyEngine::getQmlObjectById(const QString& id) {
QV4::ExecutionEngine *v4 = QV8Engine::getV4(m_JSEngine);
QV4::Scope scope(const_cast<QV4::ExecutionEngine *>(v4));
QV4::ScopedString name(scope, v4->newString(id));
return QJSValue(v4, v4->currentContext->getProperty(name));
}
Based on the answer from Phrogz, if you don't want an explicit map, you can assign the components to properties and then reference these with the [] operator:
Item {
id: page
property Component foo: Rectangle { color: "yellow" }
property Component bar: Item { }
Component.onCompleted: console.log(page["foo"], page["bar"])
//qml: QQuickRectangle(0x...) QQuickItem(0x...)
}
I'm using QML for my project, I want to know if am instantiating a file in another file, is it like instantiating object for a c++ class?
File.qml
Rectangle {
id: idRect1
.
.
}
File2.qml
Rectangle {
id: idRect2
File1 {
id:idFile1
.
.
}
}
In File2.qml i have initialized File1, does it means i have created an object of type File1? Please share some knowledge(links) on how all this mechanism works. Thanks in Advance
In QML when creating a file with first letter uppercase, you're creating a component. Components are implemented using OOP aggregation (not subclassing). That mean if I write
// MyButton.qml
import QtQuick 2.0;
Rectangle {
id: base;
width: 120;
height: 40;
color: "lightgray";
Text {
text: "foobar";
anchors.centerIn: parent;
}
}
... I haven't subclassed Rectangle, I just created a component that contains a Rectangle as root object, and configurates it in a certain way, and adds a Text object in it.
As soon as a component is created, it can be instanciated by simply writing :
MyComponent { id: myNewInstance; }
Because that's a way it works in QML.
The component name is a kind of class (but not in the C++ or JS way to define it) and it can also be used as a type for a property :
property MyComponent theComponent : myNewInstance;
Then it can hold the ID of an object created with the given component, acting somewhat like a C/C++ pointer : the property holds a link to the actual object.
But because of the way QML was designed, even if it's more aggregating than subclassing, a property of the type of the root object of a custom component can also hold ID of a derived component, in my case :
property Rectangle theComponent : myNewInstance;
Will work, but if I try to put an ID of an Image or Text or something else, QML engine will throw incompatible types error.
I hope it helps you.
The answer provided by #TheBootroo here: link
provides a way to load and change between QML files/screens/views. But when doing it like this how can one use signal and slots?
One can access the items created by the repeater by using the Repeater::itemAt(index) method, but since I don't know in what order the items are loaded I don't know what index screen2, screen3, screen4 etc. is at.
Is there any way to solve this or do one have to instantiate all the screens in memory at start up?
My code below:
main.qml:
//List of screens
property variant screenList: [
"main",
"screen2",
"screen3",
...
]
//Set this to change screen
property string currentScreen: "main"
Repeater {
id: screens
model: screenList
delegate: Loader {
active: false;
asynchronous: true
anchors.fill: parent
source: "%1.qml".arg(modelData)
visible: (currentScreen === modelData)
onVisibleChanged: {
loadIfNotLoaded()
}
Component.onCompleted: {
loadIfNotLoaded()
}
function loadIfNotLoaded () {
//To load start screen
if(visible && !active) {
active = true;
}
}
}
}
Connections {
target: screens.itemAt(indexHere)
//screen is here the string passed with the signal from screen2.
onChangeScreen: currentScreen = screen
}
Button {
id: button1
text: "Go To Screen 2"
onClicked: {
currentScreen = "screen2"
}
}
And in screen2.qml:
signal changeScreen(string screen)
Button {
text: "Go To Main"
onClicked: {
changeScreen("main")
}
}
One can access the items created by the repeater by using the Repeater::itemAt(index) method, but since I don't know in what order the items are loaded I don't know what index screen2, screen3, screen4 etc. is at.
The order of the items the Repeater instantiates is actually defined - the items will be instantiated in the order of the model, so in your case "main" will be the first, then "screen2" and so on. Now, each item inside of the Repeater is a Loader - the Repeater will create 3 Loaders in a defined order. The Loaders themselves load their source item on demand.
I think the only thing missing in your code is that the Connections refers to the Loader, but you need it to refer to the item the Loader creates. So change the Connections to
Connections {
target: screens.itemAt(indexHere).item
onChangeScreen: currentScreen = screen
}
Notice the additional .item.
After fixing this, there is one additional problem though: The Repeater hasn't yet instantiated its items when the Connections element is created, so screens.itemAt(indexHere) will return null. One way to fix this would be to use a currentIndex property instead of a currentScreen property and use that in the binding for target, and set currentIndex after the Repeater has instantiated its items:
property int currentIndex: -1
...
Connections {
target: screens.itemAt(currentIndex).item
...
}
Component.onCompleted: currentIndex = 0
Even easier would probably be to put the Connections element inside the Repeater's delegate, i.e. have 3 Connections instead of one (assuming you have 3 screens).