QML-Qt.point-Applications

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QML Qt.point() Applications

Introduction

QML Qt is a global object visible in QML and JavaScript namespaces. Note it is not a QML element and no need to be instantiated. It offers useful methods, properties and enumerations from Qt. To use them apply well-known dot notation. In QML documentation the object is categorized in “Utility” category.

In this article we analyze how Qt.point() method could be used to develop dynamic behaviors of QML visible elements and have some “whirl” effects.

The examples bellow are tested in a desktop setting – Win 7, Qt SDK 1.2. The complete snippets are available here.

Qt.point() Definition

Qt.point() method takes two arguments – x and y coordinates (in screen area) - and returns a point  :

point Qt::point(int x, int y)<code>

The type ''point'' is a QML [http://doc.qt.io/qt-4.8/qdeclarativebasictypes.html basic] type. A ''point'' type could be initialized in two ways – as a string or calling ''Qt.point():''

Rectangle {

width: 360
height: 360
property variant propertyTest: "30,40"
property variant anotherTest: Qt.point(100,200)
Component.onCompleted: {
console.log("This is a string definition",propertyTest.x)
console.log("This is a Qt.point()definition",anotherTest.y)
}

}

We recall that the ''variant'' type property accepts ''point'' type values. The property ''propertyTest'' (''anotherTest'') is of ''point'' type, has two attributes (''x'' and ''y'') and which are accessible through the dot notation.

Note the difference in the ''x'' and ''y'' definition in ''Qt.point()'' and an ''Item'' element for example. In the first case ''x'' and ''y'' arguments are of ''int'' type whereas in an ''Item'' definition they are of ''real'' type.

== Carousel Sample ==

There is a nice carousel [http://wiki.qt.io/Qt_Quick_Carousel example], which use ''PathView'' element in implementation. We will demonstrate the similar effects using ''Qt.point()'' method.

The idea is as follows. A rectangle will be moved over a predefined trajectory, which is described as a sequence of ''Qt.point()'' points. The rectangle to be moved is defined as a component file – ''RunArea.qml''. The points sequence is stored in a JavaScript array – variable ''path''. A timer periodically fires an ''onTrrigered'' signal after which the rectangle is moved to a new position. Firstly, ''the Loader'' ''test'' loads the component ''RunArea.qml'' and then changes its position.

Item{

width:400;height:400
property int ind:0
Loader{id:test}
Timer {
interval: 500; running: true; repeat: true
onTriggered: {
var path=[Qt.point(100,30),Qt.point(130,60),Qt.point(160,90),
Qt.point(130,120),Qt.point(100,150),Qt.point(70,120),
Qt.point(40,90),Qt.point(70,60)]
test.source="RunArea.qml"
test.item.x=path[ind].x
test.item.y=path[ind].y
ind=ind+1
if(ind==8)
ind=0
}
}

}

== Whirling Text ==

We will animate a text (the word ''COOL'') rotating it around an origin. Each symbol of the text is putted in a rectangle (''rec1, rec2, rec3, rec4'') and controlled individually. To get an access to ''color'' property of the ''Text'' element for each rectangle we define an alias ''col''. The collection of these rectangles is stored in property ''cloud'' that is of type ''list''. Five states are defined and they map five different positions of the text symbols. In the programming code these states are represented by JavaScript variables ''state1, state2, state3, state4'' and ''state5''.

The changes of the states are initiated by a ''Timer'' element. Each state is composed by QML points stored in corresponding state variables. You may experiment altering ''interval'' property of the ''Timer'' element. Additional effects you could have changing the ''color'' property of the rectangles – in the snippets it is commented.

Rectangle {

width: 360
height: 360
property int ind:0
Rectangle {id:rec1;x:100; y:100;width:20;height:20;
// color:"yellow"
property alias col:symbol1.color
Text {id:symbol1;text:"C"}
}
Rectangle {id:rec2;x:110;y:100;width:20;height:20;
// color:"yellow"
property alias col:symbol2.color
Text {id:symbol2;text:"O"}
}
Rectangle {id:rec3;x:120;y:100;width:20;height:20;
// color:"yellow"
property alias col:symbol3.color
Text {id:symbol3;text:"O"}
}
Rectangle {id:rec4;x:130;y:100;width:20;height:20;
// color:"yellow"
property alias col:symbol4.color
Text {id:symbol4;text:"L"}
}
property list<Item> cloud
Timer {
interval: 100; running: true; repeat: true
onTriggered: {
cloud=[rec1,rec2,rec3,rec4]
var state1=[Qt.point(100,100),Qt.point(110,90),Qt.point(120,80),
Qt.point(130,70)]
var state2=[Qt.point(100,100),Qt.point(110,110),Qt.point(120,120),
Qt.point(130,130)]
var state3=[Qt.point(100,100),Qt.point(100,120),Qt.point(100,140),
Qt.point(100,160)]
var state4=[Qt.point(100,100),Qt.point(90,120),Qt.point(80,140),
Qt.point(70,160)]
var state5=[Qt.point(100,100),Qt.point(90,80),Qt.point(80,60),
Qt.point(70,40)]
var temp;
if(ind0)
          {
            temp=state1
            for(var i1=0;i1<=3;i1=i1+1) {
                   cloud[i1].x=temp[i1].x
                   cloud[i1].y=temp[i1].y
                   cloud[i1].col="red"
               }
              ind=ind+1;
            }
            else if(ind1)
{
temp=state2
for(var i2=0;i2<=3;i2=i2+1){
cloud[i2].x=temp[i2].x
cloud[i2].y=temp[i2].y
}
ind=ind+1
}
else if(ind2)
           {
               temp=state3
               for(var i3=0;i3<=3;i3=i3+1){
                   cloud[i3].x=temp[i3].x
                   cloud[i3].y=temp[i3].y
                   cloud[i3].col="purple"
               }
               ind=ind+1
           }
           else if(ind3)
{
temp=state4
for(var i4=0;i4<=3;i4=i4+1){
cloud[i4].x=temp[i4].x
cloud[i4].y=temp[i4].y
cloud[i4].col="darkmagenta"
}
ind=ind+1
}
else if(ind==4)
{
temp=state5
for(var i5=0;i5<=3;i5=i5+1){
cloud[i5].x=temp[i5].x
cloud[i5].y=temp[i5].y
cloud[i5].col="navy"
}
ind=0
}
else {console.log("error")}
}
}

}

== Generating of Random Trajectories ==

Now we want to change the coordinates of the symbols of the word “FINE” in a random manner. For simplicity we assume that only ''y'' coordinates will be changed with a random offset. The random numbers we get with the JavaScript function ''random()''. It generates random numbers (of type ''real'') in the interval (0,1). With the JavaScript function ''floor()'' we round the returned value from ''random()'' to its downwards nearest integer. The both functions are methods of the JavaScript object ''Math'' so we call them like that - ''Math.random()'' and ''Math.floor()''. We could use a normalizing factor to extend the interval of returned random numbers as is illustrated in the next line:

Math.floor((Math.random()*10)+1)

The above call returns numbers between 1 and 10.

The point is if the method Qt.point() accepts functions calls as arguments like that Qt.point(Math.floor(Math.random()),Math.floor(math.random()))? The next code fragment gives a positive answer to this question:

//The next code fragment tests function substitution
Item {width:400;height:400
 property variant test:Qt.point(30,getPoint())

//The call to getPoint()is used as Qt.point() argument
 function getPoint()
 {
 return 200
 }

Rectangle { id:dummy
 Component.onCompleted: {
 console.log("testY=",test.y)
 }
 }
}//end Item

The code implementation of the “twisted” word uses the same QML constructs as in the previous section. It is as follows:

import QtQuick 1.1
Rectangle {
 width: 360
 height: 360
 color:"azure"
 Rectangle {id:rec1;x:100; y:100;width:20;height:20;
 color:"transparent"
 property alias col:symbol1.color
 Text {id:symbol1;text:"F";color:"red"}
 }
 Rectangle {id:rec2;x:110;y:100;width:20;height:20;
 color:"transparent"
 property alias col:symbol2.color
 Text {id:symbol2;text:"I";color:"blue"}
 }
 Rectangle {id:rec3;x:120;y:100;width:20;height:20;
 color:"transparent"
 property alias col:symbol3.color
 Text {id:symbol3;text:"N";color:"black"}
 }
 Rectangle {id:rec4;x:130;y:100;width:20;height:20;
 color:"transparent"
 property alias col:symbol4.color
 Text {id:symbol4;text:"E";color:"magenta"}
 }
 property list<Item> cloud
 Timer {
 interval: 1000; running: true; repeat: true
 onTriggered: {
 cloud=[rec1,rec2,rec3,rec4]
 var state1=[Qt.point(100,Math.floor((Math.random()*100)+10)),
 Qt.point(120,Math.floor((Math.random()*100)+10)),
 Qt.point(140,Math.floor((Math.random()*100)+10)),
 Qt.point(160,Math.floor((Math.random()*100)+10))
 ]
 for(var i1=0;i1<=3;i1=i1+1) {
 cloud[i1].y=state1[i1].y+100
 }//end for
 }//end of onTriggered
 }//end Timer
}

The next screenshots illustrate the running code:

Frame1 second frame