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In this process we will learn about some of the basics of using Declarative UI, such as \list \o Basic drawing \o States and Transitions \o Reuse of components \o Models and Views \endlist An existing knowledge of Qt is not required. The tutorial's source code is located in Qt's \c examples/declarative/tutorials/contacts directory. It is split up into a number of sub directories, and within each sub directory the files are numbered in an order of increasing features. The code in this example is not compiled, but interpreted at run time. This means you should use the qmlviewer application provided with Qt to run the examples. \list \o \l{tutorials/declarative/contacts/part1}{Drawing and Animation} \o \l{tutorials/declarative/contacts/part2}{Reusing QML Components} \o \l{tutorials/declarative/contacts/part3}{Models, Views and Delegates} \endlist */ /*! \page tutorials-declarative-contacts-part1.html \contentspage {Declarative UI Tutorial}{Contents} \nextpage {tutorials/declarative/contacts/part2}{Chapter 2} \example tutorials/declarative/contacts/part1 \title Drawing and Animation \tableofcontents The first part of this tutorial covers basic drawing of elements on the screen and causing them to animate. \section1 Drawing In this first chapter we will build a button that indicates something can be removed and asks for confirmation. When clicked it will expand from a small button with a trash can icon, to a wide button with a confirm icon on the left, the text "Remove" in the middle, and a cancel icon on the right. \image declarative-removebutton.gif Because Declarative UI is declarative, you don't pass instructions on what to paint in a sequential manner as you may be used to. Instead elements and how they appear on the screen are declared in much the same was as elements on a web page are declared. This is done using the Qt Markup Language which we will refer to by the abbreviation QML for the remainder of the tutorial. We will start by drawing a simple red rectangle with rounded corners. \image declarative-roundrect.png \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/1/RemoveButton.qml 0 This is one of the simplest of QML components. It describes a rectangle with some simple properties. In QML all components start with a capital letter, and their properties with lower case letters. Apart from the properties all QML components share, the \l{Rectangle}{Rectangle} component has the properties \list \o color - The background color of the rectangle \o tintColor - The overlay color of the rectangle \o gradientColor - The color at the base of the rectangle to blend upwards \o pen - The description of how to draw the border of the rectangle \o radius - The corner radius used to draw rounded rectangles. \endlist There are also a number of properties all QML components shares, described in the \l{Item}{Item} element reference documentation. The rectangle drawn in the above code uses the properties; \list \o id - An identifier of the component \o width - the width of the component when drawn \o height - the height of the component when drawn \endlist Currently we have described a rectangle with a width and height of 30 pixels, filled in with the color red and with rounded corners using a radius of 5. \section1 Layout The next step of the tutorial adds an image over the rectangle. We will do this by adding an \l{Image}{Image} component as a child of the \l{Rectangle}{Rectangle} component. All QML components have a list of children which are drawn in order after the parent component has been drawn. By having the \l{Image}{Image} component as a child of the \l{Rectangle}{Rectangle} component we ensure it is drawn over the \l{Rectangle}{Rectangle} component. Children also are affected by the opacity of the parent component and calculate their position in within the bounds of the parent component. \image declarative-removebutton-close.png \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/2/RemoveButton.qml 0 The trashIcon image is added as a child of the Rectangle. In this case the children property isn't explicitly used because the default property of the \l{Rectangle}{Rectangle} component is its children. Some elements often don't have children and use some other default component. When this is the case its possible to explicitly list the sub component as a child as follows: \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/2a/RemoveButton.qml 0 The \l{Image}{Image} element allows loading an image file for display. The source specified is a URL, and in this case refers to a portable network graphics file in a relative directory to where the QML file was loaded from. Also new in this code is the use of anchors. In QML components can either have their position and size specified explicitly using x, y, width and height, or they can instead specify the size and position in relation to either parent or sibling elements. The \l{Image}{Image} component uses a combination of both styles. It has a fixed size, but specifies its position to align to the right of its parent and for its vertical center to align with the vertical center of its parent. Setting a property by the identifier of a separate property binds them. This means that if while running the example the position of the \l{Rectangle}{Rectangle} component's vertical center changed, so to would the vertical center of the \l{Image}{Image} component. The parent value is a special identifier that always refers to the parent component of a component. Anchors are most useful when the size of items might change based on the component state or contents. However they are limited in that they must always refer to a parent or sibling component. See \l{anchor-layout}{Anchor-based Layout} for more information on using anchors in QML. At this point the initial state of the RemoveButton is complete. A small rounded rectangle with a trash icon. Next we will design the open state for the button. \image declarative-removebutton-open.png This is a wider rectangle with two images and some text. The code to draw this state of the button could be written as follows: \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/3/RemoveButton.qml 0 The rectangle width is now wider by 200 pixels. Also the trashIcon has been replaced. Normally we wouldn't remove the trashIcon when developing an alternate state of the RemoveButton, however since this is a tutorial its been done so that its easier to understand the alternate state we are aiming for and how it relates to transitioning between states. We also introduce the \l{Text}{Text} element. Left and Right anchors are specified in terms of the neighboring icons because we want text to fill the space between the icons. This means as the parent removeButton gets wider, so will the text component. It also means that if we animate a width change on the removeButton, any bindings, that is the values specified by an expression such as \c{parent.left} will be evaluated and animated as well. \section1 Defining States When designing a component with multiple states, it should be developed in the initial state and the changes that would be made specified as an additional state. Its not normally possible to add new children to an element when changing state This means that all possible child components should be included in the initial state, and those that should not be visible in the initial state should have their opacity set to zero. Thus for the RemoveButton we specify the starting size of the RemoveButton and hide any items that should not initially be visible. The code snippet below shows what the start of the duel state specification might look like. \code Rectangle { id: removeButton width: 30 height: 30 color: "red" radius: 5 Image { id: trashIcon width: 22 height: 22 anchors.right: parent.right anchors.rightMargin: 4 anchors.verticalCenter: parent.verticalCenter src: "../../shared/pics/trash.png" opacity: 1 } Image { id: cancelIcon width: 22 height: 22 anchors.right: parent.right anchors.rightMargin: 4 anchors.verticalCenter: parent.verticalCenter src: "../../shared/pics/cancel.png" opacity: 0 } \endcode The code above includes components from both states of the RemoveButton, but by setting opacity="0" for the cancelIcon it means that the components of the second state won't be drawn yet. The base state of a component always has an empty name, however new states can be added that describe how a component and its children should be changed. For the RemoveButton there is only one non-base state required. In this tutorial we will name it the 'opened' state. \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/4/RemoveButton.qml states In the opened state the width of the button itself changes from the base width of 30 to the new width of 230. Also the opacity of the children are changed so that the trash icon is now hidden and the other elements are now visible. \section1 Changing States To trigger the change we will react to the 'clicked' signal of a MouseRegion component. \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/4/RemoveButton.qml mouse region MouseRegion components handle mouse actions within their geometry. This geometry behaves the same way as painted components, such that children cover their parents and later siblings will cover earlier siblings and all the children of the earlier sibling, should they overlap. When a component has a signal, such as clicked, the action for the signal can be specified using \c{onSignalName}, as is done above. In this case when the clicked signal is emitted by the MouseRegion component, a function called \c{toggle()} is called. It might also have been written \code onClicked: { removeButton.state='opened' } \endcode However in this case we are using a function because it allows multiple mouse regions to use the same functionality, and also makes it easier to specify complex behavior in response to a signal. An alternative would be to explicitly state the connection: \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/4a/RemoveButton.qml mouse region This will connect to the \c{clicked()} signal of the trashMouseRegion component and execute the associated script. The \c{toggle()} function is a new function specified as part of the remove button element. \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/4/RemoveButton.qml script Any QML component can have a set of resources specified. One of those resources is any Script that might be needed. See the \l{QtScript Module} for more information on how to write script code in Qt. It is possible to refer to identified QML components within the script. Hence the function for our RemoveButton will check if the state is already open to determine what the new state should be. \section1 Animation Currently the RemoveButton is functional, but snaps between our two states. Fortunately making the transition between states smooth is very simple. We only need one more bit of code at the end of our removeButton component. \snippet declarative/tutorials/contacts/1_Drawing_and_Animation/5/RemoveButton.qml transition All QML components have a transitions property. This describes how properties of items within the component should change. In this case we specify that if the x, width or opacity of the removeButton or its children change due to a change in state, that they should take 200ms to complete their transition. \omit TODO More on types of animation, e.g. ColorAnimation, Behaviors. \endomit In the next chapter we will show how we can use the remove button in other QML components. */ /*! \page tutorials-declarative-contacts-part2.html \contentspage {Declarative UI Tutorial}{Contents} \previouspage {tutorials/declarative/contacts/part1}{Chapter 1} \nextpage {tutorials/declarative/contacts/part3}{Chapter 3} \example tutorials/declarative/contacts/part2 \title Reusing QML Components \tableofcontents The second part of this tutorial covers how to reuse QML components and have them interact with each other. The RemoveButton developed in the previous chapter is intended to be part of a more complex control for editing a field of our contact. This ContactField in turn is intended to be used in a contact editing control. \image declarative-reuse-3.png \section1 Loading QML Components Reusing the RemoveButton itself is very simple. When parsing a QML file if a Component is referred to that isn't already in the system, Qt will try to load it from a file of the same name with the ".qml" extension. \snippet declarative/tutorials/contacts/2_Reuse/1/ContactField.qml load The above QML code will attempt to load the RemoveButton component from a file with the name "RemoveButton.qml" from the following search paths. \list \o Any imported directories. These are listed at the start of the file using \c { import "path" }. \o the directory of the QML code file \endlist All the properties of the button are accessible and can be overridden from defaults. The loaded component can also refer to elements further up in the tree, so that code within RemoveButton.qml could refer to the contactField component. Only properties of the top level element in RemoveButton.qml are visible to the contact field. There are also two other ways to reuse components in QML. A component can be reused from within the same QML file using Component and ComponentInstance elements. The next code snippet produces three red rounded rectangles within a large blue rectangle. \image declarative-reuse-bluerect.png \snippet declarative/tutorials/contacts/2_Reuse/1b/BlueRect.qml all This can be useful when the component is not complex enough to justify its own file. The third way to reuse components allows for delaying loading of the QML until some later event. \l{Loader}{Loader} includes a special child, item, which has its definition provided by the contents of the source property of the loader. \snippet declarative/tutorials/contacts/2_Reuse/1a/ContactField.qml load This last method is useful if the contents of a item need to change at run time or if the initial complexity of the loaded QML needs to be reduced in order to improve the time it takes to start the application. In chapter three this method is used to improve performance of scrolling through very large numbers of items. Because of its simplicity, the first method is the recommended in most cases and will be the focus of the remainder of this chapter. \section1 Properties and Signals The next task is to be able to control aspects of the RemoveButton from the components that use it. In particular controlling how far it expands and how it reacts when the user clicks on the confirm icon of the remove button. When reusing a component in a separate QML file only the attributes of the root element are visible. To allow controlling attributes of child elements within an imported component we need to define some properties and signals for the RemoveButton. \snippet declarative/tutorials/contacts/2_Reuse/2/RemoveButton.qml define properties and signals The children of the remove button can use these properties and signals. The opened state can now bind the expanded width to the expandedWidth property. \snippet declarative/tutorials/contacts/2_Reuse/2/RemoveButton.qml use width Also when the confirm icon is clicked, as well as toggling the state it will emit the confirmed signal of the RemoveButton component. \snippet declarative/tutorials/contacts/2_Reuse/2/RemoveButton.qml use signal These properties and signals can also be accessed from the contact field the same way standard system component properties and signals are accessed. \snippet declarative/tutorials/contacts/2_Reuse/2/ContactField.qml use properties and signals Now when the remove button is expanded, it will expand to the width of the contact field. Also when the user confirms the remove action, the text section of the contact field will be cleared. \section1 States Its also possible to access the state of included components. The FieldText component we will use in this tutorial is also been written specifically for our contacts application. In this case we want it to expand when editing. One way to do this would be to anchor the field text component to the center of its parent and then let its own width change push the remove button away, however that would make it difficult to have the remove button also push the field text to the left when the remove button expands. Instead we will anchor the right edge of the field text to the left edge of the remove button and use a state change in the contact field itself to move the remove button and the field icon out of view. \snippet declarative/tutorials/contacts/2_Reuse/3/ContactField.qml all Apart from accessing the fieldText.state, the above code also uses the when attribute of its own editingText state. This is an alternative to using a signal to change state. When the value of the expression for the when attribute changes, Qt will detect if the contactField needs to enter that state. In the FieldText element a similar approach is used to fade out the label of the FieldText when the user enters some text of their own. \snippet declarative/tutorials/contacts/2_Reuse/3/FieldText.qml behavior \c{fieldText} is the enclosing component and \c{textEdit} is a TextEdit element provided by Qt. In the QML code above, the opacity of the textLabel is only 1 if the text for the textEdit is empty. This is a form of short cut to using states for an element, useful if only one property is changing as it is for the textLabel. To animate a property change is similar to animating a state change. Using the Behavior element we can specify how the property changes if it does change state, allowing for a smooth transition. \section1 Key and Mouse Focus Setting focus to true on a component does not always mean that the component has focus. This is due to the declarative nature of QML, and can be affected by multiple components both indicating focus to be true. At the time of writing this tutorial both key and mouse focus handling are still being improved. Hence we will only lightly cover the topic. For an item to have key focus in QML it is required that: \list \o If there is a FocusScope ancestor of the component that it has focus as well. \o That it is the most recent component within the focus realms descendent's to receive focus \endlist The read-only property activeFocus can be used to determine whether a component will receive key input. Any un-handled keys will be passed to the components parent, which in turn will pass keys it doesn't handle up to its own ancestors. Some components such as ListView components are also FocusScope components, as they handle focus among the child list items. At this stage of the tutorial it is sufficient to use the setting of 'focus' as we only have a list of line edits and only one should be active at any given time. Currently if multiple contact fields were put into our contact editor, any of the FieldText components could be clicked and opened, and any of the RemoveButton components could be clicked and opened, all at the same time. This leads to situations where the users actions are ambiguous \image declarative-reuse-focus.png To counteract this we will add a property of the root element to indicate when an element has 'grabbed' mouse interaction, preventing other clickable elements from reacting. \snippet declarative/tutorials/contacts/2_Reuse/4/Contact.qml grab property The code that we want to disable then simply needs to check this property before acting. \snippet declarative/tutorials/contacts/2_Reuse/4/RemoveButton.qml grab \note Handling Key and Mouse focus in QML is quite likely to change before the Qt 4.6 release. */ /*! \page tutorials-declarative-contacts-part3.html \contentspage {Declarative UI Tutorial}{Contents} \previouspage {tutorials/declarative/contacts/part2}{Chapter 2} \example tutorials/declarative/contacts/part3 \title Models, Views and Delegates \tableofcontents In the previous chapters we designed a component to display and edit a contact. The next step is to display a list of those contacts and allow the user to expand individual contacts for editing. As the previous elements will not be changed in this section, they have been moved to a lib directory for this tutorial and the relevant import path has been used. \section1 Simple List View Displaying lists requires three components. A model that holds the data displayed, a delegate to indicate how elements are drawn and a view to arrange the elements. \image declarative-tutorial-list.gif For the purposes of this tutorial we will be using an SQL query as our data model. This can be declared in the resources section of the parent item. \snippet declarative/tutorials/contacts/3_Collections/1/ContactView.qml model The SqlConnection component describes how to connect to the database in much the same ways as the QSqlDatabase::addDatabase() function is used. In this case an SQLite database is used as it can be connected to as a file, reducing complexity in setting up a database server or credentials. The SqlQuery component allows various forms of queries to be described. When the query is a select statement, the component also acts as a model allowing it to provide data to a ListView component. The query above retrieves the fields recid, label, email and phone from a contacts table, and orders the results by the label of the contact first, and then by the recid for any contacts with equivalent labels. The ListView component is suitable for displaying models and is declared much like any other QML component. The ListView component also has a delegate property that defines how to construct components for items in the list. \snippet declarative/tutorials/contacts/3_Collections/1/ContactView.qml delegate Unlike a child element, this describes a template on how to build the component for each element, much in the same way that components are loaded from files such as RemoveButton.qml. The are constructed or destroyed as items scroll into our out of the visible area of the list. The entire view component will look like: \snippet declarative/tutorials/contacts/3_Collections/1/ContactView.qml view This gives us a list of contacts that the user can flick through. \section1 Animating Delegates The next step is to allow the user to click on a contact to edit the contact. We will take advantage of QML to open a Contact component in the list rather than as a new dialog or view. This is very similar to how the contents of the FieldText and RemoveButton components are swapped in and out. \image declarative-tutorial-list-open.gif \snippet declarative/tutorials/contacts/3_Collections/2/ContactView.qml components The first step is to have two children of our delegate component that can be swapped between. The plain Text component and the Contact component built in the previous chapters. We also add a MouseRegion that can be clicked upon to change the state of the delegate component. \snippet declarative/tutorials/contacts/3_Collections/2/ContactView.qml states This defines the open state of the delegate. It changes the height of the delegate component to that of the whole list view, pushing the other items off each end of the list. It sets the listview's scroll viewportY of the ListView to the y value of the delegate so that the top of the delegate matches the top of the list view. The next step is to lock the list view. This prevents the user being able to flick the list view. The final to properties that are set should be familiar from previous chapters, setting the opacity of the items such that the new item is visible and the old item hidden. We then add a transition so that this becomes animated: \snippet declarative/tutorials/contacts/3_Collections/2/ContactView.qml transitions This allows the user to click on an item to enter the open state. Elsewhere on our contact view we add a button so that the user can leave the detailed view of the contact. \snippet declarative/tutorials/contacts/3_Collections/2/ContactView.qml button And connect it's clicked value to some script to set the state of the delegate back to its default state. \snippet declarative/tutorials/contacts/3_Collections/2/ContactView.qml connections Something worth noting at this point is that every delegate created has this connection. It is important to check whether the delegate is the one in the open state, and taking some effort to ensure only one is, before acting on the signal from the button. \section1 Performance Considerations We have now made a contact application that can view a list of contacts, open one, and close it again. Its now time to take a moment and consider the implications of a list view delegate. It is created for each and every item in the list, and while the list cleans up after itself and only has delegate components constructed for visible items and any single point of animation, the list can scroll very quickly. This means potentially thousands of delegate components will be constructed and destroyed when the user is flicking through the list. Its important then to try and minimize the complexity of the delegate. This can be done by delaying the loading of the component. By using the qml property of the \l{Item}{Item} component, we can delay building the Contact.qml item until the user attempts to open the list. \snippet declarative/tutorials/contacts/3_Collections/3/ContactView.qml setting qml \l{Loader}{Loader} has a source property that represents the filename for the contents of a special item child of the \l{Loader}{Loader}. By setting the source property of the Details component on clicking the mouse region, the more complex component isn't loaded until needed. The down side about this though is the properties of Contact cannot be set until the item is loaded. This requires using the Bind element. \snippet declarative/tutorials/contacts/3_Collections/3/ContactView.qml binding Unlike binding a value to the property of a component directly, the Bind element allows both the target and the property set to themselves be to dynamic values. This means that when the source property is set, it will change the item property of the Details component. This in turn triggers the Bind elements to set the required properties of the item, which is now an instance of the Contact component. */