advtutorial.qdoc

Go to the documentation of this file.

// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\page qml-advtutorial.html
\title QML Advanced Tutorial
\brief A more advanced tutorial, showing how to use QML to create a game.
\nextpage QML Advanced Tutorial 1 - Creating the Game Canvas and Blocks
 
This tutorial walks step-by-step through the creation of a full application using QML.
It assumes that you already know the basics of QML (for example, from reading the
\l{QML Tutorial}{simple tutorial}).
 
In this tutorial we write a game, \e {Same Game}, based on the Same Game application
included in the declarative \c examples directory, which looks like this:
 
\image declarative-samegame.png
 
We will cover concepts for producing a fully functioning application, including
JavaScript integration, using QML  \l{State}{Qt Quick States} and \l{Behavior}{Behaviors} to
manage components and enhance your interface, and storing persistent application data.
 
An understanding of JavaScript is helpful to understand parts of this tutorial, but if you don't
know JavaScript you can still get a feel for how you can integrate backend logic to create and
control QML types.
 
 
Tutorial chapters:
 
\list 1
\li \l {QML Advanced Tutorial 1 - Creating the Game Canvas and Blocks}{Creating the Game Canvas and Blocks}
\li \l {QML Advanced Tutorial 2 - Populating the Game Canvas}{Populating the Game Canvas}
\li \l {QML Advanced Tutorial 3 - Implementing the Game Logic}{Implementing the Game Logic}
\li \l {QML Advanced Tutorial 4 - Finishing Touches}{Finishing Touches}
\endlist
 
All the code in this tutorial can be found in Qt's \c examples/quick/tutorials/samegame
directory.
*/
 
/*!
\title QML Advanced Tutorial 1 - Creating the Game Canvas and Blocks
\previouspage QML Advanced Tutorial
\nextpage QML Advanced Tutorial 2 - Populating the Game Canvas
 
\example tutorials/samegame/samegame1
 
\section2 Creating the Application Screen
 
The first step is to create the basic QML items in your application.
 
To begin with, we create our Same Game application with a main screen like this:
 
\image declarative-adv-tutorial1.png
 
This is defined by the main application file, \c samegame.qml, which looks like this:
 
\snippet tutorials/samegame/samegame1/samegame.qml 0
 
This gives you a basic game window that includes the main canvas for the
blocks, a "New Game" button and a score display.
 
One item you may not recognize here
is the \l SystemPalette item. This provides access to the Qt system palette
and is used to give the button a more native look-and-feel.
 
Notice the anchors for the \c Item, \c Button and \c Text types are set using
group (dot) notation for readability.
 
\section2 Adding \c Button and \c Block Components
 
The \c Button item in the code above is defined in a separate component file named \c Button.qml.
To create a functional button, we use the QML types \l Text and \l MouseArea inside a \l Rectangle.
Here is the \c Button.qml code:
 
\snippet tutorials/samegame/samegame1/Button.qml 0
 
This essentially defines a rectangle that contains text and can be clicked. The \l MouseArea
has an \c onClicked() handler that is implemented to emit the \c clicked() signal of the
\c container when the area is clicked.
 
In Same Game, the screen is filled with small blocks when the game begins.
Each block is just an item that contains an image. The block
code is defined in a separate \c Block.qml file:
 
\snippet tutorials/samegame/samegame1/Block.qml 0
 
At the moment, the block doesn't do anything; it is just an image. As the
tutorial progresses we will animate and give behaviors to the blocks.
We have not added any code yet to create the blocks; we will do this
in the next chapter.
 
We have set the image to be the size of its parent Item using \c {anchors.fill: parent}.
This means that when we dynamically create and resize the block items
later on in the tutorial, the image will be scaled automatically to the
correct size.
 
Notice the relative path for the Image type's \c source property.
This path is relative to the location of the file that contains the \l Image type.
Alternatively, you could set the Image source to an absolute file path or a URL
that contains an image.
 
You should be familiar with the code so far. We have just created some basic
types to get started. Next, we will populate the game canvas with some blocks.
*/
 
 
/*!
\title QML Advanced Tutorial 2 - Populating the Game Canvas
\previouspage QML Advanced Tutorial 1 - Creating the Game Canvas and Blocks
\nextpage QML Advanced Tutorial 3 - Implementing the Game Logic
 
\example tutorials/samegame/samegame2
 
\section2 Generating the Blocks in JavaScript
 
Now that we've written some types, let's start writing the game.
 
The first task is to generate the game blocks. Each time the New Game button
is clicked, the game canvas is populated with a new, random set of
blocks. Since we need to dynamically generate new blocks for each new game,
we cannot use \l Repeater to define the blocks. Instead, we will
create the blocks in JavaScript.
 
Here is the JavaScript code for generating the blocks, contained in a new
file, \c samegame.js. The code is explained below.
 
\snippet tutorials/samegame/samegame2/samegame.js 0
 
The \c startNewGame() function deletes the blocks created in the previous game and
calculates the number of rows and columns of blocks required to fill the game window for the new game.
Then, it creates an array to store all the game
blocks, and calls \c createBlock() to create enough blocks to fill the game window.
 
The \c createBlock() function creates a block from the \c Block.qml file
and moves the new block to its position on the game canvas. This involves several steps:
 
\list
 
\li \l {QtQml::Qt::createComponent()}{Qt.createComponent()} is called to
   generate a type from \c Block.qml.  If the component is ready,
   we can call \c createObject() to create an instance of the \c Block
   item.
 
\li If \c createObject() returned null (i.e. if there was an error
   while loading the object), print the error information.
 
\li Place the block in its position on the board and set its width and
   height.  Also, store it in the blocks array for future reference.
 
\li Finally, print error information to the console if the component
   could not be loaded for some reason (for example, if the file is
   missing).
 
\endlist
 
 
\section2 Connecting JavaScript Components to QML
 
Now we need to call the JavaScript code in \c samegame.js from our QML files.
To do this, we add this line to \c samegame.qml which imports
the JavaScript file as a \l{QML Modules}{module}:
 
\snippet tutorials/samegame/samegame2/samegame.qml 2
 
This allows us to refer to any functions within \c samegame.js using "SameGame"
as a prefix: for example, \c SameGame.startNewGame() or \c SameGame.createBlock().
This means we can now connect the New Game button's \c onClicked handler to the \c startNewGame()
function, like this:
 
\snippet tutorials/samegame/samegame2/samegame.qml 1
 
So, when you click the New Game button, \c startNewGame() is called and generates a field of blocks, like this:
 
\image declarative-adv-tutorial2.png
 
Now, we have a screen of blocks, and we can begin to add the game mechanics.
 
*/
 
/*!
\title QML Advanced Tutorial 3 - Implementing the Game Logic
\previouspage QML Advanced Tutorial 2 - Populating the Game Canvas
\nextpage QML Advanced Tutorial 4 - Finishing Touches
 
\example tutorials/samegame/samegame3
 
\section2 Making a Playable Game
 
Now that we have all the game components, we can add the game logic that
dictates how a player interacts with the blocks and plays the game
until it is won or lost.
 
To do this, we have added the following functions to \c samegame.js:
 
\list
\li \c{handleClick(x,y)}
\li \c{floodFill(xIdx,yIdx,type)}
\li \c{shuffleDown()}
\li \c{victoryCheck()}
\li \c{floodMoveCheck(xIdx, yIdx, type)}
\endlist
 
As this is a tutorial about QML, not game design, we will only discuss \c handleClick() and \c victoryCheck() below since they interface directly with the QML types. Note that although the game logic here is written in JavaScript, it could have been written in C++ and then exposed to QML.
 
\section3 Enabling Mouse Click Interaction
 
To make it easier for the JavaScript code to interface with the QML types, we have added an Item called \c gameCanvas to \c samegame.qml. It replaces the background as the item which contains the blocks. It also accepts mouse input from the user.  Here is the item code:
 
\snippet tutorials/samegame/samegame3/samegame.qml 1
 
The \c gameCanvas item is the exact size of the board, and has a \c score property and a \l MouseArea to handle mouse clicks.
The blocks are now created as its children, and its dimensions are used to determine the board size so that
the application scales to the available screen size.
Since its size is bound to a multiple of \c blockSize, \c blockSize was moved out of \c samegame.js and into \c samegame.qml as a QML property.
Note that it can still be accessed from the script.
 
When clicked, the \l MouseArea calls \c{handleClick()} in \c samegame.js, which determines whether the player's click should cause any blocks to be removed, and updates \c gameCanvas.score with the current score if necessary. Here is the \c handleClick() function:
 
\snippet tutorials/samegame/samegame3/samegame.js 1
 
Note that if \c score was a global variable in the \c{samegame.js} file you would not be able to bind to it. You can only bind to QML properties.
 
\section3 Updating the Score
 
When the player clicks a block and triggers \c handleClick(), \c handleClick() also calls \c victoryCheck() to update the score and to check whether the player has completed the game. Here is the \c victoryCheck() code:
 
\snippet tutorials/samegame/samegame3/samegame.js 2
 
This updates the \c gameCanvas.score value and displays a "Game Over" dialog if the game is finished.
 
The Game Over dialog is created using a \c Dialog type that is defined in \c Dialog.qml. Here is the \c Dialog.qml code. Notice how it is designed to be usable imperatively from the script file, via the functions and signals:
 
\snippet tutorials/samegame/samegame3/Dialog.qml 0
 
And this is how it is used in the main \c samegame.qml file:
 
\snippet tutorials/samegame/samegame3/samegame.qml 2
 
We give the dialog a \l {Item::z}{z} value of 100 to ensure it is displayed on top of our other components. The default \c z value for an item is 0.
 
 
\section3 A Dash of Color
 
It's not much fun to play Same Game if all the blocks are the same color, so we've modified the \c createBlock() function in \c samegame.js to randomly create a different type of block (for either red, green or blue) each time it is called. \c Block.qml has also changed so that each block contains a different image depending on its type:
 
\snippet tutorials/samegame/samegame3/Block.qml 0
 
 
\section2 A Working Game
 
Now we now have a working game! The blocks can be clicked, the player can score, and the game can end (and then you can start a new one).
Here is a screenshot of what has been accomplished so far:
 
\image declarative-adv-tutorial3.png
 
This is what \c samegame.qml looks like now:
 
\snippet tutorials/samegame/samegame3/samegame.qml 0
 
The game works, but it's a little boring right now. Where are the smooth animated transitions? Where are the high scores?
If you were a QML expert you could have written these in the first iteration, but in this tutorial they've been saved
until the next chapter - where your application becomes alive!
 
*/
 
/*!
\title QML Advanced Tutorial 4 - Finishing Touches
\previouspage QML Advanced Tutorial 3 - Implementing the Game Logic
 
\example tutorials/samegame/samegame4
 
\section2 Adding Some Flair
 
Now we're going to do two things to liven up the game: animate the blocks and add a High Score system.
 
In anticipation of the new block animations, \c Block.qml file is now renamed to \c BoomBlock.qml.
 
\section3 Animating Block Movement
 
First we will animate the blocks so that they move in a fluid manner. QML has a number of methods for adding fluid
movement, and in this case we're going to use the \l Behavior type to add a \l SpringAnimation.
In \c BoomBlock.qml, we apply a \l SpringAnimation behavior to the \c x and \c y properties so that the
block will follow and animate its movement in a spring-like fashion towards the specified position (whose
values will be set by \c samegame.js).Here is the code added to \c BoomBlock.qml:
 
\snippet tutorials/samegame/samegame4/BoomBlock.qml 1
 
The \c spring and \c damping values can be changed to modify the spring-like effect of the animation.
 
The \c {enabled: spawned} setting refers to the \c spawned value that is set from \c createBlock() in \c samegame.js.
This ensures the \l SpringAnimation on the \c x is only enabled after \c createBlock() has set the block to
the correct position. Otherwise, the blocks will slide out of the corner (0,0) when a game begins, instead of falling
from the top in rows. (Try commenting out \c {enabled: spawned} and see for yourself.)
 
\section3 Animating Block Opacity Changes
 
Next, we will add a smooth exit animation. For this, we'll use a \l Behavior type, which allows us to specify
a default animation when a property change occurs. In this case, when the \c opacity of a Block changes, we will
animate the opacity value so that it gradually fades in and out, instead of abruptly changing between fully
visible and invisible. To do this, we'll apply a \l Behavior on the \c opacity property of the \c Image
type in \c BoomBlock.qml:
 
\snippet tutorials/samegame/samegame4/BoomBlock.qml 2
 
Note the \c{opacity: 0} which means the block is transparent when it is first created. We could set the opacity
in \c samegame.js when we create and destroy the blocks,
but instead we'll use \l{Qt Quick States}{states}, since this is useful for the next animation we're going to add.
Initially, we add these States to the root type of \c{BoomBlock.qml}:
\code
    property bool dying: false
    states: [
        State{ name: "AliveState"; when: spawned == true && dying == false
            PropertyChanges { target: img; opacity: 1 }
        },
        State{ name: "DeathState"; when: dying == true
            PropertyChanges { target: img; opacity: 0 }
        }
    ]
\endcode
 
Now blocks will automatically fade in, as we already set \c spawned to true when we implemented the block animations.
To fade out, we set \c dying to true instead of setting opacity to 0 when a block is destroyed (in the \c floodFill() function).
 
\section3 Adding Particle Effects
 
Finally, we'll add a cool-looking particle effect to the blocks when they are destroyed. To do this, we first add a \l ParticleSystem in
\c BoomBlock.qml, like so:
 
\snippet tutorials/samegame/samegame4/BoomBlock.qml 3
 
To fully understand this you should read \l {Using the Qt Quick Particle System}, but it's important to note that \c emitRate is set
to zero so that particles are not emitted normally.
Also, we extend the \c dying State, which creates a burst of particles by calling the \c burst() method on the particles type. The code for the states now look
like this:
 
\snippet tutorials/samegame/samegame4/BoomBlock.qml 4
 
Now the game is beautifully animated, with subtle (or not-so-subtle) animations added for all of the
player's actions. The end result is shown below, with a different set of images to demonstrate basic theming:
 
\image declarative-adv-tutorial4.gif
 
The theme change here is produced simply by replacing the block images. This can be done at runtime by changing the \l Image \c source property, so for a further challenge, you could add a button that toggles between themes with different images.
 
\section2 Keeping a High Scores Table
 
Another feature we might want to add to the game is a method of storing and retrieving high scores.
 
To do this, we will show a dialog when the game is over to request the player's name and add it to a High Scores table.
This requires a few changes to \c Dialog.qml. In addition to a \c Text type, it now has a
\c TextInput child item for receiving keyboard text input:
 
\snippet tutorials/samegame/samegame4/Dialog.qml 0
\dots 4
\snippet tutorials/samegame/samegame4/Dialog.qml 2
\dots 4
\snippet tutorials/samegame/samegame4/Dialog.qml 3
 
We'll also add a \c showWithInput() function. The text input will only be visible if this function
is called instead of \c show(). When the dialog is closed, it emits a \c closed() signal, and
other types can retrieve the text entered by the user through an \c inputText property:
 
\snippet tutorials/samegame/samegame4/Dialog.qml 0
\snippet tutorials/samegame/samegame4/Dialog.qml 1
\dots 4
\snippet tutorials/samegame/samegame4/Dialog.qml 3
 
Now the dialog can be used in \c samegame.qml:
 
\snippet tutorials/samegame/samegame4/samegame.qml 0
 
When the dialog emits the \c closed signal, we call the new \c saveHighScore() function in \c samegame.js, which stores the high score locally in an SQL database and also send the score to an online database if possible.
 
The \c nameInputDialog is activated in the \c victoryCheck() function in \c samegame.js:
 
\snippet tutorials/samegame/samegame4/samegame.js 3
\dots 4
\snippet tutorials/samegame/samegame4/samegame.js 4
 
\section3 Storing High Scores Offline
 
Now we need to implement the functionality to actually save the High Scores table.
 
Here is the \c saveHighScore() function in \c samegame.js:
 
\snippet tutorials/samegame/samegame4/samegame.js 2
 
First we call \c sendHighScore() (explained in the section below) if it is possible to send the high scores to an online database.
 
Then, we use the \l{QtQuick.LocalStorage}{Local Storage API} to maintain a persistent SQL database unique to this application. We create an offline storage database for the high scores using \c openDatabaseSync() and prepare the data and SQL query that we want to use to save it. The offline storage API uses SQL queries for data manipulation and retrieval, and in the \c db.transaction() call we use three SQL queries to initialize the database (if necessary), and then add to and retrieve high scores. To use the returned data, we turn it into a string with one line per row returned, and show a dialog containing that string.
 
This is one way of storing and displaying high scores locally, but certainly not the only way. A more complex alternative would be to create a high score dialog component, and pass it the results for processing and display (instead of reusing the \c Dialog). This would allow a more themeable dialog that could better present the high scores. If your QML is the UI for a C++ application, you could also have passed the score to a C++ function to store it locally in a variety of ways, including a simple format without SQL or in another SQL database.
 
\section3 Storing High Scores Online
 
You've seen how you can store high scores locally, but it is also easy to integrate a web-enabled high score storage into your QML application. The implementation we've done her is very
simple: the high score data is posted to a php script running on a server somewhere, and that server then stores it and
displays it to visitors. You could also request an XML or QML file from that same server, which contains and displays the scores,
but that's beyond the scope of this tutorial. The php script we use here is available in the \c examples directory.
 
If the player entered their name we can send the data to the web service us
 
If the player enters a name, we send the data to the service using this code in \c samegame.js:
 
\snippet tutorials/samegame/samegame4/samegame.js 1
 
The \l XMLHttpRequest in this code is the same as the \c XMLHttpRequest() as you'll find in standard browser JavaScript, and can be used in the same way to dynamically get XML
or QML from the web service to display the high scores. We don't worry about the response in this case - we just post the high
score data to the web server. If it had returned a QML file (or a URL to a QML file) you could instantiate it in much the same
way as you did with the blocks.
 
An alternate way to access and submit web-based data would be to use QML types designed for this purpose. XmlListModel
makes it very easy to fetch and display XML based data such as RSS in a QML application (see the Flickr demo for an example).
 
 
\section2 That's It!
 
By following this tutorial you've seen how you can write a fully functional application in QML:
 
\list
\li Build your application with \l {Qt Quick QML Types}{QML types}
\li Add application logic \l{JavaScript Expressions in QML Documents}{with JavaScript code}
\li Add animations with \l {Behavior}{Behaviors} and \l{Qt Quick States}{states}
\li Store persistent application data using, for example, \l QtQuick.LocalStorage or \l XMLHttpRequest
\endlist
 
There is so much more to learn about QML that we haven't been able to cover in this tutorial. Check out all the
examples and the \l {Qt Quick}{documentation} to see all the things you can do with QML!
*/