.NET/Java PDF, Tiff, Barcode SDK Library

The size policy is accompanied by the heightForWidth(int) method and the sizeHint method returning the preferred widget size. The implementation of these methods is shown in Listing 6-21. The heightForWidth method takes a width as argument and returns the wanted height to the layout manager. The implementation used in the CircleBar class returns the given width as height, resulting in a square widget. Listing 6-21. The size handling methods int CircleBar::heightForWidth( int width ) const { return width; } QSize CircleBar::sizeHint() const { return QSize( 100, 100 ); } The methods for handing the values value() and setValue are shown in Listing 6-22. The value method is simple it simply returns m_value. The setValue method limits the value to the range 0 100 before checking whether a change has taken place. If so, m_value is updated before a call to update is made and the valueChanged signal is emitted. By calling update(), a repaint event is triggered, which causes a call to paintEvent. Remember that you can t draw the widget outside the paintEvent method. Instead, call update and then handle the painting from the paintEvent method. Listing 6-22. Handing the value of the CircleBar widget int CircleBar::value() const { return m_value; } void CircleBar::setValue( int value ) { if( value < 0 ) value = 0; if( value > 100 ) value = 100; if( m_value == value ) return; m_value = value;

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Notice how the constructor looks like a standard method declaration, except that since there s no need for a return type specifier, we leave that out. We don t even write void, like we would for a normal method that returns nothing. And it would be weird if we did; in a sense this does return something the newly created Plane it just does so implicitly. What sort of work should you do in a constructor Opinion is divided on the subject should you do everything required to make the object ready to use, or the minimum necessary to make it safe The truth is that it is a judgment call there are no hard and fast rules. Developers tend to think of a constructor as being a relatively low-cost operation, so enormous amounts of heavy lifting (opening files, reading data) might be a bad idea. Getting the object into a fit state for use is a good objective, though, because requiring other functions to be called before the object is fully operational tends to lead to bugs. We need to update our Main function to use this new constructor and to get rid of the line of code that was setting the property, as Example 3-7 shows.

static void Main(string[] args) { Plane someBoeing777 = new Plane("BA0049"); Console.WriteLine( "Your plane has identifier {0}", someBoeing777.Identifier); } Console.ReadKey();

Summary

Notice how we pass the argument to the constructor inside the parentheses, in much the same way that we pass arguments in a normal method call. If you compile and run that, you ll see the same output as before but now we have an identifier that can t be changed by users of the object.

Be very careful when you talk about properties that can t be changed because they have a private setter. Even if you can t set a property, you may still be able to modify the state of the object referred to by that property. The built-in string type happens to be immune to that because it is immutable (i.e., it can t be changed once it has been created), so making the setter on a string property private does actually prevent clients from changing the property, but most types aren t like that.

QMetaObject and contain information about the class such as its name, its super classes, its signals, its slots, and many other interesting things The important thing to know now is that the meta-object knows about the signals and slots This leads into the next implication of this feature Until now, all the examples have fitted nicely into a single file of source code It is possible to go on like this, but the process is much smoother if you separate each class into a header and a source file A Qt tool called the meta-object compiler, moc, parses the class declaration and produces a C++ implementation file from it This might sound complex, but as long as you use QMake to handle the project, there is no difference to you This new approach means that the code from Listing 1-8 goes into a file called myclassh.

Speaking of properties that might need to change, our specification requires us to know the speed at which each plane is traveling. Sadly, our specification didn t mention the units in which we were expected to express that speed. Let s assume it is miles per hour,

and add a suitable property. We ll use the floating-point double data type for this. Example 3-8 shows the code to add to Plane.

public double SpeedInMilesPerHour { get; set; }

The implementation goes into myclasscpp, and the moc generates another C++ file from the header file called moc_myclasscpp The contents from the generated file can change between Qt versions and is nothing to worry about Listing 1-9 contains the part of the implementation that has changed because of signals and slots Listing 1-9 Implementing MyClass with signals and slots void MyClass::setText( const QString &text ) { if( m_text == text ) return; m_text = text; emit textChanged( m_text ); } The changes made to emit the signal textChanged can be divided into two parts The first half is to check that the text actually has changed If you do not check this before you connect the textChanged signal to the setText slot of the same object, you will end up with an infinite loop (or as the user would put it, the application will hang).