Introduction to GUI Programming in Java

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selfrate = tkinterDoubleVar() selfrateset(50) selfyears = tkinterIntVar() selfamount = tkinterStringVar()
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Tk allows us to create variables that are associated with widgets If a variable s value is changed programmatically, the change is re ected in its associated widget, and similarly, if the user changes the value in the widget the associated variable s value is changed Here we have created two double variables (these hold float values), an integer variable, and a string variable, and have set initial values for two of them
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principalLabel = tkinterLabel(self, text="Principal $:", anchor=tkinterW, underline=0) principalScale = tkinterScale(self, variable=selfprincipal, command=selfupdateUi, from_=100, to=10000000, resolution=100, orient=tkinterHORIZONTAL) rateLabel = tkinterLabel(self, text="Rate %:", underline=0, anchor=tkinterW) rateScale = tkinterScale(self, variable=selfrate, command=selfupdateUi, from_=1, to=100, resolution=025, digits=5, orient=tkinterHORIZONTAL) yearsLabel = tkinterLabel(self, text="Years:", underline=0, anchor=tkinterW) yearsScale = tkinterScale(self, variable=selfyears, command=selfupdateUi, from_=1, to=50, orient=tkinterHORIZONTAL) amountLabel = tkinterLabel(self, text="Amount $", anchor=tkinterW) actualAmountLabel = tkinterLabel(self, textvariable=selfamount, relief=tkinterSUNKEN, anchor=tkinterE)
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This part of the initializer is where we create the widgets The tkinterLabel widget is used to display read-only text to the user Like all widgets it is created with a parent (in this case and as usual the parent is the containing widget), and then keyword arguments are used to set various other aspects of the widget s behavior and appearance We have set the principalLabel s text appropriately, and set its anchor to tkinterW, which means that the label s text is aligned west (left) The underline parameter is used to specify which character in the label should be underlined to indicate a keyboard accelerator (eg, Alt+P); further on we will see how to make the accelerator work (A keyboard accelerator is a key sequence of the form Alt+letter where letter is an underlined letter and which results in the keyboard focus being switched to the widget associated with the accelerator, most commonly the widget to the right or below the label that has the accelerator)
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For the tkinterScale widgets we give them a parent of self as usual, and associate a variable with each one In addition, we give a function (or in this case a method) object reference as their command this method will be called automatically whenever the scale s value is changed, and set its minimum (from_, with a trailing underscore since plain from is a keyword) and maximum (to) values, and a horizontal orientation For some of the scales we set a resolution (step size) and for the rateScale the number of digits it must be able to display The actualAmountLabel is also associated with a variable so that we can easily change the text the label displays later on We have also given this label a sunken relief so that it ts in better visually with the scales
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principalLabelgrid(row=0, column=0, padx=2, pady=2, sticky=tkinterW) principalScalegrid(row=0, column=1, padx=2, pady=2, sticky=tkinterEW) rateLabelgrid(row=1, column=0, padx=2, pady=2, sticky=tkinterW) rateScalegrid(row=1, column=1, padx=2, pady=2, sticky=tkinterEW) yearsLabelgrid(row=2, column=0, padx=2, pady=2, sticky=tkinterW) yearsScalegrid(row=2, column=1, padx=2, pady=2, sticky=tkinterEW) amountLabelgrid(row=3, column=0, padx=2, pady=2, sticky=tkinterW) actualAmountLabelgrid(row=3, column=1, padx=2, pady=2, sticky=tkinterEW)
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Having created the widgets, we must now lay them out The grid layout we have used is illustrated in Figure 153
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principalLabel rateLabel yearsLabel amountLabel principalScale rateScale yearsScale actualAmountLabel
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Figure 153 The Interest program s layout
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Every widget supports the grid() method (and some other layout methods such as pack()) Calling grid() lays out the widget within its parent, making it occupy the speci ed row and column We can set widgets to span multiple columns and multiple rows using additional keyword arguments (rowspan and columnspan), and we can add some margin around them using the padx (left and
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15 Introduction to GUI Programming
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right margin) and pady (top and bottom margin) keyword arguments giving integer pixel amounts as arguments If a widget is allocated more space than it needs, the sticky option is used to determine what should be done with the space; if not speci ed the widget will occupy the middle of its allocated space We have set all of the rst column s labels to be sticky tkinterW (west) and all of the second column s widgets to be sticky tkinterEW (east and west), which makes them stretch to ll the entire width available to them All of the widgets are held in local variables, but they don t get scheduled for garbage collection because the parent child relationships ensure that they are not deleted when they go out of scope at the end of the initializer, since all of them have the main window as their parent Sometimes widgets are created as instance variables, for example, if we need to refer to them outside the initializer, but in this case we used instance variables for the variables associated with the widgets (selfprincipal, selfrate, and selfyears), so it is these we will use outside the initializer
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principalScalefocus_set() selfupdateUi() parentbind("<Alt-p>", lambda *ignore: principalScalefocus_set()) parentbind("<Alt-r>", lambda *ignore: rateScalefocus_set()) parentbind("<Alt-y>", lambda *ignore: yearsScalefocus_set()) parentbind("<Control-q>", selfquit) parentbind("<Escape>", selfquit)
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At the end of the initializer we give the keyboard focus to the principalScale widget so that as soon as the program starts the user is able to set the initial amount of money We then call the selfupdateUi() method to calculate the initial amount Next, we set up a few key bindings (Unfortunately, binding has three different meanings variable binding is where a name, that is, an object reference, is bound to an object; a key binding is where a keyboard action such as a key press or release is associated with a function or method to call when the action occurs; and bindings for a library is the glue code that makes a library written in a language other than Python available to Python programmers through Python modules) Key bindings are essential for some disabled users who have dif culty with or are unable to use the mouse, and they are a great convenience for fast typists who want to avoid using the mouse because it slows them down The rst three key bindings are used to move the keyboard focus to a scale widget For example, the principalLabel s text is set to Principal $: and its underline to 0, so the label appears as Principal $:, and with the rst keyboard binding in place when the user types Alt+P the keyboard focus will switch to the principleScale widget The same applies to the other two bindings Note that we do not bind the focus_set() method directly This is because when functions or methods are called as the result of an event binding they are given the event
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