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CHAPTER 12 ■ GRAPHICAL USER INTERFACES 277 bkg = wx.Panel(win) loadButton = wx.Button(bkg, label='Open') saveButton = wx.Button(bkg, label='Save') filename = wx.TextCtrl(bkg) contents = wx.TextCtrl(bkg, style=wx.TE_MULTILINE | wx.HSCROLL) hbox = wx.BoxSizer() hbox.Add(filename, proportion=1, flag=wx.EXPAND) hbox.Add(loadButton, proportion=0, flag=wx.LEFT, border=5) hbox.Add(saveButton, proportion=0, flag=wx.LEFT, border=5) vbox = wx.BoxSizer(wx.VERTICAL) vbox.Add(hbox, proportion=0, flag=wx.EXPAND | wx.ALL, border=5) vbox.Add(contents, proportion=1, flag=wx.EXPAND | wx.LEFT | wx.LEFT | wx.LEFT, border=5) bkg.SetSizer(vbox) win.Show() app.MainLoop() This code gives the same result as the previous program, but instead of using lots of absolute coordinates, I am now placing things in relation to one another. The constructor of the wx.BoxSizer takes an argument determining whether it’s horizontal or vertical (wx.HORIZONTAL or wx.VERTICAL), with horizontal being the default. The Add method takes several arguments. The proportion argument sets the proportions according to which space is allocated when the window is resized. For example, in the horizontal box sizer (the first one), the filename widget gets all of the extra space when resizing. If each of the three had had their proportion set to 1, each would have gotten an equal share. You can set the proportion to any number. The flag argument is similar to the style argument of the constructor; you construct it by using bitwise OR between symbolic constants (integers that have special names). The wx.EXPAND flag makes sure the component will expand into the allotted space. The wx.LEFT, wx.RIGHT, wx.TOP, wx.BOTTOM, and wx.ALL flags determine on which sides the border argument applies, and the border arguments gives the width of the border (spacing). And that’s it. I’ve got the layout I wanted. One crucial thing is lacking, however. If you click the buttons, nothing happens. ■Tip For more information on sizers, or anything else related to wxPython, check out the wxPython demo. It has sample code for anything you might want to know about, and then some. If that seems daunting, check out the wxPython Web site, http://wxpython.org.
278 CHAPTER 12 ■ GRAPHICAL USER INTERFACES Event Handling In GUI lingo, the actions performed by the user (such as clicking a button) are called events. You need to make your program notice these events somehow, and then react to them. You accomplish this by binding a function to the widget where the event in question might occur. When the event does occur (if ever), that function will then be called. You link the event handler to a given event with a widget method called Bind. Let’s assume that you have written a function responsible for opening a file, and you’ve called it load. Then you can use that as an event handler for loadButton as follows: loadButton.Bind(wx.EVT_BUTTON, load) This is pretty intuitive, isn’t it? I’ve linked a function to the button—when the button is clicked, the function is called. The symbolic constant wx.EVT_BUTTON signifies a button event. The wx framework has such event constants for all kinds of events, from mouse motion to keyboard presses and more. WHAT’S THIS LOAD STUFF ABOUT? There is nothing magical about my choice to use loadButton and load as the button and handler names— even though the button text says “Open.” It’s just that if I had called the button openButton, open would have been the natural name for the handler, and that would have made the built-in file-opening function open unavailable. While there are ways of dealing with this, I found it easier to use a different name. The Finished Program Let’s fill in the remaining blanks. All you need now are the two event handlers, load and save. When an event handler is called, it receives a single event object, which holds information about what happened, but let’s ignore that here because you’re only interested in the fact that a click occurred. Even though the event handlers are the meat of the program, they are surprisingly simple. Let’s take a look at the load function first. It looks like this: def load(event): file = open(filename.GetValue()) contents.SetValue(file.read()) file.close() The file opening/reading part ought to be familiar from Chapter 11. As you can see, the file name is found by using filename’s GetValue method (where filename is the small text field, remember?). Similarly, to put the text into the text area, you simply use contents.SetValue. The save function is just as simple: It’s the exact same as load—except that it has a 'w' and a write for the file-handling part, and GetValue for the text area:
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Beginning Python From Novice to Pro
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Index ■Symbols - operator 558 !=
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