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selection is modified, and our user would see the appropriate feedback. Of course, as our program changes, there might be other commands or user interactions that alter the display mode of the image. A better approach would ensure that the display modes are checked or unchecked as they are displayed to the user. This approach is more robust in the face of future changes, creating an application that users, documenters, and testers will appreciate for years to come. The Popup event is designed for just this purpose. This event occurs just before a submenu is displayed, so that its appearance or contents can be modified and then immediately displayed to the user. In Visual Studio, a Popup event handler is added from the Properties window much like we added a Click event in the previous section.
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IMPLEMENT A POPUP HANDLER FOR IMAGE MENU Action 1 Add a Popup event handler for the Image menu.
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Result A Popup event handler is added for the menuImage object. The beginning of this code is shown here:
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protected void menuImage_Popup (object sender, System.EventArgs e) {
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a. Display the events for the Image menu in the Properties window. b. Double-click the Popup entry 2 3 4 Verify that the sender is a MenuItem object. Determine if an image has been loaded into the application. Set the Enabled and Checked properties for each submenu item.
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if (sender is MenuItem) { { bool bImageLoaded = (imgPhoto.Image != null); foreach (MenuItem mi in ((MenuItem)sender).MenuItems) { mi.Enabled = bImageLoaded; mi.Checked = (this._selectedImageMode == mi.Index); } } }
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Our new handler downcasts the sender object to a MenuItem instance similar to the menuImage_ChildClick handler we already discussed. The handler is repeated below so we can note a few points in the code.
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protected void menuImage_Popup (object sender, System.EventArgs e) { if (sender is Menu) { Determine if an image is loaded bool bImageLoaded = (pbxPhoto.Image != null);
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Menu parentMenu = (Menu)sender; foreach (MenuItem mi in parentMenu.MenuItems) {
c Iterate over each
submenu item
MENUS
mi.Enabled = bImageLoaded; mi.Checked = (this._selectedImageMode == mi.Index); } } }
Note that the parentMenu variable here could be defined as a MenuItem object. The Menu type is a base class and allows our handler to accommodate other Menu types in the future. In addition, a couple of C# keywords we have not seen before are worth a special mention. Unlike C and C++, C# has a built-in boolean type called bool. As a result, boolean expressions such as the one here evaluate to true or false, rather than 0 or 1 as in C. In this case, the bImageLoaded variable will be set to true only after an image has been assigned to the Image property of the pbxPhoto object. In addition to the for loop used in C and other languages, C# also defines a foreach loop. A foreach loop iterates over the objects in an array or other container object, with the advantage that you don t have to worry about the starting or ending index, or whether the container is empty. The language ensures that each entry in the given container is passed to the loop code. In this case, the loop executes for each MenuItem contained in the given menuImage menu. Within the loop, each MenuItem is enabled only if an image has been loaded, and a check mark is set using the Checked property based on whether the index of the menu item matches the selected image mode. You may also notice that there is nothing in this handler to indicate that these menu items are part of a specific menu structure. This will be useful in our upcoming discussion on context menus. Compile and run the application to verify that the menus work correctly, and the display mode of the image changes depending on the menu selection. Figure 3.7 shows the application with an image displayed in Actual Size mode. Unfortunately, this figure reveals another problem with our PictureBox control. In the figure, the image is larger than the display area, but there is no way to see the rest of the image without resizing the window. While this is possible when the image is small enough, a high-resolution image may contain more pixels than our screen. Ideally, the application should display scroll bars here. Since the PictureBox control does not support scroll bars, this is not possible. You may be wondering about a book that teaches you how to build an application that doesn t quite work, and you should. Be patient until chapter 7, where we will get rid of our not-quite-right PictureBox control in order to fix this problem.
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