Custom Views in Java Integrating barcode standards 128 in Java Custom Views

Custom Views using servlet togenerate barcode code 128 in web,windows application Radio-frequency identification self.emit(SIGNAL(" barcode code 128 for Java clicked(QModelIndex)"), self.model.

index(self.selectedRow, 0)) else: QWidget.keyPressEvent(self, event).

We have chosen to support just two key presses: Up Arrow and Down Arrow. If the user presses either of these, we increment or decrement the selected row, make sure that the selected row is in range, and then schedule a paint event. If the user navigates to the row above the topmost visible row or below the bottommost visible row, we tell the scroll area to make sure that the row that has been scrolled to is visible if necessary, the scroll area will scroll to achieve this.

We also emit a clicked() signal with the newly selected row s model index. It is quite conventional to use a clicked() signal in this circumstance, since in effect, the user is clicking using the keyboard after all, the signals and slots mechanism is concerned with what the user wants rather than how they asked for it, and here they just want to select a row. If we do not handle the key press ourselves, that is, for all other key presses, we pass the event on to the base class.

The water quality view widget is visually very different from the table view shown beside it, yet it did not require that much code to implement and was not too dif cult to program. We made the widget fairly ef cient by reducing the amount of unnecessary painting. We also made the painting code as simple as possible by ensuring that the widget was always exactly the size necessary to display the entire dataset.

The disadvantage of this approach is that it pushes responsibility on to the programmer using our widget to use a QScrollArea, although this saves us from having to implement scrolling ourselves. The water quality view visualizes the data in one-to-one correspondence with the data in the model, but we are not constrained to doing this. It is also possible to create custom views that show aggregated data.

In this case, for example, we could have shown one entry per day, or per hour, perhaps by averaging each day or hour s readings.. Generic Delegates As we have seen in earlier chapters, custom delegates allow us to exercise complete control over the appearance and behavior of the data items that appear in views. Although it is obvious that if we have many models, we are likely to want a custom delegate for most if not all of them, what is not so obvious, is that the custom delegates will very likely have a lot of duplicate code. .

This section is pa barcode 128 for Java rtly based on ideas from the author s whitepaper, Qt 4 s Model/View Delegates , available at

. 16. Advanced Model/View Programming Imagine that we ha Code 128 Code Set C for Java ve just four models, each with an integer ID column, some string columns holding plain text, and a description column holding HTML text, and for some of the models, one or two oating-point columns. All the models have the ID as their rst column, but the other columns don t match up, so each one requires its own custom delegate. Providing the custom delegates is not a big undertaking, but the code dealing with the integer IDs might be the same in all of them; similarly for the strings, HTML strings, and oating-point numbers.

Now imagine that we have to write custom delegates for another half dozen new models: Much of the code will again be duplicated and this will probably make maintenance more dif cult. What would be better, particularly for models that have one data type per column, like database tables, is if instead of creating a custom delegate for each model, we could compose a delegate from a set of generic components. This would mean that the maintenance would be con ned to the generic components, and a bug x in one would automatically bene t any view that used it.

In code, the effect we are after is something like this:. self.table1 = QTab leView() self.table1.

setModel(self.model1) delegate1 = GenericDelegate(self) delegate1.insertColumnDelegate(1, PlainTextColumnDelegate()) delegate1.

insertColumnDelegate(2, PlainTextColumnDelegate()) delegate1.insertColumnDelegate(3, RichTextColumnDelegate()) delegate1.insertColumnDelegate(4, IntegerColumnDelegate()) self.

table1.setItemDelegate(delegate1) self.table2 = QTableView() self.

table2.setModel(self.model2) delegate2 = GenericDelegate(self) delegate2.

insertColumnDelegate(1, PlainTextColumnDelegate()) delegate2.insertColumnDelegate(2, IntegerColumnDelegate()) delegate2.insertColumnDelegate(3, FloatColumnDelegate()) delegate2.

insertColumnDelegate(4, FloatColumnDelegate()) delegate2.insertColumnDelegate(5, RichTextColumnDelegate()) self.table2.

setItemDelegate(delegate2). Here we have two s jvm Code 128B eparate models, but both use generic delegates that are composed of prede ned column delegates that are data-type-speci c. With this approach, we only ever have to create a single plain text column delegate, a single rich text column delegate, and so on, for each data type we want to handle, such as integers, oating-point numbers, dates, times, and date/times. In addition, we might create some project-speci c column delegates to handle custom types, but for any given data type there would be only one column delegate, drastically cutting down on code duplication and ensuring that any model.

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