0050 – MVC Part III – ComboBoxText - Adding and Removing Items

Last time we got stuck into ComboBoxText widgets and this time we carry on by adding and deleting items that show up in the drop-down list.

In this first example, we’ll look at…

Adding an Item to a ComboBoxText

Results of this example:
Current example output
Current example output
Current example terminal output
Current example terminal output (click for enlarged view)

For this functionality, we need to put an extra Button in our UI and, of course, that’s done in the AppBox class:

	class AppBox : Box
	{
		DayComboBoxText dayComboBoxText;
		AddToComboButton addToComboButton;
		
		this()
		{
			super(Orientation.HORIZONTAL, 10);
			
			dayComboBoxText = new DayComboBoxText();
			packStart(dayComboBoxText, false, false, 0);
			
			addToComboButton = new AddToComboButton(dayComboBoxText);
			packEnd(addToComboButton, false, false, 0);
			
			writeln("Type something into the Entry, then hit the Add button.");
			writeln("You can also hit Enter to echo the contents of the Entry to the terminal, but this action doesn't add the contents to the list.");
	
		} // this()
	
	} // class AppBox

I’ll draw your attention to these lines:

	addToComboButton = new AddToComboButton(dayComboBoxText);
	packEnd(addToComboButton, false, false, 0);

The only unusual thing is passing in the dayComboBoxText object so the Add Button has access to its data. No worries, right? We’ve done this kind of thing a few times by now.

I used writeln() to echo a couple of lines to the terminal so you know what’s being demonstrated.

Now let’s look at…

The Add Button Class

Which looks like this:

	class AddToComboButton : Button
	{
		private:
		ComboBoxText _comboBoxText;
		Entry _entry;
		string _entryText, buttonText = "Add";
		
		public:
		this(ComboBoxText comboBoxText)
		{
			super(buttonText);
			
			_comboBoxText = comboBoxText;
			_entry = cast(Entry)_comboBoxText.getChild();
	
			addOnReleased(&doSomething);		
			
		} // this()
		
		
		void doSomething(Button b)
		{
			_entryText = _entry.getText();
			
			if(_comboBoxText.getIndex(_entryText) is -1)
			{
				_comboBoxText.appendText(_entryText);
				writeln(_entryText, " is now on the list.");
			}
			else
			{
				writeln(_entryText, " is already on the list.");
			}
			
	
		} // doSomething()
		
	} // class AddToComboButton

Because we need to grab the text from the Entry, as mentioned before, we need access and we get that in the constructor by assigning _comboBoxText and _entry. The first assignment (_comboBoxText) isn’t strictly necessary, but it lends clarity to the cast() call. And maybe we’ll think of some other reason we need this sometime down the road.

In the doSomething() function, we take up the task of grabbing the text from the Entry and appendText() adds it to the list.

A quick note: There are three ways we can add items to the list:

  • appendText() sticks it at the end of the list,
  • prependText() sticks it at the beginning, and
  • insertText() takes a second argument so you can put it wherever you want.

Remove an Item

Results of this example:
Current example output
Current example output
Current example terminal output
Current example terminal output (click for enlarged view)

This example looks pretty much the same as the last except it’s got a Remove Button that looks like this:

	class RemoveFromComboButton : Button
	{
		private:
		ComboBoxText _comboBoxText;
		Entry _entry;
		string _entryText, buttonText = "Delete";
		
		public:
		this(ComboBoxText comboBoxText)
		{
			super(buttonText);
			
			_comboBoxText = comboBoxText;
			_entry = cast(Entry) _comboBoxText.getChild();
	
			addOnReleased(&doSomething);		
			
		} // this()
		
		
		void doSomething(Button b)
		{
			int activeTextIndex;
			
			_entryText = _entry.getText();
			activeTextIndex = _comboBoxText.getIndex(_entryText);
			
			if(activeTextIndex !is -1)
			{
				_comboBoxText.remove(activeTextIndex);
				writeln(_entryText, " has been removed.");
				_comboBoxText.setActive(0);
			}
			else
			{
				writeln("Cannot complete operation. '", _entryText, "' isn't on the list.");
			}
			
	
		} // doSomething()
		
	} // class RemoveFromComboButton

Removing, as can be seen in the if() statement inside the doSomthing() function, is a two-stage process:

  • check to make sure the item is actually on the list, and
  • call remove() to remove it.

There is a third line of code that’s just as important, though:

	_comboBoxText.setActive(0);

Why? Because if we don’t reset the active item, the just-deleted text is still sitting in the Entry. Depending on circumstances, you may want to comment this line out so the text is left there to serve as a quick-n-dirty ‘undo’ function. All the user would have to do is hit the Add Button and the item goes right back on the list.

Conclusion

Next time, we’ll take a look at the ComboBox and look at populating it with various data types starting with text, then moving on to things like numbers, images, or whatever else fits into a TreeModel.

Until then…

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© Copyright 2019 Ron Tarrant