How to code a result to a selection from a combobox? [closed] - delphi

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I have a TComboBox in Delphi 10.3. I have a Combobox with over 30 items. I need to code a different action for each item of the Combobox. At the moment I'm using if-else statements. As there are 30 different items the if statements are going to be way too long. Is there a quicker way to do this?

Answer 1

This entirely depends on your situation. It is almost impossible to answer your Q without knowing your precise scenario.
Nevertheless, here are a few ideas. Maybe they are relevant to your situation, maybe they are not.
Trivial parameterisation by index
In the best case scenario, your 30 actions can be parameterised. For instance, suppose the items of the combo box are
Show 1
Show 10
Show 100
Show 1000
...
which will display a message box with the given number. In this scenario, you don't need 30 different procedures (here each represented by a simple call to ShowMessage):
procedure TForm1.btnNextClick(Sender: TObject);
begin
case ComboBox1.ItemIndex of
0:
ShowMessage('1');
1:
ShowMessage('10');
2:
ShowMessage('100');
3:
ShowMessage('1000');
// ...
end;
end;
Instead, you should use only one procedure, but with a parameter:
procedure TForm1.btnNextClick(Sender: TObject);
begin
if ComboBox1.ItemIndex <> -1 then
ShowMessage(IntPower(10, ComboBox1.ItemIndex).ToString)
end;
Parameterisation by the associated object
If the action cannot be described by the item's index alone, you can use the object pointer associated with each item. Maybe it is enough to use it to store an integer:
procedure TForm1.FormCreate(Sender: TObject);
begin
ComboBox1.Items.BeginUpdate;
try
ComboBox1.Items.Clear;
ComboBox1.Items.AddObject('Show 51', TObject(51));
ComboBox1.Items.AddObject('Show 111', TObject(111));
ComboBox1.Items.AddObject('Show 856', TObject(856));
ComboBox1.Items.AddObject('Show 1000', TObject(1000));
finally
ComboBox1.Items.EndUpdate;
end;
end;
procedure TForm1.btnNextClick(Sender: TObject);
begin
if ComboBox1.ItemIndex <> -1 then
ShowMessage(Integer(ComboBox1.Items.Objects[ComboBox1.ItemIndex]).ToString);
end;
Otherwise, you can let it be a true pointer to some object with any amount of data (integers, strings, ...).
Unrelated procedures
The examples above all require that the procedures can be parameterised, i.e. replaced by a single procedure with a parameter. If this is not the case, if the procedures are completely unrelated, you need a different approach. But again, which approach is most suitable depends on your precise situation.
Here are a few examples.
Simple case statement
At design time, set the items to Play sound, Run Notepad, and Show Start Menu.
procedure PlaySound;
begin
MessageBeep(MB_ICONINFORMATION);
end;
procedure RunNotepad;
begin
ShellExecute(Form1.Handle, nil, 'notepad', nil, nil, SW_SHOWNORMAL)
end;
procedure ShowStartMenu;
begin
Form1.Perform(WM_SYSCOMMAND, SC_TASKLIST, 0)
end;
procedure TForm1.btnNextClick(Sender: TObject);
begin
case ComboBox1.ItemIndex of
0:
PlaySound;
1:
RunNotepad;
2:
ShowStartMenu;
end;
end;
Storing procedural pointers with the items
procedure PlaySound;
begin
MessageBeep(MB_ICONINFORMATION);
end;
procedure RunNotepad;
begin
ShellExecute(Form1.Handle, nil, 'notepad', nil, nil, SW_SHOWNORMAL)
end;
procedure ShowStartMenu;
begin
Form1.Perform(WM_SYSCOMMAND, SC_TASKLIST, 0)
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
ComboBox1.Items.BeginUpdate;
try
ComboBox1.Items.Clear;
ComboBox1.Items.AddObject('Play sound', TObject(#PlaySound));
ComboBox1.Items.AddObject('Run notepad', TObject(#RunNotepad));
ComboBox1.Items.AddObject('ShowStartMenu', TObject(#ShowStartMenu));
finally
ComboBox1.Items.EndUpdate;
end;
end;
procedure TForm1.btnNextClick(Sender: TObject);
begin
if ComboBox1.ItemIndex <> -1 then
TProcedure(ComboBox1.Items.Objects[ComboBox1.ItemIndex])();
end;
Benefit: no risk of confusing the indices; the actions are "attached" to the items.
Using a dictionary of commands
Maybe your application has a global set of commands, denoted by English words. Then you might want to use a dictionary to get the procedure associated with a word. This can be used for the combo box as well. At design time, let there be three items: beep, write, and start:
type
TForm1 = class(TForm)
ComboBox1: TComboBox;
btnNext: TButton;
procedure btnNextClick(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
private
FCommands: TDictionary<string, TProcedure>;
public
end;
procedure PlaySound;
begin
MessageBeep(MB_ICONINFORMATION);
end;
procedure RunNotepad;
begin
ShellExecute(Form1.Handle, nil, 'notepad', nil, nil, SW_SHOWNORMAL)
end;
procedure ShowStartMenu;
begin
Form1.Perform(WM_SYSCOMMAND, SC_TASKLIST, 0)
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
FCommands := TDictionary<string, TProcedure>.Create;
FCommands.Add('beep', PlaySound);
FCommands.Add('write', RunNotepad);
FCommands.Add('start', ShowStartMenu);
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
FCommands.Free;
end;
procedure TForm1.btnNextClick(Sender: TObject);
var
Cmd: TProcedure;
begin
if
(ComboBox1.ItemIndex <> -1)
and
FCommands.TryGetValue(ComboBox1.Items[ComboBox1.ItemIndex], Cmd)
then
Cmd();
end;