Testing, if form exists: what changed
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Title: Testing, if form exists
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| 1 | + | Sometimes a [[TForm|form]] may be launched from several places in a program. If it already exists, it only needs to be brought to the front. If not, it needs to be created. | |
| 2 | + | ||
| 3 | + | This method is only needed if the form is not [[Form Tutorial|auto created]] (ie not listed under Project|Project Options|Forms|Available forms). | |
| 4 | + | ||
| 5 | + | The easiest way is: | |
| 6 | + | ``` | |
| 7 | + | if (MyForm = nil) then Application.CreateForm(TMyForm, MyForm); | |
| 8 | + | MyForm.Show; | |
| 9 | + | ``` | |
| 10 | + | ||
| 11 | + | Use `CloseAction := caFree` in the form's OnClose event, like so: | |
| 12 | + | ``` | |
| 13 | + | procedure TMyForm.FormClose(Sender: Tobject; var Closeaction: Tcloseaction); | |
| 14 | + | begin | |
| 15 | + | CloseAction := caFree; | |
| 16 | + | MyForm := nil; | |
| 17 | + | End; | |
| 18 | + | ``` | |
| 19 | + | ||
| 20 | + | This method is taken from forum discussions. | |
| 21 | + | ||
| 22 | + | [](</File:Warning-icon.png>) | |
| 23 | + | ||
| 24 | + | **Warning:** This method has some limitations: | |
| 25 | + | ||
| 26 | + | ||
| 27 | + | * **Not** _more than one instance_ of that form class should exist at any time. | |
| 28 | + | * Reference to any created form of that class should be stored in _a single global_ variable. | |
| 29 | + | ||
| 30 | + | --- | |
| 31 | + | *From the Free Pascal wiki, page "Testing, if form exists", as the Internet Archive kept it on 2024-12-06, last changed on 2020-02-29. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).* |