The only way I see is to add flag for this, but is this the best way?
When the form is destroyed and I check if(Assigned(form2)) the result is true? Why?
What is the way to do this?
You can use Form1.Showing to see if a form is closed or not.
Just closing a form does not free it unless you set Action := caFree in OnClose event. Default is caHide.
Wow, a blast from the past :)
The way that Assigned() works, is that it basically does nil check on the pointer. If you destroy form2, there will still be a memory address that form2 points to.
I has been a very long time since I've done any Delphi, but from memory, you need to manually set the form2 var to nil when it is destroyed. If you have a central place (eg. a form broker?) where you create & destroy forms, this should be quite easy.
If you use Form1.Free or Form1.Destroy, Delphi will destroy the object but wont set the object reference to nil. So instead use FreeAndNil.
For more information, check Andreas Rejbrand answer in this link
Faced with the same issue when doing some routine on closing application. In this case all forms are destroyed behind the stage but pointers are not set to nil. This code helphs me:
procedure TMyForm.FormDestroy(Sender: TObject);
begin
MyForm:=nil;
end;
So pointer becomes nil and I can check it with Assigned or compare to nil.
Just as a tip, the correct way to do it in some special cases is to create a timer that do the nil assign to the variable.
I will explain it (somehow it is complex), if you create your form within your own code MyForm:=TMyForm.Create and you have a MyFrom.Close it is very easy, just add a MyForm:=nil or also better MyForm.FreeAndNil... but sometimes the reference is not anyware.
Sample: You create inside a procedure, on a loop a lot of copies of the same form (or just one), let the form open and end that procedure, now the reference to the opened form is nowhere, so you can not assign nil or do a freeandnil, etc., in a normal way.
For that cases, the trick is to use a timer (of just one milisecond) that do it, that timer needs the reference, so you must store on a global like the reference to Self, all that can be done on the on close event.
The easiest way to do the free (when no reference anywhere) is to create a TObjectList on the main form, so it will hold all form references that needs to be free, and define a timer (one milisecond) that will go through that list doing the freeandnil; then on the onlcose you add Self to that list and enable that timer.
Now the other part, you have a normal form that is auto created on start, but you need to set it to nil and re-create it on your own code.
That case has a global that point to that form, so you only need to free and nil it, but NOT (i say it loud) on any part inside the own form code, you must do it OUT (i say if loud) of the form code.
Some times you will need to free the form, when user close it, and it is not shown in modal, this case is complex, but again the same trick is valid, on the onclose event you enable a timer (that is out of that form, normally on main form) adn that timer will free and nil. That timer interval can be set as just one milisecond, it will not be run until form has totally closed (please have in mind not using Application.ProcessMessages, that is normally a really bad idea).
If you set Self to nil, free or whatever inside the own form, you can corrupt your application memory (doing that is totally unsafe, not to mention it can eat ram).
The only way to free a form (and nil its reference), a form that is not shown as modal and is the user who close it, is to program a trigger that do that after the form is totally closed.
I know about setting action to do the free, but to set it to nil there is no other safe way.
Must say: If you use timers on your main form, run a Enabled:=False on all of them on the Onclose event... otherwise weird things may occur (not allways, but sometimes... race conditions about destroying application and running code on that timers), and of couse if some one was enabled act correctly to terminate it correctly or abort it, etc.
Your question is one of the complex things to do... free and nil a form that is closed not by code, but by user action.
For all the rest: Think like if the application has at the same time a lot of forms opened and all can interact with the user at the same time (anyone is modal), and you have code that references some of them from the others... you need to know f user has closed any form to avoid accesing that form from code. This is not trivial to be done unless you use timers.
If you have a 'central' form (like an MDI application) you can put that timer on the main MDI form, so any child form that is closed can be freed and nil, the trick is again a timer on that main form.
Only and only if you are sure you can free and nil all non visible forms, you can have a timer on the main form that goes through all forms and if Visible is false, then call FreeAndNil, i consider this way prone to errors, since if you add on a future a form that must not be freed but can stay hidden... this code will not be valid.
Allways remember that if is the user the onw who closes the form that must be freed and nil, there is no way on code to detect and act, no event is launched (after the form is totally closed) and before the form is totally closed you must not even try to free it or nil its reference, weird things can occur (more prone to that if motherboard has more than one socket, also more if your app uses threads, etc).
So, for threaded apps (and also not threaded) i use another method that works great and do not need timers, but need double checking prior to each ThatForm.*, the trick is to define a Form bolean public variable like as PleaseFreeAndNilMe on the public section on the form, then on the onclose (as last line) set it to True and on the OnCreate set it as False.
That way you will know if that form had been closed or only hidden (to hide a form never call close, just call hide).
So coded will look like (you can use this as a warper, instead of defining forms as TForm define them as TMyform, or also better, use a hack like type TForm=class(Forms.TForm) instead of TMyForm=class(TForm) just to have that variable added to all forms):
TMyForm=class(TForm)
...
public
PleaseFreeAndNilMe:=Boolean;
...
procedure TMyForm.FormCreate(Sender: TObject);
begin
PleaseFreeAndNilMe:=False;
...
end;
procedure TMyForm.FormClose(Sender: TObject; var Action: TCloseAction);
begin
...
PleaseFreeAndNilMe:=True;
end;
If you preffer hacked version:
TForm=class(Froms.TForm)
public
PleaseFreeAndNilMe:=Boolean;
end;
procedure TForm.FormCreate(Sender:TObject);
begin
inherited Create(Sender);
PleaseFreeAndNilMe:=False;
end;
procedure TForm.FormClose(Sender:TObject;var Action:TCloseAction);
begin
PleaseFreeAndNilMe:=True;
inherited FormClose(Sender,Action);
end;
But as i said, prior to access any member (or just where you do the nil compare) just call a 'global' function passign the reference (no matter if it was nil or not), coded as:
function IsNilTheForm(var TheForm: TMyForm);
begin
if nil=TheForm
then begin // The form was freed and nil
IsNilTheForm:=True; // Return value
end
else begin // The form refence is not nil, but we do not know is it has been freed or not
try
if TheForm.PleaseFreeAndNilMe
then begin // The form is not freed but wants to
try
TheForm.Free;
except
end;
try
TheForm:=Nil;
except
end;
IsNilTheForm:=True; // Return value
end
else begin // The form is not nil, not freed and do not want to be freed
IsNilTheForm:=False; // Return value
end;
except // The form was freed but not set to nil
TheForm:=Nil; // Set it to nil since it had beed freed
IsNilTheForm:=True; // Return value
end;
end;
end;
So where you do if nil=MyForm then ... you can now do if (IsNilTheForm(MyForm)) then ....
That is it.
It is better the timer solution, since form is freed as soon as possible (less ram used), with the PleaseFreeAndNilMe trick the form is not freed until IsNilTheForm is called (if you do not free it any where else).
That IsNilTheForm is so complex because it is considering all states (for a multi socket motherboard and threaded apps) and letting the code free / nil it anywhere else.
Of course, that function must be called on main thread and in atomic exclusion.
Free a Form and Nil its pointer is not a trivial thing, most when user can close it at any time (since no code out of the form is fired).
The big problem is: When a user closes a form there is no way to have a event handler that is triggered out of that form and after the form ends all things it is doing.
Imaigne now that the coder has put a lot of Application.ProcessMessages; every where on the app, also on that form, etc... and has not taken the care for race conditions... try to free and nil such a form after the user asks it to be closed... this is a nightmare, but can be solved with the hacked version of TForm that has a variable that tells that the form has not been freed but wants it.
Now imagine you use hacked TForm and want a normal TForm, just define it as ...= class(Forms.TForm), that way it will now have that extra variable., so calling IsNilTheForm will act as comparing to nil.
Hope this helps VCL coders to FIX such things, like raising an event when an object is destroyed, freed, niled, hide, etc... out of the code of that object, like on main form, etc. That would make live easier... or just fix it... Close and Free implies set to Nil all refences that point to it.
There is another thing that can be done (but i try to allways avoid it): Have multiple variables that point to the exact same form (not to copies ot it), that is prone to a lot of errors, you free one and need to nil all of them, etc... The code i show is also compatible with that.
I know the code is comples... but Free and Nil a form is more complex than my code.
Related
Core problem: I don't like the way how TcxGrid delete (that is initiated by the default Ctrl+Del shortcut key for the TcxGrid or by the Delete button of the cx-navigator) is working: it is simple and direct call to the delete of the underlying dataset, there is no TcxGrid(/DataController).OnDelete(Sender: TObject, var AHandled: Boolean) event for the cxGrid or cxGridDataView. And from https://www.devexpress.com/Support/Center/Question/Details/Q308755/tcxgrid-deleting-record I understand that I can only configure the confirmation message, but I have no control over the Delete itself, i.e., I can not implement custom Delete and therefore I don't even try to ask for the straight solution of this problem. My question is about workaround.
That is why I am refusing to use cx-navigator and I opt to introduce TAction that handles Ctrl+Del shortcut key for the entire form. I have multiple grids on the form each with its own problematic Delete procedure that should be handled in non-standard way by the custom procedure.
That is why my DeleteAction determines the ActiveControl (TcxGridSite, TcxGrid) initially and then calls relevant Delete procedure afterwards. All that is fine. But some other components (e. g. TcxDBTextEdit) have default Ctrl+Del handling as well and that handling is very, very good. But if shortcut key Ctrl+Del is handled by action (and it is marked as handled even in the case when no active grid can not be found and therefor even in the case when ActionExecute has not done anything useful) then further propagation is stopped. That can be observed empirically and that can be seen theoretically from http://edn.embarcadero.com/article/38447 .
But maybe still there is some workaround how the shortcut key that has been processed by the ActionExecute can still be propagated further to the default components if they are Active controls.
I know that TAction is for (form-wide global) menu functionality but TcxGrid is not extensible enough and that is why should try to stretch the Delphi TAction design as well.
Probably there would be various ways to achieve the processing you require, there are quite a few places you can intervene during key processing, as you've already read from Peter Below's article linked in your question.
A place to intervene that I can think of which would contain all the pieces together is the form's IsShortCut method. One probable disadvantage of such a method could be that you'd have to implement it on all the forms that you want their key handling behavior changed.
Normally key processing continues only if it's not handled by shortcuts. Below solution reports the shortcut to be not processed for that reason, but then has to cause the execution of the action manually. I tested the code with a regular edit as can be seen, but I guess it shouldn't make any difference.
function TForm1.IsShortCut(var Message: TWMKey): Boolean;
begin
if (ActiveControl is TEdit) and (Message.CharCode = VK_DELETE) and
(KeyDataToShiftState(Message.KeyData) = [ssCtrl]) then begin
Result := False; // if you don't require the action to be executed, just exist here
ActionList1.IsShortCut(Message); // executes the action, ignore result
// returning false will result in main form's shortcut handler to be called
// below is only required if this is the main form
if Application.MainForm = Self then
Message.CharCode := 0;
end else
Result := inherited IsShortCut(Message);
end;
I have this code:
procedure TForm1.FormCreate(Sender: TObject);
begin
DoSomethingWithCombobox(ComboBox1.Handle);
end;
Q: Is it guaranteed that the ComboBox1.Handle is always created on TForm.FormCreate (Form1 is the parent of ComboBox1)? Maybe on OnFormShow?
from my tests, it seems that the Handle is always available at that point.
I know that ComboBox1.Handle will call HandleNeeded at this point. but can I assume that Handle will always be available at this point?
I also know that the TWincontrol can safely access it's own handle on CreateWnd. My question is specific to a scenario where I cannot control CreateWnd of the child control and only have access to the parent events/messages.
Hope my question is clear.
If your tests show that it's OK to access the control's handle there, then it should be OK. You're the application developer, so if you later change anything to break that assumption, you'll also have the power to fix it.
Accessing a control's Handle property will either yield a valid window handle or throw an exception. You won't get a null handle. The exception would typically come when the control's parent window is unable to exist.
The handle you get at that point isn't guaranteed to be the last handle the control will ever have, because controls' underlying windows may be recreated, but since you're the application developer (as opposed to a component-library developer), you have reasonable control over how often windows will be recreated after the form has finished being created. This is because you're handling the OnCreate event. Had you been overriding the Loaded method, for example, there's be less confidence that all window-creation activity had finished.
I always try to create my Applications with memory usage in mind, if you dont need it then don't create it is the way I look at it.
Anyway, take the following as an example:
Form2:= TForm2.Create(nil);
try
Form2.ShowModal;
finally
Form2.FreeOnRelease;
end;
I actually think Form2.Destroy is probably the better option, which brings me to my question..
What is the difference between calling:
Form2.Destroy;
Form2.Free;
Form2.FreeOnRelease;
They all do the same or similar job, unless I am missing something.
And also when should any of the above be used? Obviously when freeing an Object I understand that, but in some situations is Destroy better suited than Free for example?
Form2:= TForm2.Create(nil);
This is a code-smell, because Form2 is probably the global, IDE-generated variable that would normally hold an IDE-created TForm2. You most likely want to use a local variable, and one with a better name. This is not necessary an error, just a code-smell.
Form2.Destroy vs Form2.Free
Use Form2.Free, because it calls Destroy anyway. You can CTRL+Click on the name (Free) to see it's implementation. Essentially Free calls Destroy if Self is not nil.
Form2.FreeOnRelease
As the documentation says, "It should not be necessary to call FreeOnRelease directly."
I've never actually heard of FreeOnRelease before. A quick Google search turned up the reason why. From the official documentation:
FreeOnRelease is called when an
interface implemented by the component
is released. FreeOnRelease is used
internally and calls the corresponding
interface method. It should not be
necessary to call FreeOnRelease
directly.
As for Free vs. Destroy, Free is a safety feature. It's basically implemented as if self <> nil then self.Destroy;, and it was created to make constructors and destructors safe to use. Here's the basic idea:
If you're constructing an object and an unhandled exception is raised, the destructor gets called. If your object contains other objects, they may or may not have been created yet by the time the error occurred, so you can't just try to call Destroy on all of them. But you need a way to make sure that the ones that have been created do get destroyed.
Since Delphi zeros out the address space of an object before calling the constructor, anything that hasn't been created yet is guaranteed to be nil at this point. So you could say if FSubObject <> nil then FSubObject.Destroy again and again for all the sub-objects, (and if you forget that you're going to get access violations,) or you can use the Free method, which does it for you. (This is a huge improvement over C++, where the memory space is not zeroed before the constructor is called, which requires you to wrap all your sub-objects in smart pointers and use RAII to maintain exception safety!)
It's useful in other places as well, and there's really no reason not to use it. I've never noticed that Free imposes any measurable performance penalty, and it improves the safety of your code, so it's a good idea to use it in all cases.
Having said that, when dealing with forms specifically, there's an additional variable to factor into the equation: the Windows message queue. You don't know if there are still pending messages for the form you're about to free, so it's not always safe to call Free on a form. For that, there's the Release method. It posts a message to the queue that causes the form to free itself once it's got no more messages to handle, so it's generally the best way to free a form you no longer need.
The canonical form is:
Form := TMyForm.Create(nil);
try
Form.ShowModal;
finally
Form.Free;
end;
Never call Destroy, always call Free instead.
FreeOnRelease is a total red herring. Sometimes, if there are queued messages destined for your form or its children, then you might elect to call Release although often that's indicative of design problems.
The idiomatic usage is
procedure SomeProc;
var
frm: TForm2;
begin
frm := TForm2.Create(nil);
try
frm.ShowModal;
finally
frm.Free;
end;
end;
or, unless you hate the with construct,
with TForm2.Create(nil) do
try
ShowModal;
finally
Free;
end;
You should never call Destroy, according to the documentation. In fact, Free is exactly equivalent to if Self <> nil then Destroy;. That is, it is a 'safe' version of Destroy. It doesn't crash totally if the pointer happens to be nil. [To test this, add a private field FBitmap: TBitmap to your form class, and then OnCreate (for instance), try FBitmap.Free vs. FBitmap.Destroy.]
If you create the form using the approach above, Free is perfectly safe, unless you do some strange things in the form class.
However, if you use CreateForm(TForm2, Form2) to create the form and store the form object in the global instance variable Form2 and you don't free it immediately [for instance, if you want the window to stick around next to the main form in a non-modal way for a few minutes], you should probably use Release instead of Free. From the documentation,
Release does not destroy the form
until all event handlers of the form
and event handlers of components on
the form have finished executing.
Release also guarantees that all
messages in the form's event queue are
processed before the form is released.
Any event handlers for the form or its
children should use Release instead of
Free (Delphi) or delete (C++). Failing
to do so can cause a memory access
error.
FreeOnRelease has nothing in particular do to with forms. From the docs:
It should not be necessary to call
FreeOnRelease directly.
the other way is passing caFree to Action of formonclose
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
Action := caFree;
end
I has a piece of code where I override TStringGrid's inplace editor and the hint window. For this, I created my own string grid based on TStringGrid and use a TEdit for inplace editor and a TPanel for tool tips. In TMyStringGrid.Create constructor I initialize them like this:
Constructor TMyStringGrid.Create();
Begin
inherited Create(AOwner);
MyEditor:= TEdit.Create(Self);
MyEditor.Parent := Self;
End;
In this case the owner (the main form) is freeing the controls. I used this for years and it worked.
The thing is that other people argue that the programmer should use NIL instead the Self when instantiation the child controls and later to manually free them in the Destroy destructor. It seems that the second alternative has gigantic advantage over the first one, especially when you dynamically create lots of child controls (not my case). Other problem with my code, they say, is that the child controls may be freed after an Application.ProcessMessages call while the application may still want to use them.
So, I should let my code unchanged or should I manually create and free the child controls?
There is a any Borland example of compound controls?
Delphi 7, Win XP
Reference: http://delphi.about.com/od/kbcurt/ss/dynamiccreateno.htm
Yes you can use your code without changing it.
There is a any Borland example of compound controls?
Best example is to check the implementation of TLabeledEdit.
It was creating the label in constructor
if Assigned(FEditLabel) then exit;
FEditLabel := TBoundLabel.Create(Self);
FEditLabel.FreeNotification(Self);
FEditLabel.FocusControl := Self;
There's no good reason to pass nil instead of Self in this situation. That AOwner parameter is there specifically for that reason. Take advantage of it.
There's a reason to pass nil when creating a control and manually destroy it, but for a completely different situation: if you're creating a control (typically a form) inside a function. This is a pretty common pattern, for example:
MyDialog := TMyDialog.Create(nil);
try
result := MyDialog.ShowModal;
finally
MyDialog.Free;
end;
There, you want to free it immediately instead of waiting around until the current form gets destroyed, which could be much later. But when it comes to sub-components, you usually want them to be destroyed at the same time as the parent component, not later, so pass Self to AOwner and let the VCL take care of it for you.
Considering that constructor, the grid instance is owned by Aowner (which is tipically a TForm or a TFrame). The inplace editor is owned and parented by the grid instance. I don't see how ProcessMessages would cause destruction of the child objects, since they will be destroyed in the destroying loop of TMyStringGrid. That's nothing I can see of wrong on that implementation - and I use that same design for the components I create. Ownership is there on VCL to easy our lifes when managing objects' lifetime. And is not the case where nil is recomended as owner, which is shown in Mason' answer.
In the pattern shown by Mason , the reason for the NIL is that, without a owner, the object destruction will not enter in a Notification loop. If you create a lot of components which destruction you handle manually, you want to make sure the owner is NIL, otherwise a lot of code get executed (without need) in each component construction/destruction.
Many moons ago, there was an excelent white paper on the web archive of (now defunct) eagle-software.com
I am just wondering if I'm doign something that might be bad, although it seems a very practical solution to me...
I have two forms which the user will have to walk through. The user clicks on a button and form1 pops up. The user presses OK and the second one pops up. The user click OK again and the screens are gone. Or the user clicks on Retry and the screen goes back to the first one. The two screens are completely different sizes with different information.
So I came up with this code:
Form1 := TForm1.Create(SharedData);
Form2 := TForm2.Create(SharedData);
repeat
ModalResult := Form1.ShowModal;
if (ModalResult = mrOK) then ModalResult := Form2.ShowModal;
until (ModalResult <> mrRetry);
Form1.Release;
Form2.Release;
I've tested this code and it seems to work like a charm. In this code, SharedData is an object that contains data that's manipulated by both forms. I create this object before the two forms are created and when ModalResult==mrOK I just write the data back to the database.
Problem is, while I think this is a clean solution to handle flipping between two forms, I can't remember ever seeing something like this construction before. Of course, I'm a Genius. (At least, me Ego tells me I am.) But would there be something against using this piece of code or is it just fine?
If the code does what it's supposed to, I think it's fine. I've not seen anything quite like it before either, but it's pretty obvious what it's doing... which in my book is far better than just blindly following other people's work. :-)
I don't see anything wrong with the code. I do question, however, the passing of SharedData to the constructor of both forms. Unless you've overridden the constructor (using reintroduce as well), the argument to TForm.Create() accepts an owner; that owner is responsible for freeing the objects it owns. Since you're freeing them yourself, there's no need to assign an owner; just pass nil to the call to Create() to avoid the overhead of saving the reference, accessing the list of owned objects when one is freed to remove the reference, etc.
Also, Release is designed to be called from within the event handler of the control itself, such as in a button click event. It makes sure that all pending messages are handled before the control is actually freed, in order to avoid AVs. Using it the way you are again adds unnecessary overhead, as you're not using them inside an event handler. You can safely just use Form1.Free; instead.
To clarify use of Release, it's for use in code of the Form itself. For instance, if you have a button on the form, and you want that button's click to cause the form to be freed, you use Release:
procedure TForm1.Button1Click(Sender: TObject);
begin
Self.Release;
Close;
end;
This allows the button click to free the form, but makes sure that the subsequent call to Close runs first. If you use Free instead of Release above, you could end up calling Close on a non-existent Form1. (Chances are it would still be OK, because of the silliness of the code above; the form would still be resident in memory. Still a bad idea, though.)