I have a list of TPanels in a FMX application and I want to free a panel if I click on it.
To free them directly in the Onclick-handler is not the right way, because I get an access-violation. And I don't want to use windowsmessages (recommended in How to free control inside its event handler? and Why does my program crash when I destroy a button in its own OnClick handler?) because it is a firemonkey application and I do not know how these messages work on android and mac.
Is there another solution?
Use myObject.Release:
Marks this TFmxObject object for delayed deletion.
Immediate actions in this method:
set Parent = nil
insert object into delayed delete list
Delayed action:
free object from list (vPurgatory).
Remember, that method Free (and procedure FreeAndNil) does not remove the object itself in mobile platforms:
// under ARC, this method isn't actually called since the compiler translates
// the call to be a mere nil assignment to the instance variable, which then calls _InstClear
Related
I am new to Delphi and my question may be very basic.
I created a Form in a Delphi procedure. Until now, I was using ShowModal() and then freeing all the objects that I want to avoid leaking after closing the Form.
Now, I would like to show the Form modeless, but I don't know how I can free the objects inside the OnClose event.
Does anybody know a solution for it?
Simply set the Action parameter to caFree:
procedure TMyForm.FormClose(Sender: TObject; var Action: TCloseAction);
begin
Action := caFree;
end;
Per the documentation:
The value of the Action parameter determines if the form actually closes. These are the possible values of Action:
caNone
The form is not allowed to close, so nothing happens.
caHide
The form is not closed, but just hidden. Your application can still access a hidden form.
caFree
The form is closed and all allocated memory for the form is freed.
caMinimize
The form is minimized, rather than closed. This is the default action for MDI child forms.
Setting Action to caFree will cause the Form to call Release() on itself after the OnClose handler has exited:
Destroys the form and frees its associated memory.
Use Release to destroy the form and free its associated memory.
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.
Note: Release returns immediately to the caller. It does not wait for the form to be freed before returning.
Release() posts a delayed CM_RELEASE message to the Form window. Once execution flow returns to the main message loop and the message is dispatched, the Form will free itself from memory.
If your TForm object owns other objects, they will be freed automatically when the TForm is freed.
You can also
for each dynamically created object do
Object.Free;
Current Delphi versions (Since Xe) documentation recommends to use new approach
Object.DisposeOf;
This new approach works optimized in multi-device application (different operational system)
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.
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.
Is there a failsafe way of freeing a Delphi control?
I have a TStringGrid descendent which I am "embedding" a custom control in it for an inplace editor. When the user navigates within the cells of the grid via the tab key or arrow keys, I need to create a dynamic control if the cell is editable. I have hooked the needed events and am utilizing the OnKeyDown event of my custom control to pass the navigation keys back to the parent TStringGrid.
Previously, the TStringGrid descendent would simply call FreeAndNil on the embedded control, but under some circumstances this would cause access violations inside of UpdateUIState/GetParentForm. Looking at the call stack, it appears that sometimes after the control was free'd, a WM_KEYDOWN (TWinControl.WMKeyDown) message was still happening.
I've all ready looked at and implemented the changes discussed in How to free control inside its event handler?. This appears to have resolved the issue, but I am wondering if there are any other cavet's to this approach.
In effect, this workaround has simply delayed the destruction of the control until after all the existing messages on the queue at the time the CM_RELEASE message was posted.
Would it not be possible that after the CM_RELEASE was posted, another WM_KEY* or similar message could all ready have been posted to the message queue?
My current CM_RELEASE handler looks like:
procedure TMyCustomControl.HandleRelease(var Msg: TMessage);
begin
Free;
end;
So, will this be safe in all instances or should I do something to clear any other messages from the queue? ( SendMessage(Self.Handle, WM_DESTROY, 0, 0) comes to mind )
In general you should not destroy a control in an event-handler of that control.
But since your function is a plain non virtual message handler which is never called from internal code in that control you should be ok. I don't like too too much from a style point of view, but I think it's ok for your use-case.
But a custom message might be cleaner.
Would it not be possible that after the CM_RELEASE was posted, another WM_KEY* or similar message could all ready have been posted to the message queue?
If messages in queue would cause big problems you could never safely destroy a control since messages can be posted from other threads and applications. Just make sure the correct functioning of your application doesn't depends on those messages being handles in every case.
SendMessage sends the message and wait for it to return, that's why you can not use it safely in the event handler of the control you are freeing.
PostMessage in the other hand, will send the message and will be processed after the event exits (if there are no more code in the event).
I have some code that does some setup of internal objects in the Loaded() function. However, some external objects are not completely created yet, but are AFTER the Loaded() function is complete. What function does Delphi call after it calls Loaded()?
Better yet what is the creation sequence of a component?
Basically I have a TCP Server and Client. Most people will place those two components into two separate applications, some will place them in the same application for local access.
My Client tries to fetch data from the server in OnLoaded(), but the server may not be up yet! I want to know if another function is called after all the OnLoaded()'s are called.
Loaded is called immediately after the dfm is streamed in, and shouldn't be used to access the server. Your best bet is probably to post a custom message to yourself in the constructor, and have a message handler procedure that responds to that message. Posting the message puts it into the end of the message queue, and therefore it won't get processed until all the other messages ahead of it has been handled. This should delay things long enough for your components to be fully constructed for use.
I've used breakpoints in some components and firmly established that AFTERCONSTRUCTION is called BEFORE LOADED not AFTER.
I've also done the same thing on a FORM and firmly established that AFTERCONSTRUCTION is called AFTER LOADED not BEFORE.
Bear in mind that AfterConstruction is a method in TObject, but Loaded is not. It follows that Loaded is generated by code that may not necessarily put it in a specific order relative to AfterConstruction, since Loaded is not actually part of the construction sequence of a TObject and AfterConstruction is.
Indeed, if you study the RTL source, you will see that Loaded is not even invoked by any self.method of a TComponent, but is in fact invoked by a stream reader that is reading the DFM, and that will most probably be happening under the control of an "owner" component. I strongly suggest therefore that its relationship relative to the execution of AfterConstruction is not really guaranteed. The fact that it appears in a particular order for a form is because the form is most likely the component to initiate stream reading. In other words, it smacks of a convenient accident that Loaded is before AfterConstruction in a form.
Further research shows that NON-FORM components that include the following code may never call the event handler.
procedure Txxx.AfterConstruction; override;
begin
inherited AfterConstruction;
if Assigned(FOnCreate) then FOnCreate(Self);
end;
The reason is that AfterConstruction, if invoked before properties are loaded, will find FOnCreate has not been assigned yet!
In such cases, you really HAVE to use the following:
procedure Loaded; override;
begin
inherited Loaded;
if assigned(OnLoaded) then OnLoaded(self);
end;
Like I said, this will produce different outcomes for a component owned by a form than it would for the form itself! The TForm component is usually the invoker of the DFM stream reader and it is the stream reader that calls Loaded for each component it reads from the form. This process starts (fortunately) BEFORE the form's AfterConstruction, but each component that is loaded by that reader gets its AfterConstruction method called BEFORE its loaded method.
QED.
The great irony is that the Delphi 6 help file says "The AfterConstruction method implemented in TObject does nothing. Override this method when creating a class that takes some action after the object is created. For example, TCustomForm overrides AfterConstruction to generate an OnCreate event."
What it omits to say is that if you try this on anything other than a TCustomForm (which does it already), it doesn't work! Because only a form (which has it already) will load its OnCreate property before calling AfterConstruction. Any other component won't, because the DFM reader invoked by the form calls AfterConstruction before Loaded! A clear case of Borland et. al. not understanding their own code, or at best, writing a help file entry that implies something is possible when in fact it is not.
Note, if your component is not on a form and is created at runtime (even if this is as an "owned" component), its "Loaded" method will NOT be called, because there was no stream reader involved.
Another point of interest is something that "Dr" Bob Swart wrote some time ago about AfterConstruction, namely that it represents the point where virtual methods can be invoked. Evidently this is only partly true: if a form's Loaded method is invoked BEFORE AfterConstruction, then you would not be able to invoke any virtual methods from Loaded if that were true. This is not the case (obviously) because Loaded is itself a virtual method! Evidently, Loaded for a form is called between the constructor and AfterConstruction by the stream reader. It begs the question: by what method is the stream reader actually invoked? My guess is either that it runs under control of the application (not the form) and that it deliberately invokes AfterConstruction differently for a form than for other components, OR that it is the last thing the form's constructor does after having created the VMT and hence the last thing that occurs before AfterConstruction is called in the form. Accordingly all the AfterConstruction-Loaded couplets of components owned by the form are called before the form's AfterConstruction is called. Tracing the calls also shows that mostly AfterConstruction is called for ALL of those components before ALL of their loaded methods are called. I didn't however test the case where there are hierarchical "parents" (such as panels with components on them), so there may be variations on this.
Normally you would override TObject.AfterConstruction for that purpose.
The order of execution is:
each Component.AfterConstruction in creation order
(Form or DataModule).Loaded
each Component.Loaded in creation order
(Form or DataModule).AfterConstruction
Trace:
Debug Output: button AfterConstruction Process Project2.exe (4876)
Debug Output: Form Loaded Process Project2.exe (4876)
Debug Output: button Loaded Process Project2.exe (4876)
Debug Output: Form AfterConstruction Process Project2.exe (4876)
I'm not sure what you mean with
the server may not be up yet
Anyway, if the client and the server are both on the same application form or datamodule, I see alternatives:
You may "force" the system to create the server before the client and up the server in the server's OnLoad and it will be up at the client OnLoad, because documentation says:
When the streaming system loads a form or data module from its form file, it first constructs the form component by calling its constructor, then reads its property values from the form file. After reading all the property values for all the components, the streaming system calls the Loaded methods of each component in the order the components were created. This gives the components a chance to initialize any data that depends on the values of other components or other parts of itself.
Inform the "client" whenever the server is UP to let it initialize (pull data from the server). You can use a direct method call, post a message or whatever you feel comfortable with.
Let the client stand up the server inside it's own OnLoad method.
Why not use the onCreate event of the main Form ?