What is called after Loaded() in Delphi_问答_开发者

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 f开发者_JS百科etch 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.