To avoid singletons and global variables I'd like to be able to pass parameters to a TFrame component. However since a TFrame normally is included on form at design time it is only possible to use the default constructor.
The parent form can of course set some properties in the OnCreate callback after the TFrame has been created. However this does not ensure that a property is not forgotten, and the dependencies are not as clear as using a constructor.
A nice way would be if it was possible to register a factory for creating components while the dfm file is being read. Then the required parameters could be passed to the TFrame constructor when created by the factory. Is there a way of accomplishing this?
Or does anyone have a better solution on how to pass parameters to a TFrame?
All components, including descendants of TFrame, need to be able to be constructed using the constructor inherited from TComponent. Otherwise, they can't be used properly at design time. If the restriction of design-time use is acceptable to you, then you could override that constructor and raise an exception. That would prevent the component from being placed on a form at design time. Simply provide some other constructor that requires other parameters.
Because of the design-time requirement, all components need to be able to exist with some or all of their properties still at their default values. That doesn't mean the components have to do useful things while they're in that state, but they do need to be able to stay in that state indefinitely. It should be OK, for example, to place a component on a form, save the form, and close Delphi, with the intention of resuming the form-designing at a later time. The component should allow itself to be saved and restored, even if all its properties haven't been set up for final use yet.
My preferred option is to enforce the component's rules only at run time. Check that all the properties are set to sensible values before you allow them to be used. You can use assertions to enforce the proper use of your components. Consumers of your classes will learn very quickly if they haven't finished setting up your components on their forms.
I would normally add a public, non-virtual "Initialise" or (Initialize to Americans) procedure which requires all parameters to be provided. This will then set the properties.
Make the properties protected or private if possible, so the only way they can be set is from calling Initialise(AFoo, ABar : integer).
Then in TFormXXX.FormCreate or TformXXX.Create, have:
inherited;
Initialise(foo, bar);
could you create/registercomponent your own tFrame component and
place that on the form - it's create could have anything passed to it.
If a factory could provide the parameters that you need, why don't you just override the default constructor for your frame, and ask the factory-class for parameters?
I usually make my own constructor. I don't like to create frames at designtime anyway.
a) a frame can be created dynamically when required and destroyed when not needed
b) give the frame a public property with either the parameter data type or a data structure and pass the values to the form through the property.
Example:
TAddress - a class to hold the usual elements of an address.
TAddressFra - a frame with the visual controls to display the address
populate an instance of TAddress with values
create an instance of TAddressFra
assign the TAddressFra.address property with the TAddress instance
use the procedure setAddress(o_address : TAddress) to assign the values of the TAddress attributes to the corresponding visual components on the TAddressFra
Related
I want to prevent a descendant of TPageControl from having a Style property except tsTabs.
At first glance I tried to override SetStyle, but it is declared private in the base class. anyone have ideas?
TPageControl does not natively support what you are asking for. To accomplish what you are asking for, you would have to either:
have your component re-declare the Style property with new getter/setter methods, and then have the setter exit without doing anything. However, the inherited Style property will still technically be accessible at runtime if the user of the component really wants to access it.
derive your component from TCustomTabControl instead of TPageControl, and re-implement everything that TPageControl does, but without promoting the protected Style property. However, the inherited Style property will still technically be accessible at runtime if the user of the component really wants to access it.
have your component override the virtual CreateParams() method and force the TCS_TABS window style in the TCreateParams.Style field. The Style property will still have whatever value the user assigns (which will also affect the behavior of the TabPosition property), but at least the underlying window will always behave as if the tsTabs style were being used.
use a detouring library to hook TCustomTabControl.SetStyle() directly at runtime and make it return without doing anything.
Hi I'm developing a TControl descendant, lets name it THTMLBaseControl, at runtime that control only generates and output HTML code based on the settings of that control, so all the additional properties of the base TControl class and methods, including Windows Messaging system is really not used at runtime and causes memory overhead.
I need that control to be inherited from TControl so at design time I can use all the IDE designer stuff.
But at runtime almost all of those properties that at desingtime are needed i dont need them.
I also have all my controls inherited from that THTMLBaseControl, so creating a wrapper class per control class is not an option because it will duplicate the code a lot.
So what i need is something that at runtime, before the class is instantiated I can change the parent class so it will be instantiated based on another TControl-like class, maybe named TmyBaseControl inherited from "TComponent" as TControl Does, but that will not have all that TControl memory overhead and will only have the properties and methods needed by my THTMLBaseControl.
I really dont have a GUI at rutime is a web server that will serve only HTML, is some thing that intraweb and Raudus do, but always the issue is that all are based on TControl, so they have to be created at run time and generate a lot memory and process overhead that is not used. and maybe there could a solution so any THTMlBaseControl descendant instantiated at runtime will inherit the all properties and methods from TmyBaseControl and not from TControl.
I have seen there are ways to hack the VMT but maybe there are other solution and have not seen it. I already done changing the NewInstance, ClassParent and TnstanceSize class methods but i have to specify from which class and obviously i have to do the same steps per each inherited THTMLBaseControl class
And for the sake of all:
This is just a doubt, I need the components to be controls like TEdit, TPanel, visible and editable by the designer IDE I even could create my own TControl class but I was just thinking if I can reuse the code already existing.
Regards
You cannot change the class a run time. Once an object is instantiated, its class is fixed. You could hack the object to change its VMT pointer, making it refer to a different class, but that would still not address your main concern, which is memory usage — even if you change the VMT pointer, all the memory for the object has already been allocated; changing the VMT pointer doesn't magically make the object occupy less memory.
The first thing you could do is stop descending from TControl. As you've noted, you don't need any of the things it provides. All you want is something you can drop on a form at design time to set its properties. For that, all you need is TComponent, so make that your base class instead of TControl. Then you'll get something more like TTimer, which has no GUI. Once you've done that, you no longer need TForm, either. Instead, you can put your component on a TDataModule, which is specifically designed for managing non-visual components at design time.
I have my main form. Form_Main
It creates two instances of two classes.
Candle_Data : TCandle_Data;
Indicator_2700 : TIndicator_2700;
In order for Indicator_2700 to properly compute its values it must have access to the candle data in the obect Candle_Data from inside one of its methods. Thus how can Indicator_2700 access data inside Candle_Data? Does Form_Main have to pass it as a argument at Constructor time?
Both Class declarations are in their own unit file.
You could use any of the following (non-exhaustive) methods:
Pass the object reference as a parameter to any methods that need it. Of course you need to get hold of Candle_Data so the suitability of this approach really depends who the caller is.
Pass the Candle_Data object reference to the constructor of the other object and then store it in a private member field.
Make the object reference a public property of the single instance of the main form and access it that way.
We don't really have enough information to advise you which is best but the starting point is always to prefer parameters and local variables over global state.
TIndicator_2700 could have a property to link it to the instance of TCandle_Data that is relevant to its own instance or you should supply it as an argument to the method that needs to access the data.
You could certainly pass the TCandle_Data instance into the constructor of Indicator_2700, and store a reference within the resulting instance until you needed it.
Both class declarations are in their own unit file.
That suggests that both have nothing to do with the other. But still you want one to have knowledge about the other. It sounds like a little design mixup, but that doesn't need to be the case.
There are multiple solutions, here are three of them, each with its own purpose:
Place both classes in the same unit, only if both classes have a common theme/subject (e.g. TCar and TAirplane in the unit Transport),
Use one unit in the other unit, only if both units represent different subjects, but one may depend on the other (e.g. unit Transport uses unit Fuel: TCar needs TDiesel, but TDiesel doesn't need a TCar). This only works one-way. Delphi prevents using in both ways with a compiler error: "Circular unit reference to 'Fuel'". The only workaround is to use the second unit in the implementation section, but that usually is considered a big nono.
Declare a new base-class in a new unit, only if the base-class has a common subject, but the final descendants do not (e.g. TFuel is used by all transportation classes like TCar, TAirplane and THorse, but TFood (a descendant of TFuel) is only used by THorse and TPerson).
As for how to link both classes together, see the already given answers.
Three components, working together:
* CompA, a TComponent descendant, a mastermind component knowing many things and tying things together
* CompB, a TComponent descendant, mines some data from it's CompA and crunches it. Can amongst other things feed CompC with data to present
- Has a published property of type CompA
* CompC, a TComponent descendant, a TFrame descendant drawing surface that can be set at designtime to use a CompB as data provider
- Has a published property of type CompA
- Has a published property of type CompB
I think I remember having read, even though I cannot state where, that Delphi's streaming engine reads all components from the .dfm and builds a dependency graph. This graph is then used to create all components in correct order. For the listed components it should be CompA first (since it uses none of the other ones), then the CompB (it uses CompA and must be created after) and lastly the CompC since it has properties of both the other component types.
This does not happen. CompC is created before CompB. If i rearrange the order in the .dfm file using a text editor it works. The property values are not used in any constructors, only in the Loaded procedures. But truly there must be a way to make it work no matter the order of components in the dfm?
I've been banging my head against the wall for two days straight now, I need somebody to tell me which keyword I forgot or what error in design I have.
I suspect your fault is you're trying to access other objects properties on setters for sibling pointers, forgetting that at dfm loading stage --runtime-- you can't be sure pointers to other components your component depends on are yet valid because it is possible that other component is not yet created. This works this way since Delphi 1.
Because of this, you usually deffer the reading of other component's state (for example) to your overridden Loaded method.
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.
Note: All references to sibling components are resolved by the time Loaded is called. Loaded is the first place that sibling pointers can be used after being streamed in.
Because of this, usually on a setter method for a sibling pointer property you usually perform a check of this type:
procedure TMyComponent.SetDataSource(Value: TDataSource);
begin
FDataSource := Value;
//streaming in stage
if not (csLoading in ComponentState) then
ReadDataSourceProperties;
end;
procedure TMyComponent.Loaded;
begin
ReadDataSourceProperties;
end;
Take a look at the VCL source, you'll find hundreds of examples of this.
If your components are that much dependent on creation order, you are always going to be in trouble relying on the streaming mechanism. Just one addition or removal of a(n other) component on the form/datamodule can throw your order out of whack.
To ensure proper creation order, you'd be better off creating them at run time. Just note that when you create components at run-time the Loaded method will not be called. You will either have to do it yourself or move the code to some init method that you call after you create your components.
You can right click a form/datamodule and select the "Creation order" item. It will allow you to select the creation order of "non visual" components. Visual ones should follow the tab order, but I am not really sure about that.
Update: I was wrong about the tab order, but it looks the visual controls are streamed to the .dfm in Z-order. If the controls are instantiated following the order they are in the .dfm, you can use Edit -> Bring to front/send to back (or the Control menu in the form context menu) to change the z order. As long as the controls do not overlap you should be enough free to change it.
As there is no garbage collection in Delphi, where exactly do you unload variables?
Say I have a type with a set of private vars.
Would it suffice to have a Destroy method that does this work?
Do I need to explicitly call this destroy method in my consuming classes?
The best way to organize destruction in delphi is to always think about "who will create the given vars".
If you also free them in this context (For you private vars the destroy method of the class) it's much less likely that you will encounter memory leaks.
Actually the destructor of a class is normally not called via
myInstance.Destroy();
instead the typical way of doing it is via
FreeAndNil(myInstance);
or
myInstance.Free();
As delphi will handle calling the destructor method at order
Objects need to be destroyed when you are ready with them: using .Free (which calls the destructor .Destroy) or FreeAndNil.
If you are using interfaces: they are reference counted and hence they are freed for you. The same for strings.
Most functions that you need to pass an object do not take ownership of the object so you should free it afterwards when you are done with them. If you create components on a form, please note that they are objects, are owned by the form and you should let the form free them for you.
For example: in your TLight you create a TTimer probably without an owner. In this case you should Free the timer in the destructor of the TLight class. If your TLight was a TControl or TComponent itself you could have created the Timer with Self as the Owner and it was automatically freed when your TLight instance was freed.
That's how I see it:
Primitive types (like integer, real, double, extended), arrays, sets, records: they are automatically destroyed when out of scope.
Reference-counted types (strings, interfaces types, interface references): they are automatically destroyed when their reference counter is equal to zero.
Objects from classes that fall in one of those situations: does not descend from TComponent, are in list objects that do not implement ownership or are a TComponent descendant but does not have an owner assigned to it:
When their class descend from TInterfacesObject and are accessed from a interface reference: same behavior from reference-counter types (see above)
Other cases: call Object.Free or FreeAndNil(Object) to manually destroy it (in a try..finally structure would be even better)
Objects that have an owner (TComponent descendants or are in list objects that implement ownership): the owner will take of them in proper time.
I noticed the following comment from OP:
I have noticed the concept of owner.
Yet I couldn't pass my custom class to
the TTimer.Create() method as it is
expecting a TComponent. Why do they
restrict things to be used in forms
only? Doesn't that furhter petrify the
bad habit of putting logic in your
views (forms)?
See, a TComponent descendant can be registered on the pallete to have the ability to be used in design-time (for example, in a form) . But it's doesn't means it must!!
You can pass your custom-class to TTimer.Create() if it descends from TComponent but you don't need to instantiate it in design-time - or even be owned by a TForm descendant (if you want to manage it's lifetime, you don't even need an owner!).
Ownership is a mechanism it's to help people to save time....
You keep referring to a "type with private vars" and asking about "unloading variables." I'm going to go out on a limb (as I may be have misinterpreted your message), and presume you're talking about class variables and are asking about when these class variables should be finalized when the "class unloads." (You've been doing Java or .NET programming at some point, no ;-).
The key to what you're asking is a unit finalization section. The overall layout of a unit includes 4 overall sections, two of which are optional, initialization and finalization.
unit Foo;
interface
// put things here you want other units to see
implementation
// put the *implementation* of the things from the interface here along with things
// you only want this unit to see.
initialization
// <optional> put code here that you want to execute when this unit is "loaded"
finalization
// <optional> put code here that you want to execute when this unit is "unloaded"
end.
If you're talking about instance variables of a class type, then some of the other answers you've already gotten (and are bound to get) should shed some light on that.