This is not an article about LiveBinding. I was once hoping it was going to be, but instead it has become an alternative to LiveBinding. If anything, it is about compile-time binding and quality assuring the data input from of your users.
Forms, forms, forms…
How many forms have you created? Chance is – quite a few – and what do they have in common? If people type rubbish, your data becomes rubbish. So – what do you do? You validate the input to prevent rubbish getting into the system. You do… don’t you? Sure you do!
When do you validate it? When someone clicks Submit or OK? Right – then you have to go through the input, field by field, and first ensure that what the user typed in actually is understandable in the current context – such as no funny characters in an integer – and sometimes you have to check the values against each other for logical states. If someone said they took three melons, their combined weight should at least be bigger than zero, and blue shirts don’t go well with pink pants, and what else not.
If the user typed in rubbish – you have to inform him or her so that it can be corrected.
Been there, done that
There is a certain amount of logic in this scene that we keep recreating scaffolding for. Stuffing things into listboxes, formatting and filling in the values, validation of numbers and dates, converting enumerated types into strings (and back again). If you want the dialog to be slick – you might even want to validate as you go, which means eventhandlers for focus changes, keys pressed, UI items clicked, dropped down and selected, also adding to all the scaffolding code.
Some time ago, I had to create yet another dialog. Lines and lines of housekeeping code that surround the real validation logic. And naturally I don’t have to be clearvoyant to foresee numerous more such dialogs, as it is a major part of writing applications that deal with configuration, input and control.
So – I thought to myself – can I spend a little time now, and save a lot of time later? Dangerous, innit, thinking like that… suddenly you could find yourself writing a framework, and we all know what happens to frameworks, right? They turn to endless amounts of code written with good intentions of handling the unexpected, covering functionality you won’t ever need, and at some point collapse on themselves to become a black hole of sketchily documented (since noone updated the docs as new features got added) , and hastily changed (since you always are in a hurry for that extra functionality) code. And when someone else misread your framework intentions and applied it like a hammer to a screw – it just doesn’t end well.
Narrowing down the scope
Hence – Sticking with the KISS principle, I have decided to try to make it independent of other libraries, and limit what I implement to basic functionality while attempting to allow for future expansion.
I am going to create a TInput that wraps a GUI control. To put it simply – a TInput that points to a specific TEdit, and takes care of stuffing values from the variable and into the GUI control, and vice versa. The job of that TInput is the present the value correctly, and to ensure that what ever is written into that TEdit, can be converted into an integer.
I will also create a TInputList that is a collection of TInputs, that will have the job of going through the list to fill the controls, to validate the contents, and finally – if all input is syntactically correct – semantically validate the input for logical correctness.
Some of the code that I will present here, is probably centric to the type of data that I work on. For me, an input form will typically wrap an object with a set of properties that reflect a row or set of related rows in a database. Why am I not using data aware controls? Mostly because the applications we create actually can’t write to the database themselves, except through calling stored procedures that perform more magic before, during, or after the data has been written. For that reason, the TInputList will be a TInputList, and the TInputList will have a property Current:T that I can populate, and each TInput will know that it is member of a TInputList, so that it can kick of the necessary actions for stuff to get validated.
By now you have probably thought to yourself: TEdit? What about the other controls?
Because there are a number of input types, and a number of controls, and these make a number of combinations. TEdit/Double, TEdit/Integer, TEdit/String, and TEdit/Enum is already a list, and I haven’t even mentioned TComboBox yet,- so it is obvious that TInputList has to be polymorphic.
This brings us to the first part of complicated – creating a set of generic and polymorphic classes. Generics in Delphi XE still don’t to well with forward declarations, and to create polymorphic parent/children lists, it really helps to be able to forward declare.
After some consideration, I have chosen to use an abstract class without generics as my inner base class. TAbstractInput will know nothing about the data type we want to work with, nor will it know anything about the control type. All TAbstractInput will do, is define the virtual abstract methods that will be our type agnostic operators or verbs and queries, if you like. Hence, our TInputList will use TAbstractInput as its element type.
/// TAbstractInput defines the bare minimum base class for our list of inputs
TAbstractInput = class abstract
function GetEdited: Boolean; virtual; abstract;
procedure SetEdited(const Value: Boolean); virtual; abstract;
function GetEnabled: Boolean; virtual; abstract;
procedure SetEnabled(const Value: Boolean); virtual; abstract;
function ControlValueIsValid:Boolean; virtual; abstract;
function VariableValueIsValid:Boolean; virtual; abstract;
procedure FillControl; virtual; abstract;
procedure FillVariable; virtual; abstract;
procedure SetDisabledState; virtual; abstract;
procedure SetErrorState; virtual; abstract;
procedure SetNormalState; virtual; abstract;
procedure SaveNormalState; virtual; abstract;
procedure Setup; virtual; abstract;
procedure Clear; virtual; abstract;
procedure Update; virtual; abstract;
function Validate: Boolean; virtual; abstract;
property Edited: Boolean read GetEdited write SetEdited;
property Enabled: Boolean read GetEnabled write SetEnabled;
From the outside of the list, we need TInput that expose the correct type that we want to access, so that will be our outer base class type – which knows how to set and get the value, and hence the class that we use to reference an input field.
/// TInput defines the input wrapper as we want it to be
/// visible from the outside of our list of controls
TInput = class abstract(TAbstractInput)
procedure SetOnCanGetValue(const Value: TGetValue);
function GetValue:T; virtual; abstract;
procedure SetValue(const Value:T); virtual; abstract;
function CanGetValue:Boolean; virtual; abstract;
property Value:T read GetValue write SetValue;
property OnCanGetValue: TGetValue read FOnCanGetValue write SetOnCanGetValue;
Please note that this is a simplified view of TInput class.
Inside TInputList, I will subclass TInput again, and add knowledge of the controls. In fact, I will create several subclasses that handle type conversions for each data type and control type, but instead of having the user instantiate all these different class types – I will add factory methods to the TInputList instead.
Here are some excerpts from the declaration of TInputList and the basic control wrapper.
/// TInputList is a wrapper for all our input controls.
TInputList = class(TList)
/// This is our core input control wrapper on which we base wrappers for specific controls
TInputControl = class(TInput)
Properties and Binding
This is the second part of complicated. Will I be using the XE2 LiveBinding? No. IMO, LiveBinding uses the least desirable method to bind a property for setting and getting. I lamented this in my previous article, Finding yourself in a property bind. In my opinion, LiveBinding is a good idea that is implemented in the wrong way, and in it’s current form will be vulnerable to property and variable name changes during refactoring. In addition, it appears that LiveBinding is not quite mature yet. Then there is the fact that XE and older, doesn’t have LiveBinding.
After some experimentation, I came to the conclusion that even if it appears to be more elegant to use visitors or observers and RTTI binding, I will get more flexibility, readability, and maintainability by using anonymous methods.
Anonymous methods allow me to do manipulation of the value before it is set/get, and allow the setter/getter events to have side effects. It also ensures that all references are validated compile-time. It will not guarantee protection from referencing the wrong properties and variables, but they will at least be of the right type, and actually exist.
Since my primary development platform is Windows, I am a VCL developer – and when I started this little project, I had only VCL in mind. However, as the code matured, I found that I might want to be able to use this for FireMonkey as well. That still remains to be seen as FireMonkey still smell of Baboon.
Still, the core logic is platform agnostic, and the VCL bits are separated into a unit of their own.
Here is an excerpt from the VCL implementation with complete declarations.
TInputListVCL = class(TInputList)
TInputControlVCL = class(TInputList.TInputControl)
procedure ControlEnable(const aState:Boolean); override;
function ControlEnabled:Boolean; override;
procedure ControlSetFocus(const aFocused:Boolean); override;
/// Basic wrapper for a TEdit
TEditTemplate = class abstract(TInputControlVCL)
procedure SetControlValue(const Control:TEdit; const v:String); override;
function GetControlValue(const Control:TEdit): String; override;
function ControlValueAsString:String; override;
procedure SetErrorState; override;
procedure SetNormalState; override;
procedure SaveNormalState; override;
procedure SetDisabledState; override;
procedure Clear; override;
procedure Setup; override;
/// TEdit wrapper for editing a string
TEditString = class(TEditTemplate)
function ConvertControlToVariable(const cv: String; var v:String; var ErrMsg:String):Boolean; override;
function ConvertVariableToControl(const v:String; var cv:String):Boolean; override;
/// TEdit wrapper for editing a float
TEditDouble = class(TEditTemplate)
procedure SetDecimals(const Value: Integer); override;
function GetDecimals:Integer; override;
function ConvertControlToVariable(const cv: String; var v:Double; var ErrMsg:String):Boolean; override;
function ConvertVariableToControl(const v:Double; var cv:String):Boolean; override;
Putting it to use
This will be covered in part 2. Until then, don’t forget to try out RAD Studio XE2 and join the RAD Studio World Tour presentations!