Android Tutorial Classes tutorial

Basic4android v2.00 adds support for classes modules.

Classes definition from Wikipedia:
In object-oriented programming, a class is a construct that is used to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable its instances to have state and behavior. Data field members (member variables or instance variables) enable a class instance to maintain state. Other kinds of members, especially methods, enable the behavior of a class instances. Classes define the type of their instances.

A class usually represents a noun, such as a person, place or thing, or something nominalized. For example, a "Banana" class would represent the properties and functionality of bananas in general. A single, particular banana would be an instance of the "Banana" class, an object of the type "Banana".

Example:
B4X:
'Class Person module
Sub Class_Globals
   Private FirstName, LastName As String
   Private BirthDate As Long
End Sub

Sub Initialize (aFirstName As String, aLastName As String, aBirthDate As Long)
   FirstName = aFirstName
   LastName = aLastName
   BirthDate = aBirthDate
End Sub

Public Sub GetName As String
   Return FirstName & " " & LastName
End Sub

Public Sub GetCurrentAge As Int
   Return GetAgeAt(DateTime.Now)
End Sub

Public Sub GetAgeAt(Date As Long) As Int
   Dim diff As Long
   diff = Date - BirthDate
   Return Floor(diff / DateTime.TicksPerDay / 365)
End Sub

'Main module
...
Dim p As Person
p.Initialize("John", "Doe", DateTime.DateParse("05/12/1970"))
Log(p.GetCurrentAge)

I will start by explaining the differences between classes, code modules and types.

Similar to types, classes are templates. From this template you can instantiate any number of objects.
The type fields are similar to the classes global variables. However unlike types which only define the data structure, classes also define the behavior. The behavior is defined in the classes subs.

Unlike classes which are a template for objects, code modules are collections of subs. Another important difference between code modules and classes is that code modules always run in the context of the calling sub (the activity or service that called the sub). The code module doesn't hold a reference to any context. For that reason it is impossible to handle events or use CallSub with code modules.
Classes store a reference to the context of the activity or service module that called the Initialize sub. This means that classes objects share the same life cycle as the service or activity that initialized them.

Code modules are somewhat similar to singleton or static classes.

Adding a class module
Adding a new or existing class module is done by choosing Project -> Add New Module -> Class module or Add Existing module.
Like other modules, classes are saved as files with bas extension.

Classes structure
Classes must have the following two subs:

Class_Globals - This sub is similar to the activity Globals sub. These variables will be the class global variables (sometimes referred to instance variables or instance members).

Initialize - A class object should be initialized before you can call any other sub. Initializing an object is done by calling the Initialize sub. When you call Initialize you set the object's context (the parent activity or service).
Note that you can modify this sub signature and add arguments as needed.

In the above code we created a class named Person and later instantiate an object of this type:
B4X:
Dim p As Person
p.Initialize("John", "Doe", DateTime.DateParse("05/12/1970"))
Log(p.GetCurrentAge)

Calling initialize is not required if the object itself was already initialized:
B4X:
Dim p2 As Person
p2 = p 'both variables now point to the same Person object.
Log(p2.GetCurrentAge)

Polymorphism
Polymorphism allows you to treat different types of objects that adhere to the same interface in the same way.
Basic4android polymorphism is similar to the Duck typing concept.

As an example we will create two classes named: Square and Circle.
Each class has a sub named Draw that draws the object to a canvas:
B4X:
'Class Square module
Sub Class_Globals
   Private mx, my, mLength As Int
End Sub

'Initializes the object. You can add parameters to this method if needed.
Sub Initialize (x As Int, y As Int, length As Int)
   mx = x
   my = y
   mLength = length
End Sub

Sub Draw(c As Canvas)
   Dim r As Rect
   r.Initialize(mx, my, mx + mLength, my + mLength)
   c.DrawRect(r, Colors.White, False, 1dip)
End Sub
B4X:
'Class Circle module
Sub Class_Globals
   Private mx, my, mRadius As Int
End Sub

'Initializes the object. You can add parameters to this method if needed.
Sub Initialize (x As Int, y As Int, radius As Int)
   mx = x
   my = y
   mRadius = radius
End Sub

Sub Draw(cvs As Canvas)
   cvs.DrawCircle(mx, my, mRadius, Colors.Yellow, False, 1dip)
End Sub

In the main module we create a list with Squares and Circles. We then go over the list and draw all the objects:
B4X:
Sub Process_Globals
   Dim shapes As List
End Sub

Sub Globals
   Dim cvs As Canvas  
End Sub

Sub Activity_Create(FirstTime As Boolean)
   cvs.Initialize(Activity)
   Dim sq1, sq2 As Square
   Dim circle1 As Circle
   sq1.Initialize(100dip, 100dip, 50dip)
   sq2.Initialize(2dip, 2dip, 100dip)
   circle1.Initialize(50%x, 50%y, 100dip)
   shapes.Initialize
   shapes.Add(sq1)
   shapes.Add(sq2)
   shapes.Add(circle1)
   DrawAllShapes
End Sub

Sub DrawAllShapes
   For i = 0 To shapes.Size - 1
      CallSub2(shapes.Get(i), "Draw", cvs) 'Call Draw of each object
   Next
   Activity.Invalidate
End Sub
(the example code is attached)

As you can see, we do not know the specific type of each object in the list. We just assume that it has a Draw method that expects a single Canvas argument. Later we can easily add more types of shapes.
You can use the SubExists keyword to check whether an object includes a specific sub.

You can also use the Is keyword to check if an object is of a specific type.

Self reference
The Me keyword returns a reference to the current object. 'Me' keyword can only be used inside a class module.
Consider the above example. We could have passed the shapes list to the Initialize sub and then add each object to the list from the Initialize sub:
B4X:
Sub Initialize (Shapes As List, x As Int, y As Int, radius As Int)
   mx = x
   my = y
   mRadius = radius
   Shapes.Add(Me) 'Me is used to add this object to the list
End Sub

Activity object
This point is related to the activities special life cycle. Make sure to first read the activities and processes life-cycle tutorial.

Android UI elements hold a reference to the parent activity. As the OS is allowed to kill background activities in order to free memory, UI elements cannot be declared as process global variables (these variables live as long as the process lives). Such elements are named Activity objects. The same is true for custom classes. If one or more of the class global variables is of a UI type (or any activity object type) then the class will be treated as an "activity object". The meaning is that instances of this class cannot be declared as process global variables.

Properties
Starting from B4A v2.70, classes support properties. Properties syntax can be considered a syntactic sugar.
Properties combine two methods into a single "field" like member.
For example the two following methods:
B4X:
'Gets or sets the text
Sub getText As String
   Return btn.Text
End Sub

Sub setText(t As String)
   btn.Text = t
End Sub
Are merged automatically into a single property:

SS-2013-05-12_11.42.31.png


The property can be treated like any other field:
B4X:
Dim c1 As SomeClass
c1.Text = "abc"
Log(c1.Text)

The rules for properties:
- Only relevant for classes.
- One or two subs with the format get<prop> / set<prop>. Note that get / set must be lower case.
- A property can be read-only (only get), write-only (only set) or both.
- The two subs types (parameter in the set sub and return type in the get sub) must be the same.
- Within the class you should call the methods directly. The property will not appear.
- The property cannot have the same name as a global variable.

Related links:
Built-in documentation
Variables & Objects
Variables & Subs visibility
 

Attachments

  • Draw.zip
    7.1 KB · Views: 2,033
Last edited:

Jim Brown

Active Member
Licensed User
Hi Erel

Would it be possible to have top-level 'global' variables which all instances share? Similar to Process_Globals.

To illustrate, here 'flag' is global and singular. All instances can access and modify it, but it does not belong to any one instance (if that makes sense):

B4X:
'Class module  (MyClass)
Sub Process_Globals
   Dim Flag As Boolean
End Sub

Sub Class_Globals
   Public x,y,z As Int
End Sub

Public Sub Initialize
 ' blah ..
End Sub

Public Sub Test()
  If flag Then blah...
End Sub


Example
B4X:
Dim m As MyClass
m.Initialize

MyClass.flag=False

m.Test
 

salmander

Active Member
Licensed User
Hi Erel, I am using a class to add Activity menu items. Can I capture the events of the menu items inside a class?

Thanks
 

tamadon

Active Member
Licensed User
If a class object was initialized from an activity, is it accessible from another activity that didn't initialize it?

Or is it just like a view object which is only valid while its activity is running?
 

qsrtech

Active Member
Licensed User
Class property getters/setters

Hi, are there any plans to have property getter and setters added to classes so our code can be clean vs always calling class.Getpropname or class.Setpropname in our code.
John
 

aalekizoglou

Member
Licensed User
Views inside a class

I am trying to implement a new custom control in a class. The concept is to embed all required views in that class as private, an create sub to work as getter/setter.

In the below code something is wrong. B4A stops in 'pnlMain.Top = iTop' with null pointer exception.

Any help on this?

B4X:
'Class module
Sub Class_Globals
   Private pnlMain As Panel
   Private edtText As EditText
   Private lblLabel As Label
   Private strInternalLabel, strInternalText As String
   Private iInternalInputType As Int
End Sub


Public Sub Initialize(strEventName As String, iLeft, iTop, iWidth, iHeight)

   pnlMain.Initialize(strEventName)   
   pnlMain.Top = iTop
   pnlMain.Left = iLeft
   pnlMain.Height = iHeight
   pnlMain.Width = iWidth
   edtText.Initialize(strEventName)
   edtText.Top = 0   
   edtText.Left = iLeft
   edtText.Width = iWidth
   edtText.Height = 50
   lblLabel.Initialize(strEventName)   
End Sub

Sub Label(strLabel As String)
   strInternalLabel = strLabel
   'edtText.Hint = strInternalLabel
   lblLabel.Text = strInternalLabel
End Sub

Sub Text(strText As String)
   strInternalText = strText
   edtText.Text = strInternalText
End Sub

Sub InputType(iInputType As Int)
   iInternalInputType = iInputType
   edtText.InputType = iInputType
End Sub

Sub Panel As View
   Return pnlMain
End Sub
 

aalekizoglou

Member
Licensed User

thedesolatesoul

Expert
Licensed User
OK. Makes sense.

Therefore the pnlMain.Parent cannot be set in run time, can it?
no. you need to use parent.addview(pnlMain,x,y,width,height)
i am guessing you want to pass the activity/parent as a parameter too, so you can use it to add the view.
 

aalekizoglou

Member
Licensed User
no. you need to use parent.addview(pnlMain,x,y,width,height)
i am guessing you want to pass the activity/parent as a parameter too, so you can use it to add the view.

Great,

Thanks
 

Jamie

Member
Licensed User
Erel...
It would be nice to define for a private variable a getter and a setter, but accessing them like a usual attribut.
I.e.: obj.x = 5 would call the setter setX(...), while n = obj.x would call the getter getX().

In this way you can write pretty code and have in the same time full control over the attribut.

This will probably be added in one of the next updates. It is already implemented in the libraries handling code.

I think this what I am looking for from my thread here.
http://www.basic4ppc.com/forum/basi...s/27309-passing-properties-methods-class.html

Are you still planning on implementing this feature?
Thanks
Jamie...
 

kiki78

Active Member
Licensed User
Multiple Initialize Sub

For one class I want to implement second Initialize sub with different parameter.
I try to name it Initailize2, call Initialize inside, and some other possibility but I never found how to do that without error.
Is there any solution with current Basic4Android class implementation ?

Regards
 

Avauri

New Member
Licensed User
New properties

Having spent many hours hunched over a candle lite computer trying to get these new properties to work I finally nailed it.
They work really well but there is a little trick to it, now if you are a better programmer than me you will probably say that it is obvious but it wasn't obvious to me with very little documentation to go on.
The trick is in the naming -
If you declare a variable in your class - Dim Headache as string
then create a sub like this

Sub getHeadache as string
Return Headache
End Sub

You will find it wont work, the documentation suggests that it should be a property of some sort but if you declare Headache as a label and return Headache.Text it still wont work.
However if you declare a variable - Dim Head as string
then the sub as
Sub getHeadache as string
Return Head
End Sub
It seems to work
The trick seems to be making sure the private variable has a different name to the sub name(following set/get).
I hope this will save someone from going slowly crazy
 
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