Android View and ViewGroup - android

In Android ViewGroup inherits from View. A ViewGroup is a container which holds Views.
ViewGroup (LinearLayout)
View (TextView)
Why did folks at Android defined this relationship as Inheritance instead of composition. As the ViewGroup contains Views shouldn't it be composition ?


I think you're getting too hung up on the wording.
A "ViewGroup" has every bit as much reason to inherit from a "View" as a "TextView", and "ImageView" or ... more to the point ... a "ScrollView" or a "SurfaceView" (the latter two both "contain things").
Perhaps "View" wasn't necessarily the best choice of terms ... but the class heirarchy makes complete sense. Regardless of what it's subclasses are named :)
IMHO ...


I think this is a great example of the Composite design pattern:
http://en.wikipedia.org/wiki/Composite_pattern
Even though the naming might not be the best...


Reading the official doc is the golden rule.
A ViewGroup is a special view that can contain other views (called children.) The view group is the base class for layouts and views containers. This class also defines the ViewGroup.LayoutParams class which serves as the base class for layouts parameters.
If you still do not find out what it is, search with Google image:


A ViewGroup is a (subclass of) View because it can serve as a view in important ways:
It can be an element in a layout XML file
It can be displayed on the screen (by displaying its child views, its own background color, etc.)
It gets inflated along with the rest of the view hierarchy
It can serve as an activity's content view (via setContentView())
So it really is a View.
I agree that the classname ViewGroup is a bit confusing, because it sounds like it's a group, not a view. Calling it ViewGroupView might have been more logical, if unwieldy.
Why did folks at Android define this relationship as Inheritance instead of composition? As the ViewGroup contains Views shouldn't it be composition?
In a case like this, inheritance and composition are not mutually exclusive. A ViewGroup is a View (inheritance) and a ViewGroup can contain Views (composition).


Viewgroup inherits properties of views and does more with other views and viewgroup


A ViewGroup is a special view that can contain other views.
- The view group is the base class for layouts and views containers
- For example, RelativeLayout is the ViewGroup that contains TextView(View), and other Layouts also.
refer link for info:
https://developer.android.com/reference/android/view/ViewGroup.html
A View represents the basic building block for user interface components
-It occupies rectangle on the screen and is responsible for drawing and handling events.
- Examples are EditText, Button, TextView etc
refer link for info:
https://developer.android.com/reference/android/view/View.html


All UI elements in an
Android app are built using View
and ViewGroup objects.
View :- A View is an object that draws
something on the screen that the
user can interact with.
ViewGroup :- ViewGroup is used to hold Views
and ViewGroups.

Related

Difference between ViewParent and ViewGroup?

ViewParent
Defines the responsibilities for a class that will be a parent of a
View. This is the API that a view sees when it wants to interact with
its parent.
ViewGroup
A ViewGroup is a special view that can contain other views (called
children.) The view group is the base class for layouts and views
containers.
I'm struggling to differentiate the two - they're both parents of a set of views. What does ViewParent do that ViewGroup can't (why can't ViewParent's functionality just be added to ViewGroup)?

As you can see in documentation, ViewParent is an interface.
A child view can access its parent via getParent() (doc) which returns a ViewParent.
This parent may be a ViewGroup as it implements the interface but you can't know that from the child object without a "instanceof" check.
There could be a class implementing ViewParent and not being a ViewGroup... (I didn't find it though)

View
1. View objects are the basic building blocks of User Interface(UI) elements in Android.
2.View is a simple rectangle box which responds to the user's actions.
3. Examples are EditText, Button, CheckBox etc..
4. View refers to the android.view.View class, which is the base class of all UI classes.
ViewGroup
1. ViewGroup is the invisible container. It holds View and ViewGroup.
2. For example, LinearLayout is the ViewGroup that contains Button(View), and other Layouts also.
3. ViewGroup is the base class for Layouts.

Difference between View and ViewGroup in Android

What is the difference between a View and a ViewGroup in Android programming?

View
View objects are the basic building blocks of User Interface(UI) elements in Android.
View is a simple rectangle box which responds to the user's actions.
Examples are EditText, Button, CheckBox etc..
View refers to the android.view.View class, which is the base class of all UI classes.
ViewGroup
ViewGroup is the invisible container. It holds View and ViewGroup
For example, LinearLayout is the ViewGroup that contains Button(View), and other Layouts also.
ViewGroup is the base class for Layouts.

Below image is the answer. Don't take it too complex.

A ViewGroup is a special view that can contain other views (called children.) The view group is the base class for layouts and views containers. This class also defines the ViewGroup.LayoutParams class which serves as the base class for layouts parameters.
View class represents the basic building block for user interface components. A View occupies a rectangular area on the screen and is
responsible for drawing and event handling. View is the base class
for widgets, which are used to create interactive UI components
(buttons, text fields, etc.).
Example : ViewGroup (LinearLayout), View (TextView)
Reference

View is a basic building block of UI (User Interface) in android. A view is a small rectangular box which responds to user inputs. Eg: EditText, Button, CheckBox, etc..
ViewGroup is a invisible container of other views (child views) and other viewgroups. Eg: LinearLayout is a viewgroup which can contain other views in it.
ViewGroup is a special kind of view which is extended from View as its base class. ViewGroup is the base class for layouts.
as name states View is singular and the group of Views is the ViewGroup.
more info:
http://www.herongyang.com/Android/View-ViewGroup-Layout-and-Widget.html

ViewGroup is itself a View that works as a container for other views. It extends the functionality of View class in order to provide efficient ways to layout the child views.
For example, LinearLayout is a ViewGroup that lets you define the orientation in which you want child views to be laid, that's all you need to do and LinearLayout will take care of the rest.

Viewgroup inherits properties of views and does more with other views and viewgroup.
See the Android API: http://developer.android.com/reference/android/view/ViewGroup.html

in ViewGroup you can add some other Views as child. ViewGroup is the base class for layouts and view containers.

View is the SuperClass of All component like TextView, EditText, ListView, etc..
while ViewGroup is Collection of Views(TextView, EditText, ListView, etc..), somewhat like container.

A View object is a component of the user interface (UI) like a button or a text box, and it's also called widget.
A ViewGroup object is a layout, that is, a container of other ViewGroup objects (layouts) and View objects (widgets). It's possible to have a layout inside another layout. It's called nested layout but it can increase the time needed to draw the user interface.
The user interface for an app is built using a hierarchy of ViewGroup and View objects. In Android Studio it is possible to use the Component Tree window to visualise this hierarchy.
The Layout Editor in Android Studio can be used to drag and drop View objects (widgets) in the layout. It simplifies the creation of a layout.

In simple words View is the UI element which we interact with when we use an app,like button,edit text and image etc.View is the child class of Android.view.View
While View group is the container which contains all these views inside it in addition to several othe viewgroups like linear or Frame Layout etc. Example if we design & take the root element as Linear layout now our main layout is linear layout inside it we can take another view group (i.e another Linear layout) & many other views like buttons or textview etc.

A ViewGroup describes the layout of the Views in its group. The two basic examples of ViewGroups are LinearLayout and RelativeLayout. Breaking LinearLayout even further, you can have either Vertical LinearLayout or Horizontal LinearLayout. If you choose Vertical LinearLayout, your Views will stack vertically on your screen. The two most basic examples of Views are TextView and Button. Thus, if you have a ViewGroup of Vertical LinearLayout, your Views (e.g. TextViews and Buttons) would line up vertically down your screen.
When the other posters show nested ViewGroups, what they mean is, for example, one of the rows in my Vertical LinearLayout might actually, at the lower level, be several items arranged horizontally. In that case, I'd have a Horizontal LinearLayout as one of the children of my top level Vertical LinearLayout.
Example of Nested ViewGroups:
Parent ViewGroup = Vertical LinearLayout
Row1: TextView1
Row2: Button1
Row3: Image TextView2 Button2 <-- Horizontal Linear nested in Vertical Linear
Row4: TextView3
Row5: Button3

Difference between View and Subview - Android

What is the difference between a view and a subview in Android?

there is no such thing called a 'subview', its just used to refer to a view inside another view.

View is the base class for widgets, which are used to create interactive UI components (buttons, text fields, etc.). and if we insert a view inside the another the its become Subview like a linear Layout containing a button view, here button is a subview

Like Vinay said, there is no such a thing. But you have ViewGroup that contains other Views. For example, LinearLayout, RelativeLayout etc are derived from that class.
If you wish, you can read more about it here:
http://developer.android.com/reference/android/view/ViewGroup.html

Possible to nest Android views?

Even if it's possible, is it best practice? Or should I always use ViewGroup (or one of its subclasses) as the base class for custom components that will contain other ui components?

Possible to nest Android views?
Not really.
Or should I always use ViewGroup (or one of its subclasses) as the base class for custom components that will contain other ui components?
By definition, a ViewGroup is the type of View you use to hold other Views.
In theory, you could completely rewrite ViewGroup yourself, if you really wanted.

Adding Views to an Android Viewgroup Programmatically

This seems to be a common question yet documentation is very hard to find. I'm looking for examples that show me how to create my own view group (preferably by extending an already existing one) and then add views programmaticly.
Thanks.

ViewGroup
ViewGroup is abstract, and its onLayout is abstract too. So you need to provide an implementation for onLayout where you do assign a position at every child (View) of the viewgroup.

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