Android Custom View Constructor

Question

I'm learning about using Custom Views from the following:

http://developer.android.com/guide/topics/ui/custom-components.html#modifying

The description says:

Class Initialization As always, the super is called first. Furthermore, this is not a default constructor, but a parameterized one. The EditText is created with these parameters when it is inflated from an XML layout file, thus, our constructor needs to both take them and pass them to the superclass constructor as well.

Is there a better description? I've been trying to figure out what the constructor(s) should look like and I've come up with 4 possible choices (see example at end of post). I'm not sure what these 4 choices do (or don't do), why I should implement them, or what the parameters mean. Is there a description of these?

public MyCustomView()
{
    super();
}

public MyCustomView(Context context)
{
    super(context);
}

public MyCustomView(Context context, AttributeSet attrs)
{
    super(context, attrs);
} 

public MyCustomView(Context context, AttributeSet attrs, Map params)
{
    super(context, attrs, params);
}

Similar question here: stackoverflow.com/questions/9195713/… — mbonnin
– mbonnin, Commented Sep 1, 2016 at 20:46

CommonsWare · Accepted Answer · 2010-05-22 15:07:12Z

66

You don't need the first one, as that just won't work.

The third one will mean your custom View will be usable from XML layout files. If you don't care about that, you don't need it.

The fourth one is just wrong, AFAIK. There is no View constructor that take a Map as the third parameter. There is one that takes an int as the third parameter, used to override the default style for the widget.

I tend to use the this() syntax to combine these:

public ColorMixer(Context context) {
    this(context, null);
}

public ColorMixer(Context context, AttributeSet attrs) {
    this(context, attrs, 0);
}

public ColorMixer(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    // real work here
}

You can see the rest of this code in this book example.

answered May 22, 2010 at 15:07

CommonsWare

1.0m195 gold badges2.5k silver badges2.7k bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

broot Over a year ago

super(context, attrs) and super(context, attrs, 0) works differentially for me. First one is ok, but second removes original style from a view. Is it a bug in newer versions of Android?

CommonsWare Over a year ago

Yes, my this() approach had flaws. Just do a plain chain to the superclass (e.g., super(context, attrs)) and put a call to a common private method (e.g., init();) in each constructor. See github.com/commonsguy/cwac-colormixer/blob/master/src/com/… for an example.

broot Over a year ago

Yes, I did exactly the same thing, but called this(context, null) in the first constructor. One flaw is that you have to initialize final fields in both constructors - you can't do that in init() method.

CommonsWare Over a year ago

@Brutall: True, though I tend to initialize final data members directly in the data member declaration, where possible.

jenzz Over a year ago

@vanleeuwenbram Yes, using this approach the app will crash on any device running Gingerbread or lower. That is because super(context, attrs, defStyle); requires API Level 11. For backwards compatibility use a private init() method instead.

|

yanchenko · Accepted Answer · 2012-06-02 15:39:00Z

11

Here's a my pattern (creating a custom ViewGoup here, but still):

// CustomView.java

public class CustomView extends LinearLayout {

    public CustomView(Context context) {
        super(context);
        init(context);
    }

    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context);
    }

    public CustomView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init(context);
    }

    private void init(Context ctx) {
        LayoutInflater.from(ctx).inflate(R.layout.view_custom, this, true);
            // extra init
    }

}

and

// view_custom.xml

<merge xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- Views -->
</merge>

answered Jun 2, 2012 at 15:39

yanchenko

57.4k33 gold badges150 silver badges166 bronze badges

3 Comments

greg7gkb Over a year ago

This is not as effective as the @CommonsWare solution because it does not let you have fields marked as 'final', whereas using a chain of this() calls will.

Blundell Over a year ago

@greg7gkb However @Commonsware solution is flawed. You have to call each separate super and use an init()

Ran Over a year ago

No need to pass the Context to init(), you can just call getContext()

Community · Accepted Answer · 2017-05-23 11:45:38Z

8

When you are adding your custom View from xml like :

 <com.mypack.MyView
      ...
      />

you will need the public constructor MyView(Context context, AttributeSet attrs), otherwise you will get an Exception when Android tries to inflate your View.

And when you add your View from xml and also specify the android:style attribute like :

 <com.mypack.MyView
      style="@styles/MyCustomStyle"
      ...
      />

you will also need the third public constructor MyView(Context context, AttributeSet attrs,int defStyle) .

The third constructor is usually used when you extend a style and customize it, and then you would like to set that style to a given View in your layouts

Edit Details

public MyView(Context context, AttributeSet attrs) {
            //Called by Android if <com.mypack.MyView/> is in layout xml file without style attribute.
            //So we need to call MyView(Context context, AttributeSet attrs, int defStyle) 
            // with R.attr.customViewStyle. Thus R.attr.customViewStyle is default style for MyView.
            this(context, attrs, R.attr.customViewStyle);
    }

See this

edited May 23, 2017 at 11:45

CommunityBot

11 silver badge

answered Jan 8, 2015 at 11:54

Zar E Ahmer

34.5k20 gold badges244 silver badges306 bronze badges

11 Comments

Zvi Over a year ago

I have few custom views in the same layout. How can I pass to the constructor an index which tells it which of the custom views it belongs to?

Zar E Ahmer Over a year ago

You should read TypedArray and styleable attributes. stackoverflow.com/questions/11039829/…

Zvi Over a year ago

@Nepstar, I read that post and I did not understand how will it solve my problem, because styleable attributes, as far as I understood, are static properties defined before the app is running. Thus all the instances of my custom components will get the same data, whereas I need a unique id for each running one.

Zvi Over a year ago

To clarify, each of my components is part of a list and I need to know the position of the component in the list, as well as the data it contains. Moreover, onClick will not do because the custom component is a numberPicker which gets many clicks. Only after the user clicks a SAVE button do I need to collect the data in each numberPicker of each row.

Zar E Ahmer Over a year ago

you should create a method like setIndex in your customView and then you can change it programatically like (textView.setText("DummyText"); 101apps.co.za/index.php/articles/creating-custom-views.html this tutorial shows you how to change circleColor at runtime(programmatically)

|

Collectives™ on Stack Overflow

Android Custom View Constructor

3 Answers 3

7 Comments

3 Comments

11 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

7 Comments

3 Comments

11 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related