How to create constructor of custom view with Kotl

问题:

I'm trying to use Kotlin in my Android project. I need to create custom view class. Each custom view has two important constructors:

public class MyView extends View {
    public MyView(Context context) {
        super(context);
    }

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

MyView(Context) is used to instantiate view in code, and MyView(Context, AttributeSet) is called by layout inflater when inflating layout from XML.

Answer to this question suggests that I use constructor with default values or factory method. But here's what we have:

Factory method:

fun MyView(c: Context) = MyView(c, attrs) //attrs is nowhere to get
class MyView(c: Context, attrs: AttributeSet) : View(c, attrs) { ... }

or

fun MyView(c: Context, attrs: AttributeSet) = MyView(c) //no way to pass attrs.
                                                        //layout inflater can't use 
                                                        //factory methods
class MyView(c: Context) : View(c) { ... }

Constructor with default values:

class MyView(c: Context, attrs: AttributeSet? = null) : View(c, attrs) { ... }
//here compiler complains that 
//"None of the following functions can be called with the arguments supplied."
//because I specify AttributeSet as nullable, which it can't be.
//Anyway, View(Context,null) is not equivalent to View(Context,AttributeSet)

How can this puzzle be resolved?

---

UPDATE: Seems like we can use View(Context, null) superclass constructor instead of View(Context), so factory method approach seems to be the solution. But even then I can't get my code to work:

fun MyView(c: Context) = MyView(c, null) //compilation error here, attrs can't be null
class MyView(c: Context, attrs: AttributeSet) : View(c, attrs) { ... }

or

fun MyView(c: Context) = MyView(c, null) 
class MyView(c: Context, attrs: AttributeSet?) : View(c, attrs) { ... }
//compilation error: "None of the following functions can be called with 
//the arguments supplied." attrs in superclass constructor is non-null

回答1:

Kotlin supports multiple constructors since M11 which was released 19.03.2015. The syntax is as follows:

class MyView : View {
    constructor(context: Context, attrs: AttributeSet, defStyle: Int) : super(context, attrs, defStyle) {
        // ...
    }

    constructor(context: Context, attrs: AttributeSet) : this(context, attrs, 0) {}
}

More info here and here.

Edit: you can also use @JvmOverloads annotation so that Kotlin auto-generates the required constructors for you:

class MyView @JvmOverloads constructor(
    context: Context, 
    attrs: AttributeSet? = null, 
    defStyle: Int = 0
) : View(context, attrs, defStyle)

Beware, though, as this approach may sometimes lead to the unexpected results, depending on how the class you inherit from defines its constructors. Good explanation of what might happen is given in that article.

回答2:

You should use annotation JvmOverloads (as it looks like in Kotlin 1.0), you can write code like this:

class CustomView @JvmOverloads constructor(
    context: Context, 
    attrs: AttributeSet? = null, 
    defStyle: Int = 0
) : View(context, attrs, defStyle)

This will generate 3 constructors just as you most likely wanted.

Quote from docs:

For every parameter with a default value, this will generate one additional overload, which has this parameter and all parameters to the right of it in the parameter list removed.

回答3:

Custome View with kotlin here's sample code.

class TextViewLight : TextView {

constructor(context: Context) : super(context){
    val typeface = ResourcesCompat.getFont(context, R.font.ccbackbeat_light_5);
    setTypeface(typeface)
}

constructor(context: Context, attrs : AttributeSet) : super(context,attrs){
    val typeface = ResourcesCompat.getFont(context, R.font.ccbackbeat_light_5);
    setTypeface(typeface)
}

constructor(context: Context,  attrs: AttributeSet , defStyleAttr : Int) : super(context, attrs, defStyleAttr){
    val typeface = ResourcesCompat.getFont(context, R.font.ccbackbeat_light_5);
    setTypeface(typeface)
}

}

回答4:

This does seem to be an issue. I've never run into this because my custom views have either been created only in xml or only in code, but I can see where this would come up.

As far as I can see, there are two ways around this:

1) Use constructor with attrs. Using the view in xml will work fine. In code, you need to inflate an xml resource with the desired tags for your view, and convert it to an attribute set:

val parser = resources.getXml(R.xml.my_view_attrs)
val attrs = Xml.asAttributeSet(parser)
val view = MyView(context, attrs)

2) Use the constructor without attrs. You can't place the view directly in your xml, but it's easy about to place a FrameLayout in the xml and add the view to it through code.

回答5:

There are several ways to override your constructors,

When you need default behavior

class MyWebView(context: Context): WebView(context) {
    // code
}

When you need multiple version

class MyWebView(context: Context, attr: AttributeSet? = null): WebView(context, attr) {
    // code
}

When you need to use params inside

class MyWebView(private val context: Context): WebView(context) {
    // you can access context here
}

When you want cleaner code for better readability

class MyWebView: WebView {

    constructor(context: Context): super(context) {
        mContext = context
        setup()
    }

    constructor(context: Context, attr: AttributeSet? = null): super(context, attr) {
        mContext = context
        setup()
    }
}

回答6:

Given this Java code:

public final class MyView extends View {
    public MyView(Context context) {
        super(context);
    }

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

its Kotlin equivalent would use secondary constructors:

class MyView : View {
    constructor(context: Context) : super(context)

    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)
}

That syntax is useful when you really want to call different super-class constructors depending on whether the view is created in code or inflated from XML. The only case that I know of for this to be true is when you are extending the View class directly.

You can use a primary constructor with default arguments and a @JvmOverloads annotation otherwise:

class MyView @JvmOverloads constructor(
        context: Context,
        attrs: AttributeSet? = null
) : View(context, attrs)

And if you only inflate views from XML, then you can just go with the simplest:

class MyView(context: Context, attrs: AttributeSet?) : View(context, attrs)

---

If your class is open for extension and you need to retain the style of the parent, you want to go back to the first variant that uses secondary constructors only:

open class MyView : View {
    constructor(context: Context) : super(context)
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)
    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr)
    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int, defStyleRes: Int) : super(context, attrs, defStyleAttr, defStyleRes)
}

But if you want an open class that overrides the parent style and lets its subclasses override it too, you should be fine with @JvmOverloads:

open class MyView @JvmOverloads constructor(
        context: Context,
        attrs: AttributeSet? = null,
        defStyleAttr: Int = R.attr.customStyle,
        defStyleRes: Int = R.style.CustomStyle
) : View(context, attrs, defStyleAttr, defStyleRes)

回答7:

super() gives me Primary constructor call expected (Kotlin 1.3.61).

class SomeView(
    context: Context,
    attrs: AttributeSet?,
    defStyleAttr: Int,
    defStyleRes: Int
): View(context, attrs, defStyleAttr, defStyleRes) {
    constructor(context: Context) : this(context, null, 0, 0)
    constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, 0, 0)
    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : this(context, attrs, defStyleAttr, 0)
}

回答8:

You can try new Library Anko for Kotlin from JetBrains (also you can contribute on github). Currently it is in beta, but you can create views with such code

    button("Click me") {
         textSize = 18f
         onClick { toast("Clicked!") }
    }

Have a look at this library