Android Kotlin-如何扩展ConstraintLayout？

Question

I'd like my ConstaintLayout to carry extra additional properties, but I have trouble extending it. More precisely I have trouble putting correct constructor into

class myCL(): ConstraintLayout(???) {
}

Answer 1

To make sure you don't get any quirks in behavior, you should implement it like this.

class myCL: ConstraintLayout {
    constructor(context: Context) : super(context)
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)
    constructor(context: Context, attrs: AttributeSet?,
                @AttrRes defStyleAttr: Int) : super(context, attrs, defStyleAttr)
}

Answer 2

The constructor you really need is the one with all arguments:

class myCL(context: Context?, attrs: AttributeSet?, defStyleAttr: Int) :
    ConstraintLayout(context, attrs, defStyleAttr) {
}

if you want to implement all three constructors without much hassle you can use @JvmOverloads and use sensible defaults.

class myCL @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0) : ConstraintLayout(context, attrs, defStyleAttr) {
}

see https://developer.android.com/reference/android/support/constraint/ConstraintLayout

Answer 3

You can also do something like this to restict to only one super call --

class MyConstraintLayout: ConstraintLayout {

   constructor(context: Context) : this(context, null)
   constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, -1)
   constructor(context: Context, attrs: AttributeSet?, @AttrRes defStyleAttr: Int) : super(context, attrs, defStyleAttr)
}