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
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.
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.
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)
}
}
TL;DR most of the time, it should be enough to just define your custom view as:
class MyView(context: Context, attrs: AttributeSet?) : FooView(context, attrs)
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)
You don't need #JvmOverloads constructor if you don't plan to call it from Java.
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)
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.
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()
}
}
Added a complete example of creating a custom view by inflating XML layout with multiple constructors
class MyCustomView : FrameLayout {
private val TAG = MyCustomView ::class.simpleName
constructor(context: Context): super(context) {
initView()
}
constructor(context: Context, attr: AttributeSet? = null): super(context, attr) {
initView()
}
constructor(
context: Context,
attrs: AttributeSet?,
defStyleAttr: Int
): super(context, attrs, defStyleAttr) {
initView()
}
/**
* init View Here
*/
private fun initView() {
val rootView = (context
.getSystemService(Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater)
.inflate(R.layout.layout_custom_view, this, true)
// Load and use rest of views here
val awesomeBG= rootView.findViewById<ImageView>(R.id.awesomeBG)
}
in XML add your layout_custom_view view file
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="#+id/awesomeBG"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:contentDescription="#string/bg_desc"
android:fitsSystemWindows="true"
android:scaleType="centerCrop" />
<!--ADD YOUR VIEWs HERE-->
</FrameLayout>
It seems, constructor parameters are fixed by type and order, but we can add own like this:
class UpperMenu #JvmOverloads
constructor( context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0,parentLayout: Int,seeToolbar: Boolean? = false)
: Toolbar(context, attrs, defStyleAttr) {}
where parentLayout ,seeToolbar are added to it so :
val upper= UpperMenu (this,null,0,R.id.mainParent, true)
When you have some view (BottomSheetDialog) that already can show text and want to add formatted string, you should add two constructors.
class SomeDialog : BottomSheetDialog {
private val binding = DialogSomeBinding.inflate(layoutInflater)
// Base constructor that cannot be called directly
private constructor(
context: Context,
title: CharSequence
) : super(context) {
setContentView(binding.root)
binding.title.text = title
}
// Constructor with simple CharSequence message
constructor(
context: Context,
title: CharSequence,
message: CharSequence
) : this(context, title) {
binding.message.text = message
}
// Constructor with formatted SpannableString message
constructor(
context: Context,
title: CharSequence,
message: SpannableString
) : this(context, title) {
binding.message.text = message
}
}
Usage:
val span = SpannableString(getString(R.string.message, name))
...
SomeDialog(
context = requireContext(),
title = getString(R.string.title),
message = span
).show()
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
Related
I am new to kotlin and I have added a parameter in the constructor and it throws this error? how to figure out the problem I don't understand. Any help would be appreciatable
Error public constructor AppView(context: Context, _listener: OnFragmentInteractionListener, _position: Int)defined in com.views.home.AppView #JvmOverloads public constructor AppView(mlist: StoreViewMap, context: Context, attrs: AttributeSet? = ..., defStyle: Int = ...) defined in com.views.home.AppView
class AppView #JvmOverloads constructor(mlist: StoreViewMap, context: Context, attrs: AttributeSet? = null, defStyle: Int = 0) :
LinearLayout(context, attrs, defStyle) {
private lateinit var listener: OnFragmentInteractionListener
private var position = 0
private val mainView: View
var mlistener: StoreViewMap = mlist
constructor(context: Context, _listener: OnFragmentInteractionListener, _position: Int) : this(context) {
listener = _listener
position = _position
initFeed()
}
init {
val layoutInflater: LayoutInflater = context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater
mainView = layoutInflater.inflate(R.layout.view_home_feed, this)
}
private fun initFeed() {
mainView.homeSwipeLayout.setOnRefreshListener { fetchSlots() }
loadContentSlots(DataCaching(context).getContentSlots())
}
}
You have to add a default value to mList in your first constructor or add a StoreViewMapparameter in your second constructor
You're calling your own constructor by calling this(context), which mean that if you define your constructor parameters calling the constructor will ignore them.
constructor(context: Context) : this(context, null)
constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, 0)
constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs)
Here the first constructor is calling the second and the second is calling the third, but the third is calling the constructor of the inherited class LinearLayout in your class.
The solution is to create a forth constructor and adding to it the params you want ex:
constructor(context: Context, mlist: StoreViewMap, _listener: OnFragmentInteractionListener, _position: Int) : this(context){
// your code
}
This constructor will call the first constructor
I want to implement an OnLongClickListener to some of my TextViews, but I don't want to repeat the same code everywhere, so I want to extend TextView and implement the OnLongClickListener just once.
class LongClickToCopyTextView : TextView {
constructor(context: Context) : this(context, null, 0)
constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, 0)
constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr)
init {
setOnLongClickListener {
val clipboard = context?.getSystemService(Context.CLIPBOARD_SERVICE) as ClipboardManager?
val clip = ClipData.newPlainText(context?.packageName, text)
clipboard?.primaryClip = clip
true
}
}
}
The implementation of the listener is used copy the text of the TextView into the clipboard when a user long presses it.
The problem is the text of the custom TextView is not shown. But if I use regular TextView the text is displayed correctly.
XML
<com.dzboot.myips.custom.LongClickToCopyTextView
android:id="#+id/simNumber"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="14sp"
android:text="00"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintTop_toTopOf="parent" />
The issue with setting default parameters for defStyleAttr is, that the base class might do the same to actually handle styles and states. Your initialisation happens in init {} anyhow.
class LongClickToCopyTextView : TextView {
constructor(context: Context) : super(context)
constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)
constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr)
init {
setOnLongClickListener {
val clipboard = context?.getSystemService(Context.CLIPBOARD_SERVICE) as ClipboardManager?
val clip = ClipData.newPlainText(context?.packageName, text)
clipboard?.primaryClip = clip
true
}
}
}
Also you might want to extend fro AppCompatTextView instead. It has some newer features backported.
I want to create a base class for all of my custom views. It can be a different type, for example, RelativeLayout or NavigationView.
So I've created abstract class with generics which is implementing the deepest class of these views which is connecting them - View. This is what I've got:
abstract class MyCustomView<VS : ViewState, V : View<VS>, P : Presenter<VS, V, *>>(
context: Context,
attrs: AttributeSet? = null,
defStyleAttr: Int = 0,
defStyleRes: Int = 0
) : View(context, attrs, defStyleAttr, defStyleRes) { ... }
But the problem is that now I'm able to inherit only from the View. How can I build somehow generic that must be a child of the View so I could implement it and still to be able inside my base class to override methods like onAttachedToWindow?
Try to declare your class like this:
abstract class MyCustomView<VS : ViewState, V : View<VS>, P : Presenter<VS, V, *>>() : 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)
#TargetApi(Build.VERSION_CODES.LOLLIPOP)
constructor(context: Context, attrs: AttributeSet, defStyleAttr: Int, defStyleRes: Int) : super(context, attrs, defStyleAttr, defStyleRes)
(...)
}
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(???) {
}
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)
}
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
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)
}
I have a custom layout as below
class CustomComponent : FrameLayout {
constructor(context: Context?) : super(context)
constructor(context: Context?, attrs: AttributeSet?) : super(context, attrs) {
initAttrs(attrs)
}
constructor(context: Context?, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {
initAttrs(attrs)
}
constructor(context: Context?, attrs: AttributeSet?, defStyleAttr: Int, defStyleRes: Int) : super(context, attrs, defStyleAttr, defStyleRes) {
initAttrs(attrs)
}
init {
LayoutInflater.from(context).inflate(R.layout.view_custom_component, this, true)
}
fun initAttrs(attrs: AttributeSet?) {
val typedArray = context.obtainStyledAttributes(attrs, R.styleable.custom_component_attributes, 0, 0)
my_title.text = resources.getText(typedArray
.getResourceId(R.styleable.custom_component_attributes_custom_component_title, R.string.component_one))
typedArray.recycle()
}
}
Now for each constructor, I have to explicitly call initAttrs(attrs) as I can't find way to access attrs in my init function.
Is there a way I could access attrs in init function, so I could call initAttrs(attrs) from init instead of having to explicitly call it in each of the constructor?
Use a constructor with default arguments:
class CustomComponent #JvmOverloads constructor(
context: Context,
attrs: AttributeSet? = null,
defStyle: Int = 0
) : FrameLayout(context, attrs, defStyle) {
fun init {
// Initialize your view
}
}
The #JvmOverloads annotation tells Kotlin to generate three overloaded constructors so they can be called in Java as well.
In your init function, attrs becomes available as a nullable type:
fun init {
LayoutInflater.from(context).inflate(R.layout.view_custom_component, this, true)
attrs?.let {
val typedArray = context.obtainStyledAttributes(it, R.styleable.custom_component_attributes, 0, 0)
my_title.text = resources.getText(typedArray
.getResourceId(R.styleable.custom_component_attributes_custom_component_title, R.string.component_one))
typedArray.recycle()
}
}
Note the usage of it in the let block.