I have an Base64Util class with amongst others an extension function decodeBase64ToByteArray :
class Base64Util {
companion object {
fun String.decodeBase64ToByteArray(): ByteArray {
return Base64.getUrlDecoder().decode(this)
}
}
}
Now I would like to test my Base64Util via Base64IUtilTest. I can access non extension functions of course, but how can I access/test the String.decodeBase64ToByteArray() from Base64UtilTest ?
You cannot simply access member extension functions such as decodeBase64ToByteArray from outside. This is only possible if you get into the scope of the object it is defined in:
with(Base64Util.Companion){ //.Companion could be removed
"123fsad123".decodeBase64ToByteArray()
}
It could make sense to define your util functions as top-level functions in a file base64Utils.kt for example. This way, they can be called in a static way without any issue.
Related
The following trivial Kotlin code snippet
fun main() {}
compiles just fine, but the following
val main : () -> Unit = {}
makes the compiler complain that "No main method found in project.", while I was expecting them to be equivalent (I expect a programming language to be as conceptually uniform as possible).
Why does this happen? Is it related only to main, or does this behaviour concern a larger class of functions? Is there some subtle difference between declaring functions with "fun" and declaring them as lambdas?
Conceptually, they are different things. To see that, let's take a look at roughly what the equivalent Java would be. I'll use JVM for examples in this answer, but the same principles apply to all of the other Kotlin backends.
object Foo {
fun main() { ... }
}
This is roughly
class Foo {
public static void main() { ... }
}
Again, roughly. Technically, you'll get a singleton object and a method on it unless you use #JvmStatic (I assume there's some special handling for main that produces a static function on JVM, but I don't know that for a fact)
On the other hand,
object Foo {
val main: () -> Unit = { ... }
}
Here, we're declaring a property, which in Java is going to get implemented as a getter-setter pair
class Foo {
// Singleton instance
public static Foo instance = new Foo();
public Supplier<Void> main;
Foo() {
main = new Supplier<Void>() {
Void get() {
...
}
}
}
}
That is, there isn't actually a main method. There's a main field which, deep down somewhere, has a function inside of it. In my example above, that function is called get. In Kotlin, it's called invoke.
The way I like to think of it is this. Methods in Kotlin (i.e. the things you define on objects that designate their behavior) are not themselves first-class objects. They're second-class citizens which exist on an object. You can convert them to first-class objects by making them into functions. Functions are ordinary objects, like any other. If you take an ordinary object, which may or may not be a function, and call it with (), then you're actually invoking the method .invoke(...) on it. That is, () is an operator on objects which really ends up calling a method. So in Kotlin, functions are really just objects with a custom invoke and a lot of syntax sugar.
Your val defines a field which is a function. Your fun defines a method. Both of these can be called with (), but only one is a genuine method call; the other is secretly calling .invoke on another object. The fact that they look syntactically the same is irrelevant.
As the old adage goes, functions are a poor man's objects, and objects are a poor man's functions.
There is a subtle (or more than subtle) difference. Declaring it with val means that main is a property containing a reference to an anonymous function (which you defined with the lambda). If you define it with val, then when you call main(), you are actually calling the getter of the main property, and then using the invoke() operator to call invoke() on the return value of the property (the anonymous function).
I'd like to use ES6 public class fields:
class Superclass {
constructor() {
// would like to write modular code that applies to all
// subclasses here, or similarly somewhere in Superclass
this.example++; // does NOT WORK (not intialized)
//e.g. doStuffWith(this.fieldTemplates)
}
}
class Subclass extends Superclass {
example = 0
static fieldTemplates = [
Foo,
function() {this.example++},
etc
]
}
Problem:
ES6 public fields are NOT initialized before the constructors, only before the current constructor. For example, when calling super(), any child field will not yet have been defined, like this.example will not yet exist. Static fields will have already been defined. So for example if one were to execute the code function(){this.example++} with .bind as appropriate, called from the superclass constructor, it would fail.
Workaround:
One workaround would be to put all initialization logic after all ES6 public classes have been properly initialized. For example:
class Subclass extends Superclass {
example = 0
lateConstructor = (function(){
this.example++; // works fine
}).bind(this)()
}
What's the solution?
However, this would involve rewriting every single class. I would like something like this by just defining it in the Superclass.constructor, something magic like Object.defineProperty(this, 'lateConstructor', {some magic}) (Object.defineProperty is allegedly internally how es6 static fields are defined, but I see no such explanation how to achieve this programatically in say the mozilla docs; after using Object.getOwnPropertyDescriptor to inspect my above immediately-.binded-and-evaluated cludge I'm inclined to believe there is no way to define a property descriptor as a thunk; the definition is probably executed after returning from super(), that is probably immediately evaluated and assigned to the class like let exampleValue = eval(...); Object.defineProperty(..{value:exampleValue})). Alternatively I could do something horrible like do setTimeout(this.lateConstructor,0) in the Superclass.constructor but that would break many things and not compose well.
I could perhaps try to just use a hierarchy of Objects everywhere instead, but is there some way to implement some global logic for all subclasses in the parent class? Besides making everything lazy with getters? Thanks for any insight.
References:
Run additional action after constructor -- (problems: this requires wrapping all subclasses)
Can I create a thunk to run after the constructor?
No, that is not possible.
How to run code after class fields are initialized, in a sane way?
Put the code in the constructor of the class that defines those fields.
Is there some way to implement some global logic for all subclasses in the parent class?
Yes: define a method. The subclass can call it from its constructor.
Just thought of a workaround (that is hierarchically composable). To answer my own question, in a somewhat unfulfilling way (people should feel free to post better solutions):
// The following illustrates a way to ensure all public class fields have been defined and initialized
// prior to running 'constructor' code. This is achieved by never calling new directly, but instead just
// running Someclass.make(...). All constructor code is instead written in an init(...) function.
class Superclass {
init(opts) { // 'constructor'
this.toRun(); // custom constructor logic example
}
static make() { // the magic that makes everything work
var R = new this();
R.init(...arguments);
return R;
}
}
class Subclass extends Superclass {
subclassValue = 0 // custom public class field example
init(toAdd, opts) { // 'constructor'
// custom constructor logic example
this.subclassValue += toAdd; // may use THIS before super.init
super.init(opts);
// may do stuff afterwards
}
toRun() { // custom public class method example
console.log('.subclassValue = ', this.subclassValue);
}
}
Demo:
> var obj = Subclass.make(1, {});
.subclassValue = 1
> console.log(obj);
Subclass {
subclassValue: 1
__proto__: Superclass
}
I cannot reach class parameter (application) in class function.
Explain please what's the reasoning behind that? Why parameter is accessable within class body but isn't from class function?
class VM_MainActivity(application: Application) : AndroidViewModel(application) {
val taskEntities:List<TaskEntity>?= Repository(application.applicationContext).taskEntities //"application" is reachable
fun fillWithSampleData() {
Repository(**application**.applicationContext).fillDBwithSampleData() //error: Unresolved refernce: application
}
}
In your code, application is a parameter passed to the constructor. It's only available while an instance is being constructed: while running any init { } blocks, and initialising any properties. (The latter is why you can use it to initialise taskEntities.) It doesn't persist after that.
If you want your instance to ‘remember’ the value of the parameter, add val (or var) before it, to make it a property:
class VM_MainActivity(val application: Application) // …
Say we have:
class MyClass {
myProperty: string
}
Is there any built in function or easy way to get JSON like this?:
{
"myProperty": "string"
}
EDIT: My end goal is I want to dynamically print typed class definitions to a web view, in some kind of structured object syntax like JSON. I'm trying to make a server API that will return the schema for various custom classes - for example http://myserver.com/MyClass should return MyClass's properties and their types as a JSON string or other structured representation.
Evert is correct, however a workaround can look like this
class MyClass {
myProperty: string = 'string'
}
JSON.stringify(new MyClass) // shows what you want
In other words, setting a default property value lets TS compile properties to JS
If the above solution is not acceptable, then I would suggest you parsing TS files with your classes with https://dsherret.github.io/ts-simple-ast/.
Typescript class properties exist at build-time only. They are removed from your source after compiling to .js. As such, there is no run-time way to get to the class properties.
Your code snippet compiles to:
var MyClass = /** #class */ (function () {
function MyClass() {
}
return MyClass;
}());
As you can see, the property disappeared.
Based on your update, I had this exact problem. This is how I solved it.
My JSON-based API uses json-schema across the board for type validation, and also exposes these schemas for clients to re-use.
I used an npm package to automatically convert json-schema to Typescript.
This works brilliantly.
I have the following groovy code :
class FileWalker {
private String dir
public static void onEachFile(String dir,IAction ia) {
new File(dir).eachFileRecurse {
ia.perform(it)
}
}
}
walker = new FileWalker()
walker.onEachFile(args[0],new PrintAction())
I noticed that if I place a def in front of walker , the script works. Shouldn't this work the way it is now ?
You don't need a def in groovyConsole or in a groovy script. I consider it good programming practice to have it, but the language will work without it and add those types of variables to the scripts binding.
I'm not sure about the rest of your code (as it won't compile as you've posted it). But you either have a really old version of groovy or something else is wrong with your config or the rest of your code.
With the addition of a stub for the missing IAction interface and PrintAction class, I'm able to get it to run without modification:
interface IAction {
def perform(obj)
}
class PrintAction implements IAction{
def perform(obj) {
println obj
}
}
class FileWalker {
private String dir
public static void onEachFile(String dir,IAction ia) {
new File(dir).eachFileRecurse {
ia.perform(it)
}
}
}
walker = new FileWalker()
walker.onEachFile(args[0],new PrintAction())
I created a dummy directory with "foo/bar" and "foo/baz" files.
If I save it to "walkFiles.groovy" and call it from the command line with
groovy walkFiles.groovy foo
It prints:
foo/bar
foo/baz
This is with the latest version of groovy:
groovy -v
Groovy Version: 1.6-RC-3 JVM: 1.5.0_16
In scripting mode (or via "groovyConsole"), you need a declaration of walker with "def" before using it. A Groovy script file is translated into a derivative class of class Script before it get compiled. So, every declaration needs to be done properly.
On the other hand, when you're running a script in "groovysh" (or using an instance of class GroovyShell), its mechanism automatically binds every referencing object without the need of declaration.
updated:
My above answer would be wrong as I decompiled a .class of Groovy and found that it's using a binding object inside the script as well. Thus my first paragraph was indeed wrong.