I have class and two objects. I want to delete 1st object. How I can delete it?
I tried just delete() (I found it on kotlinlangcom) but it doesn't work. I have red light bulb what recommend: "Create member function Person.delete", "Rename reference" and "Create extension function Person.delete".
fun main() {
// copy object in object
data class Person (var name: String = "Orig", var type: String = "piece",
var age: Int = 18, var high: Double = 25.7, var code: Int = 1522)
{
var info: String = "0"
get() = "Name: $name Age: $age Type: $type High: $high Code: $code"
}
val ann: Person = Person("Ann", "man", 10, 0.5, 1408) // 1st object with some properties
var bob: Person = Person("Bob", "girl", 20, 15.0, 1239) // 2nd object without prop
println(ann.info)// props 1st object
println(bob.info)// props 2nd object
print(" ---- ")
bob = ann.copy() // copy 1st in 2nd
println("Bob has Anns' props: ")
print("final " + bob.info) // new props 2nd object
bob.delete()
}
You don't need to thing about deleting objects like in other languages like c++/c ... the garbage collector of the JVM is taking care of it (if you use kotlin with jvm)
All you need to know is keeping no references on the object
So if you have a collection (list,map ...) where you put the object in, you also have to put it out if the collection is a property of a long living class like a model or something ... thats the only possiblity to getting into trouble within kotlin, putting a reference into a collection which is referenced by a static or long living object.
Within a function there is no need to delete the objects created withing.
Keep in mind that the garbage collector (GC) is not running instantly after finishing the method. There are different strategies depending on the age of the object and the garbage collector itself. If you would like to see the GC in action, this tool (visualgc) https://www.oracle.com/technetwork/java/visualgc-136680.html has some pretty nice visualisations.
You could also find much more details about garbage collection here: https://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html
Related
I am making a dynamic UI using kotlin and Jetpack compose and storing the information in an object box database.
The aim is that i will have a composable that starts off with 1 initial item that is empty and when the contents of the textbox have been filled in would allow the red "+" button to be clicked and then another textfield would appear. These values will need to be able to be edited constantly all the way until the final composable value is stored. The button changes colour currently and the states are fine with the button so i can add and remove rows
The data comes in as a string and is converted into a Hashmap<Int, String>. The int is used to store the position in the map being edited and the string would be the text value.
Using log messages i see that the information is updated in the list and for recomp sake i instantly store the value of the edited list in a converted json string.
At the moment:
When i scroll past the composable it resets and looks like the initial state (even if i have added multiple rows)
Log messages show that my hashmap has the values from before e.g. {"0":"asdfdsa"} but the previous positions are ignored and as the previous information would still be present but not shown on the UI when i enter it into the first field again (the others are not visible at the time) {"0":"asdfdsa","0":"hello"}. This would later cause an error when trying to save new data to the list because of the duplicate key
In the composables my hashmap is called textFields and is defined like this. Number is used to determine how many textfields to draw on the screen
val textFields = remember { getDataStringToMap(data.dataItem.dataValue) }
val number = remember { mutableStateOf(textFields.size) }
the method to getDataStringToMap is created like this
private fun getDataMapToString(textFieldsMap: HashMap<Int, String>): String {
val gson = Gson()
val newMap = hashMapOf<Int, String>()
for (value in textFieldsMap){
if (value.value .isNotBlank()){
newMap[value.key] = value.value
}
}
return gson.toJson(newMap)
}
and the method to getDataStringToMap is created like this (I explicitly define the empty hashmap type because its more readable for me if i can see it)
private fun getDataStringToMap(textsFieldsString: String): HashMap<Int, String> {
val gson = Gson()
return if (textsFieldsString.isBlank()) {
hashMapOf<Int, String>(0 to "")
} else {
val mapType = HashMap<Int, String>().javaClass
gson.fromJson(textsFieldsString, mapType)
}
the composables for the textfields are called like this
items(number.value) { index ->
listItem(
itemValue = textFields[index].orEmpty(),
changeValue = {
textFields[index] = it
setDataValue(getDataMapToString(textFields))
},
addItem = {
columnHeight.value += itemHeight
scope.launch {
scrollState.animateScrollBy(itemHeight)
}
},
deleteItem = {
columnHeight.value -= itemHeight
scope.launch {
scrollState.animateScrollBy(-itemHeight)
}
},
lastItem = index == number.value - 1,
index = index
)
}
Edited 30/12/2022
Answer from #Arthur Kasparian solved issues. Change to rememberSaveable retains the UiState even on scroll and recomp.
Now just to sort out which specific elements are removed and shown after :D
The problem is that remember alone does not save values on configuration changes, whereas rememberSaveable does.
You can read more about this here.
I know I can use get a notification when the value of a variable has changed, Code A is OK.
I hope to monitor the change of a normal variable such as var myFilename="OK" in Code B, but myObserv isn't launching when myFilename has changed.
Can I observe the change fo a variable in Kotlin?
Code A
var savedFilename: String by Delegates.observable("") {
prop, old, new -> ...
}
Code B
var myFilename="OK"
var myObserv String by Delegates.observable(myFilename) {
prop, old, new -> ...
}
How to populate the value of this variable:
private val _urlList = MutableLiveData<List<Url>>()
of type Url:
data class Url(
val imgSrcUrl: String
)
with the incoming list of url strings from a firebase call?
Here is where the magic happens:
private fun getData(){
viewModelScope.launch {
try {
getImagesUrl {
"Here where I need to set the value of the variable to a listOf(it) with it being strings
of urls retrieved from firebase storage"
}
}catch (e: Exception){
"Handling the error"
}
}
}
Edit
The map function #dominicoder provided solved my problem, answer accepted.
Thank you all for your help
Your question is unclear because you're showing a live data of a single Url object but asking to stuff it with a list of strings. So first, your live data object needs to change to a list of Urls:
private val _urlList = MutableLiveData<List<Url>>()
Then, assuming getImagesUrl yields a list of strings, if I understood you correctly, then you would map that to a list of Urls:
getImagesUrl { listOfImageUrlStrings ->
_urlList.value = listOfImageUrlStrings.map { imageUrlString -> Url(imageUrlString) }
}
If that does not answer your question, you really need to review it and clarify.
You can set values on the MutableLiveDataObject in two ways (depends on what you're doing).
Setting the value as normal from the UI thread can be done with:
myLiveData.value = myobject
If you're setting it from a background thread like you might in a coroutine with a suspended function or async task etc then use:
myLiveData.postValue(myObject)
It's not clear from your question whether the LiveData is meant to hold a list as you mention both lists and single values. But your LiveData holds a set the values as a collection like a list, set or map. It's can be treated as a whole object so adding a value later needs to have the whole collection set again like:
myLiveData.value = mutableListOf<Url>()
//Response received and object created
myLiveData.value = myLiveData.value.apply {
add(myObject)
}
Or if the value is mutable updating the existing value (preferred as it's cleaner):
myLiveData.value.add(myObject)
The problem with that approach is you're exposing the map as a mutable/writeable object. Allowing accessors to change the values which you might not want.
first, I create empty Array(Kotlin) instance in companion object.
companion object {
var strarray: Array<String> = arrayOf()
var objectarray: LinkedHashMap<Int, List<Any>> = LinkedHashMap<Int, List<Any>>()
}
and I expected that I use empty array instance when read textString from CSV File.
fun csvFileToString():String {
val inputStream = File(Paths.get("").toAbsolutePath().toString()
.plus("/src/main/SampleCSVFile_2kb.csv")).inputStream()
val reader = inputStream.bufferedReader()
var iterator = reader.lineSequence().iterator()
var index:Int = 1;
while (iterator.hasNext()){
var lineText:String = iterator.next()
strarray.set(index, lineText)
index++
}
return ""
}
but when I run that source code
a.csvFileToString()
println(CsvParser.strarray)
occured exception
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1
strarray.set(index, lineText) <<<<<<<<< because of this line
can I use Array(from kotlin collection) like ArrayList(from java collection)?
You can add a new item to an array using +=, for example: item += item
private var songs: Array<String> = arrayOf()
fun add(input: String) {
songs += input
}
Size of Array is defined at its creation and cannot be modified - in your example it equals 0.
If you want to create Array with dynamic size you should use ArrayList.
arrayOf gives you an array. Arrays have fixed length even in Java.
listOf gives you an immutable list. You cannot add or remove items in this list.
What you're looking for is mutableListOf<String>.
In your current approach, reusing a member property, don't forget to clear the list before every use.
Your code can be further simplified (and improved) like so:
out.clear()
inputStream.bufferedReader().use { reader -> // Use takes care of closing reader.
val lines = reader.lineSequence()
out.addAll(lines) // MutableList can add all from sequence.
}
Now imagine you wanted to consume the output list but needed to parse another file at the same time.
Consider working towards a pure function (no side effects, for now no accessing member properties) and simplifying it even further:
fun csvFileToString(): String { // Now method returns something useful.
val inputStream = File(Paths.get("").toAbsolutePath().toString()
.plus("/src/main/SampleCSVFile_2kb.csv")).inputStream()
inputStream.bufferedReader().use {
return it.lineSequence().joinToString("\n")
}
}
In this case we can totally skip the lists and arrays and just read the text:
inputStream.bufferedReader().use {
return it.readText()
}
I'm assuming that's what you wanted in the first place.
Kotlin has a lot of useful extension functions built-in. Look for them first.
Dojo tookit version: 1.9.3
I have started learning dojo/store/memory from here.
https://dojotoolkit.org/documentation/tutorials/1.8/intro_dojo_store/
I tried to run the example in the tutorial with some modification.
Ran the get() (var oldjim = employeeStore.get("Jim");) call to check the value in the memory store before making a put() call.
I can see that the data has already been changed.
// retrieve object with the name "Jim"
var jim = employeeStore.get("Jim");
// show the department property
console.log("Jim's department is " + jim.department);
// iterate through all the properties of jim:
for(var i in jim){
console.log(i, "=", jim[i]);
}
// update his department
jim.department = "engineering";
// START *** Modified code
// Get the old data for "jim"
var oldjim = employeeStore.get("Jim");
// Displays the OLD data before making a put() call to the store.
console.log("oldJim's department is " + oldjim.department);
// output "oldJim's department is engineering"
// END *** Modified code
// and store the change
employeeStore.put(jim);
Is this the behaviour of the dojo/store/memory?
This is more behaviour of JavaScript than dojo/store/Memory.
Have a look at the implementation of dojo/store/Memory::get(id) method. It returns an object stored in data property array. In JavaScript objects are passed by reference and that means jim and oldjim both points to the same place in the memory. You can easily check it (jsFiddle):
console.log(jim === oldjim); // true
To avoid this behaviour you can modify dojo/store/Memory class so it returns a copy (clone) of the object:
require(["dojo/_base/declare", "dojo/_base/lang", "dojo/store/Memory"], function(declare, lang, Memory){
var ModifiedMemory = declare(Memory, {
get: function(id) {
return lang.clone(this.data[this.index[id]]);
}
});
});
In this case (using ModifiedMemory instead of Memory), jim === oldjim will evaluate to false, because they're not the same object anymore (i.e. there are 2 copies in the memory now) and therefore dojo/store/Memory::put() will work as you expect it to work.
See ModifiedMemory class in action at jsFiddle.