In flex, how to check if a variable exists? I have tried using
if (this['some_variable'] != undefined) {
//do something
}
There is a run time error saying the property some_variable does not exists. I have checked with null instead of undefined, still the same error.
please help.
[EDIT]
Based on the replies I have used this.hasOwnProperty('variable_name'). I found that its returning true if variable_name is a public but false if its private/protected. How to check for a private variable?
There are two ways for that:
if ("some_variable" in this) {
//do something
}
It uses in operator.
And:
if (this.hasOwnProperty("some_variable")) {
//do something
}
See documentation about hasOwnProperty().
What about getting information about private/protected properties the situation is that you can't get this info with the current state of Flash Player. The only possible way, I suppose, is some kind of runtime bytecode manipulation. But as far as I know nobody implemented it yet.
But I have a question about getting info about private/protected properties: for what purpose you need it? The nature of these properties/methods is you can't call them. Even if you know about their existence.
You can use
if (this. hasOwnProperty("some_variable")) {
//access the variable inside
}
if (this.hasOwnProperty('some_variable')) DO_IT_!()
Explanation:
this['some_variable'] tries to evaluate the value of the instance property some_variable. If there is no such a property, you will get this error.
To test if a property exists for a particular object use hasOwnProperty or wrap your condition in a try/catch block or use if ('some_variable' in this).
Usually you create an object property in a class file:
public class MyClass {
public var myProperty : String = "ich bin hier";
}
Then you refer to that property within the class:
trace (myProperty);
trace (this.myProperty);
Using the array syntax [] is also possible but will throw the error if the property is not defined.
trace (this['myProperty']);
And finally! If you declare your class to be dynamic you might use the array syntax even if the property does not exist.
public dynamic class MyClass {
public function MyClass() {
trace (this["never_declared_property"]);
}
}
Related
I've been trying some stuff from kotlin.reflection during my project, and got stuck on something what occurs to me as hard to understand, I have declared object as follows:
object WebsiteMapping
{
const val ADMIN = "/admin"
}
once I call:
Arrays
.stream(WebsiteMapping::class.java.declaredFields)
.forEach { field -> println(field.type) }
what I get is:
class java.lang.String
class mapping.WebsiteMapping
When I looked a little bit into what is behind declaredFields invocation I grasped why it works as it is, but is there any convenient way of taking only declared consts within that object without getting also root of the whole structure?
The field with the type class mapping.WebsiteMapping is, basically, not the root of the structure but a special field generated in the object type that holds the reference to the singleton object.
In Kotlin, this field is named INSTANCE by convention. You can therefore filter the fields that you get from the class as follows:
WebsiteMapping::class.java.declaredFields
.filter { it.name != "INSTANCE" }
.forEach { println(it.type) }
Another solution is to switch from java.reflect.* to the Kotlin reflection API kotlin.reflect (needs a dependency on the kotlin-reflect module), which automatically filters the property:
WebsiteMapping::class.memberProperties
.forEach { println(it.returnType) }
I have read Access property delegate in Kotlin which is about accessing a delegate from an instance. One can use KProperty::getDelegate since Kotlin 1.1, however this will return the instance of the delegate and therefore needs an instance of the class first.
Now I want to get the type of the delegate without having an instance of the class. Consider a library with a custom delegate type CustomDelegate that want's to get all properties of a class that are delegated to an instance of CustomDelegate:
class Example
{
var nonDelegatedProperty = "I don't care about this property"
var delegatedProperty1 by lazy { "I don't care about this too" }
var delegatedProperty2 by CustomDelegate("I care about this one")
}
How can I, given I have KClass<Example>, but not an instance of Example, get all properties delegated to CustomDelegate?
How can I, given I have KClass<Example>, but not an instance of
Example, get all properties delegated to CustomDelegate?
You can do it in two ways depending on your needs.
First of all, you have to include the kotlin-reflect dependency in your build.gradle file:
compile "org.jetbrains.kotlin:kotlin-reflect:1.1.51"
In my opinion, you should use the first solution if you can, because it's the most clear and optimized one. The second solution instead, can handle one case that the first solution can't.
First
You can loop an the declared properties and check if the type of the property or the type of the delegate is CustomDelegate.
// Loop on the properties of this class.
Example::class.declaredMemberProperties.filter { property ->
// If the type of field is CustomDelegate or the delegate is an instance of CustomDelegate,
// it will return true.
CustomDelegate::class.java == property.javaField?.type
}
There's only one problem with this solution, you will get also the fields with type CustomDelegate, so, given this example:
class Example {
var nonDelegatedProperty = "I don't care about this property"
val delegatedProperty1 by lazy { "I don't care about this too" }
val delegatedProperty2 by CustomDelegate("I care about this one")
val customDelegate = CustomDelegate("jdo")
}
You will get delegatedProperty2 and customDelegate. If you want to get only delegatedProperty2, I found an horrible solution that you can use if you need to manage this case.
Second
If you check the source code of KPropertyImpl, you can see how a delegation is implemented. So, you can do something like this:
// Loop on the properties of this class.
Example::class.declaredMemberProperties.filter { property ->
// You must check in all superclasses till you find the right method.
property::class.allSuperclasses.find {
val computeField = try {
// Find the protected method "computeDelegateField".
it.declaredFunctions.find { it.name == "computeDelegateField" } ?: return#find false
} catch (t: Throwable) {
// Catch KotlinReflectionInternalError.
return#find false
}
// Get the delegate or null if the delegate is not present.
val delegateField = computeField.call(property) as? Field
// If the delegate was null or the type is different from CustomDelegate, it will return false.
CustomDelegate::class.java == delegateField?.type
} != null
}
In this case, you will get only delegatedProperty2 as result.
I use this pattern to test for undefined and null values in ActionScript/Flex :
if(obj) {
execute()
}
Unfortunately, a ReferenceError is always thrown when I use the pattern to test for child objects :
if(obj.child) {
execute()
}
ReferenceError: Error #1069: Property child not found on obj and there is no default value.
Why does testing for child objects with if statements throw a ReferenceError?
Thanks!
You're getting this error because the obj's type does not have the child property in it. You need to do something like this:
if((obj) && (obj.hasOwnProperty('child') && (obj.child)){
execute()
}
More info on the hasOwnProperty method in the Object class:
http://livedocs.adobe.com/flex/3/langref/Object.html#hasOwnProperty%28%29
This happens when obj is a strongly typed object but it doesn't have a child field.
You can test to see if a field exists on any object using the in operator:
if ("foo" in obj && obj.foo)
execute();
I've also written a utility function to make this process easier:
function getattr(obj:Object, field:*, dflt:*=undefined):* {
if (field in obj && obj[field])
return obj[field];
return dflt;
}
You can avoid reference errors by using array notation:
if([obj.name][child.name]){
execute();
}
The important thing to realize is that simply avoiding the error can cause issues down the track - debugging will be harder in larger applications.
Of course, the ideal approach is to completely avoid the situation by using validator functions to ensure you have the right data, instead of testing for null when the data is required. :)
Say I have a property like...
public object MyObject
{
get { return (object)ViewState["myobject"]; }
set { ViewState["myobject"] = value; }
}
I modify the object like so...
object myObjCopy = MyObject;
myObjCopy.ChangeSomething();
MyObject = myObjCopy;
Is this the correct method? It just feels really clunky and I wonder if I'm missing something. Is there some clever paradigm which enables modifying viewstate more intuitively without using temporary copys everywhere in my code.
With the property you have defined, you should not need to do any copying like what you have. I'm not sure what ChangeSomething() does, but you should be able to call it directly on the property. I would normally not pull it out as an object... It's been a while since I did pure webforms development, but my ViewState helper properties usually looked more like:
public string CurrentUsername
{
get
{
if (ViewState["Username"] is string)
return (string)ViewState["Username"];
return null;
}
set { ViewState["Username"] = value; }
}
Edit: Thinking about it, I guess the copy is probably there just to remove the potential performance overhead of casting every time you reference the property. I don't think this is a valid optimization in most cases, but if you feel strongly about it, you could hide it with something like this:
private string m_CurrentUsername;
public string CurrentUsername
{
get
{
if (m_CurrentUsername == null && ViewState["Username"] is string)
m_CurrentUsername = (string)ViewState["Username"];
return m_CurrentUsername;
}
set { ViewState["Username"] = m_CurrentUsername = value; }
}
Like I said though - I wouldn't recommend this.
The correct answer is that you shouldn't be modifying viewstate at all. Your controller should create the models and populate the viewstate only just before returning the view. If you're writing this code in the view, you've got an even bigger problem. in general, the view should contain little (if any) code that changes or does things.
Edit: Oops, I think I just got ViewData (in asp.net mvc) confused with viewdata. sorry ... to answer your real question, yes, that is just about the only way :-) it's not really clunky when you are dealing with a "bag" API like the viewstate is.
In Action Script 3, you can write a class that defines a dynamic object (MovieClip and Object are two examples), this objects can be modified in run-time. What I want to know if is there some way (in run-time, of course) to know if certain object is dynamic or not.
PS: Without making something like this:
function isDynamic(object) {
try {
object.newProperty = 'someValue'
} catch (e) {
return false
}
return true
}
CookieOfFortune has the right idea, but unfortunately the code itself has problems, isDynamic is an attribute, and the returned value will be a XMLList with a value of a String that reflects a true or false value, not a child node that directly returns a Boolean. It should look more like this:
function isDynamic(object) : Boolean
{
var type:XML = describeType(object);
return type.#isDynamic.toString() == "true";
}
Be careful!
Anytime you want to use the describeType() function, please please please use the variation:
import mx.utils.DescribeTypeCache;
var typeDesc:XML = DescribeTypeCache.describeType(object).typeDescription;
Performance of making repeated calls to the runtime reflective machinery will absolutely suck. That's why Adobe invented the DescribeTypeCache class.
You can use describeType from flash.utils to describe the object in XML form. Here's the reference to the API: flash.utils.describeType
function isDynamic(object) {
var type:XML = describeType(object);
if (type.#isDynamic == "true") return true;
return false;
}
This is a very old post, but I'll add an option for those future searchers.
AS3 has a built in way of doing this:
mx.utils.ObjectUtil.isDynamicObject(yourObject);
Read more about it here.