I was wondering how to bind values where the source of the bind could be null.
I have a property:
private ObjectProperty<Operation> operation = new SimpleObjectProperty<>(null);
I also have a text field:
#FXML
private Text txtCurrentOperation;
I would like to bind the textProperty of the field to the value of the operation object.
My first thought was to use FluentAPI with its when/then/otherwise construct, but it is eagerly evaluated so the solution:
Bindings.when(operation.isNotNull())
.then("null")
.otherwise(operation.get().getName()));
will throw a NPE, because the parameter of otherwise is evaluated no matter what the result of the when.
My next idea was to use lambda somehow:
txtCurrentOperation.textProperty().bind(() ->
new SimpleStringProperty(
operation.isNotNull().get() ? "Null" : operation.get().getName()
));
But the bind has no lambda enabled solution. (Later I realized that it couldn't have, becasue the real work goes backward: the change of the binded object (operation) will trigger the update of the binder (the field text property).)
Some articles I found suggested to use an "extremal" value for the property instead of null. But Operation is a complex and heavy weight component so it is not trivial to construct an artifical instance to represent null. Even more, this seems to me boilercode, something the binding mechanism is designed to help eliminating.
My next try was to logically swap the binding direction and add listener to the operation property and let it update the field programatically. It works and rather simple as long as the need of update only depends the operation object instances:
operation.addListener((e) -> {
txtCurrentOperation.setText(operation.isNull().get() ?
"Null" : operation.get().getName());
});
operation.set(oper);
It is relatively simple, but doesn't work: it throws "A bound value cannot be set." exception and I don't see why is the text property of the control regarded as bound.
I ran out of ideas. After much searching, I still cannot solve the simple problem to update a text field differently based on whether the source is null or not.
This seems so simple and everyday problem, that I am sure I missed the solution.
If a 3rd party library is an option, check out EasyBind. Try something like this:
EasyBind.select(operation)
.selectObject(Operation::nameProperty)
.orElse("null");
There's also a JavaFX JIRA issue for the type of functionality provided by EasyBind. If you don't want to use a 3rd party library, try Bindings.select:
Bindings.when(operation.isNotNull())
.then("null")
.otherwise(Bindings.select(operation, "name"));
Be aware the null checking in Bindings.select isn't super efficient. There's a JIRA issue for it.
Just in case if somebody using not Java itself but Kotlin.
It is a good idea to use wonderful tornadofx library.
There you can just use operation.select{it.name}. Although, this feature seems not to be documented yet, so it took some time to discover it.
Related
I have some code and when I run it produces an error, saying:
NoSuchMethod: the method 'XYZ' was called on null
What does that mean and how do I fix it?
Why do I get this error?
Example
As a real world comparison, what just happened is this conversation:
Hey, how much gas is left in the tank of the car?
What are you talking about, we don't have a car.
That is exactly what is happening in your program. You wanted to call a function like _car.getGasLevel(); but there is no car, the variable _car is null.
Obviously, in your program it might not be a car. It could be a list or a string or anything else really.
Technical explanation
You are trying to use a variable that is null. Either you have explicitly set it to null, or you just never set it at all, the default value is null.
Like any variable, it can be passed into other functions. The place where you get the error might not be the source. You will have to follow the leads from the actual null value to where it originally came from, to find what the problem is and what the solution might be.
null can have different meanings: variables not set to another value will be null, but sometimes null values are used by programmers intentionally to signal that there is no value. Databases have nullable fields, JSON has missing values. Missing information may indeed be the information itself. The variable bool userWantsPizzaForDinner; for example might be used for true when the user said yes, false when the user declined and it might still be null when the user has not yet picked something. That's not a mistake, it's intentionally used and needs to be handled accordingly.
How do I fix it?
Find it
Use the stack trace that came with the error message to find out exactly which line the error was on. Then set a breakpoint on that line. When the program hits the breakpoint, inspect all the values of the variables. One of them is null, find out which one.
Fix it
Once you know which variable it is, find out how it ended up being null. Where did it come from? Was the value never set in the first place? Was the value another variable? How did that variable got it's value. It's like a line of breadcrumbs you can follow until you arrive at a point where you find that some variable was never set, or maybe you arrive at a point where you find that a variable was intentionally set to null. If it was unintentional, just fix it. Set it to the value you want it to have. If it was intentional, then you need to handle it further down in the program. Maybe you need another if to do something special for this case. If in doubt, you can ask the person that intentionally set it to null what they wanted to achieve.
simply the variable/function you are trying to access from the class does not exist
someClass.xyz();
above will give the error
NoSuchMethod: the method 'xyz' was called on null
because the class someClass does not exist
The following will work fine
// SomeClass created
// SomeClass has a function xyz
class SomeClass {
SomeClass();
void xyz() {
print('xyz');
}
}
void main() {
// create an instance of the class
final someClass = SomeClass();
// access the xyz function
someClass.xyz();
}
I have some code and when I run it produces an error, saying:
NoSuchMethod: the method 'XYZ' was called on null
What does that mean and how do I fix it?
Why do I get this error?
Example
As a real world comparison, what just happened is this conversation:
Hey, how much gas is left in the tank of the car?
What are you talking about, we don't have a car.
That is exactly what is happening in your program. You wanted to call a function like _car.getGasLevel(); but there is no car, the variable _car is null.
Obviously, in your program it might not be a car. It could be a list or a string or anything else really.
Technical explanation
You are trying to use a variable that is null. Either you have explicitly set it to null, or you just never set it at all, the default value is null.
Like any variable, it can be passed into other functions. The place where you get the error might not be the source. You will have to follow the leads from the actual null value to where it originally came from, to find what the problem is and what the solution might be.
null can have different meanings: variables not set to another value will be null, but sometimes null values are used by programmers intentionally to signal that there is no value. Databases have nullable fields, JSON has missing values. Missing information may indeed be the information itself. The variable bool userWantsPizzaForDinner; for example might be used for true when the user said yes, false when the user declined and it might still be null when the user has not yet picked something. That's not a mistake, it's intentionally used and needs to be handled accordingly.
How do I fix it?
Find it
Use the stack trace that came with the error message to find out exactly which line the error was on. Then set a breakpoint on that line. When the program hits the breakpoint, inspect all the values of the variables. One of them is null, find out which one.
Fix it
Once you know which variable it is, find out how it ended up being null. Where did it come from? Was the value never set in the first place? Was the value another variable? How did that variable got it's value. It's like a line of breadcrumbs you can follow until you arrive at a point where you find that some variable was never set, or maybe you arrive at a point where you find that a variable was intentionally set to null. If it was unintentional, just fix it. Set it to the value you want it to have. If it was intentional, then you need to handle it further down in the program. Maybe you need another if to do something special for this case. If in doubt, you can ask the person that intentionally set it to null what they wanted to achieve.
simply the variable/function you are trying to access from the class does not exist
someClass.xyz();
above will give the error
NoSuchMethod: the method 'xyz' was called on null
because the class someClass does not exist
The following will work fine
// SomeClass created
// SomeClass has a function xyz
class SomeClass {
SomeClass();
void xyz() {
print('xyz');
}
}
void main() {
// create an instance of the class
final someClass = SomeClass();
// access the xyz function
someClass.xyz();
}
I'm learning about reactive programming in Meteor:
https://stephenwalther.com/archive/2014/12/05/dont-do-react-understanding-meteor-reactive-programming
I believe that the idea behind Session.equals(key, value) is to remember an association between the reactive variable and the desired value so that updates only propagate to the surrounding code if the equality changes. That way if we have hundreds of views that depend on the variable, only the old and new views get their update code triggered when the value changes.
Note that this would not be the case if we called Session.get(key) === value because every view's code would be called when the variable changes. This is discussed further under the Session.get versus Session.equals() section of the article.
But I found an inconsistency under the Using Reactive Variables section where it says:
Notice that a reactive variable, unlike the Session object, does not have an equals() method. Yes, that is a shame.
So reactive-var is missing equals() but reactive-dict has ReactiveDict.equals().
I can't really see a conceptual reason to exclude ReactiveVar.equals(). Maybe they had no context for storing the association, or maybe there is some scoping or other issue with Javascript that prevents this that I don't fully understand.
So my question is: is this a bug?
Should I just always use reactive-dict? In which case I would change everything from:
let myReactiveVar = new ReactiveVar();
...
if(myReactiveVar.get() === 'myValue')
To the more verbose (but performant):
let myReactiveDict = new ReactiveDict();
...
if(myReactiveDict.equals('myReactiveVar', 'myValue'))
Which would match the functionality provided by Session.equals().
Another option would be to extend the ReactiveVar prototype with my own equals() method or inherit it in a child class and provide a MyReactiveVar.equals() method. Kudos if someone can provide examples to do either of these workarounds that we could submit as a pull request to the Meteor maintainers.
Update: I forgot to mention that ReactiveVar does take an equalsFunc optional parameter in its constructor. It might be possible to hack that as a reactive code block to partially implement equals() functionality without extending the class. Also, here is a related issue on GitHub.
Update: to save time, here is the relevant source code for ReactiveVar and ReactiveDict.equals(). I believe that the value parameter gets converted to serializedValue and is then added as a dependency in ReactiveDict, but I still don't see why it wouldn't be possible to do something similar for ReactiveVar.
The reason there's no equals method for ReactiveVar is because set only invalidates the computations is the new value differs from the current value.
Sets the current value of the ReactiveVar, invalidating the Computations that called get if newValue is different from the old value.
const example = new ReactiveVar(0);
Tracker.autorun(() => {
console.log(example.get());
});
example.set(1); // logs 1
example.set(0); // logs 0
example.set(0); // doesn't log
This is similar behaviour to ReactiveDict's equals method.
Note that set on ReactiveDict does not behave this way. Calling set broadcasts that the value has changed. If you want to prevent the computation from invalidating, that is when you would use equals.
Set a value for a key in the ReactiveDict. Notify any listeners that the value has changed (eg: redraw templates, and rerun any Tracker.autorun computations, that called ReactiveDict.get on this key.)
Sometimes, we need to update one field of entry.value.
the thread safe way to do that is construct a new entry.value and
use put method to update. that is said, I need to make deep copy
of original value even though I just do some little modification.
Can I do this update like
map[key].field = fieldValue;
Hash map returns an entry safely, but assignment of a field is out of scope of the map. So, you should do something here for thread safety. Your code is equivalent to:
Entry entry = map[key];
entry.field = fieldValue;
Obviously the field assignment operator doesn't know anything about the map.
Maybe a lock for the entry itself is needed. If it is just an assignment, then volatile.
i have a Flex tree control and im trying to select a tree node 3 levels down right after the dataProvider is assigned with a collection object like the following.
basically treeItem1, treeItem2, treeItem3 are the nodes in the tree and treeitem3 is a child of treeItem2 which is a child of treeItem1. Assume these treeItem(1,2,3) are referenced correctly from the collection items.
my problem is that if i wait for the whole component to load completely then select the nodes, it open/select/scrolltoIndex correctly. However, if i were to select the node right after the dataProvider is assigned, then it doesn't even open or select (basically the this.treeService.selectedItem is always null).
can anyone point out what i did wrong? is there anything needs to happen after the dataProvider is assigned?
thanks
this.treeService.dataProvider = oPricingHelper.getCurrentPricingSercicesTreeSource();
this.treeService.expandItem(treeItem1, true);
this.treeService.expandItem(treeItem2, true);
this.treeService.selectedItem = treeItem3;
this.treeService.scrollToIndex(this.treeService.selectedIndex);
I have used the updateComplete event to know when a component (such as a DataGroup or List) has completed rendering after performing a simple task (such as updating the dataProvider reference). Of course, you have to be careful and remove listening to updateComplete because it can run a lot, unless you have a need for it to run.
Something like:
//...some function...
this.treeService.addEventListener(FlexEvent.UPDATE_COMPLETE, onTreeUpdateComplete);
this.treeService.dataProvider = oPricingHelper.getCurrentPricingSercicesTreeSource();
//...rest of some function...
private function onTreeUpdateComplete(event:FlexEvent):void {
this.treeService.removeEventListener(FlexEvent.UPDATE_COMPLETE, onTreeUpdateComplete);
this.treeService.expandItem(treeItem1, true);
this.treeService.expandItem(treeItem2, true);
this.treeService.selectedItem = treeItem3;
this.treeService.scrollToIndex(this.treeService.selectedIndex);
}
I'm not positive your experiencing the same issue but I seem to have the same type of problem with using the advanced data grid, it appears in these cases where the dataprovider is acceptable as multiple types, the components do some extra work in the background to wrap things up into something Hierarchical (HierarchicalData or HierarchicalCollectionView) and in doing so the dataprovider setter call is not synchronous (so it will return before actually having assigned the internal property storing the dataprovider). I've used callLater in this case with moderate success, callLater is generally a bad practice but basically adds a function to a list of functions to call once background processing is done, so this is assuming that something in the dataprovider setter called UIComponent.suspendBackgroundProcessing() and that it will subsequently call UIComponent.resumeBackgroundProcessing() and then it will execute the list of functions added by using callLater. Alternatively you could use setTimeout(someFunction,1000).
These are both "hacks" the real solution is to dig into the framework code and see what it's really doing when you tell it to set the dataprovider. Wherever you see that it actually has set the dataprovider you could extend that class and dispatch an event that you could listen for to run the function to do the selections after this point.
If anyone has a better solution please by all means correct me (I would love to have a better answer than this)