endless loop on property sync code

[Richard Bair]

I would have thought so. WIth an InvalidationListener we cannot make this check, because we don't actually know what the new value is unless we ask for the new value -- which would make everything eager. But for a change event we should only actually send a change event notification if it has actually changed. Sounds like a bug.

Richard

On Jan 17, 2012, at 5:35 AM, Tom Eugelink wrote:

> 
> In an attempt to make some code "cheaper" I decided it could be a good idea to redundantly store the index next to the value it pointed to. Since this could save a fairly costly "getIndex" call.
> 
> So I ended up with two properties; one public value and one private index. Basically both denote the same value, so I needed to add sychronisation code:
> 
>        // react to changes of the value
>        this.valueObjectProperty.addListener(new ChangeListener<T>()
>        {
>            @Override
>            public void changed(ObservableValue<? extends T> property, T oldValue, T newValue)
>            {
>                // get the value of the new index
>                Object lIdx = getDataProvider().getIndex(newValue);
>                if (lIdx == null) throw new IllegalArgumentException("Value does not exist " + newValue);
> 
>                // set the value
>                indexObjectProperty.setValue(lIdx);
>            }
>        });
> 
>        // react to changes of the index
>        this.indexObjectProperty.addListener(new ChangeListener<Object>()
>        {
>            @Override
>            public void changed(ObservableValue<? extends Object> property, Object oldIndex, Object newIndex)
>            {
>                // get the value of the new index
>                T lValue = (T)getDataProvider().getValue(newIndex);
> 
>                // set the value
>                valueObjectProperty.setValue(lValue);
>            }
>        });
> 
> 
> This code results in an endless loop:
> Exception in Application start method
> Exception in thread "main" java.lang.RuntimeException: Exception in Application start method
>    at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
>    at com.sun.javafx.application.LauncherImpl.access$000(Unknown Source)
>    at com.sun.javafx.application.LauncherImpl$1.run(Unknown Source)
>    at java.lang.Thread.run(Thread.java:662)
> Caused by: java.lang.StackOverflowError
>    at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(Unknown Source)
>    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.set(Unknown Source)
>    at javafx.beans.property.ObjectProperty.setValue(Unknown Source)
>    at jfxtras.scene.control.SpinnerX$1.changed(SpinnerX.java:147)
>    at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(Unknown Source)
>    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.set(Unknown Source)
>    at javafx.beans.property.ObjectProperty.setValue(Unknown Source)
>    at jfxtras.scene.control.SpinnerX$2.changed(SpinnerX.java:167)
>    at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(Unknown Source)
>    at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(Unknown Source)
>    at javafx.beans.property.ObjectPropertyBase.markInvalid(Unknown Source)
> ...
> 
> 
> To solve this, I have to explicitly check if the value changed before the setValue (see "equals" below). Isn't Property suppose to do this?
> 
>        // react to changes of the value
>        this.valueObjectProperty.addListener(new ChangeListener<T>()
>        {
>            @Override
>            public void changed(ObservableValue<? extends T> property, T oldValue, T newValue)
>            {
>                // get the value of the new index
>                Object lIdx = getDataProvider().getIndex(newValue);
>                if (lIdx == null) throw new IllegalArgumentException("Value does not exist " + newValue);
> 
>                // set the value
>                if (SpinnerX.equals(indexObjectProperty.getValue(), lIdx) == false)
>                {
>                    indexObjectProperty.setValue(lIdx);
>                }
>            }
>        });
> 
>        // react to changes of the index
>        this.indexObjectProperty.addListener(new ChangeListener<Object>()
>        {
>            @Override
>            public void changed(ObservableValue<? extends Object> property, Object oldIndex, Object newIndex)
>            {
>                // get the value of the new index
>                T lValue = (T)getDataProvider().getValue(newIndex);
> 
>                // set the value
>                if (SpinnerX.equals(valueObjectProperty.getValue(), lValue) == false)
>                {
>                    valueObjectProperty.setValue(lValue);
>                }
>            }
>        });
> 
>