Re: why read ony ComponentEventException

[Bob Harner <bo...@gmail.com>]

This is a frequently-occurring user mistake, especially by developers
new to tapestry. And sometimes the error message is even more
misleading:

    org.apache.tapestry5.ioc.util.UnknownValueException: "Could not
find a coercion from type java.lang.String to type com.example.Foo"

I wonder if there's something that can be done to help, like making
the exception message more verbose about the common causes, or maybe
emitting a warning whenever ${...} is used in a component parameter in
a tml file and that parameter has a default binding prefix of "prop:"
or "var:"

On Sun, Sep 18, 2011 at 5:44 PM, Howard Lewis Ship <hl...@gmail.com> wrote:
> ${...} doesn't do what you think it does.  It specifically extracts
> the expression and converts it to a String, and the expression is
> read-only.
>
> Bind it like this:
>
>  itemsPerPage="itemsPerPage"
>
> this binds the component parameter field directly to the containing
> page's property, so it's read/write.
>
> The use of ${...} was intended for attributes  ordinary
> (non-component) elements.
>
> On Sun, Sep 18, 2011 at 1:39 PM, Ken in Nashua <kc...@live.com> wrote:
>>
>> Folks,
>>
>> I have a page feeding read/write property
>>
>> Home.JAVA
>>    @Property(read = true, write = true)                                // no getter/setter needed
>>    @Persist("session")
>>    private int itemsPerPage;
>>
>> to a component parameter
>>
>> Gallery.JAVA
>>    @Property(read = true, write = true)
>>    @Parameter(required = true, cache = true, defaultPrefix = "50")
>>    private int itemsPerPage;
>>
>>    @BeginRender
>>
>>    public void beginRender()
>>
>>    {
>>
>>        //itemsPerPage = 50;
>>
>>        //tableColumns = 3;
>>
>>    }
>>
>> org.apache.tapestry5.runtime.ComponentEventExceptionFailure
>>  writing parameter 'value' of component
>> Home:gallerywidget.itemsperpageselect: Failure writing parameter
>> 'itemsPerPage' of component Home:gallerywidget: Binding
>> org.apache.tapestry5.internal.services.AttributeExpansionBinding@3e963f38
>>  is read-only.
>>
>> here are some sources... to possbly indicate the issue but I cannot spot why.
>>
>> Any ideas ? I would think that the annotations provided are sufficient? Also if I try to set a value to this parameter within the component I receive the same thing...
>>
>>
>> HOME.TML
>> ...
>>    <t:form t:id="galleryForm" t:name="galleryForm" clientValidation="true">
>>        <t:errors/>
>>
>>        <t:Gallery t:id="GalleryWidget"
>>            collection="${collection}"
>>            itemsPerPage="${itemsPerPage}"
>>            tableColumns="${tableColumns}"
>>            cursor="${cursor}"
>>            />
>>    </t:form>
>>
>> </t:layout>
>>
>>
>>
>
>
>
> --
> Howard M. Lewis Ship
>
> Creator of Apache Tapestry
>
> The source for Tapestry training, mentoring and support. Contact me to
> learn how I can get you up and productive in Tapestry fast!
>
> (971) 678-5210
> http://howardlewisship.com
>
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