My second Hurdles article is continuing with Apache Pivot, and the topic of this article is finding named objects which means if there is a container or a control somewhere in your window that has a name or bxml:id set, how can I find a reference to that object using the identifying attribute?
My specific example is that I have a Dialog object that contains a TextInput. When the Dialog is closed I want to find that TextInput control, read the Text value that has been provided, and act on it. To do this I have a very simple setup, inside my dialog I have created a TablePane to structure my layout and within the TablePane I have a Label, the TextInput (with an attached validator) and two buttons, a "Submit" button that on a ButtonPressed event does "dialog.close(true)" and a "Cancel" button that does "dialog.close(false)". I have also configured a DialogCloseListener in code that will process the close event, check to see if the Dialog has a result, and perform an action with the TextInput value.
I was eventually able to find two solutions to this problem, the preferred solution would depend on the situation and specific implementation, but I will present both solutions here. There may be additional solutions to this particular problem that I am not aware of, but my goal here was to get an object reference with minimal code and in a generic fashion.
Option 1: Named Component Traversal
Unfortunately in Apache Pivot container tree traversal is not as natural, convenient, or consistent as I expected. It certainly is not as powerful as an XML DOM parser or as a Java File object. Unless you are using position-based object location, the component traversal has a few requirements:
- Every component in the XML tree must have a name attribute set (although name is not a required attribute)
- The name attribute must be unique among the set of children for a common parent
- Each node must be traversed in sequence from parent to child to find the intended descendant, there does not appear to be any kind of path-definition or recursive lookup available
- getNamedComponent returns a Component object which does not have getNamedComponent as a method. This method is in the Container subclass of Component so each traversal step requires at least a Cast operation. Because there does not appear to be any kind of "getAllChildren" method, I do not know if there is any way to do a tree exploration or blind traversal (which would require reflection as well as a Cast operation)
So given the following BXML structure:
<Dialog bxml:id="dialog" title="Dialog" modal="true"
xmlns:bxml="http://pivot.apache.org/bxml"
xmlns="org.apache.pivot.wtk">
<TablePane name="table">
<columns>
<TablePane.Column width="1*"/>
</columns>
<TablePane.Row height="1*">
<Label text="Enter number:"
styles="{horizontalAlignment:'center', verticalAlignment:'center'}"/>
<TextInput text="0" name="numberInput" bxml:id="numberInput">
<validator>
<IntValidator xmlns="org.apache.pivot.wtk.validation"/>
</validator>
</TextInput>
</TablePane.Row>
<TablePane.Row height="-1">
<PushButton buttonData="Submit"
ButtonPressListener.buttonPressed="dialog.close(true)"/>
<PushButton buttonData="Cancel"
ButtonPressListener.buttonPressed="dialog.close(false)"/>
</TablePane.Row>
</TablePane>
</Dialog>
The code required to locate the "numberInput" TextInput may look something like the following:
dialog.open(window,
new DialogCloseListener() {
public void dialogClosed(Dialog arg0, boolean arg1) {
if(arg0.getResult()) {
TablePane tp = (TablePane)arg0.getNamedComponent("table");
TextInput ti = (TextInput)tp.getNamedComponent("numberInput")
System.out.println(ti.getText());
}
}
});
Option 2: BXMLSerializer Lookup
The BXMLSerializer approach is the polar opposite of the traversal approach. This approach also has a uniqueness constraint aspect to it but it is supported by the framework because violation of this constraint will result in a SerializationException being thrown.
The BXMLSerializer requires that your target component has a bxml:id attribute set. All components with a bxml:id attribute get deposited into the Namespace map of the definition file that was processed by the Serializer. However it requires that a reference to the BXMLSerializer instance that was used to parse the BXML file must be kept, and it also must be accessible to the appropriate Handler/Listener that needs to use it.
Taking the example BXML file in Option 1 the following code could be used to access the TextInput control:
private BXMLSerializer bxmlSerializer;
public void startup(Display display, Map<String, String> properties)
throws Exception {
bxmlSerializer = new BXMLSerializer();
Dialog dialog = (Dialog)bxmlSerializer.readObject(Main.class, "bxml/dialog.bxml");
dialog.open(window,
new DialogCloseListener() {
public void dialogClosed(Dialog arg0, boolean arg1) {
if(arg0.getResult()) {
TextInput ti = (TextInput)bxmlSerializer.getNamespace().get("numberInput");
System.out.println(ti.getText());
}
}
});
}
Note that in this sample there is no hierarchy connection between the Dialog itself and the "numberInput" control, however Pivot provides a convenient way to reverse the process as it provides both "getAncestor" and "getParent" methods in the Component class that allow quick traversal up the tree once you have figured out how to get the child.
If you have an alternate method to access an arbitrary component within a window that is an improvement to any of the methods described here, please send me an email. My approaches described above were learned through trial and error because specific documentation on how to do this was lacking online and if there are any better approaches I will post them here as a follow-up.
Here's another
interesting "gotcha" involving Teradata v13.1 and how it handles
metadata for views, this time when using multiple layered views and a custom function. We encountered this error when creating a summary-style view in our data warehouse to push data aggregation into the database layer instead of building the query in Cognos Framework Manager.
The exact Teradata error that we received while attempting to create a new View was:
REPLACE VIEW Failed. 3822: Cannot resolve column 'DATE_ENROLLED'. Specify table or view.
A very generic error message, but what is unique about this particular column in the View is that it is the only one that is being operated on by a function.
The structure of the View query is (very abbreviated) as follows:
REPLACE VIEW view_db.new_view
(DAY_KEY)AS
SELECT
COMMON_DB.CONVERT_TIMESTAMP_TO_KEY(DATE_ENROLLED) AS DAY_KEY
FROM
view_db.person
The query by itself runs without any problems and returns the correct results. But as soon as I put it into a CREATE or REPLACE VIEW statement it failed. Please note that the source of the query (view_db.person) is a View as well that merges records together from a source table (data_db.person) to produce an accurate list of current people.
Now during our investigation we discovered that replacing the source of the query (view_db.person) by referencing the source table of that view (data_db.person) we did not see the error anymore. The column name was unaffected because DATE_ENROLLED is a field in both the View and Table that is unmodified. However this was not a solution because it defeated the purpose of building the view_db.person View in the first place, and shoe-horning the query into a sub-select would be very complex and nearly unmaintainable.
So the structure of our new view and the column source was structured as follows:
view_db.new_view [DAY_KEY]
- COMMON_DB.CONVERT_TIMESTAMP (function)
- view_db.person [DATE_ENROLLED]
- data_db.person [DATE_ENROLLED]
The Solution
We suspect that the Teradata database is encountering problems resolving the source of the DATE_ENROLLED column because of the identical naming between the view_db.person and the data_db.person which is confounded by applying the function call.
We were able to resolve the issue by creating a special sub-select query on the view_db.person that has the sole purpose of renaming the DATE_ENROLLED column.
The resulting fixed REPLACE VIEW statement is as follows:
REPLACE VIEW view_db.new_view
(DAY_KEY)
AS
SELECT
COMMON_DB.CONVERT_TIMESTAMP_TO_KEY(DATE_ENROLLED_VIEW) AS DAY_KEY
FROM
(SELECT DATE_ENROLLED AS DATE_ENROLLED_VIEW FROM view_db.person) a
This allowed the DATE_ENROLLED column to be correctly resolved and the view to be created successfully.
Learning new technologies, frameworks, and languages can be hard. This is especially true when working alone from online documentation when the subject matter is either new, not widely adopted, or the documentation is a work in progress. Minor hurdles can derail the best intentioned learner because the solution is so blindingly obvious to anyone with knowledge of the subject that a solution is never stated.
I have started this post series titled "Hurdles" to track the minor, obvious, but frustrating issues I encounter when learning new things. Perhaps someone, sometime will find one of these posts useful, but if not it will at least be a log of lessons learned.
This article is about Apache Pivot, an open-source Java UI library specifically targeted towards creating rich interface applications for the web or standalone, along with a number of supporting libraries that simplify things like creating REST-ful services. After reading up on Pivot it intrigued me enough to give it a try to evaluate it and perhaps find a use for it on personal projects or at work.
My first impressions, the BXML definition and binding structures have strong flavours of WPF. There are significant differences of course, but the feel of familiarity and the relative ease of putting together a simple application based on the tutorials that had a rich interface layer and used simple web services gave me a good first impression.
The first real hurdle came when I was designing my first input form and wanted to attach a validator to a TextInput control using BXML. The APIs make attaching a validator a trivial exercise in Java code, the TextInput object contains a method called "setValidator" and taking a "org.apache.pivot.wtk.validation.Validator" type. The available validators themselves are simple but varied and selecting an IntValidator for this task was easy to do.
However, adding a new IntValidator instance into a TextInput tag in BXML was not as simple as it seemed. I tried a variety of [validator="obj"] attributes, using dereference, parameter, and variable syntax. All I got for my trouble was a heap of error messages and invalid cast exceptions.
I eventually found my solution in the Apache Pivot - Users nabble forum (for reference, here is the link to the Apache Pivot - Developers forum too) in a topic titled "Hi, ". The conclusion to my problem was to create a child tag for the property that I wanted to set within the TextInput definition, and then create a child tag of that tag with the validator instance definition. This is the standard process for setting Collection property instances, but also applies to single Object property instances as well. Below is the simplest BXML source to illustrate the example.
<TextInput>
<validator>
<IntValidator xmlns="org.apache.pivot.wtk.validation"/>
</validator>
</TextInput>
