Monday, January 21, 2013

Hurdles #2 - Apache Pivot - Finding Named Objects

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:
  1. Every component in the XML tree must have a name attribute set (although name is not a required attribute)
  2. The name attribute must be unique among the set of children for a common parent
  3. 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
  4. 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"
    <TablePane name="table">
            <TablePane.Column width="1*"/>

        <TablePane.Row height="1*">
            <Label text="Enter number:"
                styles="{horizontalAlignment:'center', verticalAlignment:'center'}"/>
            <TextInput text="0" name="numberInput" bxml:id="
                    <IntValidator xmlns="org.apache.pivot.wtk.validation"/>

        <TablePane.Row height="-1">
            <PushButton buttonData="Submit"
            <PushButton buttonData="Cancel"
The code required to locate the "numberInput" TextInput may look something like the following:,
   new DialogCloseListener() {
       public void dialogClosed(Dialog arg0, boolean arg1) {
           if(arg0.getResult()) {
               TablePane tp = (TablePane)arg0.getNamedComponent("table");
               TextInput ti = (TextInput)tp.getNamedComponent("numberInput")
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");,
            new DialogCloseListener() {
                public void dialogClosed(Dialog arg0, boolean arg1) {
                    if(arg0.getResult()) {
                        TextInput ti = (TextInput)bxmlSerializer.getNamespace().get("numberInput");

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.