GEOG 863:
Web Application Development for Geospatial Professionals

7.2.2 Dropdown List

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As you saw in the w3schools tutorial, a dropdown list is created in HTML using a select element. This Esri sample shows a simple usage of a select element to provide the user a list of floors in a building. The user selecting one of the floor options causes only the features from that floor to be displayed.

First, have a look at the HTML. As with the earlier samples, a div is created to hold the UI element (here given an id of "optionsDiv"). Within the div is the select element and its child option elements. Each option has a value attribute (which can be accessed using JS) and the text that the user sees (the text between the start and end tags). In many cases, those two strings are the same. Here, the value attribute is assigned an expression in which the floor number is just part of a larger string. We’ll come back to that expression in a moment.

As we saw in the previous sample, the addEventListener() method is used here to set up a handler for an event associated with a form element.  In the previous sample, an anonymous callback function was embedded directly within the addEventListener() statement.  In this case, the name of a function defined elsewhere (showFloors) is specified instead.  This function will be executed whenever the floorSelect element has its change event triggered. 

The showFloors() function is defined on lines 136-151. The same expression we saw earlier (event.target) is used to get a reference to the element the listener is attached to. Unlike the checkbox sample, where the checked property was read, here the value property is read to obtain the select element’s value (e.g., "FLOOR = '1'", "FLOOR = '2'", etc.).

The logic behind the display of the selected floor’s features is pretty clever. A forEach() loop is used to iterate through each of the layers in the scene. The entire "Building Wireframe" layer is meant to always be visible, so line 148 basically says to ignore that layer. For all other layers, the definitionExpression property (discussed in Lesson 6) is modified to show just the features from the selected floor.

One wrinkle in setting the definitionExpression is that the building identifying field is not the same in all the layers. Part of the solution to this problem is the buildingQuery variable defined on lines 64-69. This object variable is defined having the layer names as the keys and the corresponding expressions needed to select building Q as the values. The definitionExpression has two parts: the first, built by retrieving the appropriate building Q selection expression from the buildingQuery variable (using layer.title as the key); the second, built using the value of the selected option in the dropdown list.

An interesting point to note is the way that the "All" option is handled. It’s assigned a value of "1=1", which may seem strange at first glance. However, it makes sense when you stop to think about it. Let’s say that the loop is processing the Walls layer. That layer will have its definitionExpression set to "BUILDINGKEY = 'Q' AND 1=1". In deciding whether a feature should be included in the layer, each side of the expression will be evaluated as either true or false. The AND operator indicates that both sides of the expression must be true. The expression 1=1 is always true, which gives the desired result of all features being displayed, regardless of their FLOOR value.