23. Managing Constraints using Constraint Sets

Up until this point in the book, all user interface design tasks have been performed using the Android Studio Layout Editor tool, either in text or design mode. An alternative to writing XML resource files or using the Android Studio Layout Editor is to write Java code to directly create, configure and manipulate the view objects that comprise the user interface of an Android activity. Within the context of this chapter, we will explore some of the advantages and disadvantages of writing Java code to create a user interface before describing some of the key concepts such as view properties and the creation and management of layout constraints.

In the next chapter, an example project will be created and used to demonstrate some of the typical steps involved in this approach to Android user interface creation.

23.1 Java Code vs. XML Layout Files

There are a number of key advantages to using XML resource files to design a user interface as opposed to writing Java code. In fact, Google goes to considerable lengths in the Android documentation to extol the virtues of XML resources over Java code. As discussed in the previous chapter, one key advantage to the XML approach includes the ability to use the Android Studio Layout Editor tool, which, itself, generates XML resources. A second advantage is that once an application has been created, changes to user interface screens can be made by simply modifying the XML file, thereby avoiding the necessity to recompile the application. Also, even when hand writing XML layouts, it is possible to get instant feedback on the appearance of the user interface using the preview feature of the Android Studio Layout Editor tool. In order to test the appearance of a Java created user interface the developer will, inevitably, repeatedly cycle through a loop...

23.2 Creating Views

As previously established, the Android SDK includes a toolbox of view classes designed to meet most of the basic user interface design needs. The creation of a view in Java is simply a matter of creating instances of these classes, passing through as an argument a reference to the activity with which that view is to be associated.

The first view (typically a container view to which additional child views can be added) is displayed to the user via a call to the setContentView() method of the activity. Additional views may be added to the root view via calls to the object’s addView() method.

When working with Java code to manipulate views contained in XML layout resource files, it is necessary to obtain the ID of the view. The same rule holds true for views created in Java. As such, it is necessary to assign an ID to any view for which certain types of access will be required in subsequent Java code. This is achieved via a call to the setId() method of...

23.3 View Attributes

Each view class has associated with it a range of attributes. These property settings are set directly on the view instances and generally define how the view object will appear or behave. Examples of attributes are the text that appears on a Button object, or the background color of a ConstraintLayout view. Each view class within the Android SDK has a pre-defined set of methods that allow the user to set and get these property values. The Button class, for example, has a setText() method which can be called from within Java code to set the text displayed on the button to a specific string value. The background color of a ConstraintLayout object, on the other hand, can be set with a call to the object’s setBackgroundColor() method.

23.4 Constraint Sets

While property settings are internal to view objects and dictate how a view appears and behaves, constraint sets are used to control how a view appears relative to its parent view and other sibling views. Every ConstraintLayout instance has associated with it a set of constraints that define how its child views are positioned and constrained.

The key to working with constraint sets in Java code is the ConstraintSet class. This class contains a range of methods that allow tasks such as creating, configuring and applying constraints to a ConstraintLayout instance. In addition, the current constraints for a ConstraintLayout instance may be copied into a ConstraintSet object and used to apply the same constraints to other layouts (with or without modifications).

A ConstraintSet instance is created just like any other Java object:

ConstraintSet set = new ConstraintSet();

Once a constraint set has been created, methods can be called on the instance to perform...

23.5 Summary

As an alternative to writing XML layout resource files or using the Android Studio Layout Editor tool, Android user interfaces may also be dynamically created in Java code.

Creating layouts in Java code consists of creating instances of view classes and setting attributes on those objects to define required appearance and behavior.

How a view is positioned and sized relative to its ConstraintLayout parent view and any sibling views is defined through the use of constraint sets. A constraint set is represented by an instance of the ConstraintSet class which, once created, can be configured using a wide range of method calls to perform tasks such as establishing constraint connections, controlling view sizing behavior and creating chains.

With the basics of the ConstraintSet class covered in this chapter, the next chapter will work through a tutorial that puts these features to practical use.