A Kotlin-based Introduction to Compound Components on Android — Part 1
Creating awesome custom views on Android by leveraging the power of Compound Components.
Series Roadmap
Android provides us with a huge set of views that already cater to our basic needs. Whether it’s a TextView to display the status of a launched transaction, or an ImageView to showcase a retrieved image from a URL, all of these views are amazing, and we can always change few attributes handed to us by Android to customize the already existing views to our needs.
But that is not always the case.
At the same time, designing an application that’s unique across android’s “design” scheme, and does not necessarily depend on existing android views to mock its look and feel is a skill that is very great to have up our sleeves. In this light, we are presented with Custom Views.
Custom Views are views that subclass existing View or ViewGroup implementations to provide absolute control over the appearance and functions of the element.
Custom Views in android can be created in two ways:
- Extending a View: This process entails creating a subclass of the View class or any of its subclasses, giving us implementation data present in a basic android view to work with, after which overriding can then follow.
- Extending a ViewGroup: Views that accommodate other views as children are referred to as ViewGroups — Layouts. Examples are LinearLayout, ConstraintLayout, RelativeLayout and a lot more. These ViewGroups can also be extended by grouping various views with different functions into a single ViewGroup, and then referencing the ViewGroup and its sub-elements with a single view name — Compound Components.
Sounds a bit obscure? don’t worry, hang in there.
This article discusses Compound Components, and how they can be used in isolating a certain number of elements, whose functionality is closely related, and would be needed more than one time across the whole application. Most times, you might not need to extend a basic view and provide custom implementations to it, the design you’re looking for might be attained by joining a set of views already provided by the Android Framework, and Voila!, your compound component is ready!
Now that you understand what Compound Components are, let us jump right in!
P.S: If the whole thing still feels blurry, you can always check out what the smart folks at Google had to say about the whole topic here. Cheers!
What would we be building?
At the end of this three-part tutorial on compound components, we should come up with this application right here! 👇🏾
The view to be built is what I refer to as a File Descriptor (not so fancy?, I know). It is a compound component that provides the functionality of displaying information about selected files present on the Android File System, and these include a preview of the file itself, its name, the date it was last modified, size of the file, and a bit more!
Excited? let’s dive right in!
To be able to follow this article closely, I expect you to at least be familiar with Android development and the Android SDK. Also, some Kotlin-specific features would be used in this tutorial, but that should not be a problem.
The starter code for this project can be found here. It basically is a new android project with an empty activity, it also contains a set of vector drawables and images that would be used across the entire project.
Whew!, this is going to be a long ride! you might want to grab a cup of coffee☕
Creating Our Compound Component
To create our compound component, we would need to put two important pieces together — layout and logic. The layout contains the views we would be joining together to create our compound component, while the logic would contain specific functions the views in the compound component could execute.
The Layout
Remember, a compound component has to extend an existing ViewGroup implementation, so in this article, we would be extending the ConstraintLayout. ConstraintLayout is an amazing ViewGroup provided to us by Android that lays out its items by attaching constraints on one View to another in the ViewGroup or the parent itself. This helps hugely in eliminating nested view hierarchies.
To create the layout, navigate to res > layout, and create a new file labelled file_descriptor.xml, below is the code for the layout.
In the code above, there are five main views, the first ImageView whose purpose is to show a preview of the file (image/video) selected, the small image with id: file_type_image aligned to the mid-left of the component denotes the type of file selected. There are also two text views that display the name and information about the file selected respectively. Then, we have two icons whose functions are to share and toggle the information state.
Before we move any further, if you downloaded the starter code for this article, you’d have in the drawables folder, a set of icons that would be used in the application, so you have nothing to worry about!.
Our layout is almost ready!, but there is a teeny-tiny issue, remember when I said a compound component is actually a ViewGroup accommodating views with closely-related functions, yes! exactly!. The idea here is to create an independent layout and then connect the views to an implementation of a certain ViewGroup — ConstraintLayout. But the issue here is, the layout created above has a ConstraintLayout as its parent, so doesn’t that mean that we are done? not quite😤!.
That is not the behaviour we want, if we go on with that layout, we would end up adding the parent of that layout — ConstraintLayout as a child of our Compound Component which happens to be a ConstraintLayout also, so our Compound Component would end up having one and only one direct child which would be a ViewGroup. This does not quite give us direct access to our inner views, so how do we go about this?
The Merge Tag
Android provides us with the merge tag. The merge tag takes a group of views and merges them together. So, instead of ordering them under a ViewGroup, they are independent and without a parent. Following this method, our views would be added directly as children of our Compound Component.
So, our file_descriptor.xml file can be modified as such:
At this point, your layout is probably looking disorganized, don’t worry, we’d be done in a sec.
The Logic
Now that the layout is all set, what’s left is to connect it to a ViewGroup implementation. Go ahead and create a file called FileDescriptor.kt, this class would contain our logic for implementing our compound component.
As a reminder, we would be extending the existing implementation of the ConstraintLayout class, so now would be a good time to see how the class looks like.
Let’s start by comprehending the anatomy of our ConstraintLayout class, it has three constructors.
- ConstraintLayout(Context context): this constructor takes in one parameter — the context of the activity inflating the layout
- ConstraintLayout(Context context, AttributeSet attrs): this constructor accepts two parameters — the context (see above), and the set of attributes defined for this layout
- ConstraintLayout(Context context, AttributeSet attrs, int defStyleAttr): this constructor takes in one additional parameter defStyleAttr, this denotes the default style to be used for this layout, usually defaults to 0.
Extending the ConstraintLayout class demands we provide adequate calls to the constructors of the superclass, so we don’t get caught “off-guard”. So, we have to provide constructors in our Compound Component that effectively invokes all three of the constructors present in the superclass. So, let us start by doing that! Below is the code for our FileDescriptor class.
In the code above, we declare explicitly, three constructors matching the ones present in the superclass. Amazing!.
But if you are a fan of Kotlin (like me), this approach isn’t really the best. Kotlin provides optional and default function parameters that allow us to specify parameters that might be included or omitted in the function invocation and default values for those that weren’t included. One more thing is: the class being extended is written in Java, and we are about to use some Kotlin-specific features in here, so there would be issues as Java does not know about default or optional parameters.
So, here comes the @JvmOverloads annotation. This annotation carefully converts the default and optional parameters used in the function declaration to independent functions with separate parameters when transpiled to Java. Here is how the class looks after using default and optional parameters.
By adding the @JvmOverloads annotation, we must provide a secondary constructor that takes in all the arguments supposed to be passed into the ConstraintLayout constructors. As you can see above, only the Context object ctx is compulsory, since all constructors of the ConstraintLayout class accept it. The second parameter attributeSet is optional since only two of the constructors accept it, and it defaults to null if none is passed in originally. The final parameter defStyleAttr also defaults to 0 if none is provided. The annotation takes care of creating the constructors in the Java equivalent of this class.
At this point, we are one step away from showing our layout on the screen. We are going to take advantage of the init block given to us by Kotlin, it serves as a block of code that runs whenever any of the constructors is used to create an object of the class.
So, what exactly do we want to do in the init block?
Inflate the layout
Yeah!, you heard that right. Inflating layouts is a very common concept in android development, as it processes the tags created in our XML layouts and converts them to their equivalent objects in code — Inflation. This concept is widely used when creating fragments or instantiating viewholders for a recyclerview.
This process entails getting the views declared in the XML for this component and then inflating them as children of this compound component.
To inflate the layout for the compound component into this class, we go through two steps.
- Get the layout inflater service — the object that does the inflating
- Inflate!
The first step is to get the inflater service from the android system by invoking getSystemService() on the context object and casting it to a type of LayoutInflater. We then inflate the layout into our compound component and we’re done!
Finally, we set the background of our component by calling setBackground and passing in our drawable resource. And that’s actually it!.
Showing on-screen
After all the setup, we can now call our custom view using its fully-qualified name in our XML layout by inserting the code below.
Android studio might not know how to render the view straightaway in your layout preview, so you might want to rebuild the project, so it records the changes made to the component.
And you should come up with this!
What’s next?
We are not done with the application yet, though we have learnt a lot, we still have to wire up the functions of the view, and we would cover that in the next part right here!
See you on the other side!😃
