Today I’m going to introduce several fundamental concepts of Android custom view. I will create an Android custom view to implement circular
SeekBar like the mockup below, make it as a widget library, open source at Github, and publish it to JCenter so that other developers can download and use it.
In this series of posts, I will cover two major topics:
- Fundamental concepts about Android custom view. (Part 1)
- The steps to publish this custom view as a library. (Part 2)
(It would be nice to give my project a star, thank you.)
Before you start…
Try to understand the requirement or design specifications, take a look at this widget first. Here is some requirements:
- The user can touch the indicator icon through this arc to set the current progress level.
- The center text display the current progress in integer.
- The indicator icon will stop dragging when exceeding max or min progress.
It works like an extension of circular
SeekBar with customized appearances and motion event.
All the android view widgets are based on
View, to implement custom view, you will start a subclass of
View and override several lifecycle callback methods of view, so you need to understand the view lifecycle at first. The following diagram shows some important methods to override:
To get started, the first thing we have to do is to create a class that extends
View, and provide two default constructors which allows us to create the view programmatically (1st constructor) or in XML layout (2nd constructor). The
SwagPoint(Context context, AttributeSet attrs) constructor is more important here because it is used when Android inflates the view from XML layout file, otherwise you will get exception.
Then, there are several things we have to control or modify in our custom view:
- Attributes: What things are customizable in your view? Determine custom attributes that allow developer to change its appearance and behavior in XML layout file according to their design.
- Size: Determine the dimensions of the view and every components in this custom view on the screen.
- Drawing: Determine how the view and components to render on the screen which contains the shape, location, appearance.
- Touch: Determine the way the user can interact with the view by touching.
Here we provide several customizable attributes for developer: the initial progress
points, the range of progress
max/min, the interval when user change the progress
step, the color and size of progress/arc/text. To define custom attributes, we create
res/values/attrs.xml file and define custom attributes for your view in a
<declare-styleable> resource element as below.
res/values/attrs.xml file, we use
TypedArray to retrieve attribute value in Java class and define instance variables (the following variables with
m as prefix name) to store. Here we add a init() method to put inside the constructor:
In order to control the view dimension, we have to override
View.onMeasure()method and calculate the size of each components. Here we have to define the arc radius according to the width/height of our view.
Give you a pen and a paper, you can draw what you want. To draw the view, you have to override the
onDraw(Canvas canvas) method.
Before that, we have to know what to draw and how to draw. Android provides two classes to do this job:
- What to draw, handled by
Canvaswhich is provided as parameter in
- How to draw, handled by
Before you use
Canvas to draw anything, it’s necessary to create
Paintobjects. For performance optimization, creating the
Paint objects ahead is quite important, because the
onDraw() method is called when redrawn is needed at anytime, we won’t create the
Paint objects inside the
onDraw()method, we should avoid any allocation in
onDraw() method. Here we define these objects (one for arc, another for progress and the other for text) as instance variables and initialize at
Once we have
Paint objects defined, we can start to implement the
onDraw(Canvas canvas) method, here we’re going to draw the text to display current progress number, the arc and current progress:
onDraw() method, there is one more important method about drawing to introduce:
invalidate(), this method is used when any redrawing is needed, we won’t call
onDraw() directly, we just call this method instead, you can use this method anywhere inside your custom view, however, for performance optimization, make sure it’s called as infrequently as possible.
When the user touch the screen, Android calls the
onTouchEvent() method, so we override
View.onTouchEvent() to handle the user input gestures:
There are several things we have to control when user touches the indicator or other place of view:
- Update the indicator icon position and progress text.
- Draw the current progress on the arc.
- Stop when reaching max or min.
To know the indicator icon position and the current progress to draw, we have to convert the touch coordinate on the screen to the angle of arc.
In our custom view, we consider the center of arc as origin
(0, 0), and use trigonometric functions to transform the touch coordinate into the angle of the arc in degree
(0, 360), and map to the current progress value of given range
Here we add a method to convert from touch coordinate to the arc angle:
It would be nice to star my project, thank you!!