Android Custom View: PlayStation Knob - Part 1: Getting Started

This is part 1 of the series "Android Custom View: PlayStation Knob", in which I will be creating an Android Custom View that acts just like the PlayStation Analogue Knob:


In this part, you will be able to:

  1. Create the Custom View
  2. Create Custom Attributes
  3. Use your View in an Activity
  4. Capture Touch Events to move the knob

Note: The code demonstrated here can be directly fetched from the git repository WidgyWidgets. You need to clone the repository and checkout to tags/knobview-part1 (Instruction are at the end of this post)

To start with this, you want to decide on whether to extend the base View class or to extend one of the widgets available in android.
In our case, I want to create a view that somehow shows in its center a circle that can be dragged by the user to any of the edges. One way to go would be to extend one of the available ViewGroups, add the knob in its center, and implement this dragging functionality. Another way would be to start the view from scratch by extending View.
To make things more interesting, I will, in this post, extend View. By the way, I will call it KnobView.

As a general practice I usually create a class that extends View and directly create a function called init and override the 3 constructors of View calling init() in all of them:

public class KnobView extends View {
 
 public KnobView(Context context) {
  super(context);
  init(context, null);
 }

 public KnobView(Context context, AttributeSet attrs) {
  super(context, attrs);
  init(context, attrs);
 }

 public KnobView(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  init(context, attrs);
 }

 private void init(Context context, AttributeSet attrs) {
  // TODO Auto-generated method stub
 }
}

If you do not know, the AttributeSet parameter in the constructor contains the attributes specified in the xml. To demonstrate, I will create an attribute called knob that will reference a drawable used for the knob in the center. To do so, we need to declare this attribute in the res folder. I will create a file called attr_knobview.xml in the res/values folder: this file will include the attributes of our KnobView. At this stage, we will only declare the attribute called knob of type integer since it is a reference to a drawable:
 
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="KnobView">
        <attr name="knob" format="integer" />
    </declare-styleable>
</resources>

Now in my init function I will get this drawable. (If you are working step by step, build your project after each change - e.g. NOW). To get the drawable:

private void init(Context context, AttributeSet attrs) {
 // TODO Auto-generated method stub
 TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.KnobView);
 int drawable = a.getResourceId(R.styleable.KnobView_knob, 0);
 if(drawable != 0) {
  mKnobDrawable = a.getDrawable(R.styleable.KnobView_knob);
 }
}

In this code, I try to get the integer which would be specified by the attribute knob (KnobView_knob). If this integer is not 0, I get the drawable and assign it to the field mKnobDrawable. You can add the field mKnobDrawable at the top of the class using this line: private Drawable mKnobDrawable;

What if there is no knob attribute specified in xml? I will simply create my own round black circle. This can be done in the else statement by creating a ShapeDrawable and assigning it to mKnobDrawable. Now my init function will assign a drawable to mKnobDrawable whether knob was specified or not.

private void init(Context context, AttributeSet attrs) {
 // TODO Auto-generated method stub
 TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.KnobView);
 int drawable = a.getResourceId(R.styleable.KnobView_knob, 0);
 if(drawable != 0) {
  mKnobDrawable = a.getDrawable(R.styleable.KnobView_knob);
 }
 else {
  mKnobDrawable = new ShapeDrawable(new OvalShape());
  ShapeDrawable s = (ShapeDrawable) mKnobDrawable;
         s.getPaint().setColor(Color.BLACK);
 }
}

Now we want to take care of the size of this knob. For now, I will center it in the view and let it take half the width and half the height. I will update the Bounds of this mKnobDrawable each time the size of our KnobView changes. This can be captured in onSizeChanged:

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
 super.onSizeChanged(w, h, oldw, oldh);
 mKnobDrawable.setBounds(w/2-w/4, h/2-h/4, w/2+w/4, h/2+h/4);
}

Notice that I am always updating the bounds of the knob drawable when the size changed. These bounds determine the actual rectangle in which my drawable will be drawn. I set the bounds to be the rectangle that is exactly half the size of the view and exactly located in the center of our view. It is pretty straight-forward.

Finally, I want to draw this Knob. This is simply calling the Drawable's draw function on our canvas in the onDraw function of the KnobView:

@Override
protected void onDraw(Canvas canvas) {
 super.onDraw(canvas);
 mKnobDrawable.draw(canvas);
}

For testing, I created an activity with the following xml layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res/mobi.sherif.widgywidgetstest"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
    <mobi.sherif.widgywidgets.KnobView
        android:id="@+id/knob1"
        android:background="#f00"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="1" />
    <mobi.sherif.widgywidgets.KnobView
        android:id="@+id/knob2"
        android:background="#0f0"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="1"
        app:knob="@drawable/ic_launcher" />
</LinearLayout>

Notice:

  1. If you are doing your own project, you probably want to specify your package name instead of mobi.sherif.widgywidgetstest (second line)
  2. Due to layout_height="0dp" and layout_weight="1", each KnobView will take half the screen.
  3. The first KnobView does not specify the knob attribute while the second does.
  4. Each of the KnobView has a differnt background (red and blue).
Anyway, if you run this activity, you will get the output that is shown in the previous image.

With some modifications, int the drawables used for the background and the knob, I was able to get the following KnobView:

I modified the first KnobView in the layout (notice the background and the knob values)

    <mobi.sherif.widgywidgets.KnobView
        android:id="@+id/knob1"
        android:background="@drawable/bg_knobview"
        app:knob="@drawable/bg_knob"
        android:layout_width="100dip"
        android:layout_height="100dip"
        android:layout_marginTop="100dip"
        android:layout_marginBottom="100dip" />

I also created res/bg_knobview
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="oval" >
    <solid android:color="#aaaaaa" />
</shape>

andres/bg_knob
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="oval" >
    <solid android:color="#333333" />
</shape>

Now it is time to move our knob. It is a very simple thing to: We will capture the touches on our view using the function onTouchEvent and when an ACTION_DOWN or ACTION_MOVE is detected, we move the knob to the location of the event. How do we do so? It's pretty easy: we use the setBounds function that we used in the onSizeChanged.

@Override
public boolean onTouchEvent(MotionEvent event) {
 final int action = MotionEventCompat.getActionMasked(event);
 if(action == MotionEvent.ACTION_DOWN || action == MotionEvent.ACTION_MOVE) {
  int w = getWidth();
  int h = getHeight();
  int x = (int) event.getX();
  int y = (int) event.getY();
  mKnobDrawable.setBounds(x-w/4, y-h/4, x+w/4, y+h/4);
  invalidate();
 }
 return true;
}
Notice that we only did two things if the action is ACTION_DOWN or ACTION_MOVE:

  1. Set the bounds of our knob drawable based on the location of the event: We kept its width = w/2 and its height h/2 but we translated it to (x,y), the location of the event
  2. Invalidated the view using invalidate() to force our view to redraw -i.e. to move the knob.
The last natural thing to do is move the knob back when the user stops his touches. That is almost the same thing but with x and y set to the midpoint of the view -i.e. (w/2, h/2). Therefore our final onTouchEvent will look something like:

@Override
public boolean onTouchEvent(MotionEvent event) {
 final int action = MotionEventCompat.getActionMasked(event);
 if(action == MotionEvent.ACTION_DOWN || action == MotionEvent.ACTION_MOVE) {
  int w = getWidth();
  int h = getHeight();
  int x = (int) event.getX();
  int y = (int) event.getY();
  mKnobDrawable.setBounds(x-w/4, y-h/4, x+w/4, y+h/4);
  invalidate();
 }
 else if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
  int w = getWidth();
  int h = getHeight();
  int x = w/2;
  int y = h/2;
  mKnobDrawable.setBounds(x-w/4, y-h/4, x+w/4, y+h/4);
  invalidate();
    }
 return true;
}

Notice that the only difference is that x and y are not set to w/2 and h/2 respectively.

At the end of this part, your knob should be able to move when touched and return to its original place when released. Clone the WidgyWidget repository to try it yourself.

Have fun (:

Note: If you want to get the code of this part only, clone and checkout tags/knobview-part1 using the following commands (If you ommit folder_name, it will automatically be cloned into folder WidgyWidgets) :

git clone https://github.com/sherifelkhatib/WidgyWidgets.git folder_name
cd folder_name
git checkout tags/knobview-part1
... Try it and when you are done ...
git checkout master

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