Creare un disegno personalizzato

@ColorInt
private var textColor    // Obtained from style attributes.

@Dimension
private var textHeight   // Obtained from style attributes.

private val textPaint = Paint(ANTI_ALIAS_FLAG).apply {
    color = textColor
    if (textHeight == 0f) {
        textHeight = textSize
    } else {
        textSize = textHeight
    }
}

private val piePaint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
    style = Paint.Style.FILL
    textSize = textHeight
}

private val shadowPaint = Paint(0).apply {
    color = 0x101010
    maskFilter = BlurMaskFilter(8f, BlurMaskFilter.Blur.NORMAL)
}

private Paint textPaint;
private Paint piePaint;
private Paint shadowPaint;

@ColorInt
private int textColor;       // Obtained from style attributes.

@Dimension
private float textHeight;    // Obtained from style attributes.

private void init() {
   textPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
   textPaint.setColor(textColor);
   if (textHeight == 0) {
       textHeight = textPaint.getTextSize();
   } else {
       textPaint.setTextSize(textHeight);
   }

   piePaint = new Paint(Paint.ANTI_ALIAS_FLAG);
   piePaint.setStyle(Paint.Style.FILL);
   piePaint.setTextSize(textHeight);

   shadowPaint = new Paint(0);
   shadowPaint.setColor(0xff101010);
   shadowPaint.setMaskFilter(new BlurMaskFilter(8, BlurMaskFilter.Blur.NORMAL));
   ...
}

private val showText    // Obtained from styled attributes.
private val textWidth   // Obtained from styled attributes.

override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
    super.onSizeChanged(w, h, oldw, oldh)
    // Account for padding.
    var xpad = (paddingLeft + paddingRight).toFloat()
    val ypad = (paddingTop + paddingBottom).toFloat()

    // Account for the label.
    if (showText) xpad += textWidth.toFloat()
    val ww = w.toFloat() - xpad
    val hh = h.toFloat() - ypad

    // Figure out how big you can make the pie.
    val diameter = Math.min(ww, hh)
}

private Boolean showText;    // Obtained from styled attributes.
private int textWidth;       // Obtained from styled attributes.

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);
    // Account for padding.
    float xpad = (float)(getPaddingLeft() + getPaddingRight());
    float ypad = (float)(getPaddingTop() + getPaddingBottom());

    // Account for the label.
    if (showText) xpad += textWidth;

    float ww = (float)w - xpad;
    float hh = (float)h - ypad;

    // Figure out how big you can make the pie.
    float diameter = Math.min(ww, hh);
}

override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
    // Try for a width based on your minimum.
    val minw: Int = paddingLeft + paddingRight + suggestedMinimumWidth
    val w: Int = View.resolveSizeAndState(minw, widthMeasureSpec, 1)

    // Whatever the width is, ask for a height that lets the pie get as big as
    // it can.
    val minh: Int = View.MeasureSpec.getSize(w) - textWidth.toInt() + paddingBottom + paddingTop
    val h: Int = View.resolveSizeAndState(minh, heightMeasureSpec, 0)

    setMeasuredDimension(w, h)
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
   // Try for a width based on your minimum.
   int minw = getPaddingLeft() + getPaddingRight() + getSuggestedMinimumWidth();
   int w = resolveSizeAndState(minw, widthMeasureSpec, 1);

   // Whatever the width is, ask for a height that lets the pie get as big as it
   // can.
   int minh = MeasureSpec.getSize(w) - (int)textWidth + getPaddingBottom() + getPaddingTop();
   int h = resolveSizeAndState(minh, heightMeasureSpec, 0);

   setMeasuredDimension(w, h);
}

private val data = mutableListOf<Item>() // A list of items that are displayed.

private var shadowBounds = RectF()       // Calculated in onSizeChanged.
private var pointerRadius: Float = 2f    // Obtained from styled attributes.
private var pointerX: Float = 0f         // Calculated in onSizeChanged.
private var pointerY: Float = 0f         // Calculated in onSizeChanged.
private var textX: Float = 0f            // Calculated in onSizeChanged.
private var textY: Float = 0f            // Calculated in onSizeChanged.
private var bounds = RectF()             // Calculated in onSizeChanged.
private var currentItem: Int = 0         // The index of the currently selected item.

override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)

    canvas.apply {
        // Draw the shadow.
        drawOval(shadowBounds, shadowPaint)

        // Draw the label text.
        drawText(data[currentItem].label, textX, textY, textPaint)

        // Draw the pie slices.
        data.forEach {item ->
            piePaint.shader = item.shader
            drawArc(
                bounds,
                360 - item.endAngle,
                item.endAngle - item.startAngle,
                true,
                piePaint
            )
        }

        // Draw the pointer.
        drawLine(textX, pointerY, pointerX, pointerY, textPaint)
        drawCircle(pointerX, pointerY, pointerRadius, textPaint)
    }
}

// Maintains the state for a data item.
private data class Item(
    var label: String,      
    var value: Float = 0f,

    @ColorInt
    var color: Int = 0,

    // Computed values.
    var startAngle: Float = 0f,
    var endAngle: Float = 0f,

    var shader: Shader
)

private List<Item> data = new ArrayList<Item>();  // A list of items that are displayed.

private RectF shadowBounds;                       // Calculated in onSizeChanged.
private float pointerRadius;                      // Obtained from styled attributes.
private float pointerX;                           // Calculated in onSizeChanged.
private float pointerY;                           // Calculated in onSizeChanged.
private float textX;                              // Calculated in onSizeChanged.
private float textY;                              // Calculated in onSizeChanged.
private RectF bounds;                             // Calculated in onSizeChanged.
private int currentItem = 0;                      // The index of the currently selected item.

protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    // Draw the shadow.
    canvas.drawOval(
            shadowBounds,
            shadowPaint
    );

    // Draw the label text.
    canvas.drawText(data.get(currentItem).label, textX, textY, textPaint);

    // Draw the pie slices.
    for (int i = 0; i < data.size(); ++i) {
        Item it = data.get(i);
        piePaint.setShader(it.shader);
        canvas.drawArc(
                bounds,
                360 - it.endAngle,
                it.endAngle - it.startAngle,
                true, 
                piePaint
        );
    }

    // Draw the pointer.
    canvas.drawLine(textX, pointerY, pointerX, pointerY, textPaint);
    canvas.drawCircle(pointerX, pointerY, pointerRadius, textPaint);
}

// Maintains the state for a data item.
private class Item {
    public String label;
    public float value;
    @ColorInt
    public int color;

    // Computed values.
    public int startAngle;
    public int endAngle;

    public Shader shader;
}