Android: Looking for a `drawArc()` Method with Inner & Outer Radius
The Need for Inner and Outer Radius
Android’s `Canvas.drawArc()` method allows you to draw an arc segment of a circle. However, it lacks the functionality to specify an inner radius, limiting it to drawing solid pie slices. Many applications require the ability to draw hollow arcs, like donut charts or circular progress bars.
Exploring Alternatives
While `drawArc()` alone doesn’t offer inner radius support, several workarounds can achieve the desired result:
* **Multiple `drawArc()` calls:** One approach involves drawing two arcs, one for the outer boundary and one for the inner boundary, then filling the space between them with a color. This requires precise positioning and alignment for both arcs.
* **Custom Path:** Building a custom `Path` using `moveTo()`, `lineTo()`, and `arcTo()` can provide more control and flexibility, allowing you to define a hollow arc with an inner and outer radius.
* **Specialized libraries:** Libraries like MPAndroidChart or WilliamChart offer specialized drawing capabilities, including drawing arcs with inner and outer radii.
Comparing Approaches
| Approach | Pros | Cons |
|—|—|—|
| Multiple `drawArc()` calls | Relatively simple to implement. | Requires precise positioning and alignment. Can be inefficient for complex shapes. |
| Custom Path | Flexible and precise control over the shape. | More code complexity. |
| Specialized libraries | Simplified API for advanced drawing. | Increased project dependencies. |
Example using Custom Path
“`java
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Path;
public class HollowArc {
public void drawHollowArc(Canvas canvas, float centerX, float centerY, float outerRadius, float innerRadius, float startAngle, float sweepAngle, Paint paint) {
Path path = new Path();
path.moveTo(centerX + outerRadius * (float) Math.cos(Math.toRadians(startAngle)), centerY + outerRadius * (float) Math.sin(Math.toRadians(startAngle)));
path.arcTo(centerX – outerRadius, centerY – outerRadius, centerX + outerRadius, centerY + outerRadius, startAngle, sweepAngle, true);
path.lineTo(centerX + innerRadius * (float) Math.cos(Math.toRadians(startAngle + sweepAngle)), centerY + innerRadius * (float) Math.sin(Math.toRadians(startAngle + sweepAngle)));
path.arcTo(centerX – innerRadius, centerY – innerRadius, centerX + innerRadius, centerY + innerRadius, startAngle + sweepAngle, -sweepAngle, true);
path.close();
canvas.drawPath(path, paint);
}
}
“`
“`java
// Example usage
Canvas canvas; // Your canvas object
Paint paint = new Paint();
paint.setColor(Color.RED);
HollowArc hollowArc = new HollowArc();
hollowArc.drawHollowArc(canvas, 100, 100, 50, 20, 0, 270, paint);
“`
Conclusion
While `drawArc()` itself lacks the desired functionality, several workarounds and alternative solutions exist. Choosing the best approach depends on your project’s complexity, performance requirements, and desired level of control over the arc’s shape. Libraries like MPAndroidChart can simplify drawing complex charts and graphs with inner and outer radii, providing a convenient option for complex data visualization needs.