| id | title | brief | sdk | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
69740674-4B3C-6D23-134B-4FFA3C17FE0D |
Fill A Polygon |
This recipe shows one way of drawing a filled polygon using Android.Graphics.Path and Android.Graphics.Canvas. |
|
The Canvas class in Android can be used to perform specialized drawing of graphics. A canvas provides a context that holds the various “drawing” calls. To draw on a Canvas, three things are required:
- A Paint object that describes the colors and styles being drawn
- A drawing primitive such as a Rect , Path , or a Bitmap to that describes what is being drawn.
- A bitmap or a view to hold the pixels that will be drawn.
The following class provides an example of how to draw a polygon and fill it.
A Path object is instantiated. The Path is initiated with a call to .MoveTo–
this sets the starting location of the Path. Then corresponding calls to the
.LineTo method are made.
For example, consider the following collection of points:
private readonly PointF[] _points = new[]
{
new PointF(100, 100),
new PointF(200, 200),
new PointF(200, 500),
new PointF(600, 600),
new PointF(400, 200),
new PointF(100, 100)
};
The example assembles a Path object that holds the primitives that make up the polygon, as shown in the following snippet:
var path = new Path();
// Set the first point, that the drawing will start from.
path.MoveTo(_points[0].X, _points[0].Y);
for (var i = 1; i < _points.Length; i++)
{
// Draw a line from the previous point in the path to the new point.
path.LineTo(_points[i].X, _points[i].Y);
}
The next step is to describe the paint for the polygon:
var paint = new Paint
{
Color = Color.White
};
// We can use Paint.Style.Stroke if we want to draw a "hollow" polygon,
// But then we had better set the .StrokeWidth property on the paint.
paint.SetStyle(Paint.Style.Fill);
Finally, the path and the paint are used in together to draw the polygon on a Canvas:
canvas.DrawPath(path, paint);
For this example, the above snippets have been encapsulate in a class called
FilledPolygon, which is a subclass of a View:
public class FilledPolygon : View
{
private readonly PointF[] _points = new[]
{
new PointF(100, 100),
new PointF(200, 200),
new PointF(200, 500),
new PointF(600, 600),
new PointF(400, 200),
new PointF(100, 100)
};
public FilledPolygon(Context context)
: base(context)
{
}
protected override void OnDraw(Canvas canvas)
{
base.OnDraw(canvas);
var path = new Path();
path.MoveTo(_points[0].X, _points[0].Y);
for (var i = 1; i < _points.Length; i++)
{
path.LineTo(_points[i].X, _points[i].Y);
}
var paint = new Paint
{
Color = Color.White
};
// We can use Paint.Style.Stroke if we want to draw a "hollow" polygon,
// But then we had better set the .StrokeWidth property on the paint.
paint.SetStyle(Paint.Style.Fill);
canvas.DrawPath(path, paint);
}
}
In this case, when the view is render, the OnDraw method will be invoked, and
the hosting activity will pass in a canvas instance.
When the application is run it will look like:
