1 运行过程
我们通过实例来讲述SharpMap的运行过程和渲染（绘制）机制。首先打开Simple.aspx，可知此页面有一组单选框（3个，分别是放大、缩小和漫游）和一个图像按钮，用于显示地图。它的代码在Simple.aspx.cs中。
打开Simple.aspx.cs，在Page_Load函数中是页面初始化代码。可知地图生成分两步：
1.1 初始化地图
//Set up the map. We use the method in the App_Code folder for initializing the map
myMap = MapHelper.InitializeMap(new
System.Drawing.Size((int)imgMap.Width.Value,(int)imgMap.Height.Value));
我们找到MapHelper.InitializeMap函数，发现地图的初始化分为以下几步：
（1）创建地图，创建图层
//Initialize a new map of size 'imagesize'
SharpMap.Map map = new SharpMap.Map(size);
//Set up the countries layer
SharpMap.Layers.VectorLayer layCountries = new SharpMap.Layers.VectorLayer("Countries");
//Set the datasource to a shapefile in the App_data folder
layCountries.DataSource = new
SharpMap.Data.Providers.ShapeFile(HttpContext.Current.Server.MapPath(@"~\App_data\countries.shp"), true);
（2）基本的图层显示设置
//Set fill-style to green
layCountries.Style.Fill = new SolidBrush(Color.Green);
//Set the polygons to have a black outline
layCountries.Style.Outline = System.Drawing.Pens.Black;
layCountries.Style.EnableOutline = true;
layCountries.SRID = 4326;
（3）加入图层到地图
//Add the layers to the map object.
//The order we add them in are the order they are drawn, so we add the rivers last to put them on top
map.Layers.Add(layCountries);
map.Layers.Add(layRivers);
map.Layers.Add(layCities);
map.Layers.Add(layLabel);
map.Layers.Add(layCityLabel);
（4）地图放缩、背景、中心等设置
//limit the zoom to 360 degrees width
map.MaximumZoom = 360;
map.BackColor = Color.LightBlue;
map.Zoom = 360;
map.Center = new SharpMap.Geometries.Point(0,0);
1.2 绘制并生成地图
我们回到Simple.aspx.cs的Page_Load函数，发现下一步调用GenerateMap();
//This is the initial view of the map. Zoom to the extents of the map:
//myMap.ZoomToExtents();
//or center on 0,0 and zoom to full earth (360 degrees)
//myMap.Center = new SharpMap.Geometries.Point(0,0);
//myMap.Zoom = 360;
//Create the map
GenerateMap();
在同一文件中，GenerateMap()包含以下两步：
（1）保存当前地图状态
//Save the current mapcenter and zoom in the viewstate
ViewState.Add("mapCenter", myMap.Center);
ViewState.Add("mapZoom", myMap.Zoom);
（2）渲染地图
//Render map
System.Drawing.Image img = myMap.GetMap();
string imgID = SharpMap.Web.Caching.InsertIntoCache(1, img);
imgMap.ImageUrl = "getmap.aspx?ID=" + HttpUtility.UrlEncode(imgID);
它的核心就是通过myMap.GetMap()创建了一个地图图片。我们来看看这个GetMap()函数。
在Map.cs文件中找到GetMap()函数，它分为以下几步：
①创建图像，得到图像绘制环境参数
System.Drawing.Image img = new System.Drawing.Bitmap(this.Size.Width, this.Size.Height);
System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(img);
g.Transform = this.MapTransform;
g.Clear(this.BackColor);
g.PageUnit = System.Drawing.GraphicsUnit.Pixel;
②绘制图层
int SRID = (Layers.Count > 0 ? Layers[0].SRID : -1); //Get the SRID of the first layer
for (int i = 0; i < _Layers.Count; i++)
{
if (_Layers[i].Enabled && _Layers[i].MaxVisible >= this.Zoom && _Layers[i].MinVisible < this.Zoom)
_Layers[i].Render(g, this);
}
关键在于_Layers[i].Render(g, this)，它是ILayer接口的一个函数，我们找VectorLayer的实现来看看，它分为两种，一种是专题地图，一种是非专题地图绘制。
对于专题地图，它针对SharpMap.Data.FeatureDataTable的每一个feature进行绘制，先绘制其外框，再绘制内部几何体。对于非专题地图，它针对this.DataSource.GetGeometriesInView(envelope)得到的几何体进行绘制，也是先绘制其外框，再绘制内部几何体。最后的绘制工作都调用SharpMap.Rendering.VectorRenderer类的函数完成。SharpMap.Rendering.VectorRenderer类包含绘制注记、线串、多线串、多点、多多边形、点、多边形等。
绘制好图层后将激发LayerRendered事件：
if(LayerRendered!=null) LayerRendered(this, g); //Fire event
绘制好地图后将激发Map类的MapRendered 事件：
if (MapRendered != null) MapRendered(g); //Fire render event
2 SharpMap.Rendering.VectorRenderer的各个函数具体分析
现在我们对渲染地图的全过程已经有了个总体概念和初步理解，下面来具体看一下SharpMap.Rendering.VectorRenderer的各个函数：
2.1 DrawLabel
/// <summary>
/// Renders a label to the map.
/// </summary>
/// <param name="g">Graphics reference</param>
/// <param name="LabelPoint">Label placement</param>
/// <param name="Offset">Offset of label in screen coordinates</param>
/// <param name="font">Font used for rendering</param>
/// <param name="forecolor">Font forecolor</param>
/// <param name="backcolor">Background color</param>
/// <param name="halo">Color of halo</param>
/// <param name="rotation">Text rotation in degrees</param>
/// <param name="text">Text to render</param>
/// <param name="map">Map reference</param>
public static void DrawLabel(System.Drawing.Graphics g, System.Drawing.PointF LabelPoint, System.Drawing.PointF Offset, System.Drawing.Font font, System.Drawing.Color forecolor, System.Drawing.Brush backcolor, System.Drawing.Pen halo, float rotation, string text, SharpMap.Map map)
这里面要注意的是这是绘制使用的函数是DrawPath和FillPath，而且是作者从DrawString改进的，可能是为了绘制过程的统一，因为其它几何体的具体绘制也使用DrawPath。
if (halo != null)
g.DrawPath(halo, path);
g.FillPath(new System.Drawing.SolidBrush(forecolor), path);
//g.DrawString(text, font, new System.Drawing.SolidBrush(forecolor), LabelPoint.X, LabelPoint.Y);
2.2 DrawLineString
/// <summary>
/// Renders a LineString to the map.
/// </summary>
/// <param name="g">Graphics reference</param>
/// <param name="line">LineString to render</param>
/// <param name="pen">Pen style used for rendering</param>
/// <param name="map">Map reference</param>
public static void DrawLineString(System.Drawing.Graphics g, Geometries.LineString line, System.Drawing.Pen pen, SharpMap.Map map)
这个函数很简单，也是采用DrawPath进行具体绘制。
2.3 DrawMultiLineString
/// <summary>
/// Renders a MultiLineString to the map.
/// </summary>
/// <param name="g">Graphics reference</param>
/// <param name="lines">MultiLineString to be rendered</param>
/// <param name="pen">Pen style used for rendering</param>
/// <param name="map">Map reference</param>
public static void DrawMultiLineString(System.Drawing.Graphics g,
Geometries.MultiLineString lines, System.Drawing.Pen pen, SharpMap.Map map)
它通过循环调用DrawLineString来进行绘制。
2.4 DrawMultiPoint
/// <summary>
/// Renders a <see cref="SharpMap.Geometries.MultiPoint"/> to the map.
/// </summary>
/// <param name="g">Graphics reference</param>
/// <param name="points">MultiPoint to render</param>
/// <param name="symbol">Symbol to place over point</param>
/// <param name="symbolscale">The amount that the symbol should be scaled. A scale of '1' equals to no scaling</param>
/// <param name="offset">Symbol offset af scale=1</param>
/// <param name="rotation">Symbol rotation in degrees</param>
/// <param name="map">Map reference</param>
public static void DrawMultiPoint(System.Drawing.Graphics g, Geometries.MultiPoint points, System.Drawing.Bitmap symbol, float symbolscale, System.Drawing.PointF offset, float rotation, SharpMap.Map map)
它通过循环调用DrawPoint来进行绘制。
2.5 DrawMultiPolygon
/// <summary>
/// Renders a multipolygon byt rendering each polygon in the collection by calling DrawPolygon.
/// </summary>
/// <param name="g">Graphics reference</param>
/// <param name="pols">MultiPolygon to render</param>
/// <param name="brush">Brush used for filling (null or transparent for no filling)</param>
/// <param name="pen">Outline pen style (null if no outline)</param>
/// <param name="clip">Specifies whether polygon clipping should be applied</param>
/// <param name="map">Map reference</param>
public static void DrawMultiPolygon(System.Drawing.Graphics g, Geometries.MultiPolygon pols, System.Drawing.Brush brush, System.Drawing.Pen pen, bool clip, SharpMap.Map map)
它通过循环调用DrawPolygon来进行绘制。
2.6 DrawPoint
/// <summary>
/// Renders a point to the map.
/// </summary>
/// <param name="g">Graphics reference</param>
/// <param name="point">Point to render</param>
/// <param name="symbol">Symbol to place over point</param>
/// <param name="symbolscale">The amount that the symbol should be scaled. A scale of '1' equals to no scaling</param>
/// <param name="offset">Symbol offset af scale=1</param>
/// <param name="rotation">Symbol rotation in degrees</param>
/// <param name="map">Map reference</param>
public static void DrawPoint(System.Drawing.Graphics g, SharpMap.Geometries.Point point, System.Drawing.Bitmap symbol, float symbolscale, System.Drawing.PointF offset, float rotation, SharpMap.Map map)
其中，symbol参数为一个位图，代表点的符号，这说明SharpMap暂时只支持位图符号，不支持矢量符号绘制。最终绘制调用DrawImage。
2.7 DrawPolygon
/// <summary>
/// Renders a polygon to the map.
/// </summary>
/// <param name="g">Graphics reference</param>
/// <param name="pol">Polygon to render</param>
/// <param name="brush">Brush used for filling (null or transparent for no filling)</param>
/// <param name="pen">Outline pen style (null if no outline)</param>
/// <param name="clip">Specifies whether polygon clipping should be applied</param>
/// <param name="map">Map reference</param>
public static void DrawPolygon(System.Drawing.Graphics g, SharpMap.Geometries.Polygon pol, System.Drawing.Brush brush, System.Drawing.Pen pen, bool clip, SharpMap.Map map)
它只支持用不同的画刷和画笔来绘制多边形，也是采用DrawPath进行具体绘制。
3 总结
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