Openlayers手动绘制圆形（v2.13）

Openlayers（v2.13）自带手动绘制矩形、多边形等图形的方法，方便快捷，但是并没有绘制圆的方法，在开发时根据API中的Box编写了手动绘圆的方法。此方法绘制的圆以鼠标拖动出的矩形较短的一条边为半径，与Windows中画图工具的画圆原理类似。

该方法绘制结束后会自动清除已经绘制的圆，如需保留的话修改方法中remove的部分即可。代码如下：

	var gPolygon = null;
	if (!layer.CIRCLEHADNLER) {
		var layerCircleControl = new OpenLayers.Control();
		OpenLayers.Util.extend(layerCircleControl, {
			draw : function() {
				layer.CIRCLEHANDLER = new OpenLayers.Handler.Box(layerCircleControl, null, {
					startBox: null,
					moveBox:function(xy){
						var start = this.map.getLonLatFromPixel(this.dragHandler.start);
						var last = this.map.getLonLatFromPixel(xy);
						var width = Math.abs(start.lon-last.lon);
						var height = Math.abs(start.lat-last.lat);
						var radius,originX,originY;
						if(width <= height){
							radius = width/2;
							originX = start.lon+(last.lon-start.lon)/2;
							if(start.lat - last.lat <= 0){
								originY = start.lat + radius;
							}else{
								originY = start.lat - radius;
							}
						}else{
							radius = height/2;
							if(start.lon - last.lon <=0){
								originX = start.lon + radius;
							}else{
								originX = start.lon - radius;
							}
							originY = start.lat+(last.lat-start.lat)/2;
						}
						var origin = new OpenLayers.Geometry.Point(originX,originY);
						var feature = OpenLayers.Geometry.Polygon.createRegularPolygon(origin, radius, 40, 0);
						if(gPolygon){
							layer.destroyFeatures(gPolygon);
							gPolygon = null;
						}						gPolygon = new OpenLayers.Feature.Vector(feature, null, style);
						if(!gPolygon){
							return;
						}
						layer.addFeatures([gPolygon]);
					},
					endBox: function(){
						//将gPolygon作为参数执行业务方法					},
			    	removeBox: function(){
						if(gPolygon){
							layer.destroyFeatures(gPolygon);
							gPolygon = null;
						}
			        }
				});
			}
		});
		this.getAbstractMap().getMap().addControl(layerCircleControl);
		var handler = layer.CIRCLEHANDLER.dragHandler;
        		handler.dragstart = function(evt){
            		var propagate = true;
            		handler.dragging = false;
            		if (handler.checkModifiers(evt) && (OpenLayers.Event.isLeftClick(evt) ||
                    		OpenLayers.Event.isSingleTouch(evt))) {
            			handler.started = true;
            			handler.start = evt.xy;
            			handler.last = evt.xy;
               			OpenLayers.Element.addClass(handler.map.viewPortDiv, "olDragDown");
                			handler.down(evt);
                			handler.callback("down", [evt.xy]);
                			OpenLayers.Event.preventDefault(evt);
                			if(!handler.oldOnselectstart) {
                			handler.oldOnselectstart = document.onselectstart ? document.onselectstart : OpenLayers.Function.True;
                			}
                			document.onselectstart = OpenLayers.Function.False;
                			propagate = !handler.stopDown;
            		}
            		return propagate;
        		}
	}
	layer.ADD_CIRCLE_HANDLER.activate();