关于高德开发平台的划线功能

想实现在地图上划线的功能，于是按照api手册写了以下代码：

NSLog(@"划线");
MAMapPoint *arrayPoint = malloc(sizeof(MAMapPoint) * 2);
arrayPoint[0] = MAMapPointMake(38.7f, 117.48f);
arrayPoint[1] = MAMapPointMake(50.6f, 117.48f);
MAPolyline *line = [MAPolyline polylineWithPoints:arrayPoint count:2];
[self.mapView addOverlay: line];
free(arrayPoint);

但是地图上不显示根据这两个点所化的直线。请问这是什么原因？

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是不是应该实现overlay的代理？