让地图视图自适应线路

在 百度MapKit SDK 中的线路中有一个方法：

//根据polyline设置地图范围
- (void)mapViewFitPolyLine:(BMKPolyline *) polyLine {
    CGFloat ltX, ltY, rbX, rbY;
    if (polyLine.pointCount < 1) {
        return;
    }
    BMKMapPoint pt = polyLine.points[0];
    ltX = pt.x, ltY = pt.y;
    rbX = pt.x, rbY = pt.y;
    for (int i = 1; i < polyLine.pointCount; i++) {
        BMKMapPoint pt = polyLine.points[i];
        if (pt.x < ltX) {
            ltX = pt.x;
        }
        if (pt.x > rbX) {
            rbX = pt.x;
        }
        if (pt.y > ltY) {
            ltY = pt.y;
        }
        if (pt.y < rbY) {
            rbY = pt.y;
        }
    }
    BMKMapRect rect;
    rect.origin = BMKMapPointMake(ltX , ltY);
    rect.size = BMKMapSizeMake(rbX - ltX, rbY - ltY);
    [_mapView setVisibleMapRect:rect];
    _mapView.zoomLevel = _mapView.zoomLevel - 0.3;
}

经过调用执行该方法，可让地图自适应的显示线路，地图视图自动缩放一个较好的level来显示当前的整个线路图，而不用本身去人为设置level。