EarthManipulator::setViewpoint doesn't work in main()funciton

[smarttigerfly]

When I use EarthManipulator::setViewpoint in a simple example, I find it doesn't work.
The simple code is:
`#include <osgDB/ReadFile>
#include <osgViewer/Viewer>
#include <osg/Node>
#include <osgGA/GUIEventHandler>

#include <osgEarth/Map>
#include <osgEarth/MapNode>
#include <osgEarth/EarthManipulator>
#include <osgEarth/Viewpoint>

static osgEarth::Viewpoint VPs[] = {
osgEarth::Viewpoint("Africa", 0.0, 0.0, 0.0, 0.0, -90.0, 10e6),
osgEarth::Viewpoint("California", -121.0, 34.0, 0.0, 0.0, -90.0, 6e6),
osgEarth::Viewpoint("Europe", 0.0, 45.0, 0.0, 0.0, -90.0, 4e6),
osgEarth::Viewpoint("Washington DC", -77.0, 38.0, 0.0, 0.0, -90.0, 1e6),
osgEarth::Viewpoint("Australia", 135.0, -20.0, 0.0, 0.0, -90.0, 2e6),
osgEarth::Viewpoint("Boston", -71.096936, 42.332771, 0, 0.0, -90, 1e5),
osgEarth::Viewpoint("Beijing", 115, 45, 0.0, 0.0, -90.0, 10e6)
};

int main()
{
osg::setNotifyLevel(osg::NotifySeverity::DEBUG_INFO);

osgEarth::initialize();
osgViewer::Viewer viewer;

osgEarth::Util::EarthManipulator* manip = new osgEarth::Util::EarthManipulator();
viewer.setCameraManipulator(manip);

osg::Node* simpleNode = osgDB::readNodeFile("osgearthdata/tests/simple.earth");
osgEarth::MapNode* mapNode = osgEarth::MapNode::get(simpleNode);
viewer.setSceneData(mapNode);

	
manip->setViewpoint(Vps[0],	0.0);    


while (!viewer.done())
{
	viewer.frame();
}
return 0;
//return viewer.run();

}`
The result is:

But if I add GUIEventHandler to viewer, it can work. The code is:
`#include <osgDB/ReadFile>
#include <osgViewer/Viewer>
#include <osg/Node>
#include <osgGA/GUIEventHandler>

#include <osgEarth/Map>
#include <osgEarth/MapNode>
#include <osgEarth/EarthManipulator>
#include <osgEarth/Viewpoint>

static osgEarth::Viewpoint VPs[] = {
osgEarth::Viewpoint("Africa", 0.0, 0.0, 0.0, 0.0, -90.0, 10e6),
osgEarth::Viewpoint("California", -121.0, 34.0, 0.0, 0.0, -90.0, 6e6),
osgEarth::Viewpoint("Europe", 0.0, 45.0, 0.0, 0.0, -90.0, 4e6),
osgEarth::Viewpoint("Washington DC", -77.0, 38.0, 0.0, 0.0, -90.0, 1e6),
osgEarth::Viewpoint("Australia", 135.0, -20.0, 0.0, 0.0, -90.0, 2e6),
osgEarth::Viewpoint("Boston", -71.096936, 42.332771, 0, 0.0, -90, 1e5),
osgEarth::Viewpoint("Beijing", 115, 45, 0.0, 0.0, -90.0, 10e6)
};

struct FlyToViewpointHandler : public osgGA::GUIEventHandler
{
FlyToViewpointHandler(osgEarth::EarthManipulator* manip) : _manip(manip), isFirst(true) { }

bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa)
{
	if (ea.getEventType() == ea.FRAME)
	{
		if(isFirst)
		{
			_manip->setViewpoint(VPs[0], 4.0);
			isFirst = false;
		}
			
	}

	if (ea.getEventType() == ea.KEYDOWN && ea.getKey() >= '1' && ea.getKey() <= '7')
	{
		_manip->setViewpoint(VPs[ea.getKey() - '1'], 4.0);
		aa.requestRedraw();
	}
	return false;
}

osg::observer_ptr<osgEarth::EarthManipulator> _manip;
bool isFirst;

};

int main()
{
osg::setNotifyLevel(osg::NotifySeverity::DEBUG_INFO);

osgEarth::initialize();
osgViewer::Viewer viewer;

    osgEarth::Util::EarthManipulator* manip = new osgEarth::Util::EarthManipulator();
viewer.setCameraManipulator(manip);
	
osg::Node* simpleNode = osgDB::readNodeFile("osgearthdata/tests/simple.earth");
osgEarth::MapNode* mapNode = osgEarth::MapNode::get(simpleNode);

viewer.setSceneData(mapNode);

viewer.addEventHandler(new FlyToViewpointHandler(manip));

while (!viewer.done())
{
	
	viewer.frame();
}
return 0;
//return viewer.run();

}`

The running result is:

The result of osgearth_version --caps is:

Someone can help me? Thank you!

[smarttigerfly]

Now, I figure it out.
I add viewer.realize(); somewhere before manip->setViewpoint(Vps[0], 0.0); and after osgViewer::Viewer viewer; and it can work.
I don't know the reason.