How can I change the zoom step length

[故因]

1. A concise explanation of the problem you're experiencing.

 I want to change the zoom step length, eg: when my mouse wheel scroll once,  camera's height minus 100km.

 Is there a way to modify the step size of the scroll wheel? mouse wheel scroll once,  camera's height minus 50km. I watched the  Cesium.ScreenSpaceCameraController  class, but there seems to be no way to change this.

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

4. The Cesium version you're using, your operating system and browser.

1.40,Windows10,Chrome68

[Omar Shehata]

Good question! Looks like it's currently a private variable in ScreenSpaceCameraController:

https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/ScreenSpaceCameraController.js#L304

You can see how the zoomFactor is used and how it changes as you get closer to the surface here:

https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/ScreenSpaceCameraController.js#L455-L468

Modifying private properties isn't recommended since the API for those might change in the future, but it could be a quick hack to allow you to control the speed. An even better solution would be to modify the class to make the property public, documenting it and making a contribution to Cesium! It should be a straightforward enough change. Check out the contributing guide here (https://github.com/AnalyticalGraphicsInc/cesium/blob/master/CONTRIBUTING.md).