Display an error message for impossible values of minZoom and maxZoom

[joswinter]

Fixes #2264

When creating a map where holds that minZoom > maxZoom the map is loaded and the user cannot zoom. The changes in this pull request fire an error when the above case holds when initiating a Transform or when setting the minZoom or maxZoom. See the following example: https://jsfiddle.net/7d365j4s/

[jfirebaugh]

Hi @joswinter, thanks for the contribution!

I see a couple issues with these changes. First off, the tests are passing by accident: Transform doesn't inherit from Evented, so this.fire('error', ...) actually throws TypeError: this.fire is not a function. TypeError inherits from Error, so t.throws passes. Note that the third argument to t.throws is not part of the assertion -- it's a diagnostic message that's printed if the assertion fails.

I think the best way to get access to fire is to validate in Map#set{Min,Max}Zoom. In fact, there already is validation there. Can you investigate why it's not working in your case?

[joswinter]

Thanks for the review @jfirebaugh!

I investigated why the validation wasn't working in the case of issue #2264. The validation only works when directly using the Map#set{Min,Max}Zoom functions however when constructing the following map createMap({minZoom:10, maxZoom:5}), no exception will be thrown. This happens because in Map#constructor the maxZoom and minZoom values are set by passing it to Transform#constructor which doesn't check whether the minZoom and maxZoom values are valid. I added a simple check to Map#constructor which checks if the minZoom and maxZoom values are valid, if these values are invalid then a Error('maxZoom must be greater than minZoom') is thrown.

Thanks @joswinter, these changes look good. Will wait for @lucaswoj to give a final review.

[lucaswoj]

There's one case that's slipping through the cracks here:

maxZoom: 0, minZoom: 1

This could be fixed by changing the test to options.minZoom != null && options.maxZoom != null

[lucaswoj]

We should test the error message mentions maxZoom and minZoom. Otherwise, we might pass because of an error line createMap is undefined.

[joswinter]

Thanks for reviewing @lucaswoj! I implemented the requested changes.