Detecting if is zoom-in or zoom-out with leaflet

问题:

How can I know when the user is zooming in or zooming out? I want to start an animation if the user zooms after some level. Is it possible to know this when the event zoomstart is triggered?

回答1:

How can I know when the user is zooming in or zooming out?

At every zoom level, calculate how much map.getZoom() has changed.

Is it possible to know this when the event zoomstart is triggered?

No.

Consider the following scenario: A user, using a touchscreen (phone/tablet).

The user puts two fingers down on the screen. Half a frame after, one of the fingers moves a couple of pixels towards the center, triggering a pinch-zoom with a tiny change in the zoom level.

Your code catches the zoomstart and zoom events that happen inmediately after. "I know!" - your code says - "The user is zooming out!".

And then the user starts moving their fingers wider and wider, zooming in. And your code gets confused.

But the user changes their mind, and then starts zooming out for whatever reason. And then in again. And then out again. And then they lift the fingers and the zoom snaps to a zoom level.

This is why you can not know what the final zoom level is going to be when you listen to a zoomstart or zoom event in Leaflet.