Show annotation on MKMapView based on user location

I've problem with Objective C at the moment with developing something special.

So - after the MKMapView is initialized and the users position is shown, I would like to implement a method to show annotations which are next to users position. I've stored all necessary points in an SQLite database in a special table. I've two fields - namely lonand lat- I think it's clear what is meant by that.

Has anybody an idea how to show JUST those annotations which are visible at the screen to be as performant as possible. After the user scrolls the map - it should call the method again to look up in database wether there are also other points which need to be shown...

Thank you very much in 开发者_运维百科advance for your help,

kind regards,

Dominik

You will need to do a few things.

1) Get the center of your map - use the centerCoordinate property of your MKMapView

2) Ask your database for all items within a certain distance from the center of the map.

3) Display them

It's (2) that's the tricky one - you need a method that will return all items near to the lat lng you have.

This link has a handy implementation of how to do that :)

Hope that helps.

A couple of points:

If you're using iOS4, the mapview behaviour has changed so that it automatically saves memory by only creating the annotations which are in the visible view.

Secondly, in the same way that you can create a tableview with thousands of cells and still have it scroll smoothly, the annotations offscreen don't affect the performance that much. You should be keeping your annotations very small so they don't consume much memory. The annotation views are the ones that take up the memory.

Your problem is going to come when the user zooms out and there are too many annotation views visible at the same time. When you're zoomed in, you don't need to worry too much about off-screen annotations.