How best to re-create markers/polylines when phone is rotated (orientation change)

问题

Background:

• Developing a native Android App that uses Android Google Map v2, uses android.support.v4.app.FragmentActivity. Runs on Android v2.2.

Objective:

• Retain Markers/Polylines "drawn" on map before the phone orientation is changed.

Question(s):

1. Can I "save" the markers/polylines as part of the bundle and simply have them re-displayed by saving them in the onSaveInstanceState using the appropriate savedInstance.put.. methods and then "restore" them in onCreate using the appropriate savedInstanceState.get.. methods.

   In reviewing the description for Marker getID() I'm confused when the Google documentation for Marker.getId() method states the following:

   When a map is restored from a Bundle, markers that were on that map are also restored. However, those markers will then be represented by different Marker objects. A marker's id can be used to retrieve the new instance of a Marker object after such restoration.

   The Google documentation (bold text above) makes it sound like the Marker's are just automagically restored without having to take any action. That isn't my experience...Maybe I'm mis-interpreting what is being stated. Or maybe you have to explicitly save map in the Bundle? Can someone clarify what this means?

2. Assuming I will have to explicitly save the Marker and Polylines to the bundle via the appropriate savedInstance.put... method should I save the entire Marker or should I save the marker id and retrieve the marker info using the marker id to re-display the marker? I couldn't find a put method that would allow me to save the entire Marker.

   I noticed that the MapFragment section of the Google Maps Android API v2 it states the following:

   Any objects obtained from the GoogleMap is associated with the view. It's important to not hold on to objects (e.g. Marker) beyond the view's life. Otherwise it will cause a memory leak as the view cannot be released.

   This statement leads me to believe that I shouldn't try and save the Marker itself but instead try and save the marker id and then re-generate the marker based on the marker object associated with the marker id. Likewise for the PolyLines. Is my assumption correct?

3. Also, should I avoid having the Marker as a class variable? My concern is that if the Marker is a class variable and the Activity Map fragment is placed on the back stack that this could cause a memory leak because it will effectively be "holding on to the object" as noted in the aforementioned documentation. Is this something I should be concerned about?

Regards.

回答1:

Can I "save" the markers/polylines as part of the bundle and simply have them re-displayed by saving them in the onSaveInstanceState using the appropriate "savedInstance.put.." methods and then "restore" them in onCreate using the appropriate "savedInstanceState.get.." methods.

No.

The Google documentation (bolded text above) makes it sound like the Marker's are just automagically restored without having to take any action. That isn't my experience...Maybe I'm mis-interpreting what is being stated. Or maybe you have to explicitly save map in the Bundle? Can someone clarify what this means?

You are not mis-interpreting anything. The documentation is incorrect.

should I save the Marker ID and retrieve the marker info using the marker ID to re-display the marker

Marker ID is not something permanent across configuration changes - it depends on the sequence of visual objects creation (first call to addMarker returns object with ID "m1", second "m2"). You cannot use this value in any way (as of API version 3.1.36) and IMHO there is really no point in its existance. I have actually sumbited an issue related to ID. There should at least be a function GoogleMap.getMarkerById(String) for Marker.getId() to make little sense.

3) Also, should I avoid having the Marker as a Class variable? My concern is that if the Marker is a class variable and the Activity Map fragment is placed on the back stack that this could cause a memory leak because it will effectively be "holding on to the object" as noted in the aforementioned documentation. Is this something I should be concerned about?

Yes. Keeping a static reference to Marker or any other visual object leads to a leak.

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Not that answering all your questions and concerns made it closer to the solution, so here are my propositions.

I assume you have a data set that Markers and Polylines are created from and probably store it in DB after being fetched from webservice. Now if you load it from DB in AsyncTask in your Activity or not even store it in DB, but only fetch in Activity directly - that's bad.

Try to make your data as accessible as possible, so it only has to be reload into memory after process is killed (or after you drop it when low on memory). If you do that - that's good. Of course not all: if you have 20000 markers and each has its image displayed on info window, it can wait...

Now that you have all the data needed to create markers in memory, just create them like you would the first time. No additional code required.

We can argue about this being good or bad idea and I will improve the answer, but this requires more info about the context:

• how many markers and polylines
• what additional data you have
• where do you keep your model
• etc.

There is another way of course: you can send MarkerOptions in onSaveInstanceState. This might work for you if you keep it up-to-date or if your Markers do not change and if there are not so many of them. I can't make sense of sending thousands of objects via IPC on every rotation or pressing HOME button.

回答2:

It seems to me that calling setRetainInstance(true); on the fragment that holds the map retains everything on the map through pauses, orientation changes, etc, without having to worry about setting and getting. Is there some reason why this isn't the recommended approach?

来源：https://stackoverflow.com/questions/16902519/how-best-to-re-create-markers-polylines-when-phone-is-rotated-orientation-chang