Android + Picasso: changing URL cache expiration

Question

I am using Picasso to download and display images in views all accros my application. Those images are changing very rarely (they are considered valid for a few months).

Is there a simple way to ask Picasso (or the underlying okHttp) to keep those images on disc for this much time?

Answer 1

Disk caching happens "below" Picasso inside the HTTP client. In fact, this process is completely transparent. We never explicitly ask for a cached-version or an internet-version, the HTTP client will make the decision internally and do the right thing.

Because we opted to leverage the HTTP client for caching, we're offered very little control over how the caching actually happens. To answer your question, no, there is no way to tell Picasso (or OkHttp) to cache an image for longer than its headers allow.

Answer 2

Before thinking about HTTP behavior, make sure you set a large max size for the disk cache:

cache = Cache(File(application.filesDir, "photos"), Long.MAX_VALUE)

(MAX_VALUE is not recommended for production.) Don't store the cache in application.cacheDir, because android can clear that whenever it wants.

Add an interceptor to set max-stale, which tells the disk cache to use all old files:

    val httpClient = OkHttpClient.Builder().cache(cache).addInterceptor { chain ->
        // When offline, we always want to show old photos.
        val neverExpireCacheControl = CacheControl.Builder().maxStale(Int.MAX_VALUE, TimeUnit.SECONDS).build()
        val origRequest = chain.request()
        val neverExpireRequest = origRequest.newBuilder().cacheControl(neverExpireCacheControl).build()

        chain.proceed(neverExpireRequest)
    }.build()

    return Picasso.Builder(application).downloader(OkHttp3Downloader(httpClient)).loggingEnabled(true).build()

I discovered this solution by debugging CacheStrategy.getCandidate(). Take a look there if this doesn't solve your problem.

Answer 3

I solved it with a Home-made cache, the trick is to add a parameter to the URL that is not used, but making each URL different every X minutes

Calendar cal2 = Calendar.getInstance();
long d = cal2.getTimeInMillis();
int extra =  (int) Math.ceil(d/ (10*60*1000));    // 10 minutes cache

Picasso.with(getBaseContext())
            .load("http://www.myurl.cat/myimage.png&extra=" + extra)
            .placeholder(R.drawable.graphicLoading)
            .error(R.drawable.graphicLoadingError)
            .into(bottomGraphic);