问题
I am loading multiple photos from Dropbox using the Core API in Android and when making requests they are serialized.
I am getting the thumbnails to show in a grid view and when the user clicks on one of them it goes to another activity it fetches the full res version from the server.
It does this fine, but the high res version only starts to be downloaded when all the others async tasks fetching the thumbnails are finished.
So what I want to know is, is this a limitation of the Dropbox Core API? Or is there a way to make the high res async task have priority over others so it is immediately downloaded and then the others can resume.
回答1:
This is due to how async tasks actually work. They don't run in parallel on 3.0+ (they used to until 3.0), because too many newbie developers weren't able to program in parallel without errors, so Google decided to change it. Instead, async tasks run on a single thread in FIFO order.
You can override this however. Instead of calling asynctask.execute(), call asynctask.executeOnExecutor() and use a THREAD_POOL_EXECUTOR. This will execute it in parallel on its own thread. I believe there's a thread cap, but it will at least make several run in parallel.
If the thread cap becomes an issue, you can always drop down to using threads instead of using async tasks. You need to do some work yourself to do an onPostExecute, but it isn't that hard. And if you're creeating your own threads you can make up to the OS limit.
来源：https://stackoverflow.com/questions/27749486/android-make-a-new-async-task-have-priority-over-others-in-background-or-dropbox