Messages and Message Handlers

Your dedicated thread will download photos, but how will it work with the RecyclerView’s adapter to display them when it cannot directly access the main thread? (Recall that background threads are not allowed to execute code that modifies the view – only the main thread is permitted do that. The main thread cannot execute long-running tasks – only a background thread is permitted to do that.)

Think back to the shoe store with two Flashes. Background Flash has wrapped up his phone call to the distributor. He needs to tell Main Flash that the shoes are back in stock. If Main Flash is busy, Background Flash cannot do this right away. He would have to wait by the register to catch Main Flash at a spare moment. ...

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