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Dan Abramov
1f04ff580d
Summary: As we saw in D15947985, and later traced down to D5623623, the `hot` option isn't used by Metro anymore. The relevant transforms _always_ run in DEV regardless of the option. Given that, it doesn't make sense that enabling or disabling Hot Reloading forces a full refresh. This significantly raises the usage barrier because **currently, you might have to wait ~20 seconds (on a large app) to just start using Hot Reloading when you're already in the middle of some screen.** So you just end up not using it. This diff changes enabling/disabling Hot Reloading to be _instant_. Here's how it works: 1. Now we always send the necessary info to the client via the new `HMRClient.setup()` function. It creates a Metro HMR client instance, but only actually sets up the socket if Hot Reloading is on. 2. The "Enable Hot Reloading" menu no longer forces a reload. Instead, it calls `HMRClient.enable()` which lazily sets up a socket (at most once). 3. The "Disable Hot Reloading" menu also doesn't trigger a refresh now. Instead, it calls `HMRClient.disable()`. We don't actually tear down the socket here because it's a pain to deal with race conditions and such. Instead, we keep the connection — but we _ignore the updates_ that come in while we're disabled. 4. As a result, it is possible to enable and disable it many times during a single session. (Updates while disabled would be ignored — which has a risk of making your running app inconsistent — but I'd argue it's expected and is worth it. You can always save a particular file to force it to update once the mode is on.) 5. In order to support "ignoring" updates, Metro's `HMRClient` (not to be confused with RN's module) now supports a `shouldApplyUpdates` field. The RN module uses it to disable handling updates when the mode is off. 6. In case there is an error that makes hot reloading unavailable (such as the server disconnecting), we surface the error only if the mode is on. If the mode is off, we stash the error message in the `_hmrUnavailableReason` variable, and display it next time you try to enable Hot Reloading. Reviewed By: rickhanlonii Differential Revision: D15958160 fbshipit-source-id: 8256fc4d5c2c3f653a78edf13b8515a5671953e4
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