I’m working on a patch<\/a> for bug 1116188<\/a> to make gathering profiles from subprocesses asynchronous. In order to do that, I’m exposing a new method on nsIProfiler called getProfileDataAsync that is returning a DOM Promise. What’s interesting about this is that I’m returning a DOM Promise from C++!\u00a01<\/a><\/sup><\/p>\n In order to construct a DOM Promise in C++, I need to hand it something that implements nsIGlobalObject. I suspect that this helps the Promise determine which memory region that it belongs to.<\/p>\n My new method gets a JSContext* because I’ve got the [implicit_jscontext] bit about the method definition in the nsIProfiler.idl file… so how do I go about turning that into an nsIGlobalObject?<\/p>\n Here’s the maneuver:<\/p>\n That will, as the name suggests, return either an nsIGlobalObject*, or a nullptr.<\/p>\n For my patch<\/a> for bug 1116188<\/a>, it’s all well and good to create a DOM Promise, but you have to resolve or reject that Promise for it to have any real value.<\/p>\n In my case, I wanted to take a string, parse it into a JS Object, and resolve with that.<\/p>\n Resolving or rejecting a DOM Promise in Javascript is pretty straight-forward<\/a> – you’re given back resolve \/ reject function, and you just need to call those with your results and you’re done.<\/p>\n In C++, things get a little hairier. As I discovered in my most recent episode of The Joy of Coding<\/a>, conditions need to be right in order for this to work out.<\/p>\n Here’s what I ended up doing (I’ve simplified the method somewhat to remove noise):<\/p>\n The key parts here are getting the AutoJSAPI on the stack, initting it, gettings its JSContext, and then putting the JSAutoCompartment on the stack. Note that I had to pass not only the JSContext, but the global JS Object for the Promise as well – I suspect that’s, again, to ensure that the right compartment is being entered. Otherwise, I start failing assertions like crazy.<\/p>\n Note that the code above is by no means perfect – I’m missing error handling functions for when the JSON parsing goes wrong. In that case, I should probably reject the Promise instead. bz pointed me to a good example of that going on here in Fetch.cpp<\/a>:<\/p>\n I’ll probably end up doing something similar in the next iteration of my patch.<\/p>\n I learned how to do that a few weeks back<\/a>. ↩<\/a><\/p><\/li><\/ol>","protected":false},"excerpt":{"rendered":" How to get an nsIGlobalObject* from a JSContext* I’m working on a patch for bug 1116188 to make gathering profiles from subprocesses asynchronous. In order to do that, I’m exposing a new method on nsIProfiler called getProfileDataAsync that is returning a DOM Promise. What’s interesting about this is that I’m returning a DOM Promise from […]<\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[874,861,110],"tags":[1188,69,125,1183,1187,1186,35,1190,1184,1189],"class_list":["post-2787","post","type-post","status-publish","format-standard","hentry","category-firefox-mozilla-2","category-mozilla-2","category-musings","tag-c","tag-dom","tag-firefox","tag-jsapi","tag-jsautocompartment","tag-jscontext","tag-mozilla","tag-nsiglobalobject","tag-promises","tag-spidermonkey"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/prmTy-IX","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/posts\/2787","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/comments?post=2787"}],"version-history":[{"count":1,"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/posts\/2787\/revisions"}],"predecessor-version":[{"id":2788,"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/posts\/2787\/revisions\/2788"}],"wp:attachment":[{"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/media?parent=2787"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/categories?post=2787"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mikeconley.ca\/blog\/wp-json\/wp\/v2\/tags?post=2787"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\/\/ Where aCX is your JSContext*:\r\nnsIGlobalObject* go = xpc::NativeGlobal(JS::CurrentGlobalOrNull(aCx));<\/pre>\n
Resolving a DOM Promise from C++ with a JS Object<\/h2>\n
void\r\nProfileGatherer::Finish()\r\n{\r\n\u00a0 AutoJSAPI jsapi;\r\n\u00a0 jsapi.Init();\r\n\u00a0 JSContext* cx = jsapi.cx();\r\n\u00a0 JSAutoCompartment ac(cx, mPromise->GlobalJSObject());\r\n\r\n\u00a0 \/\/ Now parse the JSON so that we resolve with a JS Object.\r\n\u00a0 JS::RootedValue val(cx);\r\n\u00a0 {\r\n\u00a0\u00a0\u00a0 UniquePtr<char[]> buf = mWriter.WriteFunc()->CopyData();\r\n\u00a0\u00a0\u00a0 NS_ConvertUTF8toUTF16 js_string(nsDependentCString(buf.get()));\r\n\u00a0\u00a0\u00a0 MOZ_ALWAYS_TRUE(JS_ParseJSON(cx, static_cast<const char16_t*>(js_string.get()),\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 js_string.Length(), &val));\r\n\u00a0 }\r\n\u00a0 mPromise->MaybeResolve(val);\r\n}<\/pre>\n if (!JS_ParseJSON(cx, decoded.get(), decoded.Length(), &json)) {\r\n if (!JS_IsExceptionPending(cx)) {\r\n localPromise->MaybeReject(NS_ERROR_DOM_UNKNOWN_ERR);\r\n return;\r\n }\r\n\r\n JS::Rooted<JS::Value> exn(cx);\r\n DebugOnly<bool> gotException = JS_GetPendingException(cx, &exn);\r\n MOZ_ASSERT(gotException);\r\n\r\n JS_ClearPendingException(cx);\r\n localPromise->MaybeReject(cx, exn);\r\n return;\r\n }\r\n\r\n localPromise->MaybeResolve(cx, json);\r\n return;<\/pre>\n