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B2G/Architecture: Difference between revisions
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== Gecko: DOM APIs == | == Gecko: DOM APIs == | ||
"DOM interfaces", approximately, are how web content communicates with Gecko. There's a lot more to the DOM than that, but the longer discussion is beyond the scope of this document. DOM interfaces are defined in [https://developer.mozilla.org/en/XPIDL IDL], which defines both a foreign function interface (ffi) and object model (OM) between JavaScript and C++. Again, there are more details that could be discussed here which are beyond the scope of this document. Let's learn a bit of IDL by example. | |||
The very simple vibration API is exposed to web content through an IDL interface. That interface is [https://github.com/cgjones/mozilla-central/blob/master/dom/interfaces/base/nsIDOMNavigator.idl#L66 here] | |||
<pre> | |||
[implicit_jscontext] | |||
void mozVibrate(in jsval aPattern); | |||
</pre> | |||
The <code>jsval</code> argument indicates that <code>mozVibrate</code> (our vendor-prefixed implementation of the vibration specification) accepts any JS value. Details of working with jsvals are beyond the scope of this document. The IDL compiler generates a C++ interface that's implemented [https://github.com/cgjones/mozilla-central/blob/master/dom/base/Navigator.cpp#L657 here] | |||
<pre> | |||
NS_IMETHODIMP | |||
Navigator::MozVibrate(const jsval& aPattern, JSContext* cx) | |||
{ | |||
// ... | |||
hal::Vibrate(pattern); | |||
return NS_OK; | |||
} | |||
</pre> | |||
There's quite a lot of code that's hidden here by the ellipsis ("..."), but let's ignore for it now. | |||
The call to <code>hal::Vibrate()</code> transfers control from the DOM to hal. From there, we enter the hal implementation discussed above. A key point to note here is that the DOM implementation doesn't care what platform it's running on (Gonk or Windows or OS X or ...), nor does it care whether the code is running in a "content process" or the Gecko "server process". This is all left to lower levels of the system. | |||