Athena's Parthenon Zola 2026-04-26T00:00:00+00:00 https://parthen.online/topics/uniffi/atom.xml 2026-04-26T00:00:00+00:00 2026-04-26T00:00:00+00:00 Athena https://parthen.online/notes/rust-uniffi-functional-interface/ <h2 id="functional-interfaces">Functional Interfaces</h2> <p>UniFFI has good support for callback interfaces, allowing calling code to supply its own logic to be called from Rust. How to make this work on the other-language side was relatively obvious to me: you just implement the interface. However, how to make this smooth in Rust took a moment to figure out, so I figured I'd make a note.</p> <p>A UniFFI callback interface looks (using the proc macros) like this:</p> <pre class="giallo" style="color-scheme: light dark; color: light-dark(#24292E, #EBDBB2); background-color: light-dark(#FFFFFF, #1D2021);"><code data-lang="rust"><span class="giallo-l"><span style="color: light-dark(#24292E, #A89984);">#</span><span style="color: light-dark(#24292E, #A89984);">[</span><span>uniffi</span><span style="color: light-dark(#D73A49, #8EC07C);">::</span><span>export</span><span style="color: light-dark(#24292E, #A89984);">(</span><span>callback_interface</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#24292E, #A89984);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#D73A49, #FB4934);">pub</span><span style="color: light-dark(#D73A49, #FE8019);"> trait</span><span style="color: light-dark(#6F42C1, #FABD2F);"> MyCallback</span><span style="color: light-dark(#D73A49, #8EC07C);">:</span><span style="color: light-dark(#6F42C1, #FABD2F);"> Send</span><span style="color: light-dark(#D73A49, #8EC07C);"> +</span><span style="color: light-dark(#6F42C1, #FABD2F);"> Sync</span><span style="color: light-dark(#24292E, #A89984);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#D73A49, #FB4934);"> fn</span><span style="color: light-dark(#6F42C1, #FABD2F);"> consume</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#D73A49, #8EC07C);">&amp;</span><span style="color: light-dark(#005CC5, #83A598);">self</span><span style="color: light-dark(#24292E, #A89984);">,</span><span style="color: light-dark(#24292E, #83A598);"> value</span><span style="color: light-dark(#D73A49, #8EC07C);">:</span><span style="color: light-dark(#6F42C1, #FABD2F);"> MyValue</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#24292E, #A89984);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#24292E, #A89984);">}</span></span></code></pre> <p>(where <code>MyValue</code> has <code>#[uniffi::export]</code> as well)</p> <p>and some function which takes it (boxed, because we can't know the implementing type at compile time, so it's gotta be a <code>dyn</code> rather than being able to use a generic/<code>impl Trait</code>) like so:</p> <pre class="giallo" style="color-scheme: light dark; color: light-dark(#24292E, #EBDBB2); background-color: light-dark(#FFFFFF, #1D2021);"><code data-lang="rust"><span class="giallo-l"><span style="color: light-dark(#24292E, #A89984);">#</span><span style="color: light-dark(#24292E, #A89984);">[</span><span>uniffi</span><span style="color: light-dark(#D73A49, #8EC07C);">::</span><span>export</span><span style="color: light-dark(#24292E, #A89984);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#D73A49, #FB4934);">pub</span><span style="color: light-dark(#D73A49, #FB4934);"> fn</span><span style="color: light-dark(#6F42C1, #FABD2F);"> do_it</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#24292E, #83A598);">doer</span><span style="color: light-dark(#D73A49, #8EC07C);">:</span><span style="color: light-dark(#6F42C1, #FABD2F);"> Box</span><span style="color: light-dark(#24292E, #A89984);">&lt;</span><span style="color: light-dark(#D73A49, #FB4934);">dyn</span><span style="color: light-dark(#6F42C1, #FABD2F);"> MyCallback</span><span style="color: light-dark(#24292E, #A89984);">&gt;</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#24292E, #A89984);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#24292E, #83A598);"> doer</span><span style="color: light-dark(#D73A49, #8EC07C);">.</span><span style="color: light-dark(#6F42C1, #FABD2F);">consume</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#24292E, #83A598);">some_value</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#24292E, #A89984);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#24292E, #A89984);">}</span></span></code></pre> <p>Which is all well and good. But using this from Rust is a bit of a pain - there's just one method, so it should be possible to just yunk in a closure. But if you wanna do that you gotta tell Rust that's okay, it doesn't infer it like Java's functional interfaces. Here's how you do that:</p> <pre class="giallo" style="color-scheme: light dark; color: light-dark(#24292E, #EBDBB2); background-color: light-dark(#FFFFFF, #1D2021);"><code data-lang="rust"><span class="giallo-l"><span style="color: light-dark(#D73A49, #FB4934);">impl</span><span style="color: light-dark(#24292E, #A89984);">&lt;</span><span style="color: light-dark(#6F42C1, #FABD2F);">T</span><span style="color: light-dark(#24292E, #A89984);">&gt;</span><span style="color: light-dark(#6F42C1, #FABD2F);"> MyCallback</span><span style="color: light-dark(#D73A49, #FB4934);"> for</span><span style="color: light-dark(#6F42C1, #FABD2F);"> T</span></span> <span class="giallo-l"><span style="color: light-dark(#D73A49, #FB4934);">where</span></span> <span class="giallo-l"><span style="color: light-dark(#6F42C1, #FABD2F);"> T</span><span style="color: light-dark(#D73A49, #8EC07C);">:</span><span style="color: light-dark(#6F42C1, #FABD2F);"> Fn</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#6F42C1, #FABD2F);">MyValue</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#D73A49, #8EC07C);"> -&gt;</span><span style="color: light-dark(#24292E, #A89984);"> (</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#D73A49, #8EC07C);"> +</span><span style="color: light-dark(#6F42C1, #FABD2F);"> Send</span><span style="color: light-dark(#D73A49, #8EC07C);"> +</span><span style="color: light-dark(#6F42C1, #FABD2F);"> Sync</span></span> <span class="giallo-l"><span style="color: light-dark(#24292E, #A89984);">{</span></span> <span class="giallo-l"><span style="color: light-dark(#D73A49, #FB4934);"> fn</span><span style="color: light-dark(#6F42C1, #FABD2F);"> consume</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#D73A49, #8EC07C);">&amp;</span><span style="color: light-dark(#005CC5, #83A598);">self</span><span style="color: light-dark(#24292E, #A89984);">,</span><span style="color: light-dark(#24292E, #83A598);"> value</span><span style="color: light-dark(#D73A49, #8EC07C);">:</span><span style="color: light-dark(#6F42C1, #FABD2F);"> MyValue</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#24292E, #A89984);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#005CC5, #83A598);"> self</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#24292E, #83A598);">value</span><span style="color: light-dark(#24292E, #A89984);">)</span></span> <span class="giallo-l"><span style="color: light-dark(#24292E, #A89984);"> }</span></span> <span class="giallo-l"><span style="color: light-dark(#24292E, #A89984);">}</span></span></code></pre> <p>Getting the syntax right on this took a sec but it's fairly self-explanatory if you're used to Rust. I also went down a road of trying to use <code>impl From</code>, which doesn't really work out (try it yourself if you want to know why). Anyway, now you can call <code>do_it</code> from Rust and give it a boxed, but otherwise plain, closure:</p> <pre class="giallo" style="color-scheme: light dark; color: light-dark(#24292E, #EBDBB2); background-color: light-dark(#FFFFFF, #1D2021);"><code data-lang="rust"><span class="giallo-l"><span style="color: light-dark(#6F42C1, #FABD2F);">do_it</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#6F42C1, #FABD2F);">Box</span><span style="color: light-dark(#D73A49, #8EC07C);">::</span><span style="color: light-dark(#6F42C1, #FABD2F);">new</span><span style="color: light-dark(#24292E, #A89984);">(</span><span style="color: light-dark(#D73A49, #FB4934);">|</span><span style="color: light-dark(#24292E, #83A598);">_it</span><span style="color: light-dark(#D73A49, #FB4934);">|</span><span style="color: light-dark(#24292E, #A89984);"> {</span><span style="color: light-dark(#24292E, #A89984);">}</span><span style="color: light-dark(#24292E, #A89984);">)</span><span style="color: light-dark(#24292E, #A89984);">)</span></span></code></pre>