1 files changed, 50 insertions, 0 deletions
diff --git a/src/lib/LastListener.js b/src/lib/LastListener.js
new file mode 100644
index 0000000..51600eb
--- /dev/null
+++ b/src/lib/LastListener.js
@@ -0,0 +1,50 @@
+/**
+* Wrapper around listeners on various WebExtensions
+* APIs (e.g. webRequest.on*), as a best effort to 
+* let them run last by removing and re-adding them 
+* on each call (swapping 2 copies because 
+* addListener() calls are asynchronous).
+* Note: we rely on implementation details Like
+* listeners being called in addition order; also,
+* clients should ensure they're not called twice for
+* the same event, if that's important.
+}
+*/
+
+class LastListener {
+  constructor(observed, listener, ...extras) {
+    this.observed = observed;
+    this.listener = listener;
+    this.extras = extras;
+    let ww = this._wrapped = [listener, listener].map(l => {
+      let w = (...args) => {
+        if (this.observed.hasListener(w._other)) {
+          this.observed.removeListener(w._other);
+          if (this.last === w) return this.defaultResult;
+        } else if (this.installed) { 
+          this.observed.addListener(w._other, ...this.extras);
+          this.last = w._other;
+        }
+        return this.installed ? this.listener(...args)
+          : this.defaultResult;
+      }
+      return w;
+    });
+    
+    ww[0]._other = ww[1];
+    ww[1]._other = ww[0];
+    this.installed = false;
+    this.defaultResult = null;
+  }
+
+  install() {
+    if (this.installed) return;
+    this.observed.addListener(this._wrapped[0], ...this.extras);
+    this.installed = true;
+  }
+  
+  uninstall() {
+    this.installed = false;
+    for (let l of this._wrapped) this.observed.removeListener(l);
+  }
+}