fix flicker with observedValues buffer

src/client/packages/idom-client-react/src/components.js

@@ -81,7 +81,7 @@ function UserInputElement({ model }) {
   // order to allow all changes committed by the user to be recorded in the order they
   // occur. If we don't the user may commit multiple changes before we render next
   // causing the content of prior changes to be overwritten by subsequent changes.
-  const value = props.value;
+  let value = props.value;
   delete props.value;
 
   // Instead of controlling the value, we set it in an effect.
@@ -91,6 +91,25 @@ function UserInputElement({ model }) {
     }
   }, [ref.current, value]);
 
+  // Track a buffer of observed values in order to avoid flicker
+  const observedValues = React.useState([])[0];
+  if (observedValues) {
+    if (value === observedValues[0]) {
+      observedValues.shift();
+      value = observedValues[observedValues.length - 1];
+    } else {
+      observedValues.length = 0;
+    }
+  }
+
+  const givenOnChange = props.onChange;
+  if (typeof givenOnChange === "function") {
+    props.onChange = (event) => {
+      observedValues.push(event.target.value);
+      givenOnChange(event);
+    };
+  }
+
   // Use createElement here to avoid warning about variable numbers of children not
   // having keys. Warning about this must now be the responsibility of the server
   // providing the models instead of the client rendering them.

tests/test_client.py

@@ -1,7 +1,10 @@
+import asyncio
+import time
 from pathlib import Path
 
 import idom
 from idom.testing import ServerMountPoint
+from tests.driver_utils import send_keys
 
 
 JS_DIR = Path(__file__).parent / "js"
@@ -71,14 +74,44 @@ def ButtonWithChangingColor():
 
     button = driver.find_element("id", "my-button")
 
-    assert get_style(button)["background-color"] == "red"
+    assert _get_style(button)["background-color"] == "red"
 
     for color in ["blue", "red"] * 2:
         button.click()
-        driver_wait.until(lambda _: get_style(button)["background-color"] == color)
+        driver_wait.until(lambda _: _get_style(button)["background-color"] == color)
 
 
-def get_style(element):
+def _get_style(element):
     items = element.get_attribute("style").split(";")
     pairs = [item.split(":", 1) for item in map(str.strip, items) if item]
     return {key.strip(): value.strip() for key, value in pairs}
+
+
+def test_slow_server_response_on_input_change(display, driver, driver_wait):
+    """A delay server-side could cause input values to be overwritten.
+
+    For more info see: https://github.com/idom-team/idom/issues/684
+    """
+
+    delay = 0.2
+
+    @idom.component
+    def SomeComponent():
+        value, set_value = idom.hooks.use_state("")
+
+        async def handle_change(event):
+            await asyncio.sleep(delay)
+            set_value(event["target"]["value"])
+
+        return idom.html.input({"onChange": handle_change, "id": "test-input"})
+
+    display(SomeComponent)
+
+    inp = driver.find_element("id", "test-input")
+
+    text = "hello"
+    send_keys(inp, text)
+
+    time.sleep(delay * len(text) * 1.1)
+
+    driver_wait.until(lambda _: inp.get_attribute("value") == "hello")