fix flicker with observedValues buffer (#697)

We do this by constructing a buffer of observed input values. Then, each time the server renders, we check to see if the next rendered value matches the first value in the buffer. If it does then we continue to display the last value in the buffer and shift the buffer array. If it doesn't then we clear the buffer array and display that rendered value.

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")