RN TextInput: don't let user more than maxLength when TextInput already exceeds it

Summary:
We had a classic integer underflow problem here. Before we would let you type endlessly when the text already exceeded the TextInput maxLength, now we only let you erase characters.

There is still a small problem with the TextInput: how do you handle when the value (set from JS) exceeds the maxLength? This could happen pragmatically, just by passing in a very large value or when changing maxLength (e.g. when changing from 4 to 3 digits in the case of a AMEX security code -> VISA security code).

Me and achen1 discussed firing onChange in these cases and truncating the number manually (to ensure JS's data model) was aware of the change but it seemed fraught with bugs and general weirdness in what the caller would expect to happen.

Reviewed By: javache

Differential Revision: D3991210

fbshipit-source-id: dc401c4a7aefe09fa749cd1168d36343d39dc196

Examples/UIExplorer/js/TextInputExample.ios.js

@@ -832,5 +832,41 @@ exports.examples = [
         </View>
       );
     }
-  }
+  },
+  {
+    title: 'TextInput maxLength',
+    render: function() {
+      return (
+        <View>
+          <WithLabel label="maxLength: 5">
+            <TextInput
+              maxLength={5}
+              style={styles.default}
+            />
+          </WithLabel>
+          <WithLabel label="maxLength: 5 with placeholder">
+            <TextInput
+              maxLength={5}
+              placeholder="ZIP code entry"
+              style={styles.default}
+            />
+          </WithLabel>
+          <WithLabel label="maxLength: 5 with default value already set">
+            <TextInput
+              maxLength={5}
+              defaultValue="94025"
+              style={styles.default}
+            />
+          </WithLabel>
+          <WithLabel label="maxLength: 5 with very long default value already set">
+            <TextInput
+              maxLength={5}
+              defaultValue="9402512345"
+              style={styles.default}
+            />
+          </WithLabel>
+        </View>
+      );
+    }
+  },
 ];

Libraries/Text/RCTTextFieldManager.m

@@ -41,7 +41,7 @@ RCT_EXPORT_MODULE()
   if (textField.maxLength == nil || [string isEqualToString:@"\n"]) {  // Make sure forms can be submitted via return
     return YES;
   }
-  NSUInteger allowedLength = textField.maxLength.integerValue - textField.text.length + range.length;
+  NSUInteger allowedLength = textField.maxLength.integerValue - MIN(textField.maxLength.integerValue, textField.text.length) + range.length;
   if (string.length > allowedLength) {
     if (string.length > 1) {
       // Truncate the input string so the result is exactly maxLength