Input, SmartUrlInput: Use modern React.createRef API.

Now that the `Input` component clearly doesn't use the ref on its
child `TextInput` -- only `Input`'s consumers use it; see the
previous commit -- we can straightforwardly use the new
`React.createRef` API.

We can also do so in `SmartUrlInput`.

This will smoothen some necessary changes during the upcoming RN
v0.61 -> v0.62 upgrade. In particular, it seems that the way
`TextInput` is defined has changed in an interesting way (it
certainly has changed), or else something happened in the Flow
upgrade, to cause this error in several places where we use
`TextInput` as a type:

```
Cannot use `TextInput` as a type. A name can be used as a type only
if it refers to a type definition, an interface definition, or a
class definition. To get the type of a non-class value, use
`typeof`.
```

I discovered, in this commit, that `React$Ref`, not
`React$ElementRef`, seems to be the correct type for the `ref`
attribute and the return value of `React.createRef`. Weeks ago, we
used `React$RefElement` for `this.mentionWarnings` in `ComposeBox`
and overlooked [1] the fact that it ended up being more or less
completely un-typechecked, in 20bd98a. I'll add a TODO in the next
commit, saying we should fix that.

[1] #4101 (comment)

src/common/Input.js

@@ -11,7 +11,7 @@ export type Props = $ReadOnly<{|
   ...$PropertyType<TextInput, 'props'>,
   placeholder: LocalizableText,
   onChangeText?: (text: string) => void,
-  textInputRef?: (component: ?TextInput) => void,
+  textInputRef?: React$Ref<typeof TextInput>,
 |}>;
 
 type State = {|
@@ -87,11 +87,7 @@ export default class Input extends PureComponent<Props, State> {
             underlineColorAndroid={isFocused ? BORDER_COLOR : HALF_COLOR}
             onFocus={this.handleFocus}
             onBlur={this.handleBlur}
-            ref={(component: ?TextInput) => {
-              if (textInputRef) {
-                textInputRef(component);
-              }
-            }}
+            ref={textInputRef}
             {...restProps}
           />
         )}

src/common/InputWithClearButton.js

@@ -32,7 +32,7 @@ export default class InputWithClearButton extends PureComponent<Props, State> {
     canBeCleared: false,
     text: '',
   };
-  textInput: ?TextInput;
+  textInputRef = React.createRef<TextInput>();
 
   handleChangeText = (text: string) => {
     this.setState({
@@ -46,8 +46,8 @@ export default class InputWithClearButton extends PureComponent<Props, State> {
 
   handleClear = () => {
     this.handleChangeText('');
-    if (this.textInput) {
-      this.textInput.clear();
+    if (this.textInputRef.current) {
+      this.textInputRef.current.clear();
     }
   };
 
@@ -58,9 +58,7 @@ export default class InputWithClearButton extends PureComponent<Props, State> {
       <View style={styles.row}>
         <Input
           {...this.props}
-          textInputRef={textInput => {
-            this.textInput = textInput;
-          }}
+          textInputRef={this.textInputRef}
           onChangeText={this.handleChangeText}
           value={text}
         />

src/common/SmartUrlInput.js

@@ -66,13 +66,13 @@ export default class SmartUrlInput extends PureComponent<Props, State> {
   state = {
     value: '',
   };
-  textInputRef: ?TextInput;
+  textInputRef = React.createRef<TextInput>();
   focusListener: void | NavigationEventSubscription;
 
   componentDidMount() {
     this.focusListener = this.props.navigation.addListener('didFocus', () => {
-      if (this.textInputRef) {
-        this.textInputRef.focus();
+      if (this.textInputRef.current) {
+        this.textInputRef.current.focus();
       }
     });
   }
@@ -92,9 +92,13 @@ export default class SmartUrlInput extends PureComponent<Props, State> {
 
   urlPress = () => {
     const { textInputRef } = this;
-    if (textInputRef) {
-      textInputRef.blur();
-      setTimeout(() => textInputRef.focus(), 100);
+    if (textInputRef.current) {
+      textInputRef.current.blur();
+      setTimeout(() => {
+        if (textInputRef.current) {
+          textInputRef.current.focus();
+        }
+      }, 100);
     }
   };
 
@@ -144,9 +148,7 @@ export default class SmartUrlInput extends PureComponent<Props, State> {
           underlineColorAndroid="transparent"
           onSubmitEditing={onSubmitEditing}
           enablesReturnKeyAutomatically={enablesReturnKeyAutomatically}
-          ref={(component: TextInput | null) => {
-            this.textInputRef = component;
-          }}
+          ref={this.textInputRef}
         />
         {!value && this.renderPlaceholderPart(defaultOrganization)}
         {suffix !== null && this.renderPlaceholderPart(suffix)}

src/compose/ComposeBox.js

@@ -99,28 +99,28 @@ type State = {|
   selection: InputSelection,
 |};
 
-export const updateTextInput = (textInput: ?TextInput, text: string): void => {
-  if (!textInput) {
+export const updateTextInput = (textInputInstance: TextInput | null, text: string): void => {
+  if (textInputInstance === null) {
     // Depending on the lifecycle events this function is called from,
     // this might not be set yet.
     return;
   }
 
-  textInput.setNativeProps({ text });
+  textInputInstance.setNativeProps({ text });
 
   if (text.length === 0 && TextInputReset) {
     // React Native has problems with some custom keyboards when clearing
     // the input's contents.  Force reset to make sure it works.
-    TextInputReset.resetKeyboardInput(findNodeHandle(textInput));
+    TextInputReset.resetKeyboardInput(findNodeHandle(textInputInstance));
   }
 };
 
 class ComposeBox extends PureComponent<Props, State> {
   static contextType = ThemeContext;
   context: ThemeData;
 
-  messageInput: ?TextInput = null;
-  topicInput: ?TextInput = null;
+  messageInputRef = React.createRef<TextInput>();
+  topicInputRef = React.createRef<TextInput>();
   mentionWarnings: React$ElementRef<MentionWarnings> = React.createRef();
   inputBlurTimeoutId: ?TimeoutID = null;
 
@@ -159,12 +159,12 @@ class ComposeBox extends PureComponent<Props, State> {
   };
 
   setMessageInputValue = (message: string) => {
-    updateTextInput(this.messageInput, message);
+    updateTextInput(this.messageInputRef.current, message);
     this.handleMessageChange(message);
   };
 
   setTopicInputValue = (topic: string) => {
-    updateTextInput(this.topicInput, topic);
+    updateTextInput(this.topicInputRef.current, topic);
     this.handleTopicChange(topic);
   };
 
@@ -299,8 +299,8 @@ class ComposeBox extends PureComponent<Props, State> {
       });
     }
     completeEditMessage();
-    if (this.messageInput) {
-      this.messageInput.blur();
+    if (this.messageInputRef.current !== null) {
+      this.messageInputRef.current.blur();
     }
   };
 
@@ -313,8 +313,8 @@ class ComposeBox extends PureComponent<Props, State> {
       const message = nextProps.editMessage ? nextProps.editMessage.content : '';
       this.setMessageInputValue(message);
       this.setTopicInputValue(topic);
-      if (this.messageInput) {
-        this.messageInput.focus();
+      if (this.messageInputRef.current !== null) {
+        this.messageInputRef.current.focus();
       }
     }
   }
@@ -421,9 +421,7 @@ class ComposeBox extends PureComponent<Props, State> {
                 placeholder="Topic"
                 defaultValue={topic}
                 selectTextOnFocus
-                textInputRef={component => {
-                  this.topicInput = component;
-                }}
+                textInputRef={this.topicInputRef}
                 onChangeText={this.handleTopicChange}
                 onFocus={this.handleTopicFocus}
                 onBlur={this.handleTopicBlur}
@@ -439,9 +437,7 @@ class ComposeBox extends PureComponent<Props, State> {
               underlineColorAndroid="transparent"
               placeholder={placeholder}
               defaultValue={message}
-              textInputRef={component => {
-                this.messageInput = component;
-              }}
+              textInputRef={this.messageInputRef}
               onBlur={this.handleMessageBlur}
               onChangeText={this.handleMessageChange}
               onFocus={this.handleMessageFocus}