Add composition event on macOS (#1979)

* Enable to show text when IME is active

* Remove unnecessary variable

* Enable to use IME

* fmt

* Remove println! for debug

* Fix handling of utf-8 string

* clear_marked_text should be rust function, not member function

* Store state information in ViewState

* Remove unnecessary function

* format

* Remove mut

* format

* Remove duplicate marked text

* Remove unused `is_preediting` field

Co-authored-by: Artúr Kovács <kovacs.artur.barnabas@gmail.com>

src/platform_impl/macos/util/mod.rs

@@ -3,8 +3,13 @@ mod cursor;
 
 pub use self::{cursor::*, r#async::*};
 
-use std::ops::{BitAnd, Deref};
+use std::{
+    cell::Cell,
+    ops::{BitAnd, Deref},
+    rc::Rc,
+};
 
+use block::ConcreteBlock;
 use cocoa::{
     appkit::{NSApp, NSWindowStyleMask},
     base::{id, nil},
@@ -13,8 +18,12 @@ use cocoa::{
 use core_graphics::display::CGDisplay;
 use objc::runtime::{Class, Object, Sel, BOOL, YES};
 
-use crate::dpi::LogicalPosition;
-use crate::platform_impl::platform::ffi;
+use crate::{
+    dpi::LogicalPosition,
+    platform_impl::platform::ffi::{
+        self, NSRange, NSStringEnumerationByComposedCharacterSequences,
+    },
+};
 
 // Replace with `!` once stable
 #[derive(Debug)]
@@ -104,6 +113,31 @@ pub unsafe fn ns_string_id_ref(s: &str) -> IdRef {
     IdRef::new(NSString::alloc(nil).init_str(s))
 }
 
+/// Returns the number of characters in a string.
+/// (A single character may consist of multiple UTF-32 code units.
+/// This is possible when long sequences of composing characters are present)
+///
+/// Unsafe because assumes that the `string` is an `NSString` object
+pub unsafe fn ns_string_char_count(string: id) -> usize {
+    let length: NSUInteger = msg_send![string, length];
+    let range = NSRange {
+        location: 0,
+        length,
+    };
+    let char_count = Rc::new(Cell::new(0));
+    let block = {
+        let char_count = char_count.clone();
+        ConcreteBlock::new(move || char_count.set(char_count.get() + 1)).copy()
+    };
+    let block = &*block;
+    let () = msg_send![string,
+        enumerateSubstringsInRange:range
+        options:NSStringEnumerationByComposedCharacterSequences
+        usingBlock:block
+    ];
+    char_count.get()
+}
+
 #[allow(dead_code)] // In case we want to use this function in the future
 pub unsafe fn app_name() -> Option<id> {
     let bundle: id = msg_send![class!(NSBundle), mainBundle];