Set FileReader to LOADING state while a read is in progress

FileReader's readAs* methods went straight from EMPTY to DONE: they
never set readyState to LOADING. As a result readyState was never
observably LOADING, and abort() — which only emits events when
readyState is neither EMPTY nor DONE — silently did nothing when called
during an in-progress read, so no 'abort'/'loadend' events fired.

Set the LOADING state when each read starts (via _setReadyState, which
also fires 'readystatechange'), matching the FileReader spec and making
abort() during a read work as its own code already anticipated.

Add tests for the LOADING state and for abort() dispatching abort/loadend
during a read.

Author: durvesh1992

packages/react-native/Libraries/Blob/FileReader.js

@@ -80,6 +80,8 @@ class FileReader extends EventTarget {
       );
     }
 
+    this._setReadyState(LOADING);
+
     NativeFileReaderModule.readAsDataURL(blob.data).then(
       (text: string) => {
         if (this._aborted) {
@@ -111,6 +113,8 @@ class FileReader extends EventTarget {
       );
     }
 
+    this._setReadyState(LOADING);
+
     NativeFileReaderModule.readAsDataURL(blob.data).then(
       (text: string) => {
         if (this._aborted) {
@@ -138,6 +142,8 @@ class FileReader extends EventTarget {
       );
     }
 
+    this._setReadyState(LOADING);
+
     NativeFileReaderModule.readAsText(blob.data, encoding).then(
       (text: string) => {
         if (this._aborted) {

packages/react-native/Libraries/Blob/__tests__/FileReader-test.js

@@ -46,6 +46,31 @@ describe('FileReader', function () {
     expect(e.target?.result).toBe('data:text/plain;base64,NDI=');
   });
 
+  it('should be in the LOADING state while a read is in progress', () => {
+    const reader = new FileReader();
+    expect(reader.readyState).toBe(FileReader.EMPTY);
+    reader.readAsText(new Blob());
+    // The native read resolves on a later microtask, so the reader should
+    // report LOADING synchronously after the read starts.
+    expect(reader.readyState).toBe(FileReader.LOADING);
+  });
+
+  it('should dispatch abort and loadend when aborted during a read', () => {
+    const reader = new FileReader();
+    let aborted = false;
+    let loadended = false;
+    reader.onabort = () => {
+      aborted = true;
+    };
+    reader.onloadend = () => {
+      loadended = true;
+    };
+    reader.readAsText(new Blob());
+    reader.abort();
+    expect(aborted).toBe(true);
+    expect(loadended).toBe(true);
+  });
+
   it('should read blob as ArrayBuffer', async () => {
     const e = await new Promise<Event>((resolve, reject) => {
       const reader = new FileReader();