[new feature] Uploader: before-read support promise mode (#3572)

packages/uploader/test/__snapshots__/demo.spec.js.snap

@@ -36,5 +36,13 @@ exports[`renders demo correctly 1`] = `
       </div>
     </div>
   </div>
+  <div>
+    <div class="van-uploader">
+      <div class="van-uploader__wrapper">
+        <div class="van-uploader__upload"><i class="van-icon van-icon-plus van-uploader__upload-icon">
+            <!----></i><input type="file" accept="image/*" class="van-uploader__input"></div>
+      </div>
+    </div>
+  </div>
 </div>
 `;

packages/uploader/test/index.spec.js

@@ -94,6 +94,42 @@ it('before read return false', () => {
   expect(input.element.value).toEqual('');
 });
 
+it('before read return promise and resolve', async () => {
+  const afterRead = jest.fn();
+  const wrapper = mount(Uploader, {
+    propsData: {
+      beforeRead: () => new Promise(resolve => {
+        resolve();
+      }),
+      afterRead
+    }
+  });
+
+  wrapper.vm.onChange(file);
+
+  await later();
+  expect(afterRead).toHaveBeenCalledTimes(1);
+});
+
+it('before read return promise and reject', async () => {
+  const afterRead = jest.fn();
+  const wrapper = mount(Uploader, {
+    propsData: {
+      beforeRead: () => new Promise((resolve, reject) => {
+        reject();
+      }),
+      afterRead
+    }
+  });
+
+  const input = wrapper.find('input');
+
+  await later();
+  wrapper.vm.onChange(file);
+  expect(afterRead).toHaveBeenCalledTimes(0);
+  expect(input.element.value).toEqual('');
+});
+
 test('file size overlimit', async () => {
   const wrapper = mount(Uploader, {
     propsData: {