[new feature] Checkbox: support config the maximum amount of checked options (#631)

test/unit/components/checkbox.vue

@@ -1,5 +1,5 @@
 <template>
-  <van-checkbox-group v-model="result">
+  <van-checkbox-group v-model="result" :max="max">
     <van-checkbox v-for="(item, index) in list" :key="index" :name="item" :disabled="index === 2">复选框{{item}}</van-checkbox>
   </van-checkbox-group>
 </template>
@@ -22,7 +22,8 @@ export default {
         'c',
         'd'
       ],
-      result: ['a', 'b']
+      result: ['a', 'b'],
+      max: 0
     };
   }
 };

test/unit/specs/checkbox.spec.js

@@ -67,6 +67,23 @@ describe('CheckboxGroup', () => {
     });
   });
 
+  it('click on unchecked item and checked options num beyond max', (done) => {
+    wrapper = mount(CheckboxTestComponent);
+
+    wrapper.setData({
+      'max': 2
+    });
+
+    const lastCheckboxLabel = wrapper.find('.van-checkbox')[3].find('.van-checkbox__label')[0];
+    lastCheckboxLabel.trigger('click');
+
+    wrapper.update();
+    wrapper.vm.$nextTick(() => {
+      expect(wrapper.vm.result.indexOf('d')).to.equal(-1);
+      done();
+    });
+  });
+
   it('click on disabled item', (done) => {
     wrapper = mount(CheckboxTestComponent);