perf(core): avoid rebuild all fields on FieldArray update

src/core/json-schema/src/formly-json-schema.service.ts

@@ -602,13 +602,14 @@ export class FormlyJsonschema {
 
   private isFieldValid(field: FormlyFieldConfig, schema: JSONSchema7, options: IOptions): boolean {
     const model = field.model ? clone(field.model) : (field.fieldArray ? [] : {});
-    const { form } = (field.options as any)._buildField({
-      form: Array.isArray(model) ? new FormArray([]) : new FormGroup({}),
+    const { formControl } = (field.options as any)._buildField({
+      formControl: Array.isArray(model) ? new FormArray([]) : new FormGroup({}),
       fieldGroup: [this._toFieldConfig(schema, { ...options, resetOnHide: true, ignoreDefault: true, map: null, strict: true })],
       model,
+      options: {},
     });
 
-    return form.valid;
+    return formControl.valid;
   }
 
   private mergeFields(f1: FormlyFieldConfig, f2: FormlyFieldConfig) {

src/core/src/lib/components/formly.form.ts

@@ -189,6 +189,16 @@ export class FormlyForm implements DoCheck, OnChanges, OnDestroy {
         }
       };
     }
+
+    if (!(<any> this.options)._trackModelChanges) {
+      (this.options as any)._trackModelChanges = (emitModelChange = false) => {
+        this.clearModelSubscriptions();
+        this.trackModelChanges(this.fields);
+        if (emitModelChange) {
+          this.modelChange.emit(this._modelChangeValue = clone(this.model));
+        }
+      };
+    }
   }
 
   private checkExpressionChange() {

src/core/src/lib/services/formly.form.builder.ts

@@ -80,7 +80,10 @@ export class FormlyFormBuilder {
 
     if (!options._buildField) {
       options._buildField = (field: FormlyFieldConfig) => {
-        this.buildForm(field.form, field.fieldGroup, field.model, field.options);
+        this._setOptions(field.options);
+        this._buildForm(field);
+        (field.options as FormlyFormOptionsCache)._checkField(field, true);
+
         return field;
       };
     }

src/core/src/lib/templates/field-array.type.ts

@@ -65,7 +65,7 @@ export abstract class FieldArrayType<F extends FormlyFieldConfig = FormlyFieldCo
 
     this.model.splice(i, 0, initialModel ? clone(initialModel) : undefined);
 
-    (<any> this.options)._buildForm(true);
+    this._build();
     markAsDirty && this.formControl.markAsDirty();
   }
 
@@ -75,7 +75,12 @@ export abstract class FieldArrayType<F extends FormlyFieldConfig = FormlyFieldCo
     this.field.fieldGroup.splice(i, 1);
     this.field.fieldGroup.forEach((f, key) => f.key = `${key}`);
 
-    (<any> this.options)._buildForm(true);
+    this._build();
     markAsDirty && this.formControl.markAsDirty();
   }
+
+  private _build() {
+    (<any> this.options)._buildField(this.field);
+    (<any> this.options)._trackModelChanges(true);
+  }
 }