Improve error handling

Author: jiegec

ohos/src/main/ets/components/plugin/FlutterNfcKitPlugin.ets

@@ -141,22 +141,31 @@ export default class FlutterNfcKitPlugin implements FlutterPlugin, MethodCallHan
       } else if (call.method == "transceive") {
         let tx: string = call.argument("data");
         hilog.info(0x0000, "FlutterNfcKitPlugin", "transceive: tx=%{public}s", JSON.stringify(tx));
-        this.tag!.connect();
-        this.tag!.transmit(fromHexString(tx)).then((rx) => {
-          hilog.info(0x0000, "FlutterNfcKitPlugin", "transceive: rx=%{public}s", JSON.stringify(rx));
-          result.success(toHexString(rx));
-        }).catch((err: BusinessError) => {
-          hilog.info(0x0000, "FlutterNfcKitPlugin", "Failed to transceive: err=%{public}s", JSON.stringify(err));
-          result.error("500", "Transceive failed", err);
-        });
+        if (this.tag !== null) {
+          this.tag!.connect();
+          this.tag!.transmit(fromHexString(tx)).then((rx) => {
+            hilog.info(0x0000, "FlutterNfcKitPlugin", "transceive: rx=%{public}s", JSON.stringify(rx));
+            result.success(toHexString(rx));
+          }).catch((err: BusinessError) => {
+            hilog.info(0x0000, "FlutterNfcKitPlugin", "Failed to transceive: err=%{public}s", JSON.stringify(err));
+            result.error("500", "Transceive failed", err);
+          });
+        } else {
+          result.error("406", "No tag polled", null);
+        }
       } else if (call.method == "finish") {
         // turn off reader mode
         tag.off("readerMode", elementName, (err: BusinessError, tagInfo: tag.TagInfo) => {});
+        this.tag = null;
         result.success("");
       } else if (call.method == "setIosAlertMessage") {
         // do nothing, just for compatibility
         result.success("");
+      } else if (call.method == "readNDEF") {
+        // TODO
+        result.success("[]");
       } else {
+        hilog.info(0x0000, "FlutterNfcKitPlugin", "Not implemented method: %{public}s", call.method);
         result.notImplemented();
       }
     } catch (e) {