Flutter Future 回調地獄的一種解決思路

背景

嘗試使用Flutter來開發雙端應用,實現訂閱功能. 以前用kotlin實現過一次能夠參考: koltin回調地獄的一種解決思路.

實現過程

UI

這個UI比較簡單

訂閱邏輯

一樣使用流式來實現,主動拋出異常來中斷流.

void _goBuy(SubscriptModel model) async {
    setState(() {
      _isloading = true;
    });
    // 購買前先確認是否是已經付費過,可是本地訂單丟失了
    await FlutterInappPurchase
        .instance
        // 1. 創建和GP/AppStore的鏈接
        .initConnection
        // 2. 查詢歷史緩存訂單,看看是否已是vip了. 換機器能夠restore,不必走網絡查詢
        .then((it) {
          if (it == null) {
            throw InitExection;
          }
          return FlutterInappPurchase.instance
              .getAvailablePurchases()
              .then((historyItems) => model.checkHistorySubscript(historyItems))
              .then((item) {
            if (item != null) {
              model.updatePurchased(item);
              throw VipStateException;
            }
          });
        })
        // 3. 獲取可購買產品列表
        .then((it) {
          return FlutterInappPurchase.instance
              .getSubscriptions(List.filled(1, this._productID))
              .then((items) => items
                  .firstWhere((item) => item.productId == this._productID));
        })
        // 4. 購買
        .then((item) {
          return FlutterInappPurchase.instance
              .requestSubscription(this._productID)
              .then((itemStr) => PurchasedItem.fromJSON(jsonDecode(itemStr)));
        })
        // 5.校驗
        .then((purchaseItem) => model.checkSubscript(purchaseItem))
        // 6.客戶端確認訂單
        .then((purchaseItem) {
          model.updatePurchased(purchaseItem);
          SubscriptModel.log(StatisticConstant.SUCCESS);
          FlutterInappPurchase.instance.consumePurchaseAndroid(
              purchaseItem.purchaseToken, purchaseItem.isAcknowledgedAndroid);
        })
        // 異常處理
        .catchError((e) {
          switch (e.runtimeType) {
            case PlatformException:
              String method = e.code;
              if (method == "service_error") {
                _dismissLoading(
                    '''Google services are unavailable. Install "GoogleService" and communicate again.''');
              } else if (method == "buyItemByType") {
                _dismissLoading("");
              }
              break;
            case VipStateException:
              SubscriptModel.data[StatisticConstant.REASON] = 'isVip';
              _dismissLoading('''Already is VIP. No repurchase required.''');
              break;
            case InitExection:
              _dismissLoading(
                  '''Google services are unavailable. Install "GoogleService" and communicate again.''');
              break;
          }
        })
        .whenComplete(() => {_dismissLoading("")});
  }
複製代碼

這裏實現過程和kotlin原生實現思路同樣,代碼風格都很接近,接收起來相對容易.一樣實現流式的問題也就轉化換成了:

1. 怎麼把原生代碼的回調轉化成Future流
2. 怎麼處理原生回調哪些異常場景.

把原生代碼的回調轉化成Future

在kotlin,已經試用使用通道把java回調轉化成suspend函數. 這裏也是同樣的,只是多了一層dart封裝.例如上面的initConnection方法先申明爲一個Future函數

Future<String> get initConnection async {
    if (_platform.isAndroid) {
      await _setPurchaseListener();
      String result =
          await _channel.invokeMethod('initConnection').catchError((e) {
        return null;
      });
      return result;
    } else if (_platform.isIOS) {
      await _setPurchaseListener();
      final String result =
          await _channel.invokeMethod('canMakePayments').catchError((e) {
        return null;
      });
      return result;
    }
    throw PlatformException(
        code: _platform.operatingSystem, message: "platform not supported");
  }
複製代碼

iOS和android分別實現initConnection 和 canMakePayments方法. 由於flutter是單線程模型,await和kotlin協程的await同樣是非阻塞的,原生實現功能後能夠直接利用方法通道,結束await.
方法通道的原生java代碼:

@Override
    public void onMethodCall(final MethodCall call, final Result result) {
        if (call.method.equals("initConnection")) {
            billingClient = BillingClient.newBuilder(reg.activity()).setListener(purchasesUpdatedListener)
                    .enablePendingPurchases()
                    .build();
            billingClient.startConnection(new BillingClientStateListener() {
                @Override
                public void onBillingSetupFinished(BillingResult billingResult) {
                    int responseCode = billingResult.getResponseCode();

                    if (responseCode == BillingClient.BillingResponseCode.OK) {
                        result.success("Billing client ready");
                    } else {
                        result.error(call.method, "responseCode: " + responseCode, "");
                    }
                }
            });
        }
    }
複製代碼

這裏result就是起了通道的做用.

異常機制

Future回調有個catchError函數,用法和kotlin的Exceptionhandler類似.比較下兩種代碼

.catchError((e) {
          switch (e.runtimeType) {
            case PlatformException:
              String method = e.code;
              if (method == "service_error") {
                _dismissLoading(
                    '''Google services are unavailable. Install "GoogleService" and communicate again.''');
              } else if (method == "buyItemByType") {
                _dismissLoading("");
              }
              break;
            case VipStateException:
              SubscriptModel.data[StatisticConstant.REASON] = 'isVip';
              _dismissLoading('''Already is VIP. No repurchase required.''');
              break;
            case InitExection:
              _dismissLoading(
                  '''Google services are unavailable. Install "GoogleService" and communicate again.''');
              break;
          }
        })
複製代碼

vs

when (throwable) {
                    is UserCancleException -> {
                        // 用戶取消
                    }
                    is SubscriptProductException -> {
                        // 訂閱購買異常
                    }
                    is InitException -> {
                        // GP初始化異常 通常是沒有裝GP
                    }
                    is RepeateSubscription -> {
                        // 重複購買
                    }
                    is NoProducteException -> {
                        // 找不到要購買的商品信息
                    }
                    is AcknowException -> {
                        // 訂單確認異常
                    }
複製代碼

總結

1. 基於一樣一個業務場景,總體代碼結構dart和koltin兩者類似
2. 把異步callback轉化成同步代碼的方式類似
3. 異常機制處理類似

後續

1. 把iOS端留下的問題補充好.
2. 整理下Flutter的事件模型.