如何在React Native中调用Java字符串并实时显示

Question

我用java创建了字符串：

import java.lang.Math; 
import java.io.*;
import java.util.*;

public class Test {

    public int measureConcentration(double[] means){
        return (int) ((means[3] / means[1]) * 100) ;
    } 

    public static void main(String[] args) {

        int score = measureConcentration(bandMeans);
        String ratio = Integer.toString(score);
        // Print values
        System.out.println(String.valueOf(ratio));

我想在react native中显示它。但当我尝试在powershell中编译应用程序时，由于cannot find symbol int score = measureConcentration(bandMeans);而收到一个错误

我做错了什么

Answer 1

我遵循React Native docs https://facebook.github.io/react-native/docs/native-modules-android.html#callbacks创建了Android本机模块

To expose a method to JavaScript a Java method must be annotated using @ReactMethod. The return type of bridge methods is always void. React Native bridge is asynchronous, so the only way to pass a result to JavaScript is by using callbacks or emitting events

public class TestBridge extends ReactContextBaseJavaModule {

    public TestBridge(ReactApplicationContext reactContext) {
        super(reactContext);
    }

    @Override
    public String getName() {
        return "TestBridge";
    }

    @ReactMethod
    public void getString(Callback stringCallback) {
        stringCallback.invoke("Native module String");
    }
}

及

public class TestBridgePackage implements ReactPackage {

    @Override
    public List<NativeModule> createNativeModules(ReactApplicationContext reactContext) {
        List<NativeModule> modules = new ArrayList<>();

        modules.add(new TestBridge(reactContext));

        return modules;
    }

    @Override
    public List<ViewManager> createViewManagers(ReactApplicationContext reactContext) {

        return Collections.emptyList();
    }
}

在应用程序中插入我的包

public class MainApplication extends Application implements ReactApplication {

  private final ReactNativeHost mReactNativeHost = new ReactNativeHost(this) {
    ...

    @Override
    protected List<ReactPackage> getPackages() {
      return Arrays.<ReactPackage>asList(
          new MainReactPackage(),
          new TestBridgePackage()
      );
    }

    ...
  };

  ...
}

然后用callback在React Native端获取一个字符串

import {NativeModules} from 'react-native';

export default class App extends Component<Props> {
  componentWillMount() {
    const TestBridge = NativeModules.TestBridge;
    TestBridge.getString(string => console.log(string));
  }
}

Answer 2

你试过了吗

import android.util.Log;

然后使用

Log.d("first string goes here", "second string goes here");