I tried calling Java / Objective-C native code from Flutter

I want to call the native API from Flutter

Try native code calls from Flutter to Java (Android) and Objective-C (iOS) using MethodChannel It was.

I tried running the sample code described in Writing custom platform-specific code, so make a note of the actual flow.

Click here for Kotlin / Swift version I tried calling Kotlin / Swift native code from Flutter --Qiita

Click here for sample source https://github.com/unsolublesugar/flutter-sample-custom-platform-specific-code-java-objc

Creating a sample project

First, create a sample project. The sample app implements native code to get the battery level.

In Flutter v1.9.1, the default native languages were Kotlin and Swift, so create a project in Java / Objective-C with options. $ flutter create -i objc -a java batterylevel

If you are creating a project with VSCode, you can switch the language used by selecting [Extensions]-> [Dart & Flutter] in the settings. You can access the settings in one shot by searching for "flutter create language".

Call code on the Flutter side

Add the implementation of the sample screen widget and the battery level acquisition method. Below is the main.dart file, which is an excerpt of only _MyHomePageState.

main.dart

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
...
class _MyHomePageState extends State<MyHomePage> {
  static const platform = const MethodChannel('samples.flutter.dev/battery');

  String _batteryLevel = 'Unknown battery level.';

  Future<void> _getBatteryLevel() async {
    // Get battery level.
    String batteryLevel;
    try {
      final int result = await platform.invokeMethod('getBatteryLevel');
      batteryLevel = 'Battery level at $result % .';
    } on PlatformException catch (e) {
      batteryLevel = "Failed to get battery level: '${e.message}'.";
    }

    setState(() {
      _batteryLevel = batteryLevel;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Material(
      child: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            RaisedButton(
              child: Text('Get Battery Level'),
              onPressed: _getBatteryLevel,
            ),
            Text(_batteryLevel),
          ],
        ),
      ),
    );
  }
}

The battery level acquisition method getBatteryLevel called by MethodChannel is defined on the Java and Objective-C sides, respectively.

Native code on Java side (Android)

Now let's write the Java (Android) code.

Add required import

Added import to MainActivity.java.

MainActivity.java

import android.os.Bundle;
import android.content.ContextWrapper;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.BatteryManager;
import android.os.Build.VERSION;
import android.os.Build.VERSION_CODES;

import io.flutter.app.FlutterActivity;
import io.flutter.plugins.GeneratedPluginRegistrant;
import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
import io.flutter.plugin.common.MethodChannel.Result;

Implement getBatteryLevel

Define MethodChannel in the ʻonCreate method of MainActivity and call setMethodCallHandler.

MainActivity.java

public class MainActivity extends FlutterActivity {
    private static final String CHANNEL = "samples.flutter.dev/battery";

    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        GeneratedPluginRegistrant.registerWith(this);

        new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
                new MethodCallHandler() {
                    @Override
                    public void onMethodCall(MethodCall call, Result result) {
                    }
                });
    }
}

ここで定義したチャンネル名samples.flutter.dev/batteryを、Flutter側の呼び出しで使用します。

Add code to get the battery level of your Android device.

MainActivity.java

  private int getBatteryLevel() {
    int batteryLevel = -1;
    if (VERSION.SDK_INT >= VERSION_CODES.LOLLIPOP) {
      BatteryManager batteryManager = (BatteryManager) getSystemService(BATTERY_SERVICE);
      batteryLevel = batteryManager.getIntProperty(BatteryManager.BATTERY_PROPERTY_CAPACITY);
    } else {
      Intent intent = new ContextWrapper(getApplicationContext()).
          registerReceiver(null, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
      batteryLevel = (intent.getIntExtra(BatteryManager.EXTRA_LEVEL, -1) * 100) /
          intent.getIntExtra(BatteryManager.EXTRA_SCALE, -1);
    }

    return batteryLevel;
  }

Added code to call getBatteryLevel () on MethodChannel.

MainActivity.java

@Override
public void onMethodCall(MethodCall call, Result result) {
    // Note: this method is invoked on the main thread.
    if (call.method.equals("getBatteryLevel")) {
        int batteryLevel = getBatteryLevel();

        if (batteryLevel != -1) {
            result.success(batteryLevel);
        } else {
            result.error("UNAVAILABLE", "Battery level not available.", null);
        }
    } else {
        result.notImplemented();
    }
}

Now you are ready to call Java native code for Android.

Run on emulator

If you build run the above code on the emulator, you will see a "Get Battery Level" button and "Unknown battery level." Text.

Get the battery level of your device with the tap of a button. The text display switches.

Native code on the Objective-C side (iOS)

Next, I will write the Objective-C (iOS) code. Open the ios folder in the target project and open ʻAppDelegate.m under the Runner` folder.

Implement getBatteryLevel

Defines the implementation of MethodChannel as in Android.

AppDelegate.m

#include "AppDelegate.h"
#include "GeneratedPluginRegistrant.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application
    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  FlutterViewController* controller = (FlutterViewController*)self.window.rootViewController;

  FlutterMethodChannel* batteryChannel = [FlutterMethodChannel
                                          methodChannelWithName:@"samples.flutter.dev/battery"
                                          binaryMessenger:controller];

  [batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
  }];


  [GeneratedPluginRegistrant registerWithRegistry:self];
  // Override point for customization after application launch.
  return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

@end

Added the battery level acquisition method getBatteryLevel before @end.

AppDelegate.m

- (int)getBatteryLevel {
  UIDevice* device = UIDevice.currentDevice;
  device.batteryMonitoringEnabled = YES;
  if (device.batteryState == UIDeviceBatteryStateUnknown) {
    return -1;
  } else {
    return (int)(device.batteryLevel * 100);
  }
}

Call getBatteryLevel with setMethodCallHandler.

AppDelegate.m

__weak typeof(self) weakSelf = self;
  [batteryChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
    if ([@"getBatteryLevel" isEqualToString:call.method]) {
      int batteryLevel = [weakSelf getBatteryLevel];

      if (batteryLevel == -1) {
        result([FlutterError errorWithCode:@"UNAVAILABLE"
                                  message:@"Battery info unavailable"
                                  details:nil]);
      } else {
        result(@(batteryLevel));
      }
    } else {
      result(FlutterMethodNotImplemented);
    }
  }];

When you launch iOS Simulator, you will see buttons and text just like Android. When you tap the button, getBatteryLevel () is called via MethodChannel and the text is updated. Since the battery API is not supported on iOS Simulator, the message "Battery info unavailable" is displayed. Now you can see that the code written on the Objective-C side is working.

Flutter has a terrifying amount of official documentation and is awesome.