Implement interstitial ads on Android

android/src/main/kotlin/com/shatsy/admobflutter/AdmobInterstitial.kt

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+package com.shatsy.admobflutter
+
+import android.content.Context
+import com.google.android.gms.ads.AdRequest
+import com.google.android.gms.ads.InterstitialAd
+import io.flutter.plugin.common.MethodCall
+import io.flutter.plugin.common.MethodChannel
+
+class AdmobInterstitial(private val context: Context): MethodChannel.MethodCallHandler {
+  companion object {
+    val allAds: MutableMap<Int, InterstitialAd> = mutableMapOf()
+  }
+  override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
+    when(call.method) {
+      "load" -> {
+        val id = call.argument<Int>("id")
+        val adUnitId = call.argument<String>("adUnitId")
+        val adRequest = AdRequest.Builder().build()
+
+        allAds[id!!] = InterstitialAd(context)
+        allAds[id]!!.adUnitId = adUnitId
+        allAds[id]?.loadAd(adRequest)
+        result.success(null)
+      }
+      "isLoaded" -> {
+        val id = call.argument<Int>("id")
+
+        if (allAds[id]!!.isLoaded) {
+          result.success(true)
+        } else result.success(false)
+      }
+      "show" -> {
+        val id = call.argument<Int>("id")
+
+        if (allAds[id]!!.isLoaded) {
+          allAds[id]!!.show()
+        } else result.error(null, null, null)
+      }
+      "dispose" -> {
+        val id = call.argument<Int>("id")
+
+        allAds.remove(id)
+      }
+      else -> result.notImplemented()
+    }
+  }
+}

lib/src/admob_interstitial.dart

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+import 'package:flutter/services.dart';
+import 'package:meta/meta.dart';
+
+class AdmobInterstitial {
+  static Map<int, AdmobInterstitial> _allAds = <int, AdmobInterstitial>{};
+  static const MethodChannel _channel =
+      const MethodChannel('admob_flutter/interstitial');
+
+  int id;
+  final String adUnitId;
+
+  AdmobInterstitial({@required this.adUnitId}) {
+    id = hashCode;
+    _allAds[id] = this;
+  }
+
+  Future<bool> get isLoaded async {
+    final bool result = await _channel.invokeMethod('isLoaded', <String, dynamic>{
+      'id': id,
+    });
+    return result;
+  }
+
+  void loadAd() {
+    _channel.invokeMethod('load', <String, dynamic>{
+      'id': id,
+      'adUnitId': adUnitId,
+    });
+  }
+
+  void show() async {
+    if (await isLoaded == true) {
+      _channel.invokeMethod('show', <String, dynamic>{
+        'id': id,
+      });
+    }
+  }
+
+  void dispose() async {
+    await _channel.invokeMethod('dispose', <String, dynamic>{
+      'id': id,
+    });
+
+    _allAds.remove(id);
+  }
+}