android8.0带来的新的特性—— notification channel

SDK上是这样描述channel的
A representation of settings that apply to a collection of similarly themed notifications.
简单的说，channel用来代表一系列相似的通知，即拥有相同特性的通知，使用channel可以避免创建通知产生的冗余代码

1.创建Notification Channel

单个channel

        NotificationChannel channel = new NotificationChannel("id", "name", NotificationManager.IMPORTANCE_DEFAULT);
        NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        notificationManager.createNotificationChannel(channel);

多个channel的创建

        NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        notificationManager.createNotificationChannels(Arrays.asList(
                new NotificationChannel(
                        "id",
                        "name",
                        NotificationManager.IMPORTANCE_LOW),
                new NotificationChannel(
                        "id1",
                        "name1",
                        NotificationManager.IMPORTANCE_DEFAULT),
                new NotificationChannel(
                        "id2",
                        "name2",
                        NotificationManager.IMPORTANCE_HIGH)
        ));

channel也支持其他属性的设置，例如震动模式，声音，led灯光等

2.创建通知

Notification.Builder builder = new Notification.Builder(this, "id1");
Notification notification = builder.setContentTitle("title").build();
notificationManager.notify(1, notification);

或者

Notification.Builder builder = new Notification.Builder(this);
Notification notification = builder.setChannelId("id1").setContentTitle("title").build();
notificationManager.notify(1, notification);

需要注意的是，在Android8.0(app的targetSdkVersion >= 26)上创建通知的时候必须指定channel，否则无法正常显示，见notificationmanagerservice

void enqueueNotificationInternal(final String pkg, final String opPkg, final int callingUid,
            final int callingPid, final String tag, final int id, final Notification notification,
            int incomingUserId) {
        checkCallerIsSystemOrSameApp(pkg);
        ....
        final NotificationChannel channel = mRankingHelper.getNotificationChannel(pkg,
                notificationUid, channelId, false /* includeDeleted */);
        if (channel == null) {

            doChannelWarningToast("Developer warning for package \"" + pkg + "\"\n" +
                    "Failed to post notification on channel \"" + channelId + "\"\n" +
                    "See log for more details");
            return;
        }
        ...
        mHandler.post(new EnqueueNotificationRunnable(userId, r));
    }

3.兼容方案

一般使用NotificationCompat类而不是Notification创建通知，并且要加入系统版本的判断

        NotificationCompat.Builder builder = null;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
        {
            builder = new NotificationCompat.Builder(this, CHANNEL_ID);
        } else
        {
            builder = new NotificationCompat.Builder(this);
        }
        ...

参考：https://developer.android.google.cn/reference/android/app/NotificationChannel