要编写一个CFN模板来触发Fargate任务，可以按照以下步骤进行操作：

1. 创建一个Amazon Elastic Container Service (ECS)任务定义，其中包含要在Fargate上运行的容器配置。以下是一个示例任务定义：

{
  "family": "my-task",
  "networkMode": "awsvpc",
  "containerDefinitions": [
    {
      "name": "my-container",
      "image": "your-container-image",
      "memory": 512,
      "portMappings": [
        {
          "containerPort": 80,
          "hostPort": 80,
          "protocol": "tcp"
        }
      ]
    }
  ]
}

2. 创建一个ECS服务，它使用上一步中定义的任务定义。以下是一个示例服务定义：

{
  "cluster": "my-cluster",
  "serviceName": "my-service",
  "taskDefinition": "my-task",
  "desiredCount": 1,
  "launchType": "FARGATE",
  "networkConfiguration": {
    "awsvpcConfiguration": {
      "subnets": ["subnet-12345678"],
      "securityGroups": ["sg-12345678"],
      "assignPublicIp": "ENABLED"
    }
  }
}

3. 创建一个CloudWatch事件规则，以触发Fargate任务。以下是一个示例事件规则：

{
  "name": "my-event-rule",
  "description": "Trigger Fargate task",
  "eventPattern": {
    "source": ["aws.ecs"],
    "detail-type": ["ECS Task State Change"],
    "detail": {
      "desiredStatus": ["RUNNING"],
      "lastStatus": ["PENDING"]
    }
  },
  "state": "ENABLED",
  "targets": [
    {
      "arn": "arn:aws:ecs:region:account-id:cluster/my-cluster",
      "roleArn": "arn:aws:iam::account-id:role/ecsEventsRole",
      "ecsParameters": {
        "taskDefinitionArn": "arn:aws:ecs:region:account-id:task-definition/my-task"
      }
    }
  ]
}

在上面的示例中，source字段指定了事件的来源为ECS，detail-type字段指定了事件的类型为ECS Task State Change。detail字段中的desiredStatus和lastStatus字段指定了事件的过滤条件，只有当任务的期望状态为RUNNING且最后状态为PENDING时，事件才会触发。

4. 使用CFN模板将上述定义创建为一个CloudFormation堆栈。以下是一个示例CFN模板：

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  MyTaskDefinition:
    Type: AWS::ECS::TaskDefinition
    Properties:
      Family: my-task
      NetworkMode: awsvpc
      Cpu: '256'
      Memory: '512'
      ExecutionRoleArn: !Ref MyExecutionRole
      ContainerDefinitions:
        - Name: my-container
          Image: your-container-image
          Memory: 512
          PortMappings:
            - ContainerPort: 80
              HostPort: 80
              Protocol: tcp
  MyService:
    Type: AWS::ECS::Service
    Properties:
      Cluster: my-cluster
      ServiceName: my-service
      TaskDefinition: !Ref MyTaskDefinition
      DesiredCount: 1
      LaunchType: FARGATE
      NetworkConfiguration:
        AwsvpcConfiguration:
          Subnets:
            - subnet-12345678
          SecurityGroups:
            - sg-12345678
          AssignPublicIp: ENABLED
  MyEventRule:
    Type: AWS::Events::Rule
    Properties:
      Name: my-event-rule
      Description: Trigger Fargate task
      EventPattern:
        source:
          - aws.ecs
        detail-type:
          - ECS Task State Change
        detail:
          desiredStatus:
            - RUNNING
          lastStatus:
            - PENDING
      State: ENABLED
      Targets:
        - Arn: !Sub arn:aws:ecs:${AWS::Region}:${AWS::AccountId}:cluster/my-cluster
          RoleArn: !Ref MyEventRole
          EcsParameters:
            TaskDefinitionArn: !Ref MyTaskDefinition
  MyExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicy