练习2：会攻击侵犯领地的凶狠水鸭

作者：境界

凶狠水鸭示例包下载：下载示例包 (opens new window)。

{
  "format_version": "1.16.0",
  "minecraft:entity": {
    "description": {
      "identifier": "design:green_head_duck",
      "is_spawnable": true,
      "is_summonable": true,
      "is_experimental": false,
      "runtime_identifier": "design:green_head_duck"
    },
    "component_groups": {
    },

    "components": {
      "minecraft:breathable": {
        "total_supply": 15,
        "suffocate_time": 0
      },
      "minecraft:collision_box": {
        "width": 0.6,
        "height": 1.0
      },
      "minecraft:nameable": {
      },
      "minecraft:health": {
        "value": 4,
        "max": 4
      },
      "minecraft:hurt_on_condition": {
        "damage_conditions": [
          {
            "filters": { "test": "in_lava", "subject": "self", "operator": "==", "value": true },
            "cause": "lava",
            "damage_per_tick": 4
          }
        ]
      },
      "minecraft:movement": {
        "value": 0.25
      },
      "minecraft:damage_sensor": {
        "triggers": {
          "cause": "fall",
          "deals_damage": false
        }
      },
      "minecraft:behavior.rise_to_liquid_level": {
        "priority": 0,
        "liquid_y_offset": -0.5,
        "rise_delta": 0.01,
        "sink_delta": 0.01
      },
      "minecraft:leashable": {
        "soft_distance": 4.0,
        "hard_distance": 6.0,
        "max_distance": 10.0
      },
      "minecraft:balloonable": {
        "mass": 0.5
      },
      "minecraft:navigation.walk": {
        "can_path_over_water": true,
        "can_sink": false,
        "avoid_damage_blocks": true
      },
      "minecraft:movement.basic": {
      },
      "minecraft:jump.static": {
      },
      "minecraft:can_climb": {
      },
      "minecraft:despawn": {
        "despawn_from_distance": {}
      },
      "minecraft:behavior.panic": {
        "priority": 1,
        "speed_multiplier": 1.5
      },
      "minecraft:behavior.tempt": {
        "priority": 4,
        "speed_multiplier": 1.0,
        "items": [
          "wheat_seeds",
          "beetroot_seeds",
          "melon_seeds",
          "pumpkin_seeds"
        ]
      },
      "minecraft:type_family": {
        "family": [ "duck", "mob" ]
      },
	  "minecraft:behavior.melee_attack": {
        "priority": 3,
        "speed_multiplier": 1,
        "track_target": false
      },
	  "minecraft:behavior.nearest_attackable_target": {
        "priority": 2,
        "reselect_targets": true,
		"must_see": false,
        "entity_types": [
          {
            "filters": {
              "all_of": [
                { "test": "is_family", "subject": "other", "value": "mob" },
				{ "test": "is_family", "subject": "other", "operator": "not", "value": "duck"}
              ]
            },
            "max_dist": 5
          }
        ]
      },
	  "minecraft:attack": {
		"damage": 1
	  },
      "minecraft:behavior.random_stroll": {
        "priority": 6,
        "speed_multiplier": 1.0
      },
      "minecraft:behavior.look_at_player": {
        "priority": 7,
        "look_distance": 6.0,
        "probability": 0.02
      },
      "minecraft:behavior.random_look_around": {
        "priority": 8
      },
      "minecraft:physics": {
      },
      "minecraft:pushable": {
        "is_pushable": true,
        "is_pushable_by_piston": true
      }
    },

    "events": {
    }
  }
}

攻击侵犯领地的行为，可以想象成为靠近水鸭多近会引起水鸭的攻击。因此在设置一个生物拥有近战能力的时候，需要用三个行为进行组合：

1）minecraft:attack，它的damage值可以接受一个数组，也可以接受一个整数数值。数组情况下，可以让伤害在数组内由大到小取一个随机伤害，而一个固定数值则代表的是固定伤害。

2）minecraft:behavior.nearest_attackable_target，它的entity_types接受一连串的生物目标，我们在目标选择器中，同时将目标设置为mob，但又不是duck，即所有带有mob标签却不带有duck标签的生物。max_dist代表在多远距离内会被当作目标，must_see意为是否必须看到才会当作目标，reselect_targets会在一群目标里不断选择离自己最近的目标。

3）minecraft:behavior.melee_attack,它的speed_multiplier接收一个浮点数值，即保持进攻状态时，速度会给予多少的加成，1为100%，则保持原速，1.5则是150%。track_target接收一个布尔值，若设置为true则会不停跟着目标，即使走出攻击范围，false则会在目标离开攻击范围后停止行为。

特别注意：由于minecraft:type_family组件用来标记生物的家族类型，原版鸡携带的家族标签为"mob“和"chicken"，理论上，所有原版的生物都带有这个mob标签，我们将水鸭的标签保留了mob标签，也是为了允许其他原版生物在处理一些需要判断生物标签的行为，如寻找目标的时候，能够将水鸭纳入目标范围内，更加的贴近原版的生态。除此之外，我们将chicken改为duck，一个是强调实体携带duck标签，另一个是为了在自身寻找目标的行为当中，将同为duck标签的实体剔除出目标范围。若保持原来鸡的写法，则会变成需要剔除chicken标签的实体，那样就会包括了鸡这类生物，不符合实际需求。