1 文档说 有三条Rule：
“Rule 1
A designated initializer must call a designated initializer from its immediate superclass.
Rule 2
A convenience initializer must call another initializer from the same class.
Rule 3
A convenience initializer must ultimately call a designated initializer.”
其实就是说 convennience只能横向调用，designed只能纵向调用
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2 关于自定义初始化 是区别于convennience & designed的，如果你的自定义初始化 前面加convennience，就需要调用同级的初始化，没有修饰的话就需要调用super的designed。
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参考下面代码
“class RecipeIngredient: Food {
var quantity: Int
init(name: String, quantity: Int) {
self.quantity = quantity
super.init(name: name)
}
override convenience init(name: String) {
self.init(name: name, quantity: 1)
}
}”
