Association, Composition and Aggregation in UI5, CRM, S/4HANA and C4C

UI5

UI5使用Association和Aggregation描述控件之間的關係。

Aggregation：parent和子控件在lifecycle上存在依賴關係：

When a ManagedObject is destroyed, all aggregated objects are destroyed as well and the object itself is removed from its parent. That is, aggregations won't contain destroyed objects or null/undefined.

好比UI5的轉盤控件Carousel: 一旦轉盤被析構，裏面顯示的page固然也沒有繼續存在的意義了，須要跟着被析構。

而Association描述了在lifecycle層面的一種soft dependency關係:

Managed associations also form a relationship between objects, but they don't define a lifecycle for the associated objects. They even can 'break' in the sense that an associated object might have been destroyed already although it is still referenced in an association. 

最明顯的例子就是控件和其label的關係，好比button和label：技術上來講，能夠彼此分開獨立存在。

CRM

CRM的Genil model存在三種類型的relation，能夠在doman CRM_RELATION_KIND裏查看：

區別：

• Association: Link between any kind of objects. Can also be defined across components with root or access object as target. 依賴關係最爲loose的一種relation，能夠用來鏈接跨model之間的節點。

• Aggregation: Binds child objects to a root object. Only access and dependent objects can be aggregated. 只適用於同一模型的節點之間的關聯。

• Composition: Like an aggregation, but composed child objects always exist. 特殊類型的Aggregation。目標節點的Cardinality爲1或者1..n

S/4HANA

只有兩種：association或者composition。Composition的含義同CRM裏的aggregation，而association的含義同UI5和CRM中的association一致。

C4C

只有兩種: association或者composition。C4C的這兩種relation多了一個限制：relation的目標BO必須和源BO在同一個部署單元Deployment Unit，或者目標BO位於Foundation部署單元內。

Association的語法以下圖：

association的multiplicity只支持[0,1]或者[1,1], 若是不顯式指定，默認爲[1,1].

C4C relation的一個特點是，一旦申明瞭一個子節點以後，系統會默認生成一個對應的composition。

下圖第4行代碼會自動生成一個[0,n]的從root節點到Item節點的composition。

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