西瓜視頻 Android 端內數據狀態同步方案VM-Mapping

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"背景"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"西瓜在feed、詳情頁、個人主頁有一塊功能區，包括了點贊、收藏、關注等功能。這些功能長久以來都是孤立的：多個場景下點贊、收藏、關注等狀態或數量不一致。在以往的業務迭代中，都是業務A有了需求，就加個點讚的請求，把自己業務模塊的UI更新下就完事了，業務B也自己搞一下。當西瓜開始從切面發力互動業務的時候，這些問題就凸顯出來了。線上出現了很多在頁面A點贊\/收藏完一個視頻到頁面B點贊\/收藏狀態或者點贊\/收藏數不對的case。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"例如："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/4d\/4d8733f4f5928540490b0e6285e676ad.jpeg","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"問題拆解"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在分析這塊業務時，梳理出幾種問題："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"業務上場景太分散，體現到代碼上就是在activity、scene、viewholder、自定義view等各種個樣的容器，多個業務模塊、多個端（web、flutter）上都有很相似的操作，代碼跨度很大。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"存量的代碼中有些場景是處理過同步問題的，但是處理的又不徹底，方案也不一樣，比如有的情況用了全局註冊callback，來通知所有對結果敏感的場景；有的情況用了Eventbus；有的情況是更新內存，但是卻只是個別幾個模塊通用。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"一部分問題是原來的業務邏輯，比如，使用更新後的內存變量在多個頁面或者模塊傳遞引用，由於層次比較深引用值被中間的流程篡改。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"一部分問題是服務端數據邏輯問題。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其中3、4點問題更像是邏輯bug。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"多個端的數據同步可以通過跨端事件，每個端收到事件後更新自己就行。所以最複雜最難搞的問題就是端內多場景下的數據狀態同步問題。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/c1\/c1117d743f61d77ff2621fa80065b6af.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"端內問題聚焦在幾個case："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"case1"},{"type":"text","text":":普通頁面，如Activity or Fragment上的狀態同步；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"case2"},{"type":"text","text":":feed卡片的狀態同步；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"case3"},{"type":"text","text":":feed卡片內多個複雜層級之間的狀態同步；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"case4"},{"type":"text","text":":以上的組合。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"目標"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"數據狀態同步，是要保證兩個一致性：數據一致性、UI一致性；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"方案要使用簡單，理解簡單；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"儘可能減少性能開銷。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"方案調研"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"EventBus"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個方案的本質是：監聽者收到事件->更新UI\/更新數據Model"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"對於case1"},{"type":"text","text":":如果是A頁面發起，B頁面被動接收，只需要在B頁面接收事件，更新B頁面的Model對象+UI即可。但是在收到事件之後，一定要把當前頁面的model對象更新，不然會有不一致的問題。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"對於case2"},{"type":"text","text":"："}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"eventbus註冊在ViewHolder 上：由於ViewHolder的複用，ViewHolder的數量是少於“ListData”的，那麼意味着，只在ViewHolder上監聽，會出現那些沒有和ViewHolder 建立聯繫的數據無法被更新到。如果使用黏性事件，該事件會一直在內存中，粘性事件的膨脹不可控，很可能會造成嚴重的內存問題。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"eventbus註冊在Activity or 其它頁面上，收到事件後，遍歷數據列表，更新，然後通過RecyclerView的onDataItemChanged方法局部更新。但是在很多場景，比如西瓜feed，feed框架之下的view層次非常深。很多時候Rd只關注某類卡片下的某個UI組件，Feed框架和頂層頁面容器離的很遠，修改成本高，容易出錯，對feed框架或者頂層容器的侵入比較大。另外，onDataItemChanged的局部更新是ViewHolder 對應的itemView的，這個維度比較大，並不能刷新單獨的一個點贊按鈕。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"基於k-v的監聽、通知"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以對象id爲key，某個屬性值如點贊數爲value。事件發生時，將修改值寫入k-v列表，監聽者全部監聽這個變化。當新進入一個場景時，查詢k-v列表作爲最新值。這個方案和Eventbus粘性事件很像。"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"k-v 粒度太細，一直在內存中，非常容易膨脹，沒有合適的釋放時機，導致內存浪費；一旦移除，就可能概率的數據同步失效。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"k-v列表內的狀態要使用者在合適的時機同步到業務層數據Model。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"全局共享數據Model實例"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"同一個數據Model對象，比如一個卡片Model，每次更新都是全局可見的。但是很明顯，"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對數據Model的要求很高。一個業務層數據Model類型，要全局統一，比如，一個視頻卡片業務層的類型是“ModelA”，那麼全局場景不能有“ModelB”表示卡片。在很多場景下，業務層會對原始數據Model進行包裝適配；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"內存佔用很大；可能要緩存很多個列表。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"基於註解的對象映射方案VM-Mapping"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"特點"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"以命名空間+指定字段值 爲key，匹配相同註解名的字段的映射，打平了Model類型的不同、層級嵌套的約束；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"直接更新結果到數據model（如article），與數據model視角的同步；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"打平了多個頁面、複雜view層級嵌套的差異；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"自動處理更新，使用者僅需要關心怎麼更新UI，不需要考慮數據Model的一致性；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"任意場景的支持。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"思考"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"數據狀態同步，到底同步的是什麼？"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"上述的方案中大致有幾個角色：事件、監聽者、數據Model、UI。到底誰應該是主導者？"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"基於事件的方案都需要把狀態同步給數據Model，能簡化嗎？"}]}]}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/b3\/b37b176936933210d84adff08df4c701.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個過程中有四個角色，三個操作。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"突破View層級的限制"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"從MVVM說起。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"MVVM是一種軟件設計典範，用一種業務邏輯、數據、界面顯示分離的方法組織代碼。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/98\/985f9eab5dc6ecd03ab9bf73b26f8662.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"MVVM本質上是一種數據驅動UI的理念。從這個理念看，數據狀態同步，同步的是數據Model，UI的變更是由數據的變更引起的，真正關注的點應該在數據本身上。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/25\/253ae90bdca2fc4abc45ec1500ce4681.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣，就不再需要額外一個接受事件的“容器”，來控制數據和UI了。到現在，只有三個角色，兩個操作了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"再回過頭看，爲什麼跨頁面、跨多View層級很難找到一個通用方案，是因爲總在找一個“容器”來承載事件的接受，然後再做雙份（數據和View）的同步。而且這個“容器”通常本身就是一個頁面，或者其它不同層級上的view，本身就存在很多樣化，爲這種多樣化適配，就會讓事情變得複雜。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"假如不再找額外的“容器”，直接把監聽綁定在數據上，那麼View層級的限制也就不存在了。因爲不管在什麼場景，什麼層級，真正的邏輯中心都是數據，View也是通過數據渲染出來的，View不關心自己在什麼層級，只關心數據的變化。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"突破類型的限制"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這裏有幾個類型的限制："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"數據Model的類型是否只能一成不變，假如網絡請求的原始數據是A類型，在場景1直接用了A類型，在場景2爲了適配UI對A做了包裝："}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"class A{\n val diggStatus : Int\n}\n\nclass B {\n val a : A\n val showTipEnable : Boolean\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"雖然類型不同，但是對A、B來說，都是要更新diggStatus的；"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"2","normalizeStart":"2"},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"在Android，數據Model的類型是強類型，是從網絡由二進制流反序列化出來的，那麼同一個二進流，既可以反序列化成A類型，又可以反序列化成B類型，只要滿足反序列化規則就行。但是事實上，他們的業務本質還是一個東西。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"class A{\n val diggStatus : Int\n}\n\nclass B{\n val digg_status : Int\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"3","normalizeStart":"3"},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"事件本身也是一個數據，只是它是用戶操作發起的，表象看和數據Model無關，但是一個事件既然能更新某個數據Model，那他們一定存在着對應關係。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個問題的本質是，類型約束是語言特性，但是和業務屬性無關，只要他們能確認是一個業務含義，不管他們怎麼換“馬甲”，他們總是能匹配上的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣就演變成了："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"怎麼確定兩個類型是一個業務含義；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"怎麼確定屬性的對應關係（字段匹配）。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第一個好說，主要能有唯一的業務標識，就能確定是一個業務含義；怎麼確定屬性的對應關係呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"現有的技術體系裏就有可以借鑑的思想：數據庫的使用。像jetpack 的Room組件："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"@Entity(tableName = \"users\")\ndata class User(\n @PrimaryKey(autoGenerate = true) var userId: Long,\n @ColumnInfo(name = \"user_name\")var userName: String,\n @ColumnInfo(defaultValue = \"china\") var address: String\n)\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以看到，我們只要要在應用層這麼定義一個數據Model叫User，爲它加上註解，就可以把數據庫中的字段和我們的數據對應上。那麼方案呼之欲出，註解是可以完成屬性匹配的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"於是乎整個流程就簡化成了："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/22\/222838cd40c2d34a87cb15ac9c882553.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個流程可以看到，只剩下了兩個角色，和兩個操作了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所謂數據更新UI，就是View-Model；數據映射數據，就是Data-Mapping，於是這個方案的名稱就是VM-Mapping。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"詳細設計"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"需要對上述抽象流程做實現。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"映射"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"前面說到，映射關係由註解維護，一個有三個註解："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"Mappable註解 ："}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"標註在class上，用來識別這個類是不是可以被處理。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"annotation class Mappable(val mappingSpaces: Arrary)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其中mappingSpace是命名空間，表示是“一類”數據，可以和數據庫表名對比理解，mappingSpace就是tableName。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"2","normalizeStart":"2"},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"PrimaryKey註解："}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"標記在字段上，被標記的字段作爲Model對象的唯一標識。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"mappingSpace+PrimaryKey的值，就是在映射關係中的唯一業務標識。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"@Target(AnnotationTarget.FIELD)\n@Retention(AnnotationRetention.RUNTIME)\nannotation class PrimaryKey"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"3","normalizeStart":"3"},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"MappableKey註解："}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"標註在字段上，需要被映射對應的字段"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"Target(AnnotationTarget.FIELD)\n@Retention(AnnotationRetention.RUNTIME)\nannotation class MappableKey(val value: String)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"映射關係說明："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/c4\/c47d8c6c59b4526de23b026d3f81defe.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"數據驅動UI"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Android裏有很多類似理念的東西，比如LiveData，就是數據更新通知到UI上。本質上數據驅動UI，就是在數據DataUI 之間建一個“橋樑”。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這個不過LiveData並不適合用在這裏，理由是："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"LiveData綁定的生命週期是LifecycleOwner，也就是Activity、Fragment維度，明顯我們的場景維度更細；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"直接observeForever也可以，但是由於View層級的多樣，調用方通常需要合適的時機移除；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"LiveData 強引用了數據Data，這個“橋樑”本身對數據Data的生命週期造成了影響。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"VM-Mapping做了個簡單方案。用了兩級HashMap，一級HashMap使用業務唯一標識（mappingSpace+PrimaryKey的值）爲KEY，二級使用WeakHashMap，以數據Model實例爲KEY，XGViewModel爲VALUE。維護數據Data 和 UI回調之間的關係："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/de\/de6e7b4a92c6e3cbf44d444df3b1058d.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"XGViewModel維護了通知給UI的弱引用回調合集。一個數據Model實例對應了一個XGViewModel。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當映射發生時，會通過業務標識Key，查找所有還沒有被回收的數據Model實例，然後通過對應的XGViewModel通知UI自己的變更。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"總體流程"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/db\/dbbaef048cbcb5f9e60fecc2838bc6e4.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在這個流程中，業務使用只需要關心發起映射數據和更新視圖。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲存在列表，那麼會有一個列表的維護者，就是所謂的映射中心。映射中心有兩個核心能力："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"收集需要被更新的數據Model列表；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"查找匹配。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"其它細節"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲使用了反射，爲了減少性能損耗，會對收集的數據Model類型做class和相關字段的緩存。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"列表存在膨脹現象，二級弱引用列表的key是數據Model實例本身，當它被虛擬機回收的時候，會把一級列表中的該項移除，當一級列表某個key下沒有內容時，也會把該key移除。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"移除的時機在每次添加數據Model到列表；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"移除的條件是一級列表長度達到閾值。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是注意，這個移除並不會影響VM-Mapping的能力，因爲VM-Mapping關注的是數據本身，當數據被回收的時候，不會有任何場景會用到這個數據，自然也不用關心是不是需要通知到它。"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了避免影響主線程，和多線程競爭列表的問題，映射中心操作都在單子線程中處理。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"方案對比"}]},{"type":"embedcomp","attrs":{"type":"table","data":{"content":"

方案	優勢	劣勢
Eventbus	理解成本低	事件、UI、數據Model三個角色都要保持一致，適配各種場景的成本高，不通用。
全局共享數據Model實例	使用簡單	條件苛刻；佔用內存，膨脹不可控制。
基於k-v的監聽、通知	各場景通用	粒度太細導致內存不可控制，移除策略會導致同步失效。事件需要手動同步數據Model。
VM-Mapping	使用簡單，不需要手動同步回數據Model，在所有場景下通用。	用到了反射，有一部分性能損耗。"}}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"方案收益"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"西瓜在之前遺留了大量的類似問題，一直沒有好的方案解決，要麼存在根本性缺陷，要麼實施成本高。VM-Mapping支持了在西瓜中視頻相關的核心場景快速接入，實現了線上點贊數異常問題清零。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"後續計劃"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"根據統計，由於使用運行時註解+反射，一個操作的耗時均值在10ms左右。仍然有可以優化的空間。可以考慮使用編譯時註解維護數據映射關係。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"目前訂閱數據的變化，維度是數據本身，而不是變化的字段，可以考慮通過kotlin delegate 細化監聽維度。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本文轉載自：字節跳動技術團隊（ID：BytedanceTechBlog）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"原文鏈接："},{"type":"link","attrs":{"href":"https:\/\/mp.weixin.qq.com\/s\/rutSv-Rb1Y9xK2FPoSeeVw","title":"xxx","type":null},"content":[{"type":"text","text":"西瓜視頻 Android 端內數據狀態同步方案VM-Mapping"}]}]}]}