Learn a new way to start a http server

python3 -m http.server 8000.

Little planets effect (小行星特效)

I got a task to implement this effect recently.
Before it's assigned to me, there was a fragment shader in the codebase to handle this effect.
But the view angle can not be rotated.

So I try to "google" whether there is any information about it.

Then I found this page .

It's a way to project a sphere to a plane.

It's called "stereographic projection";

Φ = asin( cos(c) sin(Φ₀) + (y sin(c) cos(Φ₀) / r )

λ = λ₀ + atan( x sin(c) / (r cos(Φ₀) cos(c) - y sin(Φ₀) sin(c) )

I can use the above equation to calculate the position correspond to the x,y coordinate on the plane. 

After I adjust the fragment shader by the equation, the user can watch this planet at any angle.

It's quite interesting to try the OpenGL shader, I can generate such amazing results by a few lines of code.

By the way, if you would like to try such kind of effect by your self, you can download  Insta360Player.

If I didn't find a way to implement the little planets effect, I will try to decompile it's apk.

Android OpenGL Texture memory occupied

I try to solve an interesting memory issue recently.

There is a viewer page in a project I owned recently.
The whole app will be closed without showing the "force close dialog" if we keep entering and leaving that page.
It seems like a memory leak , so I try to use LeakCanary to check it but found nothing related.

Then I try to use the  Memory Monitor  in Android Studio to monitor the memory usage.
But all the memory is released after the GC.

So I can only record the logs during that period and check it line by line.
Then I notice that many process is killed by Activity Manager during that period.
Even a process with OomAdj 0 (contain a foreground activity) is killed too.
It's quite abnormal that Activity Manager will kill so many process when I can't found any app spend lots of memory.
It pops into my mind there is a lesson learned I got in HTC .
In the early version of Android , the memory of a Bitmap can be allocated in the native layer.
Is it the reason it's not watched by the Memory Monitor?

So I try to survey whether there is any command I can use to check the memory usage in native layer.
Then I found the command below.

adb shell dumpsys meminfo <package_name|pid> [-d]

After I execute it, I LOL.
EGL mtrack is fucking high!!

It seems like the memory related to the OpenGL in this page is not released correctly.

After I found the hint, it's much easier to solve this issue.

Practice Line Bot

之前就看過Julian大大分享的 Line Bot 心得
所以大概知道Line Messaging API 運作的方式
不過一直都沒想到比較有意思的題目, 所以就沒寫來玩玩看

最近突然想起, 有時候忘了看女兒的聯絡簿時, 都不知道她們最近教了些什麼東西
於是就決定來寫個聯絡簿的通知小幫手

首先由於我比較熟悉的後端框架是node.js , 跟 J大用的 GO 不同
所以還是另外參考了node上的Line bot 教學文章

在 Line Business Center 申請好帳號之後
需要設定Webhook的網址
也就是 Line 會把一些 event 透過 http 發到該網址

首先npm install linebot --save 之後 ,就能安裝node下的linebot 套件
寫了最基本的程式碼後 , 就能監聽收到訊息時的event
(還得上傳上 Heroku 或類似的服務)

let app = express()
const server = require('http').Server(app)
let bot = LINEBot.create({
  channelID,
  channelSecret,
  channelToken
}, server)

bot.on('message', function(event) {
  console.log(event); //把收到訊息的 event 印出來看看
});

app.use(bot.webhook('/webhook'))

能夠知道使用者發了怎樣的訊息 , 就能利用字串比對的方法
來應答各種不同的回覆訊息

但是我準備嘗試的聯絡簿通知幫手 , 並不需要針對使用者的訊息做回應
而是定期的去爬學校的電子聯絡簿網頁 , 然後發現有新的內容時, 就發給所有使用者

所以在event的部分, 我關心的只有哪個 user 或哪個group 加了我的bot
或移除了bot , 我只要在DB維護這個 userId 跟 groupId的清單
每當有新的聯絡簿內容時 , 就發給他們即可

爬蟲的部分, 我就參考上面連結中使用的 js-crawler 跟 cheerio 兩個套件
去處理學校的聯絡簿網址 , 然後再分別拿到聯絡簿內容與當天的日期
如果發現當天沒有聯絡簿內容(可能老師還沒寫或當天是假日) ,就直接結束
如果有看到內容就將該日期與內容與 DB 內最新的內容做比對
如果日期不同, 就刷新 DB的內容, 並將內容發給所有訂閱的userId / groupId
如果日期相同, 就代表該內容已經發出去過了, 可以略過

雖然兩三年前就加減學過node.js , 但一直也沒有一個題目來認真寫
這次這個題目不僅嘗試了 Line的 Messaging API
另外也第一次在 node.js上使用 mongo db 跟爬蟲
而由於這一些非同步的指令在javascript 上都會使用callback來回傳結果
所以初版寫完之後, 馬上就遇上又臭又長的 callback hell

還好這半年有在Android 上使用 RxJava的經驗
在Javascript上學起 Promise 就較為容易上手
於就再花了些時間把code用Promise 重構 (聽說 Async / Await 更潮, 不過就下次吧)