SAP Cloud for Customer裡一個Promise的實際應用場合

i042416發表於2020-07-12
CloudPromise

There are lots of  tutorials about promise in the internet.

Recently I am studying the frontend code of SAP Cloud for Customer and I come across a real example of how promise is used there. Below is the Passcode logon view.

SAP Cloud for Customer裡一個Promise的實際應用場合

Once Passcode is entered, suppose I have already entered the system url and frontend user name in the past, they will be directly retrieved from browser storage.

SAP Cloud for Customer裡一個Promise的實際應用場合

Currently I use Chrome to access C4C and Web SQL is used as browser storage, where the system url and logon user name could be found from Chrome development tool.

SAP Cloud for Customer裡一個Promise的實際應用場合

The corresponding database initialization and table read is done by code below in file AppStatusService.js.

The series of callback functions are chained by promise API “then()” which are expected to be executed sequentially:

(1) _createTable() could only be executed after database initialization is done. (2) _getApplicationStatus could NOT be executed unless the database table which stores Application status is available – this is ensured by _createTable. (3) After application status is read from database table, _createDefaultEntries could be called to render the default value in Passcode logon view.

SAP Cloud for Customer裡一個Promise的實際應用場合

All above three steps are organized by promise to achieve the asynchronous execution mode. In order for me to understand how the above code works, I write a simplified version for illustration:

<!doctype html><html><head><script>var end;function setupDB(){
    return this.createDatabase().then(createTable).then(insertEntry).then(readEntry).then(printResult);}function createTable(){
    return new Promise(function(resovle, reject) {
        console.log("prepare to create table..." + Date.now());
        this._db.transaction(function(query){
            query.executeSql('create table if not exists user(id unique, user, passwd)');
        });
        setTimeout( _createTableOK.bind(this, resovle), 1000);
    });}function _createTableOK(resovle){
    console.log("table created successfully..." + Date.now());
    resovle();}function createDatabase(){
    return new Promise(function(resovle, reject) {
        console.log("prepare to create database..." + Date.now());
        this._db = openDatabase('mydb','1.0', 'JerryTestdb',1024);
        setTimeout( _createDatabaseOK.bind(this, resovle), 1000);
    });}function _createDatabaseOK(resovle){
    console.log("database created successfully..." + Date.now());
    resovle(this._db);}function insertEntry(){
    return new Promise(function(resolve, reject) {
        this._db.transaction(function(query){
            query.executeSql("insert into user values (1,'Jerry','1234')");
        });
        setTimeout( _insertEntryOK.bind(this, resolve), 1000);
    });}function _insertEntryOK(resolve){
    console.log("entry inserted to table successfully..." + Date.now()); 
    resolve();}function readEntry() {
    return new Promise(function(resolve, reject) {
            this._db.transaction( function(query) {
                query.executeSql('select * from user',[],function(u,results) {
                setTimeout( _readEntryOK.bind(this, resolve, results), 1000);
            }); // end of query.executeSql     } // end of function(query)     ); // end of this._db.transaction });}function _readEntryOK(resolve, oResult){
    console.log("entry readed from DB successfully..." + Date.now());
    resolve(oResult);}function printResult(oResults){
    for( var i = 0; i < oResults.rows.length; i++) {
            document.writeln('id: ' + oResults.rows[i].id);
            document.writeln('user: ' + oResults.rows[i].user);
            document.writeln('passwd: ' + oResults.rows[i].passwd);
    }
    end = true;}function work(){
    if( end ){
        clearInterval(handle);
    }
    else{
        console.log(" working..." + Date.now());
    }}setupDB();var handle = setInterval( work, 200);</script></head></html>

Open the html page with Chrome, and you can find that a database with name mydb and a table user is created with one record inserted.


SAP Cloud for Customer裡一個Promise的實際應用場合


In order to achieve the simulation that each step of webSQL is a time-consuming operation, I wrap the real logic into setTimeout with a certain time delay.

I scheduled function work to simulate the main work to do and the database related job are done in an asynchronous way organized within function module setupDB() by promise API.


SAP Cloud for Customer裡一個Promise的實際應用場合 SAP Cloud for Customer裡一個Promise的實際應用場合


The console output proves that the database operations are really executed asynchronously in exactly the same order as they are scheduled via then API of promise.


SAP Cloud for Customer裡一個Promise的實際應用場合


Note

Not all browsers support WebSQL and the specification of WebSQL is no longer in active maintenance.


SAP Cloud for Customer裡一個Promise的實際應用場合


Even in C4C frontend framework code we can see more and more usage on IndexedDB instead:


SAP Cloud for Customer裡一個Promise的實際應用場合


See the comparison on these two techniques from this link  Migrating your WebSQL DB to IndexedDB.

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SAP Cloud for Customer裡一個Promise的實際應用場合


來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/24475491/viewspace-2703963/，如需轉載，請註明出處，否則將追究法律責任。

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