javascript - Callback can not be invoked properly, if the parameter is a bound function -


when function expects callback param, suppose make sense provide bound function this

function invokecb(cb){     cb(); } function test(x){     console.log(x) } const para="xyz";  invokecb(test.bind(null,para)) //(1) invokecb(()=>{test(para)}) //(2)

i can not see wrong (1). however, when come real world, encounter unexpected behaviour

here example, in redux

store.subscribe(     ()=>{(savestate.bind(null,store.getstate()))()} )

can work while

store.subscribe(savestate.bind(null,store.getstate()))

can not correctly, i.e. store.getstate() seems never invoked properly

if need more context. here:https://egghead.io/lessons/javascript-redux-persisting-the-state-to-the-local-storage

maybe missed subtle differences between 2 form, can point out?

the differences are

  • the time @ store.getstate() evaluated
  • the return value (by using braces without return, suppress it)

let data = "abc"; function invokecb(cb){     data = "xyz";     console.log(cb()); } function test(x){     console.log(x);     return x.touppercase(); } invokecb(test.bind(null, data)); // abc abc invokecb(()=>{ test(data); }); // xyz undefined

Comments

Post a Comment

Popular posts from this blog

android - InAppBilling registering BroadcastReceiver in AndroidManifest -

i'm implementing inapp billing in android app. works well, however, i'm trying decouple broadcast receiver activity manifest. suggestion in android's trivialdrive sample: // important: dynamically register broadcast messages updated purchases. // register receiver here instead of <receiver> in manifest // because call getpurchases() @ startup, therefore can ignore // broadcasts sent while app isn't running. // note: registering listener in activity bad idea, done here // because sample. regardless, receiver must registered after // iabhelper setup, before first call getpurchases(). currently there's class extends broadcastreceiver : public class iabbroadcastreceiver extends broadcastreceiver { /** * intent action receiver should filter for. */ public static final string action = "com.android.vending.billing.purchases_updated"; private final iabbroadcastlistener mlistener; public iabbroadcastreceiver(iabbroadcastlistener listener) { m...
Read more

python Tkinter Capturing keyboard events save as one single string -

is possible store keyboard events 1 string? code below stores 1 char , prints it. card reader or bar code reader, contains collection of character/ string not 1 character @ time. goal save char pressed text variable. from tkinter import * root = tk() def key(event): text= event.char text+= event.char print ("pressed", text) def callback(event): frame.focus_set() print ("clicked at", event.x, event.y) frame = frame(root, width=100, height=100) frame.bind("<key>", key) frame.bind("<button-1>", callback) frame.pack() root.mainloop() currently, creating text variable , everytime key() function gets called, , text stores last character typed. you can define text module level variable , , use module level text inside key function - text = '' def key(event): global text text+= event.char print("pressed", text)
Read more

sql server - Why does Linq-to-SQL add unnecessary COUNT()? -

here tables: create table customer ( cid int primary key, name varchar(32) ) create table ord ( oid int primary key, cid int foreign key references customer, address varchar(20) ) here's linq-to-sql statement: var aaa = c in db.customer select new { c, o = c.ord.tolist() }; and here's inexplicable query generated linq-to-sql: select [t0].[cid] [cid], [t0].[name] [name], [t1].[oid] [oid], [t1].[cid] [cid2], [t1].[address] [address], (select count(*) [dbo].[ord] [t2] [t2].[cid] = [t0].[cid]) [value] [dbo].[customer] [t0] left outer join [dbo].[ord] [t1] on [t1].[cid] = [t0].[cid] order [t0].[cid], [t1].[oid] i want understand how rid of count(*) part. it's entirely uncalled for! linq needs count(*) in order determine number of entries returned o = c.ord.tolist() .
Read more