ForecastingStocks

Every EA is built around a few event handler functions that MetaTrader calls automatically at the right moments. You do not call them yourself — the terminal does.

The three core events

OnInit() — called once when the EA is attached or its inputs change. Validate inputs, create indicator handles, set up state. Return INIT_SUCCEEDED or fail fast.
OnTick() — called on every incoming price tick. This is the heart of the EA: read prices, evaluate the strategy, decide whether to trade. It can fire many times per second or rarely, depending on the market.
OnDeinit() — called once when the EA is removed or the terminal closes. Release handles and clean up.

A minimal EA skeleton

// input parameters are tunable from the terminal and the tester
input int    FastPeriod = 20;
input int    SlowPeriod = 50;
input double Lots       = 0.10;

int OnInit()
{
    if(FastPeriod >= SlowPeriod)
        return(INIT_PARAMETERS_INCORRECT);
    return(INIT_SUCCEEDED);
}

void OnTick()
{
    // strategy logic runs here on every tick
}

void OnDeinit(const int reason)
{
    // cleanup runs here
}

A crucial performance habit

OnTick can run extremely often. Doing heavy work on every tick — recalculating long indicators, re-reading history — is wasteful and can lag the EA. The common pattern is to act only when a new bar opens (detect a change in the bar's open time) rather than on every tick, unless your strategy genuinely needs tick-level reaction.

Other handlers you will meet

OnTimer() for periodic work, OnTrade() when a trade event fires, and OnChartEvent() for user interaction. Master the three core ones first; they cover most strategies.