MQL4自编指标学习4

市场解说员

he OnCalculate() function is called only in custom indicators when it's necessary to calculate the indicator values by the Calculate event. This usually happens when a new tick is received for the symbol, for which the indicator is calculated.

Parameters of open[], high[], low[] and close[] contain arrays with open prices, high and low prices and close prices of the current time frame. The time[] parameter contains an array with open time values, the spread[] parameter has an array containing the history of spreads (if any spread is provided for the traded security). The parameters of volume[] and tick_volume[] contain the history of trade and tick volume, respectively.

To determine the indexing direction of time[], open[], high[], low[], close[], tick_volume[], volume[] and spread[], call ArrayGetAsSeries(). In order not to depend on default values, you should unconditionally call the ArraySetAsSeries() function for those arrays, which are expected to work with.

The first rates_total parameter contains the number of bars, available to the indicator for calculation, and corresponds to the number of bars available in the chart.

We should note the connection between the return value of OnCalculate() and the second input parameter prev_calculated. During the function call, the prev_calculated parameter contains a value returned by OnCalculate() during previous call. This allows for economical algorithms for calculating the custom indicator in order to avoid repeated calculations for those bars that haven't changed since the previous run of this function.

For this, it is usually enough to return the value of the rates_total parameter, which contains the number of bars in the current function call. If since the last call of OnCalculate() price data has changed (a deeper history downloaded or history blanks filled), the value of the input parameter prev_calculated will be set to zero by the terminal.

不知道Bars 和rates_total  以及IndicatorCounted ()和prev_calculated的区别，有明白的朋友不妨指点一下。

例子使用官方给的例子

1. #property indicator_chart_window
   
2. #property indicator_buffers 1
   
3. //---- plot Line
   
4. #property indicator_label1  "Line"
   
5. #property indicator_type1   DRAW_LINE
   
6. #property indicator_color1  clrDarkBlue
   
7. #property indicator_style1  STYLE_SOLID
   
8. #property indicator_width1  1
   
9. //--- indicator buffers
   
10. double         LineBuffer[];
    
11. //+------------------------------------------------------------------+
    
12. //| Custom indicator initialization function                         |
    
13. //+------------------------------------------------------------------+
    
14. int OnInit()
    
15.   {
    
16. //--- indicator buffers mapping
    
17.    SetIndexBuffer(0,LineBuffer,INDICATOR_DATA);
    
18. //---
    
19.    return(INIT_SUCCEEDED);
    
20.   }
    
21. //+------------------------------------------------------------------+
    
22. //| Custom indicator iteration function                              |
    
23. //+------------------------------------------------------------------+
    
24. int OnCalculate(const int rates_total,
    
25.                 const int prev_calculated,
    
26.                 const datetime& time[],
    
27.                 const double& open[],
    
28.                 const double& high[],
    
29.                 const double& low[],
    
30.                 const double& close[],
    
31.                 const long& tick_volume[],
    
32.                 const long& volume[],
    
33.                 const int& spread[])
    
34.   {
    
35. //--- Get the number of bars available for the current symbol and chart period
    
36.    int bars=Bars(Symbol(),0);
    
37.    Print("Bars = ",bars,", rates_total = ",rates_total,",  prev_calculated = ",prev_calculated);
    
38.    Print("time[0] = ",time[0]," time[rates_total-1] = ",time[rates_total-1]);
    
39. //--- return value of prev_calculated for next call
    
40.    return(rates_total);
    
41.   }
    

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