如何在TradingView的Pine Script中获取趋势线值并编写交叉算法？

Hey there! Great question—working with trend lines in Pine Script is definitely trickier than moving averages because trend lines are object-based rather than a continuous series of values like SMAs. Let's break this down step by step to solve your problem.

First, let's clarify why you can't use crossover(close, lineInstance) directly:

• SMAs generate a value for every single bar, so they're a continuous series you can compare directly with price.
• Trend lines are drawing objects defined by two endpoints (x/y coordinates). They only have valid price values between those two bars—outside that range, they don't exist, and trying to get their value returns na.

Pine Script gives you the line.get_price() function to pull the price of a trend line at a specific bar index. Here's a quick example of defining a trend line and fetching its value:

// Define a trend line connecting the last two swing lows
var line swingLowTrend = na
if ta.lowest(low, 15) == low[8] and ta.lowest(low, 8) == low
    swingLowTrend := line.new(bar_index[8], low[8], bar_index, low, color=color.green, width=2)

// Get the trend line's price on the current bar
currentLinePrice = na
if not na(swingLowTrend)
    currentLinePrice := line.get_price(swingLowTrend, bar_index)

Note: If the trend line hasn't been created yet, or the current bar is outside the trend line's start/end bars, currentLinePrice will be na—always handle this in your logic.

To use crossover/crossunder like you do with SMAs, you need to convert the trend line's intermittent values into a continuous series. The easiest way is to store each bar's trend line price in an array, then convert that array to a series:

// Initialize an array to track trend line prices across all bars
var linePriceArray = array.new_float()

// Push the current bar's trend line price to the array
array.push(linePriceArray, currentLinePrice)
// Keep the array length matched to the number of bars (optional cleanup)
if array.size(linePriceArray) > bar_index + 1
    array.shift(linePriceArray)

// Convert the array to a series we can use with ta.crossover()
linePriceSeries = array.get(linePriceArray, bar_index)

// Now we can use standard crossover logic!
crossAbove = ta.crossover(close, linePriceSeries)
crossBelow = ta.crossunder(close, linePriceSeries)

// Plot signals for visibility
plotshape(crossAbove, location=location.belowbar, color=color.green, style=shape.labelup, text="BUY")
plotshape(crossBelow, location=location.abovebar, color=color.red, style=shape.labeldown, text="SELL")

Since we converted the trend line prices to a series (linePriceSeries), accessing historical values is as simple as using bracket notation, just like with price or SMAs:

// Check if the price crossed above the trend line from the previous bar
customCross = close[1] < linePriceSeries[1] and close > linePriceSeries and not na(linePriceSeries) and not na(linePriceSeries[1])

For more complex logic (like dynamically updating trend lines as new highs/lows form), combine line.set_* functions to adjust the trend line's endpoints, then repeat the value-fetching process above. Here's an example of a dynamic uptrend line:

var line dynamicUpTrend = na
// Update the trend line when a new swing high forms
if ta.highest(high, 12) == high
    if na(dynamicUpTrend)
        // Find the most recent swing low to use as the start point
        prevLowBar = ta.barssince(ta.lowest(low, 20) == low)
        dynamicUpTrend := line.new(bar_index - prevLowBar, low[prevLowBar], bar_index, high, color=color.blue)
    else
        // Update the trend line's end point to the new high
        line.set_x2(dynamicUpTrend, bar_index)
        line.set_y2(dynamicUpTrend, high)

// Fetch the updated trend line's price for each bar
dynamicLinePrice = line.get_price(dynamicUpTrend, bar_index)

• Trend lines are drawing objects, not native series—you need to manually convert their values to a series to use crossover logic.
• Always handle na values, as trend lines won't have valid prices outside their defined bar range.
• Use var when defining trend line variables to avoid re-creating the object on every bar.

内容的提问来源于stack exchange，提问作者ー PupSoZeyDe ー