前言

　　簡單實現彈幕功能，表跟我談效率，但也有用佇列控制同時彈的數量。

宣告
　　歡迎轉載，但請保留文章原始出處:)
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正文

    let DANMAKU_SPEED: CGFloat = 150    // 彈幕每秒移動速度
    let DANMAKU_SPACE_TIME: NSTimeInterval = 1    // 彈幕之間的時間間隔
    let DANMAKU_MAX_ROW = 3    // 最多同時彈幕行數
    let danmakuFont = UIFont.systemFontOfSize(18
 
)    // 彈幕字型大小
    var rowArray = Array<Array<Danmaku>>(count: 3, repeatedValue: Array<Danmaku>())    
    var danmakuQueue = NSOperationQueue()    // 佇列

    class Danmaku : NSObject{
        var msg: Msg
        var view: UILabel?
        var size = CGSize(width: 0, height: 0)
        var row 
 
= 0
        var startTime: NSDate?
        var duration: NSTimeInterval = 0
        var delay: NSTimeInterval = 0
        
        init(_ msg: Msg, _ row: Int, _ delay: NSTimeInterval = 0) {
            self.msg = msg
            self.row = row
            self.delay = delay
        }
    }
 
    func queueDanmaku(msg: Msg) {
        danmakuQueue.addOperation(NSBlockOperation(block: { [weak self] 
 
in

            if let weakself = self {
                repeat {
                    //檢測放第幾行
                    for var row = 0; row < weakself.DANMAKU_MAX_ROW; ++row {
                        let rowDanmaku = weakself.rowArray[row]
                        if rowDanmaku.count == 0 {
                            let danmaku = Danmaku(msg, weakself.danmakuFont, row)
                            weakself.rowArray[row].append(danmaku)
                            self?.performSelectorOnMainThread("sendDanmaku:", withObject: danmaku, waitUntilDone: true)
                            return
                        } else {
                            if let lastDanmaku = rowDanmaku.last {
                                if let startTime = lastDanmaku.startTime {
                                    let now = NSDate()
                                    let seconds = now.timeIntervalSinceDate(startTime)
                                    let widthDuration = Double(lastDanmaku.size.width / weakself.DANMAKU_SPEED)
                                    
                                    var delay = seconds - weakself.DANMAKU_SPACE_TIME - widthDuration
                                    if delay >= 0 {
                                        delay = 0
                                    } else {
                                        if lastDanmaku.delay > lastDanmaku.duration {
                                            continue
                                        }
                                    }
                
                                    let danmaku = Danmaku(msg, weakself.danmakuFont, row, abs(delay) + lastDanmaku.delay)
                                    weakself.rowArray[row].append(danmaku)
                                    
                                    self?.performSelectorOnMainThread("sendDanmaku:", withObject: danmaku, waitUntilDone: true)
                                    return
                                }
                            }
                        }
                    }
                    
                    sleep(1000)
                } while self != nil
            }
            
            }))
    }
    
    func sendDanmaku(danmaku: Danmaku) {
        let text = "\(danmaku.msg.user_name) : \(danmaku.msg.text)"
        let size = NSString(string: text).sizeWithAttributes([NSFontAttributeName : danmakuFont])
        let width = UIScreen.mainScreen().bounds.size.width
        let top = 54 + danmaku.row * (Int(size.height) + 10)
        let label = UILabel(frame: CGRectMake(width, CGFloat(top), size.width, size.height))
        let duration = (width + size.width) / DANMAKU_SPEED

        danmaku.view = label
        danmaku.size = size
        danmaku.startTime = NSDate()
        danmaku.duration = NSTimeInterval(duration)
        
        label.text = text
        label.font = danmakuFont
        label.textColor = UIColor.whiteColor()
        label.shadowColor = UIColor.blackColor()
        label.shadowOffset = CGSizeMake(0, -1.0)
        
        self.view.addSubview(label)
        UIView.animateWithDuration(Double(duration), delay: danmaku.delay, options: UIViewAnimationOptions.CurveLinear, animations: { () -> Void in
                label.left = -label.width
            }) { [weak self] (Bool) -> Void in
                if !(self?.rowArray[danmaku.row].isEmpty ?? true) {
                    self?.rowArray[danmaku.row].removeFirst()
                }
                label.removeFromSuperview()
        }
    }

　　　　程式碼說明：

　　　　　　程式碼控制了最多同時只能彈三行，每行最後一條如果延遲大於跑彈幕的時間（已經有一條處於完全等待狀態）就自動切到下一行，超過最大限制就等待。

　　　　　　*　　rowArray 主要用於查詢前一個彈幕的位置和時間

　　　　　　*　　別忘了在 deinit 裡面加上 danmakuQueue.cancelAllOperations()

　　　　　　*　　注意 NSBlockOperation 的 block 並不在主執行緒上