Swift UIView draw（U1;：）中的SQLite3查询不使用CATiledLayer

虽然可以读取UIView draw（View:）内的数据，但当UIView图层设置为CATiledLayer时，则无法读取数据。代码如下（使用CATiledLayer时给出“线程4:EXC\u坏访问（代码=1，地址=0x7fcc95b1d170）”：

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}我找到了答案。可能问题在于，当使用CATiledLayer时，“在一个或多个后台线程上调用层的draw（in:）方法”（），而SQLite“无法处理多线程情况”（）

class Test2: UIView{
var points = [String]()

override init(frame: CGRect){
    super.init(frame: frame)

    let fileURL = try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent("DB.sqlite")
    if sqlite3_open(fileURL.path, &db) == SQLITE_OK{
        print(fileURL.path)
    }
    else{
        print("error opening database")
    }
}
required init?(coder aDecoder: NSCoder){
    super.init(coder: aDecoder)
}

//Thread 4: EXC_BAD_ACCESS (code=1, address=0x7fcc95b1d170) when the following is used
//override class var layerClass: AnyClass{
//    return CATiledLayer.self
//}


func read(tile: String){
    var stmt:OpaquePointer?
    points = []
    let queryString = "SELECT * FROM Map WHERE tile = '\(tile)'"
    if sqlite3_prepare(db, queryString, -1, &stmt, nil) != SQLITE_OK{
        let errmsg = String(cString: sqlite3_errmsg(db)!)
        print("error preparing read: \(errmsg)")
        return
    }
    while(sqlite3_step(stmt) == SQLITE_ROW){
        points = String(cString: sqlite3_column_text(stmt, 2)).components(separatedBy: ",")
    }
}

override func draw(_ rect: CGRect){
    read(tile: "110_100")
     print(points)
}