iOS7/10地图应用中如何让ScaleBar始终显示或修改淡出时长？

我之前也碰到过类似的需求，系统MapKit默认的比例尺条确实只会在缩放操作时显示，之后自动淡出，而且苹果并没有提供公开API直接控制它的显示逻辑。不过有两种可行的方案，你可以根据需求选择：

方案一：自定义比例尺条（推荐，稳定兼容） #

依赖系统私有视图的方案容易因iOS版本更新失效，自定义的方式更可控，还能按需调整样式。步骤如下：

• 1. 创建自定义ScaleBar视图
     写一个UIView子类，核心是根据地图的region和屏幕参数计算实际距离，再转换为易读的单位（米/千米等）并绘制出来。

示例Swift代码：

import UIKit
import MapKit

class CustomMapScaleBar: UIView {
    private let scaleLabel = UILabel()
    private let scaleLine = UIView()
    private weak var mapView: MKMapView!

    init(mapView: MKMapView) {
        self.mapView = mapView
        super.init(frame: CGRect(x: 16, y: mapView.bounds.height - 80, width: 120, height: 30))
        setupUI()
        updateScale()
        // 监听地图区域变化，实时更新比例尺
        mapView.addObserver(self, forKeyPath: "region", options: [.new], context: nil)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    private func setupUI() {
        backgroundColor = UIColor.white.withAlphaComponent(0.8)
        layer.cornerRadius = 4
        clipsToBounds = true

        // 绘制比例尺线条
        scaleLine.backgroundColor = .black
        addSubview(scaleLine)
        scaleLine.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            scaleLine.topAnchor.constraint(equalTo: topAnchor, constant: 4),
            scaleLine.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 8),
            scaleLine.heightAnchor.constraint(equalToConstant: 2),
            scaleLine.widthAnchor.constraint(equalToConstant: 100)
        ])

        // 添加比例尺数值标签
        scaleLabel.font = UIFont.systemFont(ofSize: 12)
        scaleLabel.textAlignment = .center
        addSubview(scaleLabel)
        scaleLabel.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            scaleLabel.topAnchor.constraint(equalTo: scaleLine.bottomAnchor, constant: 2),
            scaleLabel.leadingAnchor.constraint(equalTo: leadingAnchor),
            scaleLabel.trailingAnchor.constraint(equalTo: trailingAnchor),
            scaleLabel.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -4)
        ])
    }

    private func updateScale() {
        let mapRect = mapView.visibleMapRect
        let scale = MKMapPointsPerMeterAtLatitude(mapView.centerCoordinate.latitude)
        let metersPerPoint = 1 / scale
        let visibleWidthInMeters = mapRect.size.width * metersPerPoint

        // 计算易读的整数值（比如100、200米，1千米这类）
        let roundedMeters = round(visibleWidthInMeters / 100) * 100
        let displayText: String
        if roundedMeters >= 1000 {
            displayText = String(format: "%.1f km", roundedMeters / 1000)
        } else {
            displayText = String(format: "%.0f m", roundedMeters)
        }

        scaleLabel.text = displayText
        // 根据计算结果调整线条宽度
        let lineWidth = (roundedMeters / visibleWidthInMeters) * 100
        scaleLine.widthAnchor.constraint(equalToConstant: lineWidth).isActive = true
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "region" {
            updateScale()
        }
    }

    deinit {
        mapView?.removeObserver(self, forKeyPath: "region")
    }
}

• 2. 将自定义视图添加到MapView
     在你的ViewController里初始化并添加这个比例尺条，固定到右下角和系统默认位置一致：

override func viewDidLoad() {
    super.viewDidLoad()
    let mapView = MKMapView(frame: view.bounds)
    view.addSubview(mapView)
    
    let scaleBar = CustomMapScaleBar(mapView: mapView)
    mapView.addSubview(scaleBar)
    
    // 设置自动布局固定位置
    scaleBar.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint.activate([
        scaleBar.trailingAnchor.constraint(equalTo: mapView.trailingAnchor, constant: -16),
        scaleBar.bottomAnchor.constraint(equalTo: mapView.bottomAnchor, constant: -40),
        scaleBar.widthAnchor.constraint(equalToConstant: 120),
        scaleBar.heightAnchor.constraint(equalToConstant: 30)
    ])
}

方案二：修改系统默认ScaleBar（不推荐，有兼容性风险） #

如果你想复用系统的ScaleBar，只能通过遍历MapView子视图找到私有类MKScaleView，再修改它的淡出动画。注意：这种方法依赖私有API，可能在iOS版本更新后失效，甚至影响App Store审核，仅作临时方案参考。

示例代码：

private func forceShowSystemScaleBar() {
    // 递归遍历子视图找到MKScaleView
    func findScaleView(in view: UIView) -> UIView? {
        if String(describing: type(of: view)) == "MKScaleView" {
            return view
        }
        for subview in view.subviews {
            if let scaleView = findScaleView(in: subview) {
                return scaleView
            }
        }
        return nil
    }

    guard let scaleView = findScaleView(in: mapView) else { return }
    // 移除所有淡出动画
    scaleView.layer.removeAllAnimations()
    // 强制保持显示状态
    scaleView.alpha = 1.0
}

// 监听地图区域变化，每次缩放后重置ScaleBar状态
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == "region" {
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
            self.forceShowSystemScaleBar()
        }
    }
}

总结来说，方案一稳定可靠，还能自定义样式；方案二适合快速临时解决，但存在兼容性和审核风险。

内容的提问来源于stack exchange，提问作者BlackOps