可在iOS 7和iOS 8上运行的警报

问题描述：

我正在得到dyld：未找到符号：_OBJC_CLASS _ $ _ UIAlertAction
当我试图让这个怪物运行时。

I'm getting dyld: Symbol not found: _OBJC_CLASS_$_UIAlertAction when I'm trying to get this monstrosity to run.

我如何弱连接8.0东西？

How do I weaklink 8.0 stuff?

var device : UIDevice = UIDevice.currentDevice()!;
var systemVersion = device.systemVersion;
var iosVerion : Float = systemVersion.bridgeToObjectiveC().floatValue;
if(iosVerion < 8.0) {
    let alert = UIAlertView()
    alert.title = "Noop"
    alert.message = "Nothing to verify"
    alert.addButtonWithTitle("Click")
    alert.show()
} else {
    var alert : UIAlertController? = UIAlertController(title: "Noop", message: "Nothing to verify", preferredStyle: UIAlertControllerStyle.Alert)
    if alert {
        let actionStyle : UIAlertActionStyle? = UIAlertActionStyle.Default;
        var alertAction : UIAlertAction? = UIAlertAction(title: "Click", style: actionStyle!, handler: nil)
        if(alertAction) {
            alert!.addAction(alertAction)
            self.presentViewController(alert, animated: true, completion: nil)
        }
    }
}
return;

已解决：UIKit必须标记为Optional而不是Required。简化版现在是：

Resolved: UIKit had to be marked Optional rather than Required. Simplified version is now:

var device : UIDevice = UIDevice.currentDevice()!;
var systemVersion = device.systemVersion;
var iosVerion : Float = systemVersion.bridgeToObjectiveC().floatValue;
if(iosVerion < 8.0) {
    let alert = UIAlertView()
    alert.title = "Noop"
    alert.message = "Nothing to verify"
    alert.addButtonWithTitle("Click")
    alert.show()
} else {
    var alert : UIAlertController = UIAlertController(title: "Noop", message: "Nothing to verify", preferredStyle: UIAlertControllerStyle.Alert)
    alert.addAction(UIAlertAction(title: "Click", style:.Default, handler: nil))
    self.presentViewController(alert, animated: true, completion: nil)
}

答

在项目构建阶段的Link Binary With Libraries部分中明确添加UIKit。
- Explicitly add UIKit in the "Link Binary With Libraries" section of your project's build phases.
  
  您可以测试UIAlertController的存在，如下所示：
  
  You can test for the existence of UIAlertController like this:
```
if NSClassFromString("UIAlertController") != nil {
    // Use it
} else {
    // Fall back
}
```
- 我写了一个包装器适用于iOS 7和iOS 8.您可以在此处找到它。它需要一个视图控制器，后跟一堆可选参数和任意数量的按钮：
- I wrote a wrapper that works on both iOS 7 and iOS 8. You can find it here. It takes a view controller followed by a bunch of optional arguments and any number of buttons:
```
showAlert(self, style: .ActionSheet, sourceView: cell, completion: {
    tableView.deselectRowAtIndexPath(indexPath, animated: true)
},
    (.Default, "Send clipboard", {
        if someCondition {
            // completion must be specified because of a Swift bug (rdar://18041904)
            showAlert(self, title: "Nothing to send", message: "The clipboard is empty.", completion: nil,
                (.Cancel, "OK", nil)
            )
        }
    }),
    (.Cancel, "Cancel", nil)
)
```

可在iOS 7和iOS 8上运行的警报

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