Overview
The Main Thread Checker is a standalone tool for Swift and C languages that detects invalid usage of AppKit, UIKit, and other APIs on a background thread. Updating UI on a thread other than the main thread is a common mistake that can result in missed UI updates, visual defects, data corruptions, and crashes.
How the Main Thread Checker Works
At app launch, the Main Thread Checker dynamically replaces the implementations of methods that should only be called on the main thread with a version that prepends the check. Methods known to be safe for use on background threads are excluded from this check.
Note
Unlike other code diagnostic tools, the Main Thread Checker doesn't require recompilation, and can be used with existing binaries. You can run it on a macOS app without the Xcode debugger, such as on a continuous integration system, by injecting the dynamic library file located at /Applications/Xcode.app/Contents/Developer/usr/lib/libMainThreadChecker.dylib.
Performance Impact
The performance impact of the Main Thread Checker is minimal, with a 1–2% CPU overhead and additional process launch time of <0.1 seconds.
Because of its minimal performance overhead, the Main Thread Checker is automatically enabled when you run your app with the Xcode debugger.
Updating UI from a Completion Handler
Long-running tasks such as networking are often executed in the background, and provide a completion handler to signal completion. Attempting to read or update the UI from a completion handler may cause problems.
代码语言：javascript复制let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if let data = data {
self.label.text = "(data.count) bytes downloaded"
// Error: label updated on background thread
}
}
task.resume()Solution
Dispatch the call to update the label text to the main thread.
代码语言：javascript复制let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if let data = data {
DispatchQueue.main.async { // Correct
self.label.text = "(data.count) bytes downloaded"
}
}
}
task.resume()
本文作者：博主： gyrojeff 文章标题：UIKit, AppKit等类的进程处理
本文地址：https://cloud.tencent.com/developer/article/1827224
版权说明：若无注明，本文皆为“gyro永不抽风！”原创，转载请保留文章出处。
许可协议:署名-非商业性使用-相同方式共享 4.0 国际 (CC BY-NC-SA 4.0) 转载请保留原文链接及作者！
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