对于Xamarin Android，而不是

Handler handler;
handler = new Handler();

只写

Handler handler;
handler = new Handler(Looper.MyLooper());

其余的代码都很好。

根据文档(https://developer.android.com/reference/android/os/Handler#Handler()):

Implicitly choosing a Looper during Handler construction can lead to bugs where operations are silently lost (if the Handler is not expecting new tasks and quits), crashes (if a handler is sometimes created on a thread without a Looper active), or race conditions, where the thread a handler is associated with is not what the author anticipated. Instead, use an Executor or specify the Looper explicitly, using Looper#getMainLooper, {link android.view.View#getHandler}, or similar. If the implicit thread local behavior is required for compatibility, use new Handler(Looper.myLooper()) to make it clear to readers.

我们应该停止使用没有Looper的构造函数，而是指定一个Looper。

使用Executor而不是handler获取更多信息。 使用ScheduledExecutorService实现post delay:

ScheduledExecutorService worker = Executors.newSingleThreadScheduledExecutor();
Runnable runnable = () -> {
    public void run() {
        // Do something
    }
};
worker.schedule(runnable, 2000, TimeUnit.MILLISECONDS);

从API级别30开始，有2个构造函数已弃用。

处理程序() 处理程序(Handler.Callback)

谷歌解释了以下原因。

Implicitly choosing a Looper during Handler construction can lead to bugs where operations are silently lost (if the Handler is not expecting new tasks and quits), crashes (if a handler is sometimes created on a thread without a Looper active), or race conditions, where the thread a handler is associated with is not what the author anticipated. Instead, use an Executor or specify the Looper explicitly, using Looper#getMainLooper, {link android.view.View#getHandler}, or similar. If the implicit thread local behavior is required for compatibility, use new Handler(Looper.myLooper(), callback) to make it clear to readers.

解决方案1:使用Executor

1. 执行主线程中的代码。

Java

// Create an executor that executes tasks in the main thread. 
Executor mainExecutor = ContextCompat.getMainExecutor(this);

// Execute a task in the main thread
mainExecutor.execute(new Runnable() {
    @Override
    public void run() {
        // You code logic goes here.
    }
});

科特林

// Create an executor that executes tasks in the main thread.
val mainExecutor = ContextCompat.getMainExecutor(this)

// Execute a task in the main thread
mainExecutor.execute {
    // You code logic goes here.
}

2. 在后台线程中执行代码

Java

// Create an executor that executes tasks in a background thread.
ScheduledExecutorService backgroundExecutor = Executors.newSingleThreadScheduledExecutor();

// Execute a task in the background thread.
backgroundExecutor.execute(new Runnable() {
    @Override
    public void run() {
        // Your code logic goes here.
    }
});

// Execute a task in the background thread after 3 seconds.
backgroundExecutor.schedule(new Runnable() {
    @Override
    public void run() {
        // Your code logic goes here
    }
}, 3, TimeUnit.SECONDS);

科特林

// Create an executor that executes tasks in a background thread.
val backgroundExecutor: ScheduledExecutorService = Executors.newSingleThreadScheduledExecutor()

// Execute a task in the background thread.
backgroundExecutor.execute {
    // Your code logic goes here.
}

// Execute a task in the background thread after 3 seconds.
backgroundExecutor.schedule({
    // Your code logic goes here
}, 3, TimeUnit.SECONDS)

注意:使用后记得关闭执行程序。

backgroundExecutor.shutdown(); // or backgroundExecutor.shutdownNow();

3.在后台线程中执行代码，在主线程中更新UI。

Java

// Create an executor that executes tasks in the main thread. 
Executor mainExecutor = ContextCompat.getMainExecutor(this);

// Create an executor that executes tasks in a background thread.
ScheduledExecutorService backgroundExecutor = Executors.newSingleThreadScheduledExecutor();

// Execute a task in the background thread.
backgroundExecutor.execute(new Runnable() {
    @Override
    public void run() {
        // Your code logic goes here.
        
        // Update UI on the main thread
        mainExecutor.execute(new Runnable() {
            @Override
            public void run() {
                // You code logic goes here.
            }
        });
    }
});

科特林

// Create an executor that executes tasks in the main thread. 
val mainExecutor: Executor = ContextCompat.getMainExecutor(this)

// Create an executor that executes tasks in a background thread.
val backgroundExecutor = Executors.newSingleThreadScheduledExecutor()

// Execute a task in the background thread.
backgroundExecutor.execute {
    // Your code logic goes here.

    // Update UI on the main thread
    mainExecutor.execute {
        // You code logic goes here.
    }
}

解决方案2:通过使用以下构造函数之一显式地指定一个Looper。

处理器(是我) 处理器(卢珀指导员回调)。

1. 执行主线程中的代码

1.1. Handler with a Looper

Java

Handler mainHandler = new Handler(Looper.getMainLooper());

科特林

val mainHandler = Handler(Looper.getMainLooper())

1.2带有循环程序和处理程序的处理程序。回调

Java

Handler mainHandler = new Handler(Looper.getMainLooper(), new Handler.Callback() {
    @Override
    public boolean handleMessage(@NonNull Message message) {
        // Your code logic goes here.
        return true;
    }
});

科特林

val mainHandler = Handler(Looper.getMainLooper(), Handler.Callback {
    // Your code logic goes here.
    true
})

2. 在后台线程中执行代码

2.1. Handler with a Looper

Java

// Create a background thread that has a Looper
HandlerThread handlerThread = new HandlerThread("HandlerThread");
handlerThread.start();

// Create a handler to execute tasks in the background thread.
Handler backgroundHandler = new Handler(handlerThread.getLooper()); 

科特林

// Create a background thread that has a Looper
val handlerThread = HandlerThread("HandlerThread")
handlerThread.start()


// Create a handler to execute tasks in the background thread.
val backgroundHandler = Handler(handlerThread.looper)

2.2. Handler with一个Looper和一个Handler。回调

Java

// Create a background thread that has a Looper
HandlerThread handlerThread = new HandlerThread("HandlerThread");
handlerThread.start();

// Create a handler to execute taks in the background thread.
Handler backgroundHandler = new Handler(handlerThread.getLooper(), new Handler.Callback() {
    @Override
    public boolean handleMessage(@NonNull Message message) {
        // Your code logic goes here.
        return true;
    }
});

科特林

// Create a background thread that has a Looper
val handlerThread = HandlerThread("HandlerThread")
handlerThread.start()


// Create a handler to execute taks in the background thread.
val backgroundHandler = Handler(handlerThread.looper, Handler.Callback {
    // Your code logic goes here.
    true
})

注意:使用后记得释放线程。

handlerThread.quit(); // or handlerThread.quitSafely();

3.在后台线程中执行代码，在主线程中更新UI。

Java

// Create a handler to execute code in the main thread
Handler mainHandler = new Handler(Looper.getMainLooper());

// Create a background thread that has a Looper
HandlerThread handlerThread = new HandlerThread("HandlerThread");
handlerThread.start();

// Create a handler to execute in the background thread
Handler backgroundHandler = new Handler(handlerThread.getLooper(), new Handler.Callback() {
    @Override
    public boolean handleMessage(@NonNull Message message) {
        // Your code logic goes here.
        
        // Update UI on the main thread.
        mainHandler.post(new Runnable() {
            @Override
            public void run() {
                
            }
        });
        
        return true;
    }
});

科特林

// Create a handler to execute code in the main thread
val mainHandler = Handler(Looper.getMainLooper())

// Create a background thread that has a Looper
val handlerThread = HandlerThread("HandlerThread")
handlerThread.start()

// Create a handler to execute in the background thread
val backgroundHandler = Handler(handlerThread.looper, Handler.Callback {
    // Your code logic goes here.

    // Update UI on the main thread.
    mainHandler.post {
        
    }
    true
})

只有无参数的构造函数已弃用，现在最好通过loop . getmainlooper()方法在构造函数中指定循环器。

用于Java

new Handler(Looper.getMainLooper()).postDelayed(new Runnable() {
    @Override
    public void run() {
        // Your Code
    }
}, 3000);

将它用于Kotlin

Handler(Looper.getMainLooper()).postDelayed({
    // Your Code
}, 3000)

来源:developer.android.com

在Kotlin中使用这种结构是个好主意

companion object Run {
   fun after(delay: Long, process: () -> Unit) {
      Handler(Looper.getMainLooper()).postDelayed({
          process()
      }, delay)
   }
}

稍后称为

Run.after(SPLASH_TIME_OUT) {
   val action = SplashFragmentDirections.actionSplashFragmentToLogin()
   v.findNavController().navigate(action)
}