Android Handler 的用法指南

// 在主线程创建Handler会自动关联主线程的Looper
Handler mainHandler = new Handler(Looper.getMainLooper()) {
    @Override
    public void handleMessage(Message msg) {
        // 处理消息
        switch (msg.what) {
            case 1:
                String text = (String) msg.obj;
                textView.setText(text);
                break;
        }
    }
};

// 发送空消息
handler.sendEmptyMessage(1);
// 发送带what的消息
Message msg = handler.obtainMessage();
msg.what = 2;
msg.obj = "Hello Handler";
handler.sendMessage(msg);
// 延迟发送
handler.sendEmptyMessageDelayed(1, 1000); // 1秒后发送
handler.sendMessageDelayed(msg, 2000); // 2秒后发送

new Thread(() -> {
    // 为当前线程创建Looper
    Looper.prepare();
    Handler threadHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            // 处理子线程消息
        }
    };
    // 开始消息循环
    Looper.loop();
}).start();

new Thread(() -> {
    // 模拟耗时操作
    try {
        Thread.sleep(1000);
        // 通过Handler发送消息到主线程更新UI
        Message msg = mainHandler.obtainMessage();
        msg.what = 1;
        msg.obj = "更新后的文本";
        mainHandler.sendMessage(msg);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}).start();

// 延迟执行
handler.postDelayed(() -> {
    Toast.makeText(this, "5秒后执行", Toast.LENGTH_SHORT).show();
}, 5000);
// 循环执行
final Runnable runnable = new Runnable() {
    @Override
    public void run() {
        // 执行任务
        Log.d("Handler", "每隔1秒执行");
        // 再次post实现循环
        handler.postDelayed(this, 1000);
    }
};
handler.postDelayed(runnable, 1000);
// 取消定时任务
handler.removeCallbacks(runnable);

// 工作线程
class WorkerThread extends Thread {
    public Handler workerHandler;
    @Override
    public void run() {
        Looper.prepare();
        workerHandler = new Handler(Looper.myLooper()) {
            @Override
            public void handleMessage(Message msg) {
                // 处理来自主线程的消息
                String task = (String) msg.obj;
                Log.d("WorkerThread", "执行任务: " + task);
                // 可以回传结果给主线程
                Message resultMsg = mainHandler.obtainMessage();
                resultMsg.what = 2;
                resultMsg.obj = task + " 完成";
                mainHandler.sendMessage(resultMsg);
            }
        };
        Looper.loop();
    }
}
// 主线程发送任务给工作线程
WorkerThread worker = new WorkerThread();
worker.start();
// 等待workerHandler初始化
new Handler().postDelayed(() -> {
    if (worker.workerHandler != null) {
        Message msg = worker.workerHandler.obtainMessage();
        msg.obj = "下载文件";
        worker.workerHandler.sendMessage(msg);
    }
}, 1000);

// 创建HandlerThread
HandlerThread handlerThread = new HandlerThread("MyHandlerThread");
handlerThread.start();
// 获取HandlerThread的Looper创建Handler
Handler threadHandler = new Handler(handlerThread.getLooper()) {
    @Override
    public void handleMessage(Message msg) {
        // 在后台线程处理消息
    }
};
// 发送消息
threadHandler.post(() -> {
    // 在HandlerThread中执行
});
// 退出时释放资源
handlerThread.quitSafely();

// 使用静态内部类+弱引用
private static class SafeHandler extends Handler {
    private final WeakReference<Activity> activityRef;
    public SafeHandler(Activity activity) {
        super(Looper.getMainLooper());
        this.activityRef = new WeakReference<>(activity);
    }
    @Override
    public void handleMessage(Message msg) {
        Activity activity = activityRef.get();
        if (activity != null && !activity.isFinishing()) {
            // 安全处理消息
        }
    }
}
// 在Activity中使用
private SafeHandler safeHandler = new SafeHandler(this);

// 复用Message对象（推荐）
Message msg = handler.obtainMessage(WHAT_ARG, obj);
handler.sendMessage(msg);
// 设置回调代替继承Handler
handler.sendMessage(Message.obtain(handler, () -> {
    // 回调处理
}));

@Override
protected void onDestroy() {
    super.onDestroy();
    handler.removeCallbacksAndMessages(null);
    if (handlerThread != null) {
        handlerThread.quitSafely();
    }
}