显示 Toast 的线程安全方式（Application Wide）

public class MainApplication extends Application {
    
    private static Context context; //application context

    private Handler mainThreadHandler;
    private Toast toast;

    public Handler getMainThreadHandler() {
        if (mainThreadHandler == null) {
            mainThreadHandler = new Handler(Looper.getMainLooper());
        }
        return mainThreadHandler;
    }

    @Override public void onCreate() {
        super.onCreate();
        context = this;
    }
       
    public static MainApplication getApp(){
        return (MainApplication) context;
    }

    /**
     * Thread safe way of displaying toast.
     * @param message
     * @param duration
     */
    public void showToast(final String message, final int duration) {
        getMainThreadHandler().post(new Runnable() {
            @Override
            public void run() {
                if (!TextUtils.isEmpty(message)) {
                    if (toast != null) {
                        toast.cancel(); //dismiss current toast if visible
                        toast.setText(message);
                    } else {
                        toast = Toast.makeText(App.this, message, duration);
                    }
                    toast.show();
                }
            }
        });
    }

记得在 manifest 中添加 MainApplication。

现在从任何线程调用它来显示 Toast 消息。

MainApplication.getApp().showToast("Some message", Toast.LENGTH_LONG);