子线程真的不能弹Toast吗？

一、背景：

做Android开发的工程师都知道一条金科玉律（在主线程操作UI的更新与变化）。我们经常在做工程业务的时候，会使用封装的ToastUtils来统一做toast。如下所示：

public class ToastUtils{

    private static Toast sToast;
    private static Handler sMainHandler;
    private ToastUtils(){
        throw new UnsupportedOperationException("it is a utils class, should not be newInstance");
    }

    public static void show(Context ctx, String content){

        //判断当前是否在UI线程
        if (Looper.myLooper() != Looper.getMainLooper()) {
            if(sMainHandler == null){
                sMainHandler = new Handler(Looper.getMainLooper());
            }

            sMainHandler.post(new Runnable(){

                @overrride
                public void run(){
                    showInner(ctx,content);
                }
            });
        }else{

            showInner(ctx,content);
        }
       
    }

    private void showInner(Context ctx, String content){
        //避免频繁创建Toast对象，也能防止Toast风暴
        if(sToast == null){
            sToast = Toast.makeText(ctx,content,Toast.LENGHT_LONG);
        }else{
            sToast.setText(content);
        }

        sToast.show();
    }

}

二、解惑

但是真的只能在主线程才能弹Toast吗？答案是NO。一般我们在子线程弹toast的时候，会得到提示："Can't toast on a thread that has not called Looper.prepare()"
我们跟踪一下代码：

• makeText()

public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
            @NonNull CharSequence text, @Duration int duration) {
        
        //这一步骤是关键
        Toast result = new Toast(context, looper);

        LayoutInflater inflate = (LayoutInflater)
                context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);
        TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message);
        tv.setText(text);

        result.mNextView = v;
        result.mDuration = duration;

        return result;
    }

• 接着看new Toast()

/**
     * Constructs an empty Toast object.  If looper is null, Looper.myLooper() is used.
     * @hide
     */
     
     //构造Toast对象，传递参数又一个Looper
    public Toast(@NonNull Context context, @Nullable Looper looper) {
        mContext = context;
        //这里是关键
        mTN = new TN(context.getPackageName(), looper);
        mTN.mY = context.getResources().getDimensionPixelSize(
                com.android.internal.R.dimen.toast_y_offset);
        mTN.mGravity = context.getResources().getInteger(
                com.android.internal.R.integer.config_toastDefaultGravity);
    }

• 再接着，我们看new TN().TN是一个binder实现类，提供给AMS使用

private static class TN extends ITransientNotification.Stub {
        ...省略

        static final long SHORT_DURATION_TIMEOUT = 4000;
        static final long LONG_DURATION_TIMEOUT = 7000;

        TN(String packageName, @Nullable Looper looper) {
            // XXX This should be changed to use a Dialog, with a Theme.Toast
            // defined that sets up the layout params appropriately.
            final WindowManager.LayoutParams params = mParams;
            params.height = WindowManager.LayoutParams.WRAP_CONTENT;
            params.width = WindowManager.LayoutParams.WRAP_CONTENT;
            params.format = PixelFormat.TRANSLUCENT;
            params.windowAnimations = com.android.internal.R.style.Animation_Toast;
            params.type = WindowManager.LayoutParams.TYPE_TOAST;
            params.setTitle("Toast");
            params.flags = WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON
                    | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                    | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;

            mPackageName = packageName;

            if (looper == null) {
                // Use Looper.myLooper() if looper is not specified.
                looper = Looper.myLooper();
         //duang duang duang，我们看到了子线程弹toast的异常提示
         //也就是如果当前线程如果没有Looper则会抛出异常。
                if (looper == null) {
                    throw new RuntimeException(
                            "Can't toast on a thread that has not called Looper.prepare()");
                }
            }
            mHandler = new Handler(looper, null) {
                @Override
                public void handleMessage(Message msg) {
                    switch (msg.what) {
                        case SHOW: {
                            IBinder token = (IBinder) msg.obj;
                            handleShow(token);
                            break;
                        }
                        case HIDE: {
                            handleHide();
                            // Don't do this in handleHide() because it is also invoked by
                            // handleShow()
                            mNextView = null;
                            break;
                        }
                        case CANCEL: {
                            handleHide();
                            // Don't do this in handleHide() because it is also invoked by
                            // handleShow()
                            mNextView = null;
                            try {
                                getService().cancelToast(mPackageName, TN.this);
                            } catch (RemoteException e) {
                            }
                            break;
                        }
                    }
                }
            };
        }

       ...省略的一些方法
        }

• 通过上面的代码分析，我们可以知道：之所以子线程弹toast抛出异常是因为没有looper。而主线程天生自带Looper。所以没有问题，也就是说只要我们给子线程创建Looper就能在子线程上弹toast了。

三、正确的子线程弹toast方法：

 public void toastInWorkerThread(Context ctx, String content){
        new Thread(new Runnable{
            @overrride
            public void run(){
                Lopper.prepare();
                Toast.makeText(ctx, content,Toast.LENGHT_LONG).show()
                Looper.loop();
            }
        }).start();
    }