Toast源码解析——toast能在子线程中弹出吗

Android人知道在安卓中不能在子线程中更新ui，为什么呢？真的不能吗？

先做个试验，我们在子线程中调用toast，看看有什么效果，按照上边一句话的意思，这里应该会报错显示不能再子线程中刷新ui吧

val thread = object : Thread() {
    override fun run() {
        Toast.makeText(this@MainActivity, "123", Toast.LENGTH_LONG).show()
    }
}
thread.start()

不出意外的崩溃了：

java.lang.RuntimeException: Can't toast on a thread that has not called Looper.prepare()
    at android.widget.Toast$TN.(Toast.java:397)
    at android.widget.Toast.(Toast.java:117)
    at android.widget.Toast.makeText(Toast.java:280)
    at android.widget.Toast.makeText(Toast.java:270)

日志可以发现，结果并不像我们想的那样，而是告诉我们需要一个looper，这个错误看着有点眼熟，了解过Handler构造函数的知道，new Handler的时候，如果没有取到当前线程的Looper也会报类似的错误。那是不是我们在toast show之前调用Looper.prepare()就能显示toast了呢？看代码：

val thread = object : Thread() {
    override fun run() {
       Looper.prepare()
       Toast.makeText(this@MainActivity, "123", Toast.LENGTH_LONG).show()
       Looper.loop()
    }
}
thread.start()

运行后发现确实没问了。当然这里执行的结果就是我们的线程不能退出了。那toast到底是怎么显示出来的呢?

---

Let’s rtfsc.

首先看看Toast

Toast.makeText(this@MainActivity, "123", Toast.LENGTH_LONG).show()

上边是调用Toast的方式，我们从makeText开始看起

public static Toast makeText(Context context, CharSequence text, @Duration int duration) {
    return makeText(context, null, text, duration);
}

public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
        @NonNull CharSequence text, @Duration int duration) {
    //构造Toast实例
    Toast result = new Toast(context, looper);

    //组装布局
    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;

    //组装后的toast实例
    return result;
}

看看Toast的构造方法

public Toast(@NonNull Context context, @Nullable Looper looper) {
    mContext = context;
    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);
}

这里有个关键的对象TN，TN是个什么呢？

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

他其实是一个binder对象，具体的咱先不说，先看看他的构造函数

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.
        //第一部分，设置window参数
    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;

    //第二部分，拿当前线程的looper，当然这里取到的looper取决于当前代码执行所在的线程
    if (looper == null) {
        // Use Looper.myLooper() if looper is not specified.
        looper = Looper.myLooper();
        if (looper == null) {
            throw new RuntimeException(
                    "Can't toast on a thread that has not called Looper.prepare()");
        }
    }

    //第三部分，创建handler关联looper
    mHandler = new Handler(looper, null) {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case SHOW: {
                    IBinder token = (IBinder) msg.obj;
                    //第四部分，显示toast
                    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;
                }
            }
        }
    };
}

上边代码好像能看出点东西了，可以看到，文章开头的错误是在这里报出来的，另外toast的显示隐藏都是在一个handler中操作的，这个handler关联的正是我们toast所在的线程的looper,这就可以看出来子线程是可以更新toast的，前提是需要提供looper。

了解过的都知道，Toast、Dialog、PopupWindow都是基于Android的Window的，那么我们遇到的说子线程不能刷新view，如在在线程中setText等操作，这是为什么呢？下一篇我们讲讲所有View的parent->ViewRootImpl