/**
 * Show the view or text notification for a short period of time.  This time
 * could be user-definable.  This is the default.
 * @see #setDuration
 */
public static final int LENGTH_SHORT = 0;

/**
 * Show the view or text notification for a long period of time.  This time
 * could be user-definable.
 * @see #setDuration
 */
public static final int LENGTH_LONG = 1;

private static class TN extends ITransientNotification.Stub {
    ...
    static final long SHORT_DURATION_TIMEOUT = 4000;
    static final long LONG_DURATION_TIMEOUT = 7000; 
    ...

    public void handleShow(IBinder windowToken) {
        ...
        mParams.hideTimeoutMilliseconds = mDuration ==
            Toast.LENGTH_LONG ? LONG_DURATION_TIMEOUT : SHORT_DURATION_TIMEOUT;
        ...
    }
    ...
}

/**
 * ...
 * ...                                        . Therefore, we do hide
 * such windows to prevent them from overlaying other apps.
 *
 * @hide
 */
public long hideTimeoutMilliseconds = -1;

long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY;

static final int LONG_DELAY = PhoneWindowManager.TOAST_WINDOW_TIMEOUT;
static final int SHORT_DELAY = 2000; // 2 seconds

/** Amount of time (in milliseconds) a toast window can be shown. */
public static final int TOAST_WINDOW_TIMEOUT = 3500; // 3.5 seconds

...

    /** @hide */
    @IntDef({LENGTH_SHORT, LENGTH_LONG})
    @Retention(RetentionPolicy.SOURCE)
    public @interface Duration {}

...

    /**
     * Set how long to show the view for.
     * @see #LENGTH_SHORT
     * @see #LENGTH_LONG
     */
    public void setDuration(@Duration int duration) {
        mDuration = duration;
        mTN.mDuration = duration;
    }

...

    /**
     * Make a standard toast that just contains a text view.
     *
     * @param context  The context to use.  Usually your {@link android.app.Application}
     *                 or {@link android.app.Activity} object.
     * @param text     The text to show.  Can be formatted text.
     * @param duration How long to display the message.  Either {@link #LENGTH_SHORT} or
     *                 {@link #LENGTH_LONG}
     *
     */
    public static Toast makeText(Context context, CharSequence text, @Duration int duration) {
        return makeText(context, null, text, duration);
    }

...

/**
 * Denotes that the annotated element of integer type, represents
 * a logical type and that its value should be one of the explicitly
 * named constants. If the {@link #flag()} attribute is set to true,
 * multiple constants can be combined.
 * ...
 */

Toast.makeText(this, "Hello", 2);

new Thread(new Runnable() {
    @Override
    public void run() {
        Toast.makeText(MainActivity.this, "Call toast on non-UI thread", Toast.LENGTH_SHORT)
                .show();
    }
}).start();

// 我们的代码里调用的 makeText 方法
public static Toast makeText(Context context, CharSequence text, @Duration int duration) {
    return makeText(context, null, text, duration);
}

// 隐藏的 makeText 方法，不能手动调用
public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
        @NonNull CharSequence text, @Duration int duration) {
    Toast result = new Toast(context, looper); // 这里的 looper 为 null
    ...

public Toast(@NonNull Context context, @Nullable Looper looper) {
    mContext = context;
    mTN = new TN(context.getPackageName(), looper); // looper 为 null
    ...
}

// looper = null
TN(String packageName, @Nullable Looper looper) {
    ...
    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()");
        }
    }
    ...
}

new Thread(new Runnable() {
    @Override
    public void run() {
        final int MSG_TOAST = 101;
        final int MSG_QUIT = 102;

        Looper.prepare();

        final Handler handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {

                switch (msg.what) {
                    case MSG_TOAST:
                        Toast.makeText(MainActivity.this, "Call toast on non-UI thread", Toast.LENGTH_SHORT)
                                .show();
                        sendEmptyMessageDelayed(MSG_QUIT, 4000);
                        return;

                    case MSG_QUIT:
                        Looper.myLooper().quit();
                        return;
                }

                super.handleMessage(msg);
            }
        };

        handler.sendEmptyMessage(MSG_TOAST);

        Looper.loop();
    }
}).start();

view.postDelayed(new Runnable() {
    @Override
    public void run() {
        Toast.makeText(MainActivity.this, "background toast", Toast.LENGTH_SHORT).show();
    }
}, 5000);

/**
 * Make a standard toast to display using the specified looper.
 * If looper is null, Looper.myLooper() is used.
 * @hide
 */
public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
        @NonNull CharSequence text, @Duration int duration) {
    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;
}

/**
 * Constructs an empty Toast object.  If looper is null, Looper.myLooper() is used.
 * @hide
 */
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);
}

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();
        if (looper == null) {
            throw new RuntimeException(
                    "Can't toast on a thread that has not called Looper.prepare()");
        }
    }
    mHandler = new Handler(looper, null) {
        ...
    };
}

/**
 * Show the view for the specified duration.
 */
public void show() {
    if (mNextView == null) {
        throw new RuntimeException("setView must have been called");
    }

    INotificationManager service = getService();
    String pkg = mContext.getOpPackageName();
    TN tn = mTN;
    tn.mNextView = mNextView;

    try {
        service.enqueueToast(pkg, tn, mDuration);
    } catch (RemoteException e) {
        // Empty
    }
}

@Override
public void enqueueToast(String pkg, ITransientNotification callback, int duration)
{
    ...

    synchronized (mToastQueue) {
        ...
        try {
            ToastRecord record;
            int index = indexOfToastLocked(pkg, callback);
            // If it's already in the queue, we update it in place, we don't
            // move it to the end of the queue.
            if (index >= 0) {
                record = mToastQueue.get(index);
                record.update(duration);
            } else {
                // Limit the number of toasts that any given package except the android
                // package can enqueue.  Prevents DOS attacks and deals with leaks.
                if (!isSystemToast) {
                    int count = 0;
                    final int N = mToastQueue.size();
                    for (int i=0; i<N; i++) {
                         final ToastRecord r = mToastQueue.get(i);
                         if (r.pkg.equals(pkg)) {
                             count++;
                             if (count >= MAX_PACKAGE_NOTIFICATIONS) {
                                 Slog.e(TAG, "Package has already posted " + count
                                        + " toasts. Not showing more. Package=" + pkg);
                                 return;
                             }
                         }
                    }
                }

                Binder token = new Binder();
                mWindowManagerInternal.addWindowToken(token, TYPE_TOAST, DEFAULT_DISPLAY);
                record = new ToastRecord(callingPid, pkg, callback, duration, token);
                mToastQueue.add(record);
                index = mToastQueue.size() - 1;
                keepProcessAliveIfNeededLocked(callingPid);
            }
            // If it's at index 0, it's the current toast.  It doesn't matter if it's
            // new or just been updated.  Call back and tell it to show itself.
            // If the callback fails, this will remove it from the list, so don't
            // assume that it's valid after this.
            if (index == 0) {
                showNextToastLocked();
            }
        } finally {
            Binder.restoreCallingIdentity(callingId);
        }
    }
}

// Limit the number of toasts that any given package except the android
// package can enqueue.  Prevents DOS attacks and deals with leaks.
if (!isSystemToast) {
    int count = 0;
    final int N = mToastQueue.size();
    for (int i=0; i<N; i++) {
         final ToastRecord r = mToastQueue.get(i);
         if (r.pkg.equals(pkg)) {
             count++;
             if (count >= MAX_PACKAGE_NOTIFICATIONS) {
                 Slog.e(TAG, "Package has already posted " + count
                        + " toasts. Not showing more. Package=" + pkg);
                 return;
             }
         }
    }
}

@GuardedBy("mToastQueue")
void showNextToastLocked() {
    ToastRecord record = mToastQueue.get(0);
    while (record != null) {
        ...
        try {
            record.callback.show(record.token);
            scheduleTimeoutLocked(record);
            return;
        } catch (RemoteException e) {
            ...
            if (index >= 0) {
                mToastQueue.remove(index);
            }
            ...
        }
    }
}

...

@GuardedBy("mToastQueue")
private void scheduleTimeoutLocked(ToastRecord r)
{
    mHandler.removeCallbacksAndMessages(r);
    Message m = Message.obtain(mHandler, MESSAGE_TIMEOUT, r);
    long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY;
    mHandler.sendMessageDelayed(m, delay);
}

private void handleTimeout(ToastRecord record)
{
    if (DBG) Slog.d(TAG, "Timeout pkg=" + record.pkg + " callback=" + record.callback);
    synchronized (mToastQueue) {
        int index = indexOfToastLocked(record.pkg, record.callback);
        if (index >= 0) {
            cancelToastLocked(index);
        }
    }
}

...

@GuardedBy("mToastQueue")
void cancelToastLocked(int index) {
    ToastRecord record = mToastQueue.get(index);
    try {
        record.callback.hide();
    } catch (RemoteException e) {
        ...
    }

    ToastRecord lastToast = mToastQueue.remove(index);
    mWindowManagerInternal.removeWindowToken(lastToast.token, true, DEFAULT_DISPLAY);

    keepProcessAliveIfNeededLocked(record.pid);
    if (mToastQueue.size() > 0) {
        // Show the next one. If the callback fails, this will remove
        // it from the list, so don't assume that the list hasn't changed
        // after this point.
        showNextToastLocked(); // 继续显示队列里的下一个 Toast
    }
}

...

private final class WorkerHandler extends Handler
{
    ...
    @Override
    public void handleMessage(Message msg)
    {
        switch (msg.what)
        {
            case MESSAGE_TIMEOUT:
                handleTimeout((ToastRecord)msg.obj);
                break;
            ...
        }
    }
}

private static class TN extends ITransientNotification.Stub {
    ...
    TN(String packageName, @Nullable Looper looper) {
        ...
        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();
                        ...
                        break;
                    }
                    ...
                }
            }
        };
    }

    /**
     * schedule handleShow into the right thread
     */
    @Override
    public void show(IBinder windowToken) {
        if (localLOGV) Log.v(TAG, "SHOW: " + this);
        mHandler.obtainMessage(SHOW, windowToken).sendToTarget();
    }

    /**
     * schedule handleHide into the right thread
     */
    @Override
    public void hide() {
        if (localLOGV) Log.v(TAG, "HIDE: " + this);
        mHandler.obtainMessage(HIDE).sendToTarget();
    }

    ...

    public void handleShow(IBinder windowToken) {
        ...
                mWM.addView(mView, mParams);
        ...
    }

    ...

    public void handleHide() {
        ...
                mWM.removeViewImmediate(mView);
        ...
    }
}