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Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

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Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged(), and it has a slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue() slot:

The emit line emits the signal valueChanged() from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged() signal to the second object's setValue() slot using QObject::connect():

Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. Then b emits the same valueChanged() signal, but since no slot has been connected to b's valueChanged() signal, the signal is ignored.

Note that the setValue() function sets the value and emits the signal only if value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

A Real Example

The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect(). There are several ways to connect signal and slots. The first is to use function pointers:

There are several advantages to using QObject::connect() with function pointers. First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments. Arguments can also be implicitly converted by the compiler, if needed.

You can also connect to functors or C++11 lambdas:

In both these cases, we provide this as context in the call to connect(). The context object provides information about in which thread the receiver should be executed. This is important, as providing the context ensures that the receiver is executed in the context thread.

The lambda will be disconnected when the sender or context is destroyed. You should take care that any objects used inside the functor are still alive when the signal is emitted.

The other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

All of these would work:

But this one won't work:

...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

Lambda expressions are a convenient way to pass custom arguments to a slot:

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

See also QLCDNumber, QObject::connect(), Digital Clock Example, Tetrix Example, Meta-Object System, and Qt's Property System.

© 2021 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

In my earlier blog, I already covered the explanations about the A/B partitions, including how it works, its advantages, etc (You could refer here: How to Flash ROMs/Magisk/TWRP/Kernels on A/B Partition Devices). This guide will have an in-depth look at How to flash various files, including Custom ROMs, TWRP, Kernels, Magisk, etc in A/B Partition Devices. Just be sure to follow each and every part of this tutorial in the exact same manner as mentioned.

For your convenience, this tutorial will be broken down into short sections, with each section having instructions for different types of files to flash.

Before beginning with the process, keep the following things in mind:

1. When you flash a ROM through Recovery, it will be flashed to the inactive slot. This is the reason why you need to switch slots every time (so as to move to the ‘inactive’ slot and in the process make it the ‘active’ one) after flashing a custom ROM and then flash everything else.
2. But until you reboot the device from Recovery, your active slot isn’t going to change and you will still be the previous OS (and not the ROM you just installed). That is why we recommend rebooting the device so as to get to the slot where the newly installed OS is present, and hence make it the active one.
3. Always flash the files in the following order:
   • TWRP
   • Custom ROM
   • Custom Kernel (optional)
   • Magisk (for root).

Prerequisites

• A complete backup of all the data on your device is extremely important. This is because, during the process, your device’s storage will completely be wiped off.
• Keep your device at a minimum of 60% charged so that it doesn’t turn off in between.
• Make sure to unlock the bootloader of your device. Refer to our guide on how to do so.
• Download and install the ADB and Platform Tools. Now head over to this platform-tools folder, type in CMD in the address bar, and hit Enter. This will launch the Command Prompt.
  
• Enable USB Debugging and OEM Unlocking on your device: Go to ‘Settings’ > ‘About Phone’ > Tap on the ‘Build Number’ 7 times > Go back to ‘Settings’ > ‘Developer Options’ > Toggle on the ‘USB Debugging’ and ‘OEM Unlocking’ switch.

Downloads

• Download the TWRP Recovery (both the image and zip file) for your device from the official site. If it isn’t officially released yet for your device, try the unofficial one. It would be available at the XDA forum.
• Next, download the Custom ROM and Kernel of your choice. Custom ROMs could be downloaded here: Top 5 Custom ROMs based on Android 9.0 Pie (2019). or you could check out the XDA site as well.
• Also, download the latest Magisk installer zip file: Magisk latest file (for root-access).
• Finally, download the GApps package corresponding to the device’s architecture and Android OS. Then choose the desired variant.

A note on GApps

• Under device architecture, select ARM64 for your device. However, if you want to double-check, download the ‘CPU-Z’ app. Under the ‘System’ tab, look for the ‘Kernel Architecture’- if it is armv7a, armv8l or related types, then it is definitely ARM64.
• If you are short on time (and also internet), then simply go for the ‘mini‘ variant. It contains the most basic yet important Google Apps and Services. The rest could be downloaded once the device boots up to the OS.
• Some ROMs like the ‘Pixel Experience’ already have Gapps included, so you wouldn’t be needing it. Always check on the ROMs’ development/support page (or ask me in the comments section) before downloading the Gapps for any ROM.

STEP 1- Setting Up Your Device

If you are coming from a stock ROM, you need to wipe your data and files. Follow the below points:

1. First off, connect your device to PC via the USB Cable.
2. Next, in the PowerShell window, type in the below code to boot your device to fastboot mode:
3. Now type the below code to wipe all of your device’s data and files:

An important point worth discussing: The custom ROM you are about to flash will be flashed to the inactive slot. So we will be starting this tutorial by taking Slot B as the active slot (hence Slot A would be the inactive slot- where the custom ROM would be installed). If you have Slot A as the active, change it right now by typing the below code in the PowerShell Window:

STEP 2: Installing TWRP Recovery

With Slot ‘B’ as the active slot, we will begin with the installation process. The process starts with booting the TWRP Recovery and then subsequently flashing it on your device. Carry out the below-mentioned steps for the same:

• Don’t Miss: How to Boot Any Android Device to TWRP Recovery

1. Rename the downloaded TWRP recovery (the .img file) to twrp (the complete name would hence be twrp.img)
2. Place it inside the folder where you have installed ADB and Fastboot Platform Tools.
3. Now we will be temporarily booting the device to recovery mode. I have already explained in my previous guide that there is no separate recovery partition for A/B partition devices. Hence TWRP could not be flashed directly by fastboot commands. As it is present inside the boot partition, therefore we will be booting into recovery first and then flashing it. Type the below code to boot into recovery:

   (If this method of booting to recovery doesn’t work out for you, refer to the FAQs section at the end of the guide).

4. Your device will boot to TWRP Recovery. Now you need to erase all the data present on the device. Go to ‘Wipe’ > Select ‘System’ and ‘Data’ > Right swipe on the ‘Swipe to Wipe’ button to wipe the selected partitions.
   
5. Next, go to ‘Mount’ and enable the ‘MTP’ option. The device will now be visible on your PC. Transfer the TWRP zip file, ROM, GApps package, Magisk, Kernel and anything else you need to flash. (If MTP didn’t work out for you, refer to the FAQs section at the end of this guide).
   
6. We will start by flashing the TWRP zip file. Go to ‘Install’, select the TWRP zip file and perform a right swipe on ‘Swipe to confirm Flash’.
7. [IMPORTANT] Now reboot your device again to TWRP Recovery. Go to the ‘Reboot’ option of TWRP and select ‘Recovery’.

That’s it. The first part of the guide on how to flash various files in A/B partition devices is now over. Head over to the next section to install a Custom ROM.

STEP 3: Installing a Custom ROM/Kernel/Magisk

1. Go to Install and select the ROM you wish to install.
2. Right swipe on the ‘Swipe to confirm Flash’ to install the selected ROM.
3. Currently, the ROM would be installed on the inactive slot (Slot A), whereas you are working on the active slot (Slot B).
4. Go to ‘Reboot’ and select ‘Slot A’ – the current inactive slot on which the Custom ROM is installed.
5. In order to make Slot A the active one, Reboot your device to Recovery. Tap the back button and select ‘Recovery’ from the TWRP ‘Reboot’ menu.
   
6. Now flash ‘GApps‘ if required by your ROM.
7. [Optional] Flash a custom kernel if you want.
8. [Optional for Custom ROM, Compulsory for Stock] Flash DM-VERITY & FORCE ENCRYPT DISABLER. This is needed to disable DM verity and encryption in the vendor partition. Most of the custom ROMs already do this. But Stock ROM doesn’t. So it is necessary to flash this file if you prefer to stay on the stock. Still, if you are using a custom ROM, consider flashing it. It wouldn’t do any harm even if isn’t required by your custom ROM.
9. Now, if you want root access, consider flashing the ‘Magisk‘ zip file. Select ‘Magisk-v19.3.zip’ and right swipe to confirm flash.
10. Finally, go to ‘Reboot’ and select ‘System’. This will reboot the device to the Android Operating System.

• DON’T MISS: [GUIDE] Flash ROMs/Magisk/TWRP/Kernels on A/B Partition Devices

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Final Thoughts

After performing the above steps, you will be able to boot your device to the TWRP Recovery. Then using the Recovery, you would have flashed a custom ROM. And finally, you rooted the device using the Magisk Installer zip file. Therefore, in a nutshell, this is what we did:

1. Temporary Booted to TWRP Recovery using Fastboot.
2. Permanently installed the TWRP Recovery using the booted TWRP.
3. Installed the Custom ROM via TWRP.
4. Installed Kernel (if you chose to).
5. Flashed DM Verity and Force Encrypt Disable (compulsory on stock, optional for custom ROMs).
6. Finally, we flashed the Magisk zip file from the TWRP recovery to get root access.

So this was the comprehensive guide on How to Flash Various Files and Mods in A/B Partition Devices. If you are having any queries with respect to any part of this guide, first have a look at FAQs below. However, if the problem still persists, do let me know in the comments section below. Happy Flashing!

FAQs

Q1) How to boot my device to TWRP recovery if the fastboot boot twrp.img command fails?
A1) If the above command fails or gives you a bootloop, ‘force shutdown’ the device and try the above fastboot command again.

Q2) “Transferring files using MTP didn’t work”.
A2) You may try any one of the following alternatives:

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• USE THE SIDELOAD METHOD: Transfer the file to be flashed to ADB and Fastboot Platform Tool’s folder. Next, under TWRP, go to ‘Advanced’ > ‘Sideload’ > ‘Swipe to Start Sideload’. Now, in the PowerShell window, enter the following command- ‘adb sideload filename.zip’.
  
• USE THE ADB PUSH METHOD: The file to be flashed should be present inside the ADB and Fastboot Tools folder. Under TWRP, go to ‘Advanced’>’File manager’.
  Now navigate to the ‘data/media’ folder. If the folder named ‘0’ is not present there, then create it. This can be done by going to ‘Advanced’>’Terminal’ and typing the below codes (codes are after the image):
  

• Next, open PowerShell window on your PC and type the below code:

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That’s all from the tutorial on How to Flash Various Files in A/B Partition Devices.