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https://github.com/woboq/qmetaobject-rs

Integrate Qml and Rust by building the QMetaObject at compile time.
https://github.com/woboq/qmetaobject-rs

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Integrate Qml and Rust by building the QMetaObject at compile time.

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README 

        
# QMetaObject crate for Rust



[![Crates.io](https://img.shields.io/crates/v/qmetaobject.svg)](https://crates.io/crates/qmetaobject)

[![Documentation](https://docs.rs/qmetaobject/badge.svg)](https://docs.rs/qmetaobject/)



A framework empowering everyone to create Qt/QML applications with Rust.

It does so by building `QMetaObject`s at compile time, registering QML types (optionally via exposing `QQmlExtensionPlugin`s) and providing idiomatic wrappers.



## Objectives



 - Rust procedural macro (custom derive) to generate a `QMetaObject` at compile time.

 - Bindings for the main Qt types using the `cpp!` macro from the [`cpp`](https://crates.io/crates/cpp) crate.

 - Users of this crate should not require to type any line of C++ or use another build system beyond cargo.

 - Performance: Avoid any unnecessary conversion or heap allocation.



Presentation Blog Post: https://woboq.com/blog/qmetaobject-from-rust.html



## Overview



```rust

use cstr::cstr;

use qmetaobject::prelude::*;



// The `QObject` custom derive macro allows to expose a class to Qt and QML

#[derive(QObject, Default)]

struct Greeter {

    // Specify the base class with the qt_base_class macro

    base: qt_base_class!(trait QObject),

    // Declare `name` as a property usable from Qt

    name: qt_property!(QString; NOTIFY name_changed),

    // Declare a signal

    name_changed: qt_signal!(),

    // And even a slot

    compute_greetings: qt_method!(fn compute_greetings(&self, verb: String) -> QString {

        format!("{} {}", verb, self.name.to_string()).into()

    })

}



fn main() {

    // Register the `Greeter` struct to QML

    qml_register_type::(cstr!("Greeter"), 1, 0, cstr!("Greeter"));

    // Create a QML engine from rust

    let mut engine = QmlEngine::new();

    // (Here the QML code is inline, but one can also load from a file)

    engine.load_data(r#"

        import QtQuick 2.6

        import QtQuick.Window 2.0

        // Import our Rust classes

        import Greeter 1.0



        Window {

            visible: true

            // Instantiate the rust struct

            Greeter {

                id: greeter;

                // Set a property

                name: "World"

            }

            Text {

                anchors.centerIn: parent

                // Call a method

                text: greeter.compute_greetings("hello")

            }

        }

    "#.into());

    engine.exec();

}

```



## Features



 - Create object inheriting from QObject, QQuickItem, QAbstractListModel, QQmlExtensionPlugin, ...

 - Export Qt properties, signals, methods, ...

 - Also support `#[derive(QGadget)]` (same as Q_GADGET)

 - Create Qt plugin (see examples/qmlextensionplugins)

 - Partial scene graph support



Requires Qt >= 5.8



## Cargo features



Cargo provides a way to enable (or disable default) optional [features](https://doc.rust-lang.org/cargo/reference/features.html).



### `log`



By default, Qt's logging system is not initialized, and messages from e.g. QML's `console.log` don't go anywhere.

The "log" feature enables integration with [`log`](https://crates.io/crates/log) crate, the Rust logging facade.



The feature is enabled by default. To activate it, execute the following code as early as possible in `main()`:



```rust

fn main() {

    qmetaobject::log::init_qt_to_rust();

    // don't forget to set up env_logger or any other logging backend.

}

```



### `chrono_qdatetime`



Enables interoperability of `QDate` and `QTime` with Rust [`chrono`](https://crates.io/crates/chrono) package.



This feature is disabled by default.



### `webengine`



Enables `QtWebEngine` functionality. For more details see the [example](./examples/webengine).



This feature is disabled by default.



## What if a wrapper for the Qt C++ API is missing?



It is quite likely that you would like to call a particular Qt function which

is not wrapped by this crate.



In this case, it is always possible to access C++ directly from your rust code

using the `cpp!` macro.



We strive to increase coverage of wrapped API, so whenever there is something

you need but currently missing, you are welcome to open a feature request on

GitHub issues or send a Pull Request right away.



### Tutorial: Adding Rust wrappers for Qt C++ API



This section teaches how to make your own crate with new Qt wrappers, and walk

through a Graph example provided with this repository.



First things first, set up your _Cargo.toml_ and _build.rs_:



1. Add `qttypes` to dependencies.

   Likely, you would just stick to recent versions published on [crates.io](versions).

   ```toml

   [dependencies]

   qttypes = { version = "0.2", features = [ "qtquick" ] }

   ```

   Add more Qt modules you need to the features array.

   Refer to [qttypes crate documentation](docs.qttypes) for a full list of supported modules.

   


   If you _absolutely need_ latest unreleased changes, use this instead of `version = "..."`:

    * `path = "../path/to/qmetaobject-rs/qttypes"` or

    * `git = "https://github.com/woboq/qmetaobject-rs"`



2. Add `cpp` to dependencies and `cpp_build` to build-dependencies.

   You can find up-to-date instructions on [`cpp` documentation](https://docs.rs/cpp) page.

   ```toml

   [dependencies]

   cpp = "0.5"



   [build-dependencies]

   cpp_build = "0.5"

   ```



3. Copy _build.rs_ script from [_qmetaobject/build.rs_](./qmetaobject/build.rs).

   It will run `cpp_build` against you package, using environment provided by

   [_qttypes/build.rs_](./qttypes/build.rs).



Now, every time you build your package, content of `cpp!` macros will be

collected in one big C++ file and compiled into a static library which will

later be linked into a final binary. You can find this _cpp_closures.cpp_

file buried inside Cargo target directory. Understanding its content might be

useful for troubleshooting.



There are two forms of `cpp!` macro.



* The one with double curly `{{` braces `}}` appends its content verbatim to

  the C++ file. Use it to `#include` headers, define C++ structs & classes etc.



* The other one is for calling expressions at runtime. It is usually written

  with `(` parenthesis `)`, it takes `[` arguments `]` list and requires an

  `unsafe` marker (either surrounding block or as a first keyword inside).



Order of macros invocations is preserved on a per-file (Rust module) basis;

but processing order of files is not guaranteed by the order of `mod`

declarations. So don't assume visibility — make sure to `#include` everything

needed on top of every Rust module.



Check out [documentation of `cpp`](https://docs.rs/cpp) to read more about how

it works internally.



Now that we are all set, let's take a look at the Graph example's code. It is

located in [_examples/graph_](./examples/graph) directory.



Before adding wrappers, we put relevant `#include` lines inside a `{{` double

curly braced `}}` macro:



```rust

cpp! {{

    #include 

}}

```



If you need to include you own local C++ headers, you can do that too! Check

out how main qmetaobject crate includes _qmetaobject_rust.hpp_ header in

every Rust module that needs it.



Next, we declare a custom QObject, just like in the [overview](#overview), but

this time it derives from `QQuickItem`. Despite its name, `#[derive(QObject)]`

proc-macro can work with more than one base class, as long as it is properly

wrapped and implements the [`QObject`](trait.QObject) trait.



```rust

#[derive(Default, QObject)]

struct Graph {

    base: qt_base_class!(trait QQuickItem),



    // ...

}

```



We wish to call [`QQuickItem::setFlag`] method which is currently not

exposed in the qmetaobject-rs API, so let's call it directly:



```rust

impl Graph {

    fn appendSample(&mut self, value: f64) {

        // ...

        let obj = self.get_cpp_object();

        cpp!(unsafe [obj as "QQuickItem *"] {

            obj->setFlag(QQuickItem::ItemHasContents);

        });

        // ...

    }

}

```



Alternatively, we could add a proper method wrapper, and call it without `unsafe`:



```rust

#[repr(C)]

enum QQuickItemFlag {

    ItemClipsChildrenToShape = 0x01,

    ItemAcceptsInputMethod = 0x02,

    ItemIsFocusScope = 0x04,

    ItemHasContents = 0x08,

    ItemAcceptsDrops = 0x10,

}



impl Graph {

    fn set_flag(&mut self, flag: QQuickItemFlag) {

        let obj = self.get_cpp_object();

        assert!(!obj.is_null());

        cpp!(unsafe [obj as "QQuickItem *", flag as "QQuickItem::Flag"] {

            obj->setFlag(flag);

        });

    }



    fn appendSample(&mut self, value: f64) {

        // ...

        self.set_flag(QQuickItemFlag::ItemHasContents);

        // ...

    }

}

```



Note that C++ method takes optional second argument, but since optional

arguments are not supported by Rust nor by FFI glue, it is always left out

(and defaults to `true`) in this case. To improve on this situation, we could

have added second required argument to Rust function, or implement

two "overloads" with slightly different names, e.g. `set_flag(Flag, bool)` &

`set_flag_on(Flag)` or `enable_flag(Flag)` etc.



Assert for not-null should not be needed if object is guaranteed to be

properly instantiated and initialized before usage. This applies to the

following situations:



- Call [`QObject::cpp_construct()`] directly and store the result in immovable

  memory location;



- Construct [`QObjectPinned`] instance: any access to pinned object or

  conversion to [`QVariant`] ensures creation of C++ object;



- Instantiate object as a QML component. They are always properly

  default-initialized by a QML engine before setting any properties or

  calling any signals/slots.



And that's it! You have just implemented a new wrapper for a Qt C++ class

method. Now send us a Pull Request. 🙂



[versions]: https://crates.io/crates/qmetaobject/versions

[trait.QObject]: https://docs.rs/qmetaobject/latest/qmetaobject/trait.QObject.html

[`QQuickItem::setFlag`]: https://doc.qt.io/qt-5/qquickitem.html#setFlag

[`QObject::cpp_construct()`]: https://docs.rs/qmetaobject/latest/qmetaobject/trait.QObject.html#tymethod.cpp_construct

[`QObjectPinned`]: https://docs.rs/qmetaobject/latest/qmetaobject/struct.QObjectPinned.html

[`QVariant`]: https://docs.rs/qmetaobject/latest/qmetaobject/struct.QVariant.html

[docs.qttypes]: https://docs.rs/qttypes/latest/qttypes/#cargo-features



## Comparison to Other Projects



The primary goal of this crate is to provide idiomatic Rust bindings for QML.

It focuses solely on QML, not QWidgets or any other non-graphical Qt API.

The aim is to eliminate the need for users to know or use C++ and other build systems.

This crate excels in achieving this goal.



* **[CXX-Qt](https://github.com/KDAB/cxx-qt/)** is an ideal solution for incorporating some Rust into an existing C++ project.

  CXX-Qt is newer than this crate and utilizes Rust features such as attribute macro,

  which didn't exist when the qmetaobject crate was designed.

  (Only derive procedural macro were available in stable Rust at that time)



* The **[Rust Qt Binding Generator](https://invent.kde.org/sdk/rust-qt-binding-generator)**

  is another project that aids in integrating Rust logic into an existing C++/Qt project.

  This project was also developed before Rust had procedural macros, so it uses an external .json file to generate C++ and Rust code.



* There are also several older crates that attempted to provide Rust binding around the Qt C++ API.

  Often automatically generated, these bindings are not idiomatic Rust, require unsafe code to use,

  and are no longer maintained.



* **[Slint](https://slint.rs)** is a project created by the author of this crate.

  Slint is not a QML or Qt binding, but a new language inspired from QML, entirely implemented in Rust.

  It shares the same objective of providing a means to add a UI to a Rust project with idiomatic Rust API, but instead of using QML for the UI, it uses its own language.\

  [![Slint Logo](https://slint.dev/logo/slint-logo-simple-light.svg)](https://slint.rs)



The qmetaobject crate is currently only being passively maintained as focus has shifted towards developing Slint.

You are encouraged to explore [Slint](https://slint.rs) as an exciting, innovative alternative for creating GUI in Rust projects.



## Applications built with this crate



 * https://github.com/gyroflow/gyroflow

   ![Gyroflow screenshot](https://github.com/gyroflow/gyroflow/blob/master/resources/screenshot.jpg)



* https://github.com/kalaksi/lightkeeper

  ![LightKeeper screenshot](https://github.com/kalaksi/lightkeeper/blob/master/doc/images/LightkeeperRM-overview.png)