Model and view of QT

introduce

Everyone does GUI Developed , Should understand the model / View architecture programming , But do you know its origin ？

Actually MVC In the framework of 1982 The year has been born since Small talk The framework of language , Now it has become a model of software design , Using a business logic 、 data 、 The interface displays the separated method organization code , Aggregation of business logic into one component , While improving and personalizing the interface and user interaction , No need to rewrite business logic .
MVC Is uniquely developed to map traditional input 、 Processing and output functions in a logical graphical user interface structure .

then MVC The frame is on fire ,Smalltalk But failed .
You may not have heard of Smalltalk, But it is actually recognized as the second object-oriented programming language in history ( The first is Simula67, I guess you haven't heard of it ) And the first truly integrated development environment (IDE), It has greatly promoted the production of many other design languages .
Yes Smalltalk I put the small knowledge of expansion at the end of my blog , You can read it as a popular science , Understand the origin of the next object

Model / View frame

• Model (Model) It's the application object , Used to represent data ;
• View (View) Is the user interface of the model , Used to display data ;
• control (controller), Defines how the user interface responds to user input .

stay MVC Before , The user interface will 3 The two components come together ,MVC To separate them , This improves flexibility and reusability .
If you combine the two components of view and control , It forms a model / View frame . This also separates the storage of data from the presentation of data to users , But it provides a simpler framework .
Data and interface are separated . It makes it possible to display the same data in multiple different views , You can also create new views , Without changing the underlying data framework .
In order to deal with user input flexibly , Delegation is also introduced （Delegate） The concept of , It can be used to customize the rendering and editing of data .

Each of these components uses an abstract class to define , It provides some general interfaces and default implementations of some functions ,
Model 、 View 、 Proxy components communicate with each other through signals and slots ：

• When the data of the data source changes , The model signals the view
• When the user interacts with the displayed component , Views signal to provide interactive information
• When editing an item , The delegate sends a signal , Tell the status of the model and view editor

Model

Qt All model classes of are based on QAbstractItemModel Virtual class , This defines the interface , Views and agents can access data .
Qt Some ready-made models are provided to deal with data items ：

• QStringListModel Used to store a simple QString A list of items ;
• QStandardItemModel Manage complex tree structured data items , Each data item can contain any data ;
• QFileSystemModel Provides information about files and directories in the local file system ;
• QSqlQueryModel、QSqlTableModel and QSqlRelationalTableModel To access the database

If Qt These standard models provided cannot meet the needs , You can also subclass QAbstractItemModel、QAbstractListModel perhaps QAbstractTableModel To create a custom model .

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In order to ensure that the representation of data is separated from the acquisition of data ,Qt The concept of model index is introduced .
Each piece of data that can be obtained through the model is represented by a model index , Both views and delegates use these indexes to request data items and display .
The model index has QModelIndex Class provides , It is a temporary reference to a piece of data , It can be used to retrieve or modify the data in the model .
Because the model can reorganize the internal structure at any time , In this way, the model index may fail , So you don't need and shouldn't store model indexes

View

Qt Several different types of views are provided :

• QListView Display data items as a list
• QTableView Displays the data in the model in a table
• QTreeView Display the data items of the model in a hierarchical list .
• These classes are based on QAbstractItemView Abstract base class , These classes can be used directly , It can also be subclassed to provide customized views .

Processing project selection

Model / The view architecture provides a very convenient method for selecting items . The information of the item selected in the view is stored in $QItemSelectionModel$ In the example , In this way, the model index of the selected item is kept in a separate model , Independent of all views . When setting up multiple models on one model , You can share choices among multiple views .
   The selection is specified by the selection range , Just record the model index at the beginning and end of each selection range , Discontinuous selection can be described using multiple selection ranges . Selection can be regarded as a set of model indexes saved in the selection model , The most recent project is called the current selection .

entrust （ agent ）

In the model / In the view frame ,QAbstractItemDelegate Is the abstract base class of delegate ,
The default delegate implementation is implemented by QStyledItemDelegate Class provides , It's also called Qt Default delegate for standard views .
However ,QStyleItemDelegate and QItemDelegate It's independent of each other . Only one of them can be selected to draw and provide an editor for items in the view . The main difference between them is ,QStyledItemDelegate Use the current style to draw the project , therefore , When you want to implement a custom delegate, you need to Qt When applied with style sheets , It is recommended to use QStyleItemDelegate As the base class .

And MVC Different modes , Model / The view structure does not contain a complete separate component to deal with the interaction with users . General , The view is used to present the data in the model to the user , Also used to process user input .
   For greater flexibility , Interactions can be performed by delegates . These components provide input functions , It is also responsible for rendering individual items in some views . The standard interface for controlling delegation is QAbstractItemDelegate Definition in class .
   Delegation is achieved by painter() and sizeHint() Function so that they can render their own content . However , Simple component-based delegates can be subclassed QItemDelegate To achieve , Instead of using QAbstractItemDelegate, In this way, you can use the default items of these functions to implement . Delegated editors can be implemented in two ways , One is to use parts to manage the editing process , The other is to deal with events directly . You can refer to Qt Provided Spin Box Delegate Example Sample program to understand the first form . If you want to inherit QAbstractItemDelegate To achieve custom rendering operations , Then you can refer to Pixelator Example The sample program . in addition , You can also use QStyledItemDelegate As the base class , In this way, you can customize the display of data , This can be referred to as Star Delegate Example The sample program .
  Qt All standard views in use QItemDelegate To provide editing functions , The default implementation of this delegate interface is QListView、QTableView and QTreeView And other standard views provide a common style of rendering for each project . The default delegate in the standard view handles all standard roles , The specific content can be found in QItemDelegate Class . have access to itemDelegate() Function to get the delegate used in a view , Use setItemDelegate() Function can install a custom delegate for a view .

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The origin of object orientation

When we talk about the origin, we have to talk about Simula67 This language , It first introduced the concept of object .
The earliest germination of the concept of object and instance in computer science can be traced back to MIT PDP-1 System . This system is probably the earliest capacity-based architecture （capability based architecture） The actual system . in addition 1963 year Ivan Sutherland Of Sketchpad The same idea is contained in the application .
Object as a programming entity dates back to 1960 Age from $Simula67$ Language introduces thinking .$Simula$ The language is $Ole-JohanDahl$ and $KristenNygaard$ Designed for simulating the environment at the Oslo computer center in Norway .

（ It is said that , They designed this language to simulate ships , And he is interested in the interaction of different ship attributes . They grouped different ships into different classes , And every object , Based on its class , You can define its own properties and behavior .）

This method is the earliest concept of analytical procedure . In analytic programs , We map real-world objects to abstract objects , It's called “ simulation ”.Simula It's not just the introduction of “ class ” The concept of , The idea of example is also applied —— This may be the earliest application of these concepts .

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20 century 70 Age Xerox $PARC$ Invented by the Institute $Smalltalk$ Language defines the concept of object-oriented programming as , In basic operations , Extensive use of objects and messages .

$Smalltalk$ The founder of was deeply $Simula67$ The main ideological influence of , but Smalltalk Objects in are completely dynamic —— They can be created 、 Modify and destroy , This is related to $Simula$ Static objects in are different .
Besides ,$Smalltalk$ The idea of inheritance is also introduced , Therefore, it goes beyond the programming model that cannot create instances and the model that does not have inheritance $Simula$.

Feeling $smalltalk$ The failure of is entirely an accident , because $sun$ The company refused to pay a high price for the installation $smalltalk80$ System , If at that time $sun$ If adopted $smalltalk$, Today's object-oriented programming is estimated to be $smalltalk$ A series of

although $smalltalk$ The original version of is not widely known , But its advantages and other variants are everywhere and some legacy technologies ,
First to $smalltalk80$ To write $jit$ The compiler brothers later wrote Google's v8 engine , This v8 Because of its good performance, the engine was later burned to the server and became $nodejs$ At the heart of , There is a startup company $smalltalk$ Compiler is $sun$ After acquisition , Become $javavirtualquasimachineOfOneMinistrybranch(hotspot)$,
The most famous is the windowed graphical user interface that Microsoft and apple have made their fortune ,$smalltalk$ The reason for the birth of the so-called object-oriented programming and graphical user interface , Even specialized just in time compilation techniques , Are derived from its own characteristics .
In the original language system , and $C$ Language and $unix$ The series is different , Its syntax is generated by the class itself , In a system that mixes data and code , The operating system is incorporated into the programming language , A class is responsible for a function , Is a programming language , A specific system ,
For example, in the initial system , No file system , No process control , Automatic memory recovery , Everything is a $image$ The world , There are a lot of objects in it , When you develop a software , You can save it directly to someone else and open it on his disk , Or you can share it online “ Live ” Object to others , This object is the code after instantiation of a class , It is essentially a function , With specific input and output , As long as there is a host environment , Parse it out and generate it , The user gets this object , He can use this object to do the work he needs , If we put $unix$ The system is compared to a country that sells factories ,$smalltalk$ It's like selling screwdrivers , You are free to choose the tools you need , Finish your work , The communication interface between these objects is completely consistent with the language standard , There is no need to encode and decode the communication between various systems .

But object - oriented languages are not a drawback , The biggest problem with objects is the confusion between data and logic , Programmers think that objects are the aggregation of logic and forget that objects are also the carrier of data .
The result is that a large number of programmers who do not understand the principles of the computer misuse objects to burst the memory , If the memory of the computer is only a few hundred MB Even a few dozen MB, Unmanaged programmers who have no concept of memory simply can't cover it .

So now the embedded system is still C The world of series , Still a paradise for experienced programmers , Learning the management of memory and the underlying principles of computer is not something that non majors can learn in a few months .
Many low code software and convenient development languages have lowered the development threshold , A person who doesn't know the code at all can develop a business system after several months' study , But the sophistication of the code is not something that these fast-paced programmers can harm .
It is no wonder that excellent design patterns and architectures are the successful experience of foreign coding , In China, there is basically no , Because excellent senior programmers are TM Changed careers ！！！
Our current coding environment is too bad , Programmers are a job that needs to be accumulated over time , Now enterprises use newcomers who don't know the code at all ,35 How many senior programmers have been harmed by the job crisis at the age of （ They also show off that the composition of enterprises is young people ）, Look at foreign programmers in their sixties and seventies everywhere , They are the pillars of the entire code world , We can't just focus on business - oriented code , Think TMD Rapid development is right ( Refactoring is troublesome , A piece of junk written in code ), We should make good code and be a good programmer ！！！

When checking the data, I suddenly saw the thinking about the domestic coding environment , Is like , I'm sorry ~