Anti-Patterns | Developer Documentation

Anti-Patterns

Introduction

The purpose of this document is to provide the guidelines for how not to write code for the KiCad project. Anti-patterns are code practices that seem to occur often but shouldn’t. If your code contains any of these patterns, please fix them.

Do not modify this document without the consent of the project leader. All changes to this document require approval.

Bare Enumerations

Bare enums are declared like this

enum LAST_LAYOUT {
    NONE,
    ALIAS,
    PARENT
};

These are problematic because they declare the simple keywords NONE, ALIAS and PARENT into the global scope. These then clash with enums or worse, defines, that exist in other header or source files used in the project.

The solution is to use scoped enums. In this form, the enum is

enum class LAST_LAYOUT {
    NONE,
    ALIAS,
    PARENT
};

LAST_LAYOUT l = LAST_LAYOUT::NONE;

switch(l)
{
    case LAST_LAYOUT::NONE   : std::cout << "NONE\n";   break;
    case LAST_LAYOUT::ALIAS  : std::cout << "ALIAS\n";  break;
    case LAST_LAYOUT::PARENT : std::cout << "PARENT\n"; break;
}

Dereferencing Pointers

Dereferencing an invalid pointer almost always results in a crash. Never assume a pointer is valid. Even if the code in question has never crashed before, that doesn’t mean that someone wont break something in the future. Use assertions to help developers catch invalid pointers during development. Even better than assertions, you can use the wxWidgets debugging macros which can raise assertions on debug builds and gracefully handle run time issues in release builds. If you are concerned about pulling in wxWidgets user interface code, there are header only C++ assertion libraries available.

Avoid the temptation to silently hide potentially bad pointers. The following code:

void SomeFunction( SomeObject* somePointer )
{
    if( somePointer )
    {
        somePointer->DeReferencedMethod();
    }
}

should be replaced with:

void SomeFunction( SomeObject* somePointer )
{
    // This will trigger a wxWidgets assertion dialog in debug builds and silently
    // return in release builds which is far better than crashing.
    wxCHECK( somePointer, /* void */ )

    somePointer->DeReferencedMethod();
}

wxUpdateUIEvent Abuse

Avoid using wxUpdateUIEvent for anything other than the methods provided by the event object. Updating controls in a wxUpdateUIEvent handler will typically trigger other control events that in turn will fire more wxUpdateUIEvents which results in an infinite event loop that can cause the user interface to become less responsive.

If you are not sure you are abusing wxUpdateUIEvent, the advanced configuration setting UpdateUIEventInterval allows you to set the time in milliseconds which the update events are generated or -1 to disable all update events. If slowing or disabling the update events makes the user interface more responsive, a wxUpdateUIEvent handler is the problem.

Translating Partial Sentences

Never break an translated sentence into multiple _() tags. This prevents translations from understanding your intention and from creating grammatically correct sentences in languages other than English.

For example the following is incorrect:

ShowReport( _( "Duplicate instances of " ) + m_changeArray[j].NewRefDes );

This should be replaced by the following:

ShowReport( wxString::Format( _( "Duplicate instances of %s" ), m_changeArray[j].NewRefDes ) );

The new formulation allows a translator to move the object of the sentence. In the original formulation, the object (the NewRefDes string) would always appear at the end of the sentence, which may not be grammatically correct in non-English languages.

Similarly, do not break sentences into parts like this:

wxString verb  = updateMode ? _( "Update" ) : _( "Change" );
wxString warning = _( "%s all items on the board?" );

wxMessageBox( wxString::Format( warning, verb ) );

In English, the word Update is both a noun and a verb. Without context, the translator will not know which version to use. Additionally, it will not be clear to the translator that the verb string is used by the warning string as they will not appear together in the translation file.

Instead, we must write the sentences explicitly:

wxString msg = updateMode ? _( "Update all items on the board?" ) :
                            _( "Change all items on the board?" );

wxMessageBox( msg );

Dialogs

Breaking Close Event Handling

Modal and Quasi-modal dialog windows closure is handled automatically when the standard button IDs (wxID_OK, wxID_CANCEL, wxID_CLOSE, etc.) as well as the close button title bar decorator. There is no need to handle wxCloseEvent directly unless the dialog has special requirement such as vetoing the event. Very few dialogs require any special handling of close event. For more information see the wxWidgets closing windows documentation. If you do not fully understand the default dialog close event handling, do not implement one in your dialog. Doing so will most likely break the default handling.

Avoid Direct Access of Parent

Avoid passing a specific parent window pointer type to the dialog and saving it for use dialog methods. At best, this is a bad coding practice as it limits the re-usability of the dialog to a given parent window type. At worst, it can cause crashes in mode-less dialog windows when accessing the point in the dialog object destructor.

If you need to pass information to the parent window, use the wxWidgets events instead.

This:

MY_DIALOG::MY_DIALOG( SCH_EDIT_FRAME* aParent )
{
    m_frame = aParent;
}

void MY_DIALOG::SomeMethod()
{
    m_frame->SomeFrameMethod();
}

should be replaced with this:

MY_DIALOG::MY_DIALOG( wxWindow* aParent )
{
}

void MY_DIALOG::SomeMethod()
{
    wxWindow* parent = GetParent();

    if( parent )
    {
        SOME_EVENT evt();

        // If the parent doesn't handle this event it goes into the bit bucket.
        parent->HandleWindowEvent( evt );
    }
}

Object Property Modifiers

Modal and Quasi-modal dialog windows should be implemented as simple object property modifiers. Avoid modifying properties of live objects. Make a copy of the object being modified to transfer property information between the dialog controls and the object properties.

MY_DIALOG::MY_DIALOG( wxWindow* aParent, const LIB_PIN& aPin )
{
    m_pinCopy( m_Pin );     // Make a copy of the LIB_PIN object to modify.
}

Handling the OK Button

Dismissing a modal or quasi-model dialog window by clicking on the "OK" button does not indicate that any of the object properties have been changed. Always compare the properties from the dialog object against the properties of the original object. Only set the document modified flag if the object properties have been modified.

MY_DIALOG dlg( this, m_somePin );

if( dlg.ShowQuasiModal() == wxID_OK && dlg.GetPin() != m_somePin )
{
    // Update the undo/redo handling here.

    m_somePin = dlg.GetPin();

    // Set modified flag here.
}

Mode Specific Coding

Avoid adding mode specific code to dialogs. The dialog cannot know how it is going to be shown in advance. Making assumptions about how the dialog is shown from within the dialog can lead to unexpected bugs.

void MY_DIALOG::OnCloseEvent( wxCloseEvent& aEvent )
{
    // The code will clean up the dialog when it is mode-less but it also will
    // completely bypass the automatic data transfer handling for modal and
    // quasi-modal dialogs.
    Destroy();
}

Mode-less Dialogs

Memory Management

wxWidgets mode-less dialogs are always created on the heap using the C `new` operator. Therefore, it is the developers responsibility to manage the memory allocated for the dialog. However, https://www.wxwidgets.org/[wxWidgets] windows object memory should never be cleaned up using the C delete operator. wxWidgets provides the Destroy() method to perform any required window tear down and then deletes the allocated memory.

Dialog Life Time

wxWidgets does not guarantee that a parent window will out live a mode-less dialog created by it. Therefore, attempting to access the parent window from the dialog’s destructor (See Avoid Direct Access of Parent) may crash. If you have to access the parent from the the dialog destructor, check to make sure the dialog has been shown in either the modal or quasi-modal mode.

Don’t do this:

MY_DIALOG::~MY_DIALOG()
{
    m_parent->SomeParentWindowMethod();   // May crash if the dialog is mode-less.
}

instead do this:

MY_DIALOG::~MY_DIALOG()
{
    // Ensures the parent is still valid.
    if( IsModal() || IsQuasiModal()
        m_parent->SomeParentWindowMethod();
}

S-Expression File Format Anti-patterns

Unquoted strings

Always quote strings in KiCad file formats, whether or not they contain user-generated data or you think they might need to be quoted.

// Do this
(generator "eeschema")

// Not this (assuming that 'eeschema' is not a token)
(generator eeschema)

Non-standard boolean flags

Use the tokens yes and no for booleans in the file format. All boolean flags should be a two- element list of the form (token [yes|no]). Do not use implicit-true booleans. Boolean flags may be omitted from formatting where it makes sense to do so (and the parser should have a defined default value that will be assumed if the flag is missing), but if the boolean token is present, an explicit value must also be present.

// Do this
(my_item
   (hide yes)
)

// Or this
(my_item
    (show no)
)

// Not this (an implicitly-true boolean)
(my_item
    (hide)
)

// And not this (a boolean outside an enclosing list)
(my_item hide yes)

Ignoring unneeded or deprecated tokens implicitly

When implementing a parser that needs to skip over tokens in a file that are not needed, always do so explicitly, by parsing the tokens and doing nothing with them. Do not do this implicitly by skipping a certain number of symbols.

// Do this
for( token = NextTok(); token != T_RIGHT && token != EOF; token = NextTok() )
{
    if( token == T_LEFT )
        token = NextTok();

    switch( token )
    {
    case T_generator:
        // Skip (generator "generatorname")
        NeedSYMBOL();
        NeedRIGHT();
        break;
    //...
    }
}

// Don't do this
for( token = NextTok(); token != T_RIGHT && token != EOF; token = NextTok() )
{
    if( token == T_LEFT )
        token = NextTok();

    switch( token )
    {
    case T_version:
        m_requiredVersion = parseInt( FromUTF8().mb_str( wxConvUTF8 ) );
        NeedRIGHT();

        // Skip (generator "generatorname")
        NeedLEFT();
        NeedSYMBOL();
        NeedSYMBOL();
        NeedRIGHT();
        break;
    //...
    }
}