michael@0: michael@0: michael@0: michael@0: michael@0: Adding a new style property - layout cookbook michael@0: michael@0: michael@0:

Adding a new style property

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Document history:
michael@0:
michael@0:
03/21/2002: Marc Attinasi (attinasi@netscape.com) created document michael@0: / implementing -moz-force-broken-image-icon for bug michael@0: 58646
michael@0:
michael@0: 6/18/2002: David Baron (dbaron@dbaron.org): corrected support michael@0: for 'inherit' and 'initial' (didn't test with code, though), michael@0: and explained what the final boolean in CheckPropertyData is for. michael@0:
michael@0:
michael@0: 11/09/2002: Christopher Aillon (caillon@returnzero.com): updated michael@0: nsCSSPropList.h macro description and added information about michael@0: nsIDOMCSS2Properties.idl. michael@0:
michael@0:
michael@0:
michael@0: NOTE: This document is still missing a few pieces. I need to michael@0: add information on adding to nsComputedDOMStyle. michael@0:
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michael@0:

Overview

michael@0: When a new style property is needed there are many places in the code that michael@0: need to be updated. This document outlines the procedure used to add a new michael@0: property, in this case the property is a proprietary one called '-moz-force-broken-image-icons' michael@0: and is used as a way for a stylesheet to force broken image icons to be displayed. michael@0: This is all being done in the context of michael@0: bug 58646. michael@0: michael@0:

Analysis

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Up front you have to decide some things about the new property:

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Questions:

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Is the property proprietary or specified by the CSS standard?
Is the property inherited?
What types of values can the property have?
Does it logically fit with other existing properties?
What is the impact to the layout of a page if that property changes?
What do you want to name it?

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Answers:

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In our specific case, we want a property that is used internally, michael@0: so it is a proprietary property.
The property is to be used for images, which are leaf elements, so michael@0: there is no need to inherit it.
The property is used simply to force a broken image to be represented michael@0: by an icon, so it only supports the values '0' and '1' as numerics.
It is hard to see how this property fits logically in with other michael@0: properties, but if we stretch our imaginations we could say that it is a michael@0: sort of UI property.
If this property changes, the image frame has to be recreated. This michael@0: is because the decision about whether to display the icon or not will impact michael@0: the decision to replace the image frame with an inline text frame for the michael@0: ALT text, so if the ALT text inline is already made, there is no image frame michael@0: left around to reflow or otherwise modify.
Finally, the name will be '-moz-force-broken-image-icons' - that michael@0: should be pretty self-describing (by convention we start proprietary property michael@0: names with '-moz-').

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Implementation

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There are several places that need to be educated about a new style property. michael@0: They are: michael@0:

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CSS Property Names and Hint Tables: the new michael@0: property name must be made formally know to the system
CSS Declaration: the declaration must be michael@0: able to hold the property and its value(s)
CSS Parser: the parser must be able to parse the michael@0: property name, validate the values, and provide a declaration for the property michael@0: and value
Style Context: the StyleContext must be able michael@0: to hold the resolved value of the property, and provide a means to retrieve the michael@0: property value. Additionally, the StyleContext has to know what kind of impact a michael@0: change to this property causes.
Rule Nodes: the RuleNodes need to know how the michael@0: property is inherited and how it is shared by other elements.
DOM: Your style should be accessible from the DOM so michael@0: users may dynamically set/get its property value.
Layout: layout has to know what to do with the michael@0: property, in other words, the meaning of the property.

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CSS Property Name / Constants / Hints

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michael@0: First, add the new name to the property list in michael@0: nsCSSPropList.h michael@0: Insert the property in the list alphabetically, using the existing michael@0: property names as a template. The format of the entry you will create is: michael@0:

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CSS_PROP(-moz-force-broken-image-icons, force_broken_image_icons, MozForceBrokenImageIcons, NS_STYLE_HINT_FRAMECHANGE) // bug 58646

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The first value is the formal property name, in other words the property michael@0: name as it is seen by the CSS parser.
michael@0: The second value is the name of the property as it will appear internally.
michael@0: The third value is the name of the DOM property used to access your style.
michael@0: The last value indicates what must change when the value of the property michael@0: changes. It should be an michael@0: nsChangeHint.

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If you need to introduce new constants for the values of the property, they michael@0: must be added to michael@0: nsStyleConsts.h michael@0: and to the appropriate keyword tables in michael@0: nsCSSProps.cpp michael@0: (note: this cookbook does not do this since the new property does not require michael@0: any new keywords for the property values). michael@0:

CSS Declaration

michael@0: Changes will need to be made to the structs and classes defined in michael@0: nsCSSDeclaration.h michael@0: and michael@0: nsCSSDeclaration.cpp michael@0:
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michael@0: First, find the declaration of the struct that will hold the new property michael@0: value (in the header file). For this example it is the struct nsCSSUserInterface michael@0: . Modify the struct declaration to include a new data member for the new michael@0: property, of the type CSSValue. Next, open the implementation file (the cpp) michael@0: and modify the struct's constructors.
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michael@0: Next, the michael@0: AppendValue michael@0: method must be updated to support your new property. The CSSParser will michael@0: call this to build up a declaration. Find the portion of that method that michael@0: deals with the other properties in the struct that you are adding your property michael@0: to (or create a new section if you are creating a new style struct). For michael@0: this example we will find the 'UserInterface' section and add our new property michael@0: there michael@0: .
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    // nsCSSUserInterface
michael@0:     case eCSSProperty_user_input:
michael@0:     case eCSSProperty_user_modify:
michael@0:     case eCSSProperty_user_select:
michael@0:     case eCSSProperty_key_equivalent:
michael@0:     case eCSSProperty_user_focus:
michael@0:     case eCSSProperty_resizer:
michael@0:     case eCSSProperty_cursor:
michael@0:     case eCSSProperty_force_broken_image_icons: {
michael@0:       CSS_ENSURE(UserInterface) {
michael@0:         switch (aProperty) {
michael@0:           case eCSSProperty_user_input:       theUserInterface->mUserInput = aValue;      break;
michael@0:           case eCSSProperty_user_modify:      theUserInterface->mUserModify = aValue;     break;
michael@0:           case eCSSProperty_user_select:      theUserInterface->mUserSelect = aValue;     break;
michael@0:           case eCSSProperty_key_equivalent: 
michael@0:             CSS_ENSURE_DATA(theUserInterface->mKeyEquivalent, nsCSSValueList) {
michael@0:               theUserInterface->mKeyEquivalent->mValue = aValue;
michael@0:               CSS_IF_DELETE(theUserInterface->mKeyEquivalent->mNext);
michael@0:             }
michael@0:             break;
michael@0:           case eCSSProperty_user_focus:       theUserInterface->mUserFocus = aValue;      break;
michael@0:           case eCSSProperty_resizer:          theUserInterface->mResizer = aValue;        break;
michael@0:           case eCSSProperty_cursor:
michael@0:             CSS_ENSURE_DATA(theUserInterface->mCursor, nsCSSValueList) {
michael@0:               theUserInterface->mCursor->mValue = aValue;
michael@0:               CSS_IF_DELETE(theUserInterface->mCursor->mNext);
michael@0:             }
michael@0:             break;
michael@0:           case eCSSProperty_force_broken_image_icons: theUserInterface->mForceBrokenImageIcon = aValue; break;
michael@0: 
michael@0:           CSS_BOGUS_DEFAULT; // make compiler happy
michael@0:         }
michael@0:       }
michael@0:       break;
michael@0:     }
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michael@0: The GetValue method must be similarly modified michael@0: :
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    // nsCSSUserInterface
michael@0:     case eCSSProperty_user_input:
michael@0:     case eCSSProperty_user_modify:
michael@0:     case eCSSProperty_user_select:
michael@0:     case eCSSProperty_key_equivalent:
michael@0:     case eCSSProperty_user_focus:
michael@0:     case eCSSProperty_resizer:
michael@0:     case eCSSProperty_cursor:
michael@0:     case eCSSProperty_force_broken_image_icons: {
michael@0:       CSS_VARONSTACK_GET(UserInterface);
michael@0:       if (nullptr != theUserInterface) {
michael@0:         switch (aProperty) {
michael@0:           case eCSSProperty_user_input:       aValue = theUserInterface->mUserInput;       break;
michael@0:           case eCSSProperty_user_modify:      aValue = theUserInterface->mUserModify;      break;
michael@0:           case eCSSProperty_user_select:      aValue = theUserInterface->mUserSelect;      break;
michael@0:           case eCSSProperty_key_equivalent:
michael@0:             if (nullptr != theUserInterface->mKeyEquivalent) {
michael@0:               aValue = theUserInterface->mKeyEquivalent->mValue;
michael@0:             }
michael@0:             break;
michael@0:           case eCSSProperty_user_focus:       aValue = theUserInterface->mUserFocus;       break;
michael@0:           case eCSSProperty_resizer:          aValue = theUserInterface->mResizer;         break;
michael@0:           case eCSSProperty_cursor:
michael@0:             if (nullptr != theUserInterface->mCursor) {
michael@0:               aValue = theUserInterface->mCursor->mValue;
michael@0:             }
michael@0:             break;
michael@0:           case eCSSProperty_force_broken_image_icons: aValue = theUserInterface->mForceBrokenImageIcons; break;
michael@0: 
michael@0:           CSS_BOGUS_DEFAULT; // make compiler happy
michael@0:         }
michael@0:       }
michael@0:       else {
michael@0:         aValue.Reset();
michael@0:       }
michael@0:       break;
michael@0:     }
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michael@0: Finally modify michael@0: the 'List' method to output the property value.
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void nsCSSUserInterface::List(FILE* out, int32_t aIndent) const
michael@0: {
michael@0:   for (int32_t index = aIndent; --index >= 0; ) fputs("  ", out);
michael@0: 
michael@0:   nsAutoString buffer;
michael@0: 
michael@0:   mUserInput.AppendToString(buffer, eCSSProperty_user_input);
michael@0:   mUserModify.AppendToString(buffer, eCSSProperty_user_modify);
michael@0:   mUserSelect.AppendToString(buffer, eCSSProperty_user_select);
michael@0:   nsCSSValueList*  keyEquiv = mKeyEquivalent;
michael@0:   while (nullptr != keyEquiv) {
michael@0:     keyEquiv->mValue.AppendToString(buffer, eCSSProperty_key_equivalent);
michael@0:     keyEquiv= keyEquiv->mNext;
michael@0:   }
michael@0:   mUserFocus.AppendToString(buffer, eCSSProperty_user_focus);
michael@0:   mResizer.AppendToString(buffer, eCSSProperty_resizer);
michael@0:   
michael@0:   nsCSSValueList*  cursor = mCursor;
michael@0:   while (nullptr != cursor) {
michael@0:     cursor->mValue.AppendToString(buffer, eCSSProperty_cursor);
michael@0:     cursor = cursor->mNext;
michael@0:   }
michael@0: 
michael@0:   mForceBrokenImageIcon.AppendToString(buffer,eCSSProperty_force_broken_image_icons);
michael@0: 
michael@0:   fputs(NS_LossyConvertUTF16toASCII(buffer).get(), out);
michael@0: }
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CSS Parser

michael@0: Next, the CSSParser must be educated about this new property so that it can michael@0: read in the formal declarations and build up the internal declarations that michael@0: will be used to build the rules. If you are adding a simple property that michael@0: takes a single value, you simply add your new property to the ParseSingleProperty michael@0: method. If a more complex parsing is required you will have to write a new michael@0: method to handle it, modeling it off of one of the existing parsing helper michael@0: methods (see michael@0: ParseBackground michael@0: , for and example). We are just adding a simple single-value property here.
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michael@0: Open nsCSSParser.cpp and look for the method michael@0: ParseSingleProperty michael@0: . This method is responsible for calling the relevant helper routine to parse michael@0: the value(s). Find an existing property that is similar to the property you michael@0: are adding. For our example we are adding a property that takes a numeric michael@0: value so we will model it after the 'height' property and call ParsePositiveVariant. michael@0: Add a new case for the new property and call the appropriate parser-helper michael@0: and make a call to ParseVariant passing the michael@0: variant flag michael@0: that makes sense for your property. In our case
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  case eCSSProperty_force_broken_image_icons:

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    return ParsePositiveVariant(aErrorCode, aValue, VARIANT_INTEGER, nullptr);

michael@0: This will parse the value as a positive integer value, which is what we want.
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michael@0:

Style Context

michael@0: Having implemented support for the new property in the CSS Parser and CSS michael@0: Declaration classes in the content module, it is now time to provide support michael@0: for the new property in layout. The Style Context must be given a new data michael@0: member corresponding to the declaration's new data member, so the computed michael@0: value can be held for the layout objects to use.
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michael@0: First look into michael@0: nsStyleStruct.h michael@0: to see the existing style strucs. Find the one that you want to store the michael@0: data on. In this example, we want to put it on the nsStyleUserInterface struct, michael@0: however there is also a class nsStyleUIReset that holds the non-inherited michael@0: values, so we will use that one (remember, our property is not inherited). michael@0: Add a data member michael@0: to hold the value: michael@0:

struct nsStyleUIReset {
michael@0:   nsStyleUIReset(void);
michael@0:   nsStyleUIReset(const nsStyleUIReset& aOther);
michael@0:   ~nsStyleUIReset(void);
michael@0: 
michael@0:   NS_DEFINE_STATIC_STYLESTRUCTID_ACCESSOR(eStyleStruct_UIReset)
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michael@0:   void* operator new(size_t sz, nsPresContext* aContext) {
michael@0:     return aContext->AllocateFromShell(sz);
michael@0:   }
michael@0:   void Destroy(nsPresContext* aContext) {
michael@0:     this->~nsStyleUIReset();
michael@0:     aContext->FreeToShell(sizeof(nsStyleUIReset), this);
michael@0:   };
michael@0: 
michael@0:   int32_t CalcDifference(const nsStyleUIReset& aOther) const;
michael@0: 
michael@0:   uint8_t   mUserSelect;      // [reset] (selection-style)
michael@0:   PRUnichar mKeyEquivalent;   // [reset] XXX what type should this be?
michael@0:   uint8_t   mResizer;         // [reset]
michael@0:   uint8_t   mForceBrokenImageIcon; // [reset]  (0 if not forcing, otherwise forcing)
michael@0: };
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michael@0: In the implementation file michael@0: nsStyleContext.cpp michael@0: add the new data member to the constructors of the style struct and the CalcDifference michael@0: method, which must return the correct style-change hint when a change to michael@0: your new property is detected. The constructor changes are obvious, but here michael@0: is the CalcDifference change for our example:
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int32_t nsStyleUIReset::CalcDifference(const nsStyleUIReset& aOther) const
michael@0: {
michael@0:   if (mForceBrokenImageIcon == aOther.mForceBrokenImageIcon) {
michael@0:     if (mResizer == aOther.mResizer) {
michael@0:       if (mUserSelect == aOther.mUserSelect) {
michael@0:         if (mKeyEquivalent == aOther.mKeyEquivalent) {
michael@0:           return NS_STYLE_HINT_NONE;
michael@0:         }
michael@0:         return NS_STYLE_HINT_CONTENT;
michael@0:       }
michael@0:       return NS_STYLE_HINT_VISUAL;
michael@0:     }
michael@0:     return NS_STYLE_HINT_VISUAL;
michael@0:   }
michael@0:   return NS_STYLE_HINT_FRAMECHANGE;
michael@0: }
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CSSStyleRule

michael@0: The nsCSSStyleRule must be updated to manage mapping the declaration to the michael@0: style struct. In the file michael@0: nsCSSStyleRule.cpp michael@0: , locate the Declaration mapping function corresponding to the style struct michael@0: you have added your property to. For example, we michael@0: update michael@0: MapUIForDeclaration:
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static nsresult
michael@0: MapUIForDeclaration(nsCSSDeclaration* aDecl, const nsStyleStructID& aID, nsCSSUserInterface& aUI)
michael@0: {
michael@0:   if (!aDecl)
michael@0:     return NS_OK; // The rule must have a declaration.
michael@0: 
michael@0:   nsCSSUserInterface* ourUI = (nsCSSUserInterface*)aDecl->GetData(kCSSUserInterfaceSID);
michael@0:   if (!ourUI)
michael@0:     return NS_OK; // We don't have any rules for UI.
michael@0: 
michael@0:   if (aID == eStyleStruct_UserInterface) {
michael@0:     if (aUI.mUserFocus.GetUnit() == eCSSUnit_Null && ourUI->mUserFocus.GetUnit() != eCSSUnit_Null)
michael@0:       aUI.mUserFocus = ourUI->mUserFocus;
michael@0:     
michael@0:     if (aUI.mUserInput.GetUnit() == eCSSUnit_Null && ourUI->mUserInput.GetUnit() != eCSSUnit_Null)
michael@0:       aUI.mUserInput = ourUI->mUserInput;
michael@0: 
michael@0:     if (aUI.mUserModify.GetUnit() == eCSSUnit_Null && ourUI->mUserModify.GetUnit() != eCSSUnit_Null)
michael@0:       aUI.mUserModify = ourUI->mUserModify;
michael@0: 
michael@0:     if (!aUI.mCursor && ourUI->mCursor)
michael@0:       aUI.mCursor = ourUI->mCursor;
michael@0: 
michael@0: 
michael@0:   }
michael@0:   else if (aID == eStyleStruct_UIReset) {
michael@0:     if (aUI.mUserSelect.GetUnit() == eCSSUnit_Null && ourUI->mUserSelect.GetUnit() != eCSSUnit_Null)
michael@0:       aUI.mUserSelect = ourUI->mUserSelect;
michael@0:    
michael@0:     if (!aUI.mKeyEquivalent && ourUI->mKeyEquivalent)
michael@0:       aUI.mKeyEquivalent = ourUI->mKeyEquivalent;
michael@0: 
michael@0:     if (aUI.mResizer.GetUnit() == eCSSUnit_Null && ourUI->mResizer.GetUnit() != eCSSUnit_Null)
michael@0:       aUI.mResizer = ourUI->mResizer;
michael@0:     
michael@0:     if (aUI.mForceBrokenImageIcon.GetUnit() == eCSSUnit_Null && ourUI->mForceBrokenImageIcon.GetUnit() == eCSSUnit_Integer)
michael@0:       aUI.mForceBrokenImageIcon = ourUI->mForceBrokenImageIcon;
michael@0:   }
michael@0: 
michael@0:   return NS_OK;
michael@0: 
michael@0: }
michael@0:

michael@0:

Rule Node

michael@0: Now we have to update the RuleNode code to know about the new property. First, michael@0: locate the PropertyCheckData array for the data that you added the new property michael@0: to. For this example, we add the following:
michael@0:

static const PropertyCheckData UIResetCheckProperties[] = {
michael@0:   CHECKDATA_PROP(nsCSSUserInterface, mUserSelect, CHECKDATA_VALUE, PR_FALSE),
michael@0:   CHECKDATA_PROP(nsCSSUserInterface, mResizer, CHECKDATA_VALUE, PR_FALSE),
michael@0:   CHECKDATA_PROP(nsCSSUserInterface, mKeyEquivalent, CHECKDATA_VALUELIST, PR_FALSE)
michael@0:   CHECKDATA_PROP(nsCSSUserInterface, mForceBrokenImageIcon, CHECKDATA_VALUE, PR_FALSE)
michael@0: };
michael@0:

michael@0: The first two arguments correspond to the structure and data member from michael@0: the CSSDeclaration, the third is the data type, the fourth indicates michael@0: whether it is a coord value that uses an explicit inherit value on the michael@0: style data struct that must be computed by layout.
michael@0:
michael@0: Next, we have to make sure the ComputeXXX method for the structure the property michael@0: was added to is updated to mange the new value. In this example we need to michael@0: modify the nsRuleNode::ComputeUIResetData method to handle the CSS Declaration michael@0: to the style struct:
michael@0:

  ...
michael@0:   // resizer: auto, none, enum, inherit
michael@0:   if (eCSSUnit_Enumerated == uiData.mResizer.GetUnit()) {
michael@0:     ui->mResizer = uiData.mResizer.GetIntValue();
michael@0:   }
michael@0:   else if (eCSSUnit_Auto == uiData.mResizer.GetUnit()) {
michael@0:     ui->mResizer = NS_STYLE_RESIZER_AUTO;
michael@0:   }
michael@0:   else if (eCSSUnit_None == uiData.mResizer.GetUnit()) {
michael@0:     ui->mResizer = NS_STYLE_RESIZER_NONE;
michael@0:   }
michael@0:   else if (eCSSUnit_Inherit == uiData.mResizer.GetUnit()) {
michael@0:     inherited = PR_TRUE;
michael@0:     ui->mResizer = parentUI->mResizer;
michael@0:   }
michael@0: 
michael@0:   // force-broken-image-icons: integer, inherit, initial
michael@0:   if (eCSSUnit_Integer == uiData.mForceBrokenImageIcons.GetUnit()) {
michael@0:     ui->mForceBrokenImageIcons = uiData.mForceBrokenImageIcons.GetIntValue();
michael@0:   } else if (eCSSUnit_Inherit == uiData.mForceBrokenImageIcons.GetUnit()) {
michael@0:     inherited = PR_TRUE;
michael@0:     ui->mForceBrokenImageIcons = parentUI->mForceBrokenImageIcons;
michael@0:   } else if (eCSSUnit_Initial == uiData.mForceBrokenImageIcons.GetUnit()) {
michael@0:     ui->mForceBrokenImageIcons = 0;
michael@0:   }
michael@0:   
michael@0:   if (inherited)
michael@0:     // We inherited, and therefore can't be cached in the rule node.  We have to be put right on the
michael@0:     // style context.
michael@0:     aContext->SetStyle(eStyleStruct_UIReset, *ui);
michael@0:   else {
michael@0:     // We were fully specified and can therefore be cached right on the rule node.
michael@0:     if (!aHighestNode->mStyleData.mResetData)
michael@0:       aHighestNode->mStyleData.mResetData = new (mPresContext) nsResetStyleData;
michael@0:     aHighestNode->mStyleData.mResetData->mUIData = ui;
michael@0:     // Propagate the bit down.
michael@0:     PropagateDependentBit(NS_STYLE_INHERIT_UI_RESET, aHighestNode);
michael@0:   }
michael@0:   ...
michael@0:

michael@0:

DOM

michael@0: Users in scripts, or anywhere outside of layout/ or content/ may need to access michael@0: the new property. This is done using the CSS OM, specifically michael@0: nsIDOMCSSStyleDeclaration and CSS2Properties. michael@0: By the magic of C++ pre-processing, the michael@0: CSS2Properties bits will be implemented automatically when you michael@0: add your property to michael@0: nsCSSPropList.h. michael@0:

Layout

michael@0: OK, finally the style system is supporting the new property. It is time to michael@0: actually make use of it now.
michael@0:
michael@0: In layout, retrieve the styleStruct that has the new property from the frame's michael@0: style context. Access the new property and get its value. It is that simple. michael@0: For this example, it looks like this, in nsImageFrame:
michael@0:

        PRBool forceIcon = PR_FALSE;
michael@0: 
michael@0:         if (StyleUIReset()->mForceBrokenImageIcon) {
michael@0:           forceIcon = PR_TRUE;
michael@0:         }
michael@0: 
michael@0:

michael@0: Create some testcases with style rules that use the new property, make sure michael@0: it is being parsed correctly. Test it in an external stylesheet and in inline michael@0: style. Test that it is inherited correctly, or not inherited as appropriate michael@0: to your property. Update this document with any further details, or correcting michael@0: any errors.
michael@0: