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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head>
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  <meta name="author" content="Marc Attinasi"><title>Adding a new style property - layout cookbook</title></head>

<h1>Adding a new style property</h1>
  <h4>Document history:</h4>
    <li>03/21/2002: Marc Attinasi ( created document
/ implementing -moz-force-broken-image-icon for bug <a href="">
      6/18/2002: David Baron ( corrected support
      for 'inherit' and '-moz-initial' (didn't test with code, though),
      and explained what the final boolean in CheckPropertyData is for.
      11/09/2002: Christopher Aillon ( updated
      nsCSSPropList.h macro description and added information about
      <b>NOTE</b>:  This document is still missing a few pieces.  I need to
      add information on adding to <code>nsComputedDOMStyle</code>.
When a new style property is needed there are many places in the code that
need to be updated. This document outlines the procedure used to add a new
property, in this case the property is a proprietary one called '-moz-force-broken-image-icons'
and is used as a way for a stylesheet to force broken image icons to be displayed.
This is all being done in the context of <a href="">
bug 58646</a>.

<p>Up front you have to decide some things about the new property:</p>

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

<p>There are several places that need to be educated about a new style property.
They are:
    <li><a href="#CSSPropList">CSS Property Names and Hint Tables</a>: the new
property name must be made formally know to the system</li>
    <li><a href="#CSSDeclaration">CSS Declaration</a>: the declaration must be
able to hold the property and its value(s)</li>
    <li><a href="#Parser">CSS Parser</a>: the parser must be able to parse the
property name, validate the values, and provide a declaration for the property
and value</li>
    <li><a href="#StyleContext">Style Context</a>: the StyleContext must be able
to hold the resolved value of the property, and provide a means to retrieve the
property value. Additionally, the StyleContext has to know what kind of impact a
change to this property causes.</li>
    <li><a href="#RuleNode">Rule Nodes</a>: the RuleNodes need to know how the
property is inherited and how it is shared by other elements.</li>
    <li><a href="#DOM">DOM</a>: Your style should be accessible from the DOM so
users may dynamically set/get its property value.</li>
    <li><a href="#Layout">Layout</a>: layout has to know what to do with the
property, in other words, the meaning of the property.</li>
  <h3><a name="CSSPropList">CSS Property Name / Constants / Hints</a></h3>

First, add the new name to the property list in <a href="">
 �Insert the property in the list alphabetically, using the existing
property names as a template. The format of the entry you will create is:
  <pre>CSS_PROP(-moz-force-broken-image-icons, force_broken_image_icons, MozForceBrokenImageIcons, NS_STYLE_HINT_FRAMECHANGE) // bug 58646</pre>

<p>The first value is the formal property name, in other words the property
name as it is seen by the CSS parser.<br>
The second value is the name of the property as it will appear internally.<br>
The third value is the name of the DOM property used to access your style.<br>
The last value indicates what must change when the value of the property
changes. It should be an
<a href="">nsChangeHint</a>.</p>
<p>If you need to introduce new constants for the values of the property, they
must be added to <a href="">
 and to the appropriate keyword tables in <a href="">
 (note: this cookbook does not do this since the new property does not require
any new keywords for the property values).
  <h3><a name="CSSDeclaration">CSS Declaration</a></h3>
Changes will need to be made to the structs and classes defined in <a href="">
nsCSSDeclaration.h </a>
and <a href="">
First, find the declaration of the struct that will hold the new property
value (in the header file). For this example it is the struct <b>nsCSSUserInterface</b>
. Modify the struct declaration to include a new data member for the new
property, of the type CSSValue. Next, open the implementation file (the cpp)
and modify the struct's constructors. <br>
Next, the <a href="">
 method must be updated to support your new property. The CSSParser will
call this to build up a declaration. Find the portion of that method that
deals with the other properties in the struct that you are adding your property
to (or create a new section if you are creating a new style struct). For
this example we will find the 'UserInterface' section and add our new property
  <a href="#AppendValueCase">there</a>
  <pre>    // nsCSSUserInterface
    case eCSSProperty_user_input:
    case eCSSProperty_user_modify:
    case eCSSProperty_user_select:
    case eCSSProperty_key_equivalent:
    case eCSSProperty_user_focus:
    case eCSSProperty_resizer:
    case eCSSProperty_cursor:
    case eCSSProperty_force_broken_image_icons: {
      CSS_ENSURE(UserInterface) {
        switch (aProperty) {
          case eCSSProperty_user_input:       theUserInterface-&gt;mUserInput = aValue;      break;
          case eCSSProperty_user_modify:      theUserInterface-&gt;mUserModify = aValue;     break;
          case eCSSProperty_user_select:      theUserInterface-&gt;mUserSelect = aValue;     break;
          case eCSSProperty_key_equivalent: 
            CSS_ENSURE_DATA(theUserInterface-&gt;mKeyEquivalent, nsCSSValueList) {
              theUserInterface-&gt;mKeyEquivalent-&gt;mValue = aValue;
          case eCSSProperty_user_focus:       theUserInterface-&gt;mUserFocus = aValue;      break;
          case eCSSProperty_resizer:          theUserInterface-&gt;mResizer = aValue;        break;
          case eCSSProperty_cursor:
            CSS_ENSURE_DATA(theUserInterface-&gt;mCursor, nsCSSValueList) {
              theUserInterface-&gt;mCursor-&gt;mValue = aValue;
<b>          <a name="AppendValueCase"></a>case eCSSProperty_force_broken_image_icons: theUserInterface-&gt;mForceBrokenImageIcon = aValue; break;
          CSS_BOGUS_DEFAULT; // make compiler happy

The GetValue method must be similarly <a href="#GetValueCase">modified</a>
  <pre>    // nsCSSUserInterface
    case eCSSProperty_user_input:
    case eCSSProperty_user_modify:
    case eCSSProperty_user_select:
    case eCSSProperty_key_equivalent:
    case eCSSProperty_user_focus:
    case eCSSProperty_resizer:
    case eCSSProperty_cursor:
    case eCSSProperty_force_broken_image_icons: {
      if (nsnull != theUserInterface) {
        switch (aProperty) {
          case eCSSProperty_user_input:       aValue = theUserInterface-&gt;mUserInput;       break;
          case eCSSProperty_user_modify:      aValue = theUserInterface-&gt;mUserModify;      break;
          case eCSSProperty_user_select:      aValue = theUserInterface-&gt;mUserSelect;      break;
          case eCSSProperty_key_equivalent:
            if (nsnull != theUserInterface-&gt;mKeyEquivalent) {
              aValue = theUserInterface-&gt;mKeyEquivalent-&gt;mValue;
          case eCSSProperty_user_focus:       aValue = theUserInterface-&gt;mUserFocus;       break;
          case eCSSProperty_resizer:          aValue = theUserInterface-&gt;mResizer;         break;
          case eCSSProperty_cursor:
            if (nsnull != theUserInterface-&gt;mCursor) {
              aValue = theUserInterface-&gt;mCursor-&gt;mValue;
<b>          <a name="GetValueCase"></a>case eCSSProperty_force_broken_image_icons: aValue = theUserInterface-&gt;mForceBrokenImageIcons; break;
          CSS_BOGUS_DEFAULT; // make compiler happy
      else {

Finally <a href="#ListCase">modify </a>
the 'List' method to output the property value.<br>
  <pre>void nsCSSUserInterface::List(FILE* out, PRInt32 aIndent) const
  for (PRInt32 index = aIndent; --index &gt;= 0; ) fputs("  ", out);

  nsAutoString buffer;

  mUserInput.AppendToString(buffer, eCSSProperty_user_input);
  mUserModify.AppendToString(buffer, eCSSProperty_user_modify);
  mUserSelect.AppendToString(buffer, eCSSProperty_user_select);
  nsCSSValueList*  keyEquiv = mKeyEquivalent;
  while (nsnull != keyEquiv) {
    keyEquiv-&gt;mValue.AppendToString(buffer, eCSSProperty_key_equivalent);
    keyEquiv= keyEquiv-&gt;mNext;
  mUserFocus.AppendToString(buffer, eCSSProperty_user_focus);
  mResizer.AppendToString(buffer, eCSSProperty_resizer);
  nsCSSValueList*  cursor = mCursor;
  while (nsnull != cursor) {
    cursor-&gt;mValue.AppendToString(buffer, eCSSProperty_cursor);
    cursor = cursor-&gt;mNext;

 <b> <a name="ListCase"></a>mForceBrokenImageIcon.AppendToString(buffer,eCSSProperty_force_broken_image_icons);</b>

  fputs(NS_LossyConvertUTF16toASCII(buffer).get(), out);

  <h3><a name="Parser">CSS Parser</a></h3>
Next, the CSSParser must be educated about this new property so that it can
read in the formal declarations and build up the internal declarations that
will be used to build the rules. If you are adding a simple property that
takes a single value, you simply add your new property to the ParseSingleProperty
method. If a more complex parsing is required you will have to write a new
method to handle it, modeling it off of one of the existing parsing helper
methods (see <a href="">
, for and example). We are just adding a simple single-value property here.<br>
Open nsCSSParser.cpp and look for the method <a href="">
. This method is responsible for calling the relevant helper routine to parse
the value(s). Find an existing property that is similar to the property you
are adding. For our example we are adding a property that takes a numeric
value so we will model it after the 'height' property and call ParsePositiveVariant.
Add a new case for the new property and call the appropriate parser-helper
and make a call to ParseVariant passing the <a href="">
variant flag</a>
 that makes sense for your property. In our case<br>
  <pre>� case eCSSProperty_force_broken_image_icons:</pre>
  <pre>��� return ParsePositiveVariant(aErrorCode, aValue, VARIANT_INTEGER, nsnull);</pre>
This will parse the value as a positive integer value, which is what we want.<br>
  <h3><a name="StyleContext">Style Context</a></h3>
Having implemented support for the new property in the CSS Parser and CSS
Declaration classes in the content module, it is now time to provide support
for the new property in layout. The Style Context must be given a new data
member corresponding to the declaration's new data member, so the computed
value can be held for the layout objects to use.<br>
First look into <a href="">
 to see the existing style strucs. Find the one that you want to store the
data on. In this example, we want to put it on the nsStyleUserInterface struct,
however there is also a class nsStyleUIReset that holds the non-inherited
values, so we will use that one (remember, our property is not inherited).
Add a <a href="#StyleContextMember">data member</a>
 to hold the value:
  <pre>struct nsStyleUIReset: public nsStyleStruct {
  nsStyleUIReset(const nsStyleUIReset&amp; aOther);


  void* operator new(size_t sz, nsPresContext* aContext) {
    return aContext->AllocateFromShell(sz);
  void Destroy(nsPresContext* aContext) {
    aContext-&gt;FreeToShell(sizeof(nsStyleUIReset), this);

  PRInt32 CalcDifference(const nsStyleUIReset&amp; aOther) const;

  PRUint8   mUserSelect;      // [reset] (selection-style)
  PRUnichar mKeyEquivalent;   // [reset] XXX what type should this be?
  PRUint8   mResizer;         // [reset]
  <b><a name="StyleContextMember"></a>PRUint8   mForceBrokenImageIcon; // [reset]  (0 if not forcing, otherwise forcing)</b>
In the implementation file <a href="">
nsStyleContext.cpp </a>
add the new data member to the constructors of the style struct and the CalcDifference
method, which must return the correct style-change hint when a change to
your new property is detected. The constructor changes are obvious, but here
is the CalcDifference change for our example:<br>
  <pre>PRInt32 nsStyleUIReset::CalcDifference(const nsStyleUIReset&amp; aOther) const
 <b> if (mForceBrokenImageIcon == aOther.mForceBrokenImageIcon) {</b>
    if (mResizer == aOther.mResizer) {
      if (mUserSelect == aOther.mUserSelect) {
        if (mKeyEquivalent == aOther.mKeyEquivalent) {
          return NS_STYLE_HINT_NONE;
        return NS_STYLE_HINT_CONTENT;
      return NS_STYLE_HINT_VISUAL;
The nsCSSStyleRule must be updated to manage mapping the declaration to the
style struct. In the file <a href="">
, locate the Declaration mapping function corresponding to the style struct
you have added your property to. For example, we <a href="">
update </a>
  <pre>static nsresult
MapUIForDeclaration(nsCSSDeclaration* aDecl, const nsStyleStructID&amp; aID, nsCSSUserInterface&amp; aUI)
  if (!aDecl)
    return NS_OK; // The rule must have a declaration.

  nsCSSUserInterface* ourUI = (nsCSSUserInterface*)aDecl-&gt;GetData(kCSSUserInterfaceSID);
  if (!ourUI)
    return NS_OK; // We don't have any rules for UI.

  if (aID == eStyleStruct_UserInterface) {
    if (aUI.mUserFocus.GetUnit() == eCSSUnit_Null &amp;&amp; ourUI-&gt;mUserFocus.GetUnit() != eCSSUnit_Null)
      aUI.mUserFocus = ourUI-&gt;mUserFocus;
    if (aUI.mUserInput.GetUnit() == eCSSUnit_Null &amp;&amp; ourUI-&gt;mUserInput.GetUnit() != eCSSUnit_Null)
      aUI.mUserInput = ourUI-&gt;mUserInput;

    if (aUI.mUserModify.GetUnit() == eCSSUnit_Null &amp;&amp; ourUI-&gt;mUserModify.GetUnit() != eCSSUnit_Null)
      aUI.mUserModify = ourUI-&gt;mUserModify;

    if (!aUI.mCursor &amp;&amp; ourUI-&gt;mCursor)
      aUI.mCursor = ourUI-&gt;mCursor;

  else if (aID == eStyleStruct_UIReset) {
    if (aUI.mUserSelect.GetUnit() == eCSSUnit_Null &amp;&amp; ourUI-&gt;mUserSelect.GetUnit() != eCSSUnit_Null)
      aUI.mUserSelect = ourUI-&gt;mUserSelect;
    if (!aUI.mKeyEquivalent &amp;&amp; ourUI-&gt;mKeyEquivalent)
      aUI.mKeyEquivalent = ourUI-&gt;mKeyEquivalent;

    if (aUI.mResizer.GetUnit() == eCSSUnit_Null &amp;&amp; ourUI-&gt;mResizer.GetUnit() != eCSSUnit_Null)
      aUI.mResizer = ourUI-&gt;mResizer;
    <a name="MapUIForDeclChange"></a>if (aUI.mForceBrokenImageIcon.GetUnit() == eCSSUnit_Null &amp;&amp; ourUI-&gt;mForceBrokenImageIcon.GetUnit() == eCSSUnit_Integer)
      aUI.mForceBrokenImageIcon = ourUI-&gt;mForceBrokenImageIcon;</b>

  return NS_OK;

  <h3><a name="RuleNode">Rule Node</a></h3>
Now we have to update the RuleNode code to know about the new property. First,
locate the PropertyCheckData array for the data that you added the new property
to. For this example, we add the following:<br>
  <pre>static const PropertyCheckData UIResetCheckProperties[] = {
  <b>CHECKDATA_PROP(nsCSSUserInterface, mForceBrokenImageIcon, CHECKDATA_VALUE, PR_FALSE)</b>
The first two arguments correspond to the structure and data member from
the CSSDeclaration, the third is the data type, the fourth indicates
whether it is a coord value that uses an explicit inherit value on the
style data struct that must be computed by layout.<br>
Next, we have to make sure the ComputeXXX method for the structure the property
was added to is updated to mange the new value. In this example we need to
modify the nsRuleNode::ComputeUIResetData method to handle the CSS Declaration
to the style struct:<br>
  <pre>  ...
  // resizer: auto, none, enum, inherit
  if (eCSSUnit_Enumerated == uiData.mResizer.GetUnit()) {
    ui-&gt;mResizer = uiData.mResizer.GetIntValue();
  else if (eCSSUnit_Auto == uiData.mResizer.GetUnit()) {
    ui-&gt;mResizer = NS_STYLE_RESIZER_AUTO;
  else if (eCSSUnit_None == uiData.mResizer.GetUnit()) {
    ui-&gt;mResizer = NS_STYLE_RESIZER_NONE;
  else if (eCSSUnit_Inherit == uiData.mResizer.GetUnit()) {
    inherited = PR_TRUE;
    ui-&gt;mResizer = parentUI-&gt;mResizer;

  <b>// force-broken-image-icons: integer, inherit, -moz-initial
  if (eCSSUnit_Integer == uiData.mForceBrokenImageIcons.GetUnit()) {
    ui-&gt;mForceBrokenImageIcons = uiData.mForceBrokenImageIcons.GetIntValue();
  } else if (eCSSUnit_Inherit == uiData.mForceBrokenImageIcons.GetUnit()) {
    inherited = PR_TRUE;
    ui-&gt;mForceBrokenImageIcons = parentUI-&gt;mForceBrokenImageIcons;
  } else if (eCSSUnit_Initial == uiData.mForceBrokenImageIcons.GetUnit()) {
    ui-&gt;mForceBrokenImageIcons = 0;
  if (inherited)
    // We inherited, and therefore can't be cached in the rule node.  We have to be put right on the
    // style context.
    aContext-&gt;SetStyle(eStyleStruct_UIReset, *ui);
  else {
    // We were fully specified and can therefore be cached right on the rule node.
    if (!aHighestNode-&gt;mStyleData.mResetData)
      aHighestNode-&gt;mStyleData.mResetData = new (mPresContext) nsResetStyleData;
    aHighestNode-&gt;mStyleData.mResetData-&gt;mUIData = ui;
    // Propagate the bit down.
    PropagateDependentBit(NS_STYLE_INHERIT_UI_RESET, aHighestNode);
  <h3><a name="DOM">DOM</a></h3>
Users in scripts, or anywhere outside of layout/ or content/ may need to access
the new property.  This is done using the CSS OM, specifically
<code>nsIDOMCSSStyleDeclaration</code>, <code>nsIDOMCSS2Properties</code>, and
<code>nsIDOMNSCSS2Properties</code>.  By the magic of C++ pre-processing, the
CSS2Properties interfaces will be implemented automatically when you
<a href="#CSSPropList">add your property</a> to <a href="">
nsCSSPropList.h</a>. Because of this, if you fail to add your property to the
DOM interface, you will be rewarded with compiler errors.  All you have to do
is modify <a href="">
nsIDOMCSS2Properties.idl</a> and add the appropriate attribute to the
<code>nsIDOMNSCSS2Properties</code> interface (lower in the file).
For example:<br>
  <pre>           attribute DOMString        MozForceBrokenImageIcon;
                                        // raises(DOMException) on setting

  <h3><a name="Layout">Layout</a></h3>
OK, finally the style system is supporting the new property. It is time to
actually make use of it now.<br>
In layout, retrieve the styleStruct that has the new property from the frame's
style context. Access the new property and get its value. It is that simple.
For this example, it looks like this, in nsImageFrame:<br>
  <pre>        PRBool forceIcon = PR_FALSE;

        const nsStyleUIReset* styleData;
        GetStyleData(eStyleStruct_UIReset, (const nsStyleStruct*&amp;) styleData);
        if (styleData-&gt;mForceBrokenImageIcon) {
          forceIcon = PR_TRUE;

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