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Function callbacks

Some Win32 APIs are asynchronous: a function call doesn't complete and return all the information requested. Dart has the async/await pattern for handling asynchronous calls within Dart libraries and packages, but the C-based Win32 API model does not have a similar construct. So calling Win32 APIs that are asynchronous is handled instead through callbacks, in which you pass a function with the first API call that Win32 executes to pass data back.


At present, Dart can only receive function callbacks from the same thread that was used to create the callback. Win32 APIs that callback from a different thread are incompatible with this model. See issue #544 for more information.

As an example of creating a callback, let's look at the EnumFontFamiliesEx function, which enumerates all uniquely-named fonts in the system that match a specified set of font characteristics. EnumFontFamiliesEx takes a LOGFONT struct which contains the

The Dart function signature for EnumFontFamiliesEx is as follows:

int EnumFontFamiliesEx(
int hdc,
Pointer<LOGFONT> lpLogfont,
Pointer<NativeFunction<EnumFontFamExProc>> lpProc,
int lParam,
int dwFlags) { ... }

Notice the third parameter, which is a pointer to the callback function. EnumFontFamExProc. This is called once for every enumerated font, and is defined as:

typedef EnumFontFamExProc = Int32 Function(Pointer<LOGFONT> lpelfe,
Pointer<TEXTMETRIC> lpntme, DWORD FontType, LPARAM lParam);

To create a callback function, define a Dart function that matches the types in the native callback function above. Replace any integer types with a simple int Dart type. Here's an example:

int enumerateFonts(
Pointer<LOGFONT> logFont, Pointer<TEXTMETRIC> _, int __, int ___) {
// Get extended information from the font
final logFontEx = logFont.cast<ENUMLOGFONTEX>();

return TRUE; // continue enumeration

A couple of details that are worthy of note:

  • Since we are only using the first parameter, we use the Dart _ convention to note that other parameter values are ignored.
  • This callback returns TRUE to indicate that the enumeration should continue. We could stop the callback from being fired for the next enumerated value by returning FALSE instead (for example, if we'd found a specific font that we were looking for).

Closures and tear-offs are not supported for function callbacks. They must be a static function (e.g. declared as a top-level function).

Now we have our function callback, we can use it to call EnumFontFamiliesEx:

void main() {
final hDC = GetDC(NULL);
final searchFont = calloc<LOGFONT>()..ref.lfCharSet = HANGUL_CHARSET;
final callback = Pointer.fromFunction<EnumFontFamExProc>(enumerateFonts, 0);

EnumFontFamiliesEx(hDC, searchFont, callback, 0, 0);


In the above function, we create a struct LOGFONT containing our required search characteristics (fonts that support the Hangul, or Korean, character set). We then create a pointer to the Dart callback function using the .fromFunction() method. Lastly, we call the EnumFontFamiliesEx API to set up the enumeration. The Dart enumerateFonts() function will now be called once for every discovered font that matches the search characteristics.

A complete version of the example here can be found in the example folder of package:win32 as fonts.dart.