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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.


Dart currently offers two ways for creating callback functions that can be invoked from native functions:

  • NativeCallable.isolateLocal: Constructs a NativeCallable that must be invoked from the same thread that created it.
  • NativeCallable.listener: Constructs a NativeCallable that can be invoked from any thread. However, this comes with a restriction — only void functions are supported.

In most cases, utilizing NativeCallable.isolateLocal should suffice. However, if you encounter a Cannot invoke native callback outside an isolate. error, it indicates that the API you're calling operates in a different thread context. In such cases, consider using NativeCallable.listener, especially if the callback is a void function. If not, there is currently no way to call that particular API.

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).

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 = NativeCallable<EnumFontFamExProc>.isolateLocal(
exceptionalReturn: 0,

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

// Close the callback when it's no longer needed


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 NativeCallable for the Dart callback function using the NativeCallable.isolateLocal constructor. 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.