Text: return a bool from AbstractFont::fillGlyphCache().

So the font implementations can actually return something reasonable on
failure, instead of asserting.

doc/changelog.dox

@@ -837,6 +837,8 @@ See also:
 -   Added a @ref Text::GlyphCache::GlyphCache(NoCreateT) and
     @ref Text::DistanceFieldGlyphCache::DistanceFieldGlyphCache(NoCreateT)
     constructor allowing to construct the object without a GL context present
+-   @ref Text::AbstractFont::fillGlyphCache() now returns a @cpp bool @ce to
+    allow font plugin implementations to gracefully report failures
 
 @subsubsection changelog-latest-changes-texturetools TextureTools library
 

src/Magnum/Text/AbstractFont.cpp

@@ -272,21 +272,22 @@ Vector2 AbstractFont::glyphAdvance(const UnsignedInt glyph) {
     return doGlyphAdvance(glyph);
 }
 
-void AbstractFont::fillGlyphCache(AbstractGlyphCache& cache, const Containers::StringView characters) {
+bool AbstractFont::fillGlyphCache(AbstractGlyphCache& cache, const Containers::StringView characters) {
     CORRADE_ASSERT(isOpened(),
-        "Text::AbstractFont::fillGlyphCache(): no font opened", );
+        "Text::AbstractFont::fillGlyphCache(): no font opened", {});
     CORRADE_ASSERT(!(features() & FontFeature::PreparedGlyphCache),
-        "Text::AbstractFont::fillGlyphCache(): feature not supported", );
+        "Text::AbstractFont::fillGlyphCache(): feature not supported", {});
 
     const Containers::Optional<Containers::Array<char32_t>> utf32 = Utility::Unicode::utf32(characters);
     CORRADE_ASSERT(utf32,
-        "Text::AbstractFont::fillGlyphCache(): not a valid UTF-8 string:" << characters, );
+        "Text::AbstractFont::fillGlyphCache(): not a valid UTF-8 string:" << characters, {});
 
-    doFillGlyphCache(cache, *utf32);
+    return doFillGlyphCache(cache, *utf32);
 }
 
-void AbstractFont::doFillGlyphCache(AbstractGlyphCache&, Containers::ArrayView<const char32_t>) {
-    CORRADE_ASSERT_UNREACHABLE("Text::AbstractFont::fillGlyphCache(): feature advertised but not implemented", );
+bool AbstractFont::doFillGlyphCache(AbstractGlyphCache&, Containers::ArrayView<const char32_t>) {
+    CORRADE_ASSERT_UNREACHABLE("Text::AbstractFont::fillGlyphCache(): feature advertised but not implemented", {});
+    return {};
 }
 
 Containers::Pointer<AbstractGlyphCache> AbstractFont::createGlyphCache() {

src/Magnum/Text/AbstractFont.h

@@ -501,8 +501,13 @@ class MAGNUM_TEXT_EXPORT AbstractFont: public PluginManager::AbstractPlugin {
          * @ref FontFeature::PreparedGlyphCache do not support partial glyph
          * cache filling, use @ref createGlyphCache() instead. Expects that a
          * font is opened and @p characters is valid UTF-8.
+         *
+         * On success returns @cpp true @ce. On failure, for example if the
+         * @p cache doesn't have expected format or the @p characters can't
+         * fit, prints a message to @relativeref{Magnum,Error} and returns
+         * @cpp false @ce.
          */
-        void fillGlyphCache(AbstractGlyphCache& cache, Containers::StringView characters);
+        bool fillGlyphCache(AbstractGlyphCache& cache, Containers::StringView characters);
 
         /**
          * @brief Create glyph cache
@@ -661,7 +666,7 @@ class MAGNUM_TEXT_EXPORT AbstractFont: public PluginManager::AbstractPlugin {
          * The string is converted from UTF-8 to UTF-32, duplicate characters
          * are *not* removed.
          */
-        virtual void doFillGlyphCache(AbstractGlyphCache& cache, Containers::ArrayView<const char32_t> characters);
+        virtual bool doFillGlyphCache(AbstractGlyphCache& cache, Containers::ArrayView<const char32_t> characters);
 
         /** @brief Implementation for @ref createGlyphCache() */
         virtual Containers::Pointer<AbstractGlyphCache> doCreateGlyphCache();

src/Magnum/Text/Test/AbstractFontTest.cpp

@@ -88,6 +88,7 @@ struct AbstractFontTest: TestSuite::Tester {
     void glyphSizeAdvanceOutOfRange();
 
     void fillGlyphCache();
+    void fillGlyphCacheFailed();
     void fillGlyphCacheNotSupported();
     void fillGlyphCacheNotImplemented();
     void fillGlyphCacheNoFont();
@@ -151,6 +152,7 @@ AbstractFontTest::AbstractFontTest() {
               &AbstractFontTest::glyphSizeAdvanceOutOfRange,
 
               &AbstractFontTest::fillGlyphCache,
+              &AbstractFontTest::fillGlyphCacheFailed,
               &AbstractFontTest::fillGlyphCacheNotSupported,
               &AbstractFontTest::fillGlyphCacheNotImplemented,
               &AbstractFontTest::fillGlyphCacheNoFont,
@@ -929,12 +931,13 @@ void AbstractFontTest::fillGlyphCache() {
         Vector2 doGlyphAdvance(UnsignedInt) override { return {}; }
         Containers::Pointer<AbstractShaper> doCreateShaper() override { return {}; }
 
-        void doFillGlyphCache(AbstractGlyphCache& cache, Containers::ArrayView<const char32_t> characters) override {
+        bool doFillGlyphCache(AbstractGlyphCache& cache, Containers::ArrayView<const char32_t> characters) override {
             CORRADE_COMPARE(cache.size(), (Vector3i{100, 100, 1}));
             CORRADE_COMPARE_AS(characters, Containers::arrayView<char32_t>({
                 'h', 'e', 'l', 'o'
             }), TestSuite::Compare::Container);
             called = true;
+            return true;
         }
 
         bool called = false;
@@ -945,10 +948,36 @@ void AbstractFontTest::fillGlyphCache() {
 
     DummyGlyphCache cache{PixelFormat::R8Unorm, {100, 100}};
 
-    font.fillGlyphCache(cache, "helo");
+    CORRADE_VERIFY(font.fillGlyphCache(cache, "helo"));
     CORRADE_VERIFY(font.called);
 }
 
+void AbstractFontTest::fillGlyphCacheFailed() {
+    struct MyFont: AbstractFont {
+        FontFeatures doFeatures() const override { return {}; }
+        bool doIsOpened() const override { return true; }
+        void doClose() override {}
+
+        UnsignedInt doGlyphId(char32_t) override { return {}; }
+        Vector2 doGlyphSize(UnsignedInt) override { return {}; }
+        Vector2 doGlyphAdvance(UnsignedInt) override { return {}; }
+        Containers::Pointer<AbstractShaper> doCreateShaper() override { return {}; }
+
+        bool doFillGlyphCache(AbstractGlyphCache&, Containers::ArrayView<const char32_t>) override {
+            return false;
+        }
+
+        bool called = false;
+    } font;
+
+    /* Capture correct function name */
+    CORRADE_VERIFY(true);
+
+    DummyGlyphCache cache{PixelFormat::R8Unorm, {100, 100}};
+
+    CORRADE_VERIFY(!font.fillGlyphCache(cache, ""));
+}
+
 void AbstractFontTest::fillGlyphCacheNotSupported() {
     CORRADE_SKIP_IF_NO_ASSERT();