wasm: support setting the font DPI from JS

We have not really been able to determine what the
default DPI should be, so make it configurable with
API on qtloader.js:

  qtLoader.setFontDpi(72);

Also lowers the default DPI to the standard value of
96 (down from Qt default 100).

Task-number: QTBUG-75510
Change-Id: Ica1164c8d80bb06519233adebf2c9e400c0991ce
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/wasm/qtloader.js

@@ -124,6 +124,8 @@
 //      Remove canvas at run-time. Removes the corresponding QScreen.
 // resizeCanvasElement
 //      Signals to the application that a canvas has been resized.
+// setFontDpi
+//      Sets the logical font dpi for the application.
 
 
 var Module = {}
@@ -237,6 +239,8 @@ function QtLoader(config)
     publicAPI.addCanvasElement = addCanvasElement;
     publicAPI.removeCanvasElement = removeCanvasElement;
     publicAPI.resizeCanvasElement = resizeCanvasElement;
+    publicAPI.setFontDpi = setFontDpi;
+    publicAPI.fontDpi = fontDpi;
 
     restartCount = 0;
 
@@ -557,6 +561,16 @@ function QtLoader(config)
             Module.qtResizeCanvasElement(element);
     }
 
+    function setFontDpi(dpi) {
+        Module.qtFontDpi = dpi;
+        if (publicAPI.status == "Running")
+            Module.qtSetFontDpi(dpi);
+    }
+
+    function fontDpi() {
+        return Module.qtFontDpi;
+    }
+
     setStatus("Created");
 
     return publicAPI;

src/plugins/platforms/wasm/qwasmintegration.cpp

@@ -50,6 +50,7 @@
 #include <QtCore/qcoreapplication.h>
 
 #include <emscripten/bind.h>
+#include <emscripten/val.h>
 
 // this is where EGL headers are pulled in, make sure it is last
 #include "qwasmscreen.h"
@@ -80,12 +81,18 @@ static void resizeCanvasElement(emscripten::val canvas)
     QWasmIntegration::get()->resizeScreen(canvasId);
 }
 
+static void qtUpdateDpi()
+{
+    QWasmIntegration::get()->updateDpi();
+}
+
 EMSCRIPTEN_BINDINGS(qtQWasmIntegraton)
 {
     function("qtBrowserBeforeUnload", &browserBeforeUnload);
     function("qtAddCanvasElement", &addCanvasElement);
     function("qtRemoveCanvasElement", &removeCanvasElement);
     function("qtResizeCanvasElement", &resizeCanvasElement);
+    function("qtUpdateDpi", &qtUpdateDpi);
 }
 
 QWasmIntegration *QWasmIntegration::s_instance;
@@ -245,4 +252,14 @@ void QWasmIntegration::resizeScreen(const QString &canvasId)
     m_screens.value(canvasId)->updateQScreenAndCanvasRenderSize();
 }
 
+void QWasmIntegration::updateDpi()
+{
+    emscripten::val dpi = emscripten::val::module_property("qtFontDpi");
+    if (dpi.isUndefined())
+        return;
+    qreal dpiValue = dpi.as<qreal>();
+    for (QWasmScreen *screen : m_screens)
+        QWindowSystemInterface::handleScreenLogicalDotsPerInchChange(screen->screen(), dpiValue, dpiValue);
+}
+
 QT_END_NAMESPACE

src/plugins/platforms/wasm/qwasmintegration.h

@@ -81,6 +81,7 @@ public:
     void addScreen(const QString &canvasId);
     void removeScreen(const QString &canvasId);
     void resizeScreen(const QString &canvasId);
+    void updateDpi();
 
 private:
     mutable QWasmFontDatabase *m_fontDb;
@@ -89,6 +90,7 @@ private:
 
     QHash<QString, QWasmScreen *> m_screens;
     mutable QWasmClipboard *m_clipboard;
+    qreal m_fontDpi = -1;
     static QWasmIntegration *s_instance;
 };
 

src/plugins/platforms/wasm/qwasmscreen.cpp

@@ -31,6 +31,7 @@
 #include "qwasmwindow.h"
 #include "qwasmeventtranslator.h"
 #include "qwasmcompositor.h"
+#include "qwasmintegration.h"
 #include <emscripten/bind.h>
 #include <emscripten/val.h>
 
@@ -100,6 +101,17 @@ QImage::Format QWasmScreen::format() const
     return m_format;
 }
 
+QDpi QWasmScreen::logicalDpi() const
+{
+    emscripten::val dpi = emscripten::val::module_property("qtFontDpi");
+    if (!dpi.isUndefined()) {
+        qreal dpiValue = dpi.as<qreal>();
+        return QDpi(dpiValue, dpiValue);
+    }
+    const qreal defaultDpi = 96;
+    return QDpi(defaultDpi, defaultDpi);
+}
+
 qreal QWasmScreen::devicePixelRatio() const
 {
     // FIXME: The effective device pixel ratio may be different from the

src/plugins/platforms/wasm/qwasmscreen.h

@@ -63,6 +63,7 @@ public:
     QRect geometry() const override;
     int depth() const override;
     QImage::Format format() const override;
+    QDpi logicalDpi() const override;
     qreal devicePixelRatio() const override;
     QString name() const override;
     QPlatformCursor *cursor() const override;