Add support for soft dependencies.

Depends items get a "required" property, module presence can be
checked via "present" property.

Change-Id: Id188a442b3b32c5b2e50e568e59cee074774f125
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

2 files changed, 32 insertions, 2 deletions

diff --git a/doc/reference/items/depends.qdoc b/doc/reference/items/depends.qdoc
index 1bb7c0142..6ace42a3f 100644
--- a/doc/reference/items/depends.qdoc
+++ b/doc/reference/items/depends.qdoc
@@ -53,6 +53,14 @@
         \li true
         \li Determines whether the dependency will actually be applied.
     \row
+        \li required
+        \li bool
+        \li \c true
+        \li Setting this property to \c false creates a "soft dependency", meaning that it is not
+            considered an error if the given module cannot be found. In such a case, an instance of
+            the respective module will be created, but only the \c present property will be
+            available for querying, and it will be set to \c false.
+    \row
         \li name
         \li string
         \li undefined
diff --git a/doc/reference/items/module.qdoc b/doc/reference/items/module.qdoc
index 0cc7707b9..35ef659cf 100644
--- a/doc/reference/items/module.qdoc
+++ b/doc/reference/items/module.qdoc
@@ -40,11 +40,26 @@
     product if the product depends on (or loads) the module.
 
     A Module can be loaded by adding a dependency to it in a product.
-    For example, the following product will load the \c cpp module:
+    For example, the following product will load the \c cpp module. In addition, it will try
+    to load modules that may or may not exist, and in the latter case use a fallback.
     \code
     Product {
         Depends { name: "cpp" }
-        ...
+        Depends {
+            name: "awesome_module"
+            required: false
+        }
+        Depends {
+            name: "adequate_module"
+            condition: !awesome_module.present
+            required: false
+        }
+        Depends {
+            name: "crappy_module"
+            condition: !awesome_module.present && !adequate_module.present
+        }
+
+        // ...
     }
     \endcode
 
@@ -55,6 +70,13 @@
             \li Default
             \li Description
         \row
+            \li present
+            \li bool
+            \li \c true
+            \li This property is read-only. Its value is \c false if and only if the respective
+                \c Depends item had its \c required property set to \false and the module was
+                not found.
+        \row
             \li setupBuildEnvironment
             \li script
             \li \c undefined