Introduce module providers

If a dependency is not found, we now search for a matching module
provider that can generate one for us.
We also provide a generic fall-back provider which uses pkg-config to
locate the dependency (but could be extended to incorporate other
methods in the future). This is the most important part of this change
for practical purposes, as it makes hundreds of popular libraries
available for use in qbs projects without users having to write any
boilerplate code.
In a future patch, a module provider could also be used to implement the
functionality of the qtprofilesetup library, relieving users of the need
to create a profile for building Qt applications.

[ChangeLog] The Depends item now falls back to pkg-config to locate
dependencies whose names do not correspond to a qbs module.

Fixes: QBS-1107
Change-Id: Ifd4f05c237cf58cd9fe707c3da648d3dbb33e82b
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

1 files changed, 59 insertions, 1 deletions

diff --git a/doc/qbs.qdoc b/doc/qbs.qdoc
index 14787ffbf..44678bc40 100644
--- a/doc/qbs.qdoc
+++ b/doc/qbs.qdoc
@@ -68,6 +68,7 @@
                \li  \l{Generators}
                \li  \l{Multiplexing}
                \li  \l{Custom Modules and Items}
+               \li  \l{Module Providers}
            \endlist
        \li  \l{How-tos}
        \li  \l{Reference}
@@ -791,6 +792,7 @@
         \li  \l{Generators}
         \li  \l{Multiplexing}
         \li  \l{Custom Modules and Items}
+        \li  \l{Module Providers}
     \endlist
 
 */
@@ -1351,7 +1353,7 @@
     \contentspage index.html
     \previouspage multiplexing.html
     \page custom-modules.html
-    \nextpage howtos.html
+    \nextpage module-providers.html
 
     \title Custom Modules and Items
 
@@ -1421,6 +1423,62 @@
 
 /*!
     \contentspage index.html
+    \previouspage custom-modules.html
+    \page module-providers.html
+    \nextpage howtos.html
+
+    \title Module Providers
+
+    There are use cases for which a pre-defined module is not flexible enough.
+    For instance, the overall set of modules related to a certain task might depend
+    on some information present on the local platform.
+
+    \note Module providers are an advanced concept that you will rarely need to use directly.
+    Reading this section is not required for most people's everyday work.
+
+    \section1 How \QBS Uses Module Providers
+
+    If \QBS encounters a \l Depends item whose name does not match a known module,
+    it checks whether such a module can be generated. This procedure works as follows:
+    \list 1
+        \li All \l{Project::qbsSearchPaths}{search paths} are scanned for a file called
+            \c {module-providers/<name>/provider.qbs}, where \c <name> is the name of the dependency
+            as specified in the \c Depends item. Multi-component names such as "a.b" are turned
+            into nested directories, and each of them is scanned, starting with the deepest path.
+            For instance, if the dependency's name is \c {a.b}, then \QBS will look for
+            \c {a/b/provider.qbs} and then \c {a/provider.qbs}.
+        \li If such a file is found, it needs to contain a \l ModuleProvider item. This item
+            is instantiated, which potentially leads to the creation of one or more modules,
+            and \QBS retrieves the search paths to find these modules from the item.
+            The details are described in the \l ModuleProvider documentation.
+        \li If a matching module provider was found and provided new search paths,
+            a second attempt will be made to locate the dependency using the new paths.
+            The search for a matching module provider ends as soon as one was found, regardless
+            of whether it created any modules or not.
+        \li If no matching module provider was found in any of the search paths, \QBS will fall back
+            to a generic module provider, which creates a module that attempts to locate the
+            dependency via \c pkg-config.
+            This fallback mechanism can be disabled in the respective
+            \l{Depends::enableFallback}{Depends} item or globally via the
+            \l{no-fallback-module-provider}{--no-fallback-module-provider} option.
+    \endlist
+
+    \section1 Parameterizing Module Providers
+
+    You can pass information to module providers from the command line, via profiles or
+    from within a product, in a similar way as you would do for modules. For instance, the
+    following invocation of \QBS passes information to two module providers \c a and \c b:
+    \code
+    $ qbs moduleProviders.a.p1:true moduleProviders.a.p2:true moduleProviders.b.p:false
+    \endcode
+    \QBS will set the properties of the respective module providers accordingly.
+    In the above example, module provider \c a needs to declare two boolean properties \c p1
+    and \c p2, and they will be set to \c true and \c false, respectively.
+
+*/
+
+/*!
+    \contentspage index.html
     \previouspage shell.html
     \page generators.html
     \nextpage multiplexing.html