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/****************************************************************************
**
** Copyright (C) 2017 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
//! [expressing os versions]
Apple platforms have a built-in way to express the OS versions that an
application supports, which allows older versions of the platforms to
automatically display a user friendly error message prompting the user to
update their OS, as opposed to crashing and displaying a stack trace.
The main concepts involved in expressing support for a particular range of
OS versions are:
\list
\li \e {Deployment target} specifies the \e hard minimum version of
\macos, iOS, tvOS, or watchOS that your application supports.
\li \e {SDK version} specifies the \e soft maximum version of \macos,
iOS, tvOS, or watchOS that your application supports.
\endlist
When you develop an application for an Apple platform, you should always
use the latest version of Xcode and the latest SDK available at the time of
development. On some platforms, like iOS, you will actually be rejected from
the App Store if you do not. Therefore, the SDK version is always
greater than or equal to the deployment target.
When you develop an application for an Apple platform, you must set the
deployment target. Various build tools within the Xcode toolchain all have
a flag which you can use to set this value, including but not limited to
the compiler and linker. By setting the deployment target value, you are
explicitly declaring that your application must work on at least that
version, and will not work with any earlier versions of the OS. It is then
up to you to ensure that your use of the system APIs matches what you have
declared. Since the compiler knows what you have declared, it can help in
enforcing that.
The SDK version is considered a \e soft maximum version of the OS that an
application is compatible with in the way that if the application is built
with an SDK, it will continue to use the behaviors of that SDK even on newer
OS versions, because the OS checks the binary's load commands and emulates
backwards compatibility with the older OS. For example, if an application is
built with the \macos 10.12 SDK, it will continue to use 10.12 behaviors
even on 10.13 and above.
However, Mach-O binaries are inherently forward compatible. For example, an
application built with the iOS 9 SDK will run just fine on iOS 10, but might
not be opted into whatever behavior changes may have been made to certain
functionality on the new release, until that application is recompiled
against that newer SDK.
The minimum OS version can be expressed to the system by the compiler and
linker flags that embed it into the Mach-O binary. In addition, the
\c LSMinimumSystemVersion key must be set in the application's app bundle.
This value must be equal to the value passed to the compiler and linker,
because on \macos it will allow the OS to display a user friendly error
dialog that says the application requires a newer version of the OS as
opposed to a crash dialog. The \c LSMinimumSystemVersion is also the key
that the App Store uses to display the required OS version; the compiler and
linker flags have no power there.
For the most part, Qt applications will work without problems. For example,
in qmake, the Qt mkspecs set \l QMAKE_IOS_DEPLOYMENT_TARGET,
\l QMAKE_MACOSX_DEPLOYMENT_TARGET, \l QMAKE_TVOS_DEPLOYMENT_TARGET, or
\l QMAKE_WATCHOS_DEPLOYMENT_TARGET to the minimum version that Qt itself
supports. Similarly, in Qbs, the Qt modules set \c cpp.minimumIosVersion,
\c cpp.minimumMacosVersion, \c cpp.minimumTvosVersion, or
\c cpp.minimumWatchosVersion to the minimum version that Qt itself supports.
However, you must take care when you manually set your own target version.
If you set it to a value higher than what Qt requires and supply your own
\c Info.plist file, you must add an \c LSMinimumSystemVersion entry to the
\c Info.plist that matches the value of the deployment target, because the
OS will use the \c LSMinimumSystemVersion value as the authoritative one.
If you specify a deployment target value lower than what Qt requires, the
application will almost certainly crash somewhere in the Qt libraries when
run on an older version than Qt supports. Therefore, make sure that the
actual build system code reflects the minimum OS version that is actually
required.
//! [expressing os versions]
*/
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