added qabstractitemmodeltester

027_tool.qabstractitemmodeltester.md

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+# Use QAbstractItemModelTester for Custom Models
+
+## Goals
+Detect/Avoid a certain set of errors in subclasses of QAbstractItemModel.
+
+## Description
+QAbstractItemModelTester can be used to detect common errors in the implementation of models based on QAbstractItemModel with very little effort and even before they appear in the form of a visible bug.
+
+## Environment
+Qt
+
+## Platform
+All
+
+## Implementation effort
+Very minimal
+
+## Applicability
+- When implementing new models
+- As part of test suites
+- In debug sessions
+
+## Caveats
+Citing the [documentation](https://doc.qt.io/qt-5/qabstractitemmodeltester.html): 
+
+> While QAbstractItemModelTester is a valid help for development and testing of custom item models, it does not (and cannot) catch all possible problems in QAbstractItemModel subclasses. Notably, it will never perform meaningful destructive testing of a model, which must be therefore tested separately.
+
+## See also
+
+## Implementation hints
+QAbstractItemModelTester can be used in two ways:
+
+In unit tests
+Directly in the running application (introduces testlib-dependency). Should be disabled for release builds. At least QAbstractItemModelTester::FailureReportingMode::Fatal should not be used in release builds
+Both ways are described in the [documentation](https://doc.qt.io/qt-5/qabstractitemmodeltester.html)