UnitTests: How to write tests

Change-Id: Ic2221a83d139e7d7ad7b4d0f40d602822fb6654c
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/README.md

@@ -11,10 +11,156 @@ All tests here depend on the [GoogleTest][1] framework.
 
 ## Best Practices
 
-We're following those patterns/approaches;
+We follow these patterns/approaches for structuring our tests:
 
-* The Arrange, Act, and Assert (AAA) Pattern
+Arrange, Act, and Assert (AAA) / Given When Then (GWT)
-* Given When Then (GWT) Pattern
+
+This pattern structures your tests into three distinct sections:
+
+    Arrange (Given): Set up the initial conditions and inputs.
+    Act (When): Execute the code being tested.
+    Assert (Then): Verify the result.
+
+The test name is descriptive and uses underlines for readability.
+In the Act block, only the code you want to test should exist. Don't
+add the setup code there. It makes the debugging harder because you have
+to step over it.
+
+```cpp
+TEST(String, default_string_is_empty)
+{
+    String text;
+
+    bool isEmpty = text.empty();
+
+    ASSERT_TRUE(isEmpty);
+}
+```
+
+If the code block gets hard to read, you can write Arrange, Act, and Assert comments.
+So the blocks are easier to identify. Don't write other comments. Use a descriptive
+test name.
+
+```cpp
+TEST(String, default_string_is_empty)
+{
+    // arrange
+    String text;
+
+    // act
+    bool isEmpty = text.empty();
+
+    // assert
+    ASSERT_TRUE(isEmpty);
+}
+```
+
+If you use a fixture, you can sometimes skip the arrange part.
+
+Fixtures should have the same name as your class. If you test functions,
+it is the namespace. Sometimes you need multiple fixtures. Then you append
+a descriptive text. You can put an underline in between.
+
+```cpp
+class String : public ::testing::Test
+{
+protected:
+    String text;
+}
+
+TEST_F(String, default_string_is_empty)
+{
+    bool isEmpty = text.empty();
+
+    ASSERT_TRUE(isEmpty);
+}
+```
+For mocks, you have to reverse the order of act and assert.
+
+```cpp
+class String : public ::testing::Test
+{
+protected:
+    NiceMock<OutputMock> outputMock;
+    Printer printer;
+}
+
+TEST_F(String, printer_appends_message_to_the_end)
+{
+    // arrange
+    String text;
+
+    // assert
+    EXPECT_CALL(outputMock, print(EndsWith(text)))
+
+    // act
+    printer.print(text);
+}
+```
+
+Don't write loops or branches in tests. Google Tests has many facilities supporting you to write clear tests.
+
+Matcher, for example, gives a much better error message.
+
+```cpp
+TEST_F(Storage, return_all_entries_with_the_name_which_starts_with_foo)
+{
+    storage.load(filepath);
+
+    auto entries = storage.entries("foo*");
+
+    ASSERT_THAT(entries, UnorderedElementsAre(IsEntry("foo", Field(&Entry::values), Contains(5)),
+                                              IsEntry("fooBar", Field(&Entry::values), IsSubset(42, 77))));
+}
+```
+
+You can even make the matcher easier to read.
+
+```cpp
+template<typename Matcher>
+auto FieldValues(const Matcher &matcher)
+{
+    return Field(&Entry::values, matcher);
+}
+
+TEST_F(Storage, return_all_entries_with_the_name_which_starts_with_foo)
+{
+    storage.load(filepath);
+
+    auto entries = storage.entries("foo*");
+
+    ASSERT_THAT(entries, UnorderedElementsAre(IsEntry("foo", FieldValues(Contains(5)),
+                                              IsEntry("fooBar", FieldValues(IsSubset(42, 77))));
+}
+```
+
+## Avoid
+
+Don't use using namespaces. It leads easily to name collisions.
+
+```cpp
+using namespace QmlDesigner;
+```
+
+If you have long namespace names, you can use:
+
+```cpp
+namespace ModelUtils = QmlDesigner::ModelUtils;
+```
+
+You can import single types too. But mind, that your tests are written
+for reading, not for writing. They are part of the code documentation.
+
+```cpp
+using QmlDesigner::ModelNode;
+using Node = QmlDesigner::ModelNode;
+```
+
+There are exceptions like literal namespaces.
+
+```cpp
+using namespace Qt::Literals;
+```
 
 ## Test Organization