Fail early and give tailored message when dot command is not present.

src/main/kotlin/com/vanniktech/dependency/graph/generator/DependencyGraphGeneratorTask.kt

@@ -1,11 +1,14 @@
 package com.vanniktech.dependency.graph.generator
 
 import com.vanniktech.dependency.graph.generator.DependencyGraphGeneratorExtension.Generator
+import org.apache.tools.ant.taskdefs.condition.Os
 import org.gradle.api.DefaultTask
+import org.gradle.api.GradleException
 import org.gradle.api.tasks.InputFile
 import org.gradle.api.tasks.OutputFile
 import org.gradle.api.tasks.TaskAction
 import java.io.File
+import java.io.OutputStream
 
 open class DependencyGraphGeneratorTask : DefaultTask() {
   lateinit var generator: Generator // TODO does this need to be an input? Quick testing shows no.
@@ -15,6 +18,23 @@ open class DependencyGraphGeneratorTask : DefaultTask() {
   @OutputFile lateinit var imageOutputFile: File
 
   @TaskAction fun run() {
+    val result = project.exec {
+      it.standardOutput = NullOutputStream // Don't print the output.
+      it.executable = "command"
+      it.args("-v", "dot")
+      it.isIgnoreExitValue = true
+    }
+
+    if (result.exitValue != 0) {
+      val message = when {
+        Os.isFamily(Os.FAMILY_MAC) -> "Please install via: brew install graphviz"
+        Os.isFamily(Os.FAMILY_UNIX) -> "Please install via: sudo apt-get install graphviz"
+        else -> "Please find a way to install it."
+      }
+
+      throw GradleException("This task requires dot from graphviz. $message")
+    }
+
     dotOutputFile.writeText(DotGenerator(project, generator).generateContent())
 
     project.exec {
@@ -29,4 +49,10 @@ open class DependencyGraphGeneratorTask : DefaultTask() {
       it.rename("\\.dot", "")
     }
   }
+
+  internal object NullOutputStream : OutputStream() {
+    override fun write(b: Int) {
+      // No-op.
+    }
+  }
 }

src/test/java/com/vanniktech/dependency/graph/generator/DependencyGraphGeneratorPluginTest.kt

@@ -0,0 +1,37 @@
+package com.vanniktech.dependency.graph.generator
+
+import com.vanniktech.dependency.graph.generator.DependencyGraphGeneratorExtension.Generator.Companion.ALL
+import org.assertj.core.api.Java6Assertions.assertThat
+import org.gradle.api.internal.project.DefaultProject
+import org.gradle.api.plugins.JavaLibraryPlugin
+import org.gradle.testfixtures.ProjectBuilder
+import org.junit.Before
+import org.junit.Test
+import java.io.File
+
+class DependencyGraphGeneratorPluginTest {
+  private lateinit var singleProject: DefaultProject
+
+  @Before fun setUp() {
+    // Casting this to DefaultProject so we can call evaluate later.
+    singleProject = ProjectBuilder.builder().withName("single").build() as DefaultProject
+    singleProject.plugins.apply(JavaLibraryPlugin::class.java)
+    singleProject.repositories.run { add(mavenCentral()) }
+    singleProject.dependencies.add("api", "org.jetbrains.kotlin:kotlin-stdlib:1.2.30")
+    singleProject.dependencies.add("implementation", "io.reactivex.rxjava2:rxjava:2.1.10")
+  }
+
+  @Test fun taskProperties() {
+    singleProject.plugins.apply(DependencyGraphGeneratorPlugin::class.java)
+
+    singleProject.evaluate() // Need to call this for afterEvaluate() to pick up.
+
+    val task = singleProject.tasks.getByName("generateDependencyGraph") as DependencyGraphGeneratorTask
+    assertThat(task.generator).isSameAs(ALL)
+    assertThat(task.group).isEqualTo("reporting")
+    assertThat(task.description).isEqualTo("Generates a dependency graph")
+    assertThat(task.inputFile).hasToString(singleProject.buildFile.toString())
+    assertThat(task.dotOutputFile).hasToString(File(singleProject.buildDir, "dependency-graph.dot").toString())
+    assertThat(task.imageOutputFile).hasToString(File(singleProject.projectDir, "dependency-graph.png").toString())
+  }
+}

src/test/java/com/vanniktech/dependency/graph/generator/dot/HeaderTest.kt

@@ -7,6 +7,10 @@ import org.junit.Test
 
 class HeaderTest {
   @Test fun syntax() {
+    assertThat(Header("foo")).hasToString("label=\"foo\" fontsize=\"24\" height=\"5\" labelloc=\"t\" labeljust=\"c\"")
+    assertThat(Header("foo", 10)).hasToString("label=\"foo\" fontsize=\"10\" height=\"5\" labelloc=\"t\" labeljust=\"c\"")
+    assertThat(Header("foo", 10, 25)).hasToString("label=\"foo\" fontsize=\"10\" height=\"25\" labelloc=\"t\" labeljust=\"c\"")
+    assertThat(Header("foo", 10, 25, BOTTOM)).hasToString("label=\"foo\" fontsize=\"10\" height=\"25\" labelloc=\"b\" labeljust=\"c\"")
     assertThat(Header("1234", 100, 5, BOTTOM, LEFT)).hasToString("label=\"1234\" fontsize=\"100\" height=\"5\" labelloc=\"b\" labeljust=\"l\"")
   }
 }