[Xamarin.Android.Build.Tasks] improve incremental deploy in IDEs

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Utilities/DeviceTest.cs

@@ -140,5 +140,29 @@ protected static void ClickButton (string packageName, string buttonName, string
 			var bounds = GetControlBounds (packageName, buttonName, buttonText);
 			RunAdbInput ("input tap", bounds.x + ((bounds.w - bounds.x) / 2), bounds.y + ((bounds.h - bounds.y) / 2));
 		}
-    }
-}
+
+		/// <summary>
+		/// Returns the first device listed via `adb devices`
+		/// 
+		/// Output is:
+		/// > adb devices
+		/// List of devices attached
+		/// 89RY0AEFA device
+		/// </summary>
+		/// <returns></returns>
+		protected static string GetAttachedDeviceSerial ()
+		{
+			var text = RunAdbCommand ("devices");
+			var lines = text.Split ('\n');
+			if (lines.Length < 2) {
+				Assert.Fail ($"Unexpected `adb devices` output: {text}");
+			}
+			var serial = lines [1];
+			var index = serial.IndexOf ('\t');
+			if (index != -1) {
+				serial = serial.Substring (0, index);
+			}
+			return serial.Trim ();
+		}
+	}
+}

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1262,7 +1262,7 @@ because xbuild doesn't support framework reference assemblies.
 			WriteOnlyWhenDifferent="true"
 	/>
 	<WriteLinesToFile
-			Condition=" '$(DesignTimeBuild)' != 'True' "
+			Condition=" '$(DesignTimeBuild)' != 'True' And ('$(AdbTarget)' != '' Or !Exists('$(_AdbPropertiesCache)')) "
 			File="$(_AdbPropertiesCache)"
 			Lines="AdbTarget=$(AdbTarget);AdbOptions=$(AdbOptions)"
 			Overwrite="true"

tests/MSBuildDeviceIntegration/Tests/InstallTests.cs

@@ -12,7 +12,7 @@ namespace Xamarin.Android.Build.Tests
 	[TestFixture]
 	[NonParallelizable] //These tests deploy to devices
 	[Category ("Commercial")]
-	public class InstallTests : BaseTest
+	public class InstallTests : DeviceTest
 	{
 		[Test]
 		public void ReInstallIfUserUninstalled ([Values (false, true)] bool isRelease)
@@ -298,5 +298,28 @@ public void ToggleFastDev ()
 				Assert.IsTrue (builder.Install (proj), "Third install should have succeeded.");
 			}
 		}
+
+		[Test]
+		public void BlankAdbTarget ()
+		{
+			if (!CommercialBuildAvailable) {
+				Assert.Ignore ("Not required on Open Source Builds");
+			}
+			if (!HasDevices) {
+				Assert.Ignore ("Test Skipped no devices or emulators found.");
+			}
+
+			var serial = GetAttachedDeviceSerial ();
+			var proj = new XamarinAndroidApplicationProject ();
+			proj.SetProperty (proj.DebugProperties, "AndroidUseSharedRuntime", true);
+			proj.SetProperty (proj.DebugProperties, "EmbedAssembliesIntoApk", false);
+
+			using (var b = CreateApkBuilder (Path.Combine ("temp", TestName))) {
+				b.Build (proj, parameters: new [] { $"AdbTarget=\"-e {serial}\"" });
+				// Build again, no $(AdbTarget)
+				b.Build (proj);
+				Assert.IsTrue (b.Output.IsTargetSkipped ("_BuildApkFastDev"), "_BuildApkFastDev should be skipped!");
+			}
+		}
 	}
 }