add documentation

Author: Max Mustermann

doc/remove-compatibility.md

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+# Remove compatibility check
+
+At first start and once per day the application asks the Dexcom server for compatibility of the used device. The following modifications remove this behaviour from the application.
+
+
+## Disable compatibility webservice
+
+The file `/src/smali/com/android/databinding/library/baseAdapters/c.smali` implements the compatibility check. The function `performAppCompatibilityServerIO` initiates the request to the Dexcom server and looks somewhat like that:
+
+```java
+public class c implements b {
+
+  // some code before...
+
+  public void performAppCompatibilityServerIO(AppRuntimeInfo var1, String var2, com.dexcom.cgm.appcompatability.a var3) {
+    this.m_validityResult = this.checkValidity(var1);
+    if (!Objects.equals(this.m_validityResult.getMessageId(), "00000000-0000-0000-0000-000000000000")) {
+       this.m_getMessageResult = this.getMessage(UUID.fromString(this.m_validityResult.getMessageId()), var2);
+    }
+  
+    if (var3 != null) {
+       var3.serverCallFinished();
+    } else {
+       throw new IllegalArgumentException("AppCompatCompleteListener was null when server call was complete");
+    }
+  }
+
+  // some code after...
+
+}
+```
+
+This modification removes the request and just returns a successful response:
+
+```java
+public class c implements b {
+
+  // some code before...
+
+  public void performAppCompatibilityServerIO(AppRuntimeInfo var1, String var2, com.dexcom.cgm.appcompatability.a var3) {
+    this.m_validityResult = new ValidityResult();
+    this.m_validityResult.setValidity("ValidEnvironment");
+    this.m_validityResult.setMessageId("00000000-0000-0000-0000-000000000000");
+    if (var3 != null) {
+       var3.serverCallFinished();
+    } else {
+       throw new IllegalArgumentException("AppCompatCompleteListener was null when server call was complete");
+    }
+  }    
+
+  // some code after...
+
+}
+```
+
+The modification translated into smali code looks like this:
+
+```smali
+.method public performAppCompatibilityServerIO(Lcom/dexcom/cgm/model/AppRuntimeInfo;Ljava/lang/String;Lcom/dexcom/cgm/appcompatability/a;)V
+    .locals 2
+
+    new-instance v0, Lcom/dexcom/cgm/model/ValidityResult;
+
+    invoke-direct {v0}, Lcom/dexcom/cgm/model/ValidityResult;-><init>()V
+
+    iput-object v0, p0, Lcom/android/databinding/library/baseAdapters/c;->m_validityResult:Lcom/dexcom/cgm/model/ValidityResult;
+
+    iget-object v0, p0, Lcom/android/databinding/library/baseAdapters/c;->m_validityResult:Lcom/dexcom/cgm/model/ValidityResult;
+
+    const-string v1, "ValidEnvironment"
+
+    invoke-virtual {v0, v1}, Lcom/dexcom/cgm/model/ValidityResult;->setValidity(Ljava/lang/String;)V
+
+    iget-object v0, p0, Lcom/android/databinding/library/baseAdapters/c;->m_validityResult:Lcom/dexcom/cgm/model/ValidityResult;
+
+    const-string v1, "00000000-0000-0000-0000-000000000000"
+
+    invoke-virtual {v0, v1}, Lcom/dexcom/cgm/model/ValidityResult;->setMessageId(Ljava/lang/String;)V
+
+    if-eqz p3, :cond_0
+
+    invoke-interface {p3}, Lcom/dexcom/cgm/appcompatability/a;->serverCallFinished()V
+
+    return-void
+
+    :cond_0
+    new-instance v0, Ljava/lang/IllegalArgumentException;
+
+    const-string v1, "AppCompatCompleteListener was null when server call was complete"
+
+    invoke-direct {v0, v1}, Ljava/lang/IllegalArgumentException;-><init>(Ljava/lang/String;)V
+
+    throw v0
+.end method
+```
+
+
+## Redirect from compatibility view
+
+The file `/src/smali/com/dexcom/cgm/activities/AppCompatabilityActivity.smali` implements the compatibility screen. It shows an error message in case the Dexcom server denied compatibility for the device.
+
+The function `serverCallFinished` is executed after the application received a response from the Dexcom server. It looks somewhat like that:
+
+```java
+public class AppCompatabilityActivity extends DexBaseActivity implements OnAppInitListener, a {
+
+  // some code before...
+
+  public void serverCallFinished() {
+    this.setNextCheckTimeToTomorrow();
+    if (Objects.equals(this.getAppCompatibilityService().getValidity().getMessageId(), "00000000-0000-0000-0000-000000000000")) {
+       this.goDirectlyToNextActivity();
+    } else {
+       this.m_validityResult = this.getAppCompatibilityService().getValidity();
+       this.m_getMessageResult = this.getAppCompatibilityService().getMessageResult();
+       this.processNewAppCompatability();
+       this.m_isFreshInstall = false;
+    }
+  }
+
+  // some code after...
+
+}
+```
+
+This modification ignores the response and redirects to the next view:
+
+```java
+public class AppCompatabilityActivity extends DexBaseActivity implements OnAppInitListener, a {
+
+  // some code before...
+
+  public void serverCallFinished() {
+    this.setNextCheckTimeToTomorrow();
+    this.goDirectlyToNextActivity();
+  }
+
+  // some code after...
+
+}
+```
+
+The modification translated into smali code looks like this:
+
+```smali
+.method public serverCallFinished()V
+    .locals 0
+
+    invoke-direct {p0}, Lcom/dexcom/cgm/activities/AppCompatabilityActivity;->setNextCheckTimeToTomorrow()V
+
+    invoke-direct {p0}, Lcom/dexcom/cgm/activities/AppCompatabilityActivity;->goDirectlyToNextActivity()V
+
+    return-void
+.end method
+```