Add the ability to get tabs by a relative position.

Made 0 be an invalid position.

README.md

@@ -96,7 +96,27 @@ Retrieve an instance of `Tab` from this `id` (return `null` if not found).
 
 #### `tabGroup.getTabByPosition(position)`
 
-Retrieve an instance of `Tab` from this `position` (return `null` if not found).
+Retrieve an instance of `Tab` from this `position` (return `null` if not found). A negative value is an offset from the right.
+
+To get the tab in the leftmost position:
+
+```javascript
+tabGroup.getTabByPosition(1);
+```
+
+To get the tab in the rightmost position:
+
+```javascript
+tabGroup.getTabByPosition(-1);
+```
+
+> Note: Position 0 does not contain a tab.
+
+#### `tabGroup.getTabByRelPosition(position)`
+
+Retrieve an instance of `Tab` from this `position` relative to the active tab (return `null` if not found).
+`tabGroup.getNextTab()` is an alias to `tabGroup.getTabByRelPosition(1)`.
+`tabGroup.getPreviousTab()` is an alias to `tabGroup.getTabByRelPosition(-1)`.
 
 #### `tabGroup.getActiveTab()`
 
@@ -146,21 +166,7 @@ Get current tab icon URL / icon.
 
 #### `tab.setPosition(newPosition)`
 
-Move tab to the specified position. A negative value is an offset from the right.
+Move tab to the specified position. If `position` is 0 then `null` is returned and nothing happens. See [`tabGroup.getTabByPosition`](#tabgroupgettabbypositionposition) for information about positions.
-
-To move a tab to the leftmost position:
-
-```javascript
-tab.setPosition(1);
-```
-
-> Note: a position of 0 also moves the tab to the leftmost position
-
-To move a tab to the rightmost position:
-
-```javascript
-tab.setPosition(-1);
-```
 
 #### `tab.getPosition(fromRight)`
 

index.js

@@ -86,9 +86,6 @@ class TabGroup extends EventEmitter {
 
     getTabByPosition (position) {
         let fromRight = position < 0;
-        if (position === 0) {
-            position = 1;
-        }
         for (let i in this.tabs) {
             if (this.tabs[i].getPosition(fromRight) === position) {
                 return this.tabs[i];
@@ -97,6 +94,22 @@ class TabGroup extends EventEmitter {
         return null;
     }
 
+    getTabByRelPosition (position) {
+        position = this.getActiveTab().getPosition() + position;
+        if (position <= 0) {
+            return null;
+        }
+        return this.getTabByPosition(position);
+    }
+
+    getNextTab () {
+        return this.getTabByRelPosition(1);
+    }
+
+    getPreviousTab () {
+        return this.getTabByRelPosition(-1);
+    }
+
     getTabs () {
       return this.tabs;
     }
@@ -231,6 +244,9 @@ class Tab extends EventEmitter {
     }
 
     setPosition (newPosition) {
+        if (newPosition === 0) {
+            return null;
+        }
         let tabContainer = this.tabGroup.tabContainer;
         let tabs = tabContainer.children;
         let oldPosition = this.getPosition() - 1;