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# 4. Multiple Document Interface

All the windows that are part of the GPS environment are under control of what is commonly called a multiple document interface (MDI for short). This is a common paradigm on windowing systems, where related windows are put into a bigger window which is itself under control of the system or the windows manager.

This means that, by default, no matter how many editors, browsers, views, ... windows you have opened, your system will still see only one window (On Windows systems, the task bar shows only one icon). However, you can organize the GPS windows exactly the way you want, all inside the GPS main window.

This section will show the various capacities that GPS provides to help you organize your workspace.

## 4.1. Selecting Windows

At any time, there is only one selected window in GPS (the active window). You can select a window either by clicking in its title bar, which will then get a different color, or by selecting its name in the menu Window.

Alternatively, windows can be selected with the keyboard. By default, the selection key is `Alt-Tab`. When you press it, a temporary dialog is popped-up on the screen, with the name of the window that will be selected when the key is released. If you press the selection key multiple times, this will iterate over all the windows currently open in GPS.

This interactive selection dialog is associated with a filter, displayed below the name of the selected window. If you maintain `Alt` pressed while pressing other keys than `Tab`, this will modify the current filter. From then on, pressing `Alt-Tab` will only iterate through those windows that match the filter.

The filter is matched by any window whose name contains the letter you have typed. For instance, if you are currently editing the files `unit1.adb` and `file.adb`, pressing `t` will only leave `unit1.adb` selectable.

## 4.2. Closing Windows

Wherever the windows are displayed, they are always closed in the same manner. In the right side of the title bar of the window, one small button is displayed, looking like a cross. Clicking on this button will close the window.

An alternative way to close the window is to double-click on the icon to the left of the title bar of the window. Not all windows have such an icon, but editors do for instance.

When a window is closed, the focus is given to the window of the same part of the MDI (each of the docks or the middle area) that previously had the focus. Therefore, if you simply open an editor as a result of a cross-reference query, you can simply close that editor to go back to where you were before.

Alternatively, you can also select the window by clicking anywhere in its title bar, and then select the menu Window->Close.

Finally, a window can be closed by right-clicking in the associated notebook tab (if the tabs are visible), and select Close in the contextual menu.

In the notebook tab (when you are in an editor), you will also find a Close all other editors menu, which, as its name implies, will keep a single editor open, the one you are clicking on.

## 4.3. Splitting Windows

Windows can be split at will, through any combination of horizontal and vertical splits. This feature requires at least two windows (text editors, browsers, ...) to be superimposed in the central area. Selecting either the Window->Split Horizontally or Window->Split Vertically menus will then split the selected window in two. In the left (resp. top) pane, the currently selected window will be left on its own. The rest of the previously superimposed windows will be put in the right (resp. bottom) pane. You can then in turn split these remaining windows to achieve any layout you want.

All split windows can be resized interactively by dragging the handles that separate them. A preference (menu Edit->Preferences) controls whether this resizing is done in opaque mode or border mode. In the latter case, only the new handle position will be displayed while the mouse is dragged.

You may want to bind the key shortcuts to the menus Window->Split Horizontally as well as Window->Split Vertically using the key manager. In addition, if you want to achieve an effect similar to e.g. the standard Emacs behavior (where `control-x 2` splits a window horizontally, and `control-x 3` splits a window vertically), you can use the key manager (The Key Manager Dialog).

Moving Windows will show how to do the splitting through drag-and-drop and the mouse, which in general is the fastest way to do.

Several editors or browsers can be put in the same area of the MDI. In such a case, they will be grouped together in a notebook widget, and you can select any of them by clicking on the corresponding tab. Note that if there are lots of windows, two small arrows will appear on the right of the tabs. Clicking on these arrows will show the remaining tabs.

In some cases GPS will change the color and size of the title (name) of a window in the notebook tab. This indicates that the window content has been updated, but the window wasn’t visible. Typically, this is used to indicate that new messages have been written in the messages or console window.

## 4.4. Floating Windows

Although the MDI, as described so far, is already extremely flexible, it is possible that you prefer to have several top-level windows under direct control of your system or window manager. This would be the case for instance if you want to benefit from some extra possibilities that your system might provide (virtual desktops, different window decoration depending on the window’s type, transparent windows, multiple screens, ...).

GPS is fully compatible with this behavior, since windows can also be floating windows. Any window that is currently embedded in the MDI can be made floating at any time, simply by selecting the window and then selecting the menu Window->Floating. The window will then be detached, and can be moved anywhere on your screen, even outside of GPS’s main window.

There are two ways to put a floating window back under control of GPS. The more general method is to select the window through its title in the menu Window, and then unselect Window->Floating.

The second method assumes that the preference Destroy Floats in the menu Edit->Preferences has been set to false. Then, you can simply close the floating window by clicking in the appropriate title bar button, and the window will be put back in GPS. If you actually want to close it, you need to click once again on the cross button in its title bar.

A special mode is also available in GPS, where all windows are floating. The MDI area in the main window becomes invisible. This can be useful if you rely on windows handling facilities supported by your system or window manager but not available in GPS. This might also be useful if you want to have windows on various virtual desktops, should your window manager support this.

This special mode is activated through a preference (menu Edit->Preferences). This preference is entitled All Floating.

## 4.5. Moving Windows

As we have seen, the organization of windows can be changed at any time by selecting a notebook containing several editors or browsers, and selecting one of the Split menus in the Window menu.

A more intuitive method is also provided, based on the drag-and-drop paradigm. The idea is simply to select a window, wherever it is, and then, by clicking on it and moving the mouse while keeping the left button pressed, drop it anywhere else inside GPS.

Selecting an item so that it can be dragged is done simply by clicking with the left mouse button in its title bar, and keep the button pressed while moving the mouse.

If the window is inside a notebook, you can also choose to select the notebook tab to start dragging the window around. In such a case, the windows within the notebook can also be reordered: select the tab, then start moving left or right to the new position the window should have. Note that your mouse must remain within the tab area, since otherwise GPS will enter in the mode where the window can be put in other notebooks.

If you want to move a window to another notebook by dragging its tab, you should first move out of the tab area (vertically in general), and then anywhere in GPS. That’s to distinguish between the mode where you want to reorder tabs and the mode where you want to move windows.

While you keep the mouse button pressed, and move the mouse around, the selected drop area is highlighted with a dashed border. This shows precisely where the window would be put if you were to release the mouse button at that point.

If you move your mouse all the way to the side of the desktop, and then drop the window, that window will occupy the full width (resp. height) of the desktop on that side.

Here are the various places where a window can be dropped:

Inside the MDI
  The location of the current window is indicated by a dashed rectangle, and the window you are dragging will be positioned at the same location as that rectangle: either on top of the window on which you dropped it (therefore they will both be put inside a notebook), or to one of the sides of that window, splitting as needed.
System window
  If you drop a window outside of GPS (for instance, on the background of your screen), the window will be floated.

If you maintain the `shift` key pressed while dropping the window, this might result in a copy operation instead of a simple move. For instance, if you are dropping an editor, a new view of the same editor will be created, resulting in two views present in GPS: the original one is left at its initial location, and a second view is created at the new location.

If you maintain the `control` key pressed while dropping the window, all the windows that were in the same notebook are moved, instead of the single one you selected. This is the fastest way to move a group of windows to a new location, instead of moving them one by one.

## 4.6. Perspectives

GPS supports the concept of perspectives. These are activity-specific desktops, each with their own set of windows, but sharing some common windows like the editors.

Depending on the activity you want to perform (debugging, version control,...) you could switch to another perspective. For instance, in the context of the debugger, the new perspective would by default contain the call stack window, the data window, the debugger consoles,... each at your favorite location. Whenever the debug starts, you therefore do not have to open these windows again.

The perspectives have names, and you switch perspectives by selecting the menu /Window/Perspectives/. You can also create a new perspective by selecting the menu /Window/Perspectives/Create New.

GPS will sometimes automatically change perspectives. For instance, if you start a debugger, it will switch to the perspective called “Debug” (if it exists). When the debugger terminates, you are switched back to the “Default” perspective (again, if it exists).

When you leave a perspective, GPS automatically saves its contents (which windows are opened, their location,...), so that when you are going back to the same perspective you find the same layout.

Likewise, when GPS exits, it will save the layout of all perspectives into a file called `perspectives.xml`, so that it can restore them when you restart GPS. This behavior is controlled by the “Save desktop on exit” preference, and can be disabled.

One of the difficulties in working with perspectives is knowing which windows will be preserved when you switch to another perspective, and which windows will be hidden. There is a central area where all preserved windows are found. Typically, it only contains editors (including if you have split them side by side for instance). If you drag and drop another window on top or to the sides of an editor, that window will be preserved when changing perspectives, unless it was already found elsewhere in the new perspective. The small tooltip that appears on the screen while you drag and drop will tell you whether the window (if dropped at the current location) will be visible in other perspectives or not.

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