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#### 20.8.1 Key Sequence Input

The command loop reads input a key sequence at a time, by calling `read-key-sequence`. Lisp programs can also call this function; for example, `describe-key` uses it to read the key to describe.

Function: read-key-sequence prompt &optional continue-echo dont-downcase-last switch-frame-ok command-loop

  This function reads a key sequence and returns it as a string or vector. It keeps reading events until it has accumulated a complete key sequence; that is, enough to specify a non-prefix command using the currently active keymaps. (Remember that a key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer.)

  If the events are all characters and all can fit in a string, then `read-key-sequence` returns a string (see Strings of Events). Otherwise, it returns a vector, since a vector can hold all kinds of events—characters, symbols, and lists. The elements of the string or vector are the events in the key sequence.

  Reading a key sequence includes translating the events in various ways. See Translation Keymaps.

  The argument prompt is either a string to be displayed in the echo area as a prompt, or `nil`, meaning not to display a prompt. The argument continue-echo, if non-`nil`, means to echo this key as a continuation of the previous key.

  Normally any upper case event is converted to lower case if the original event is undefined and the lower case equivalent is defined. The argument dont-downcase-last, if non-`nil`, means do not convert the last event to lower case. This is appropriate for reading a key sequence to be defined.

  The argument switch-frame-ok, if non-`nil`, means that this function should process a `switch-frame` event if the user switches frames before typing anything. If the user switches frames in the middle of a key sequence, or at the start of the sequence but switch-frame-ok is `nil`, then the event will be put off until after the current key sequence.

  The argument command-loop, if non-`nil`, means that this key sequence is being read by something that will read commands one after another. It should be `nil` if the caller will read just one key sequence.

  In the following example, Emacs displays the prompt ‘`?`’ in the echo area, and then the user types `C-x C-f`.

      (read-key-sequence "?")

      ---------- Echo Area ----------
      ?C-x C-f
      ---------- Echo Area ----------

           ⇒ "^X^F"

  The function `read-key-sequence` suppresses quitting: `C-g` typed while reading with this function works like any other character, and does not set `quit-flag`. See Quitting.

Function: read-key-sequence-vector prompt &optional continue-echo dont-downcase-last switch-frame-ok command-loop

  This is like `read-key-sequence` except that it always returns the key sequence as a vector, never as a string. See Strings of Events.

If an input character is upper-case (or has the shift modifier) and has no key binding, but its lower-case equivalent has one, then `read-key-sequence` converts the character to lower case. Note that `lookup-key` does not perform case conversion in this way.

When reading input results in such a shift-translation, Emacs sets the variable `this-command-keys-shift-translated` to a non-`nil` value. Lisp programs can examine this variable if they need to modify their behavior when invoked by shift-translated keys. For example, the function `handle-shift-selection` examines the value of this variable to determine how to activate or deactivate the region (see handle-shift-selection).

The function `read-key-sequence` also transforms some mouse events. It converts unbound drag events into click events, and discards unbound button-down events entirely. It also reshuffles focus events and miscellaneous window events so that they never appear in a key sequence with any other events.

When mouse events occur in special parts of a window, such as a mode line or a scroll bar, the event type shows nothing special—it is the same symbol that would normally represent that combination of mouse button and modifier keys. The information about the window part is kept elsewhere in the event—in the coordinates. But `read-key-sequence` translates this information into imaginary prefix keys, all of which are symbols: `header-line`, `horizontal-scroll-bar`, `menu-bar`, `mode-line`, `vertical-line`, and `vertical-scroll-bar`. You can define meanings for mouse clicks in special window parts by defining key sequences using these imaginary prefix keys.

For example, if you call `read-key-sequence` and then click the mouse on the window’s mode line, you get two events, like this:

    (read-key-sequence "Click on the mode line: ")
         ⇒ [mode-line
             (mouse-1
              (#<window 6 on NEWS> mode-line
               (40 . 63) 5959987))]

Variable: num-input-keys

  This variable’s value is the number of key sequences processed so far in this Emacs session. This includes key sequences read from the terminal and key sequences read from keyboard macros being executed.

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