函数原型：
public boolean onKeyDown(int keyCode, KeyEvent event);
第一个参数是用户按下键时，当前所接收到的按键代号；
第二个参数是按键事件的对象。
使用方法：
如果要使用这个方法。直接在主 acivity 中重写即可，一般使用开关语句 switch 来把keyCode 和 （event.按键类型） 对比来执行对应的操作。
下面我们来看下event 的按键属性都有哪些。
在上面的函数中，按住control 加 鼠标左键点击 KeyEvent，就可以点进去看到很多东西。
public class KeyEvent extends InputEvent implements Parcelable {
/** Key code constant: Unknown key code. */
public static final int KEYCODE_UNKNOWN = 0;
/** Key code constant: Soft Left key.
* Usually situated below the display on phones and used as a multi-function
* feature key for selecting a software defined function shown on the bottom left
* of the display. */
public static final int KEYCODE_SOFT_LEFT = 1;
/** Key code constant: Soft Right key.
* Usually situated below the display on phones and used as a multi-function
* feature key for selecting a software defined function shown on the bottom right
* of the display. */
public static final int KEYCODE_SOFT_RIGHT = 2;
/** Key code constant: Home key.
* This key is handled by the framework and is never delivered to applications. */
public static final int KEYCODE_HOME = 3;
/** Key code constant: Back key. */
public static final int KEYCODE_BACK = 4;
/** Key code constant: Call key. */
public static final int KEYCODE_CALL = 5;
/** Key code constant: End Call key. */
public static final int KEYCODE_ENDCALL = 6;
/** Key code constant: '0' key. */
public static final int KEYCODE_0 = 7;
/** Key code constant: '1' key. */
public static final int KEYCODE_1 = 8;
/** Key code constant: '2' key. */
public static final int KEYCODE_2 = 9;
/** Key code constant: '3' key. */
public static final int KEYCODE_3 = 10;
/** Key code constant: '4' key. */
public static final int KEYCODE_4 = 11;
/** Key code constant: '5' key. */
public static final int KEYCODE_5 = 12;
/** Key code constant: '6' key. */
public static final int KEYCODE_6 = 13;
/** Key code constant: '7' key. */
public static final int KEYCODE_7 = 14;
/** Key code constant: '8' key. */
public static final int KEYCODE_8 = 15;
/** Key code constant: '9' key. */
public static final int KEYCODE_9 = 16;
/** Key code constant: '*' key. */
public static final int KEYCODE_STAR = 17;
/** Key code constant: '#' key. */
public static final int KEYCODE_POUND = 18;
/** Key code constant: Directional Pad Up key.
* May also be synthesized from trackball motions. */
public static final int KEYCODE_DPAD_UP = 19;
/** Key code constant: Directional Pad Down key.
* May also be synthesized from trackball motions. */
public static final int KEYCODE_DPAD_DOWN = 20;
/** Key code constant: Directional Pad Left key.
* May also be synthesized from trackball motions. */
public static final int KEYCODE_DPAD_LEFT = 21;
/** Key code constant: Directional Pad Right key.
* May also be synthesized from trackball motions. */
public static final int KEYCODE_DPAD_RIGHT = 22;
/** Key code constant: Directional Pad Center key.
* May also be synthesized from trackball motions. */
public static final int KEYCODE_DPAD_CENTER = 23;
/** Key code constant: Volume Up key.
* Adjusts the speaker volume up. */
public static final int KEYCODE_VOLUME_UP = 24;
/** Key code constant: Volume Down key.
* Adjusts the speaker volume down. */
public static final int KEYCODE_VOLUME_DOWN = 25;
/** Key code constant: Power key. */
public static final int KEYCODE_POWER = 26;
/** Key code constant: Camera key.
* Used to launch a camera application or take pictures. */
public static final int KEYCODE_CAMERA = 27;
/** Key code constant: Clear key. */
public static final int KEYCODE_CLEAR = 28;
/** Key code constant: 'A' key. */
public static final int KEYCODE_A = 29;
/** Key code constant: 'B' key. */
public static final int KEYCODE_B = 30;
/** Key code constant: 'C' key. */
public static final int KEYCODE_C = 31;
/** Key code constant: 'D' key. */
public static final int KEYCODE_D = 32;
/** Key code constant: 'E' key. */
public static final int KEYCODE_E = 33;
/** Key code constant: 'F' key. */
public static final int KEYCODE_F = 34;
/** Key code constant: 'G' key. */
public static final int KEYCODE_G = 35;
/** Key code constant: 'H' key. */
public static final int KEYCODE_H = 36;
/** Key code constant: 'I' key. */
public static final int KEYCODE_I = 37;
/** Key code constant: 'J' key. */
public static final int KEYCODE_J = 38;
/** Key code constant: 'K' key. */
public static final int KEYCODE_K = 39;
/** Key code constant: 'L' key. */
public static final int KEYCODE_L = 40;
/** Key code constant: 'M' key. */
public static final int KEYCODE_M = 41;
/** Key code constant: 'N' key. */
public static final int KEYCODE_N = 42;
/** Key code constant: 'O' key. */
public static final int KEYCODE_O = 43;
/** Key code constant: 'P' key. */
public static final int KEYCODE_P = 44;
/** Key code constant: 'Q' key. */
public static final int KEYCODE_Q = 45;
/** Key code constant: 'R' key. */
public static final int KEYCODE_R = 46;
/** Key code constant: 'S' key. */
public static final int KEYCODE_S = 47;
/** Key code constant: 'T' key. */
public static final int KEYCODE_T = 48;
/** Key code constant: 'U' key. */
public static final int KEYCODE_U = 49;
/** Key code constant: 'V' key. */
public static final int KEYCODE_V = 50;
/** Key code constant: 'W' key. */
public static final int KEYCODE_W = 51;
/** Key code constant: 'X' key. */
public static final int KEYCODE_X = 52;
/** Key code constant: 'Y' key. */
public static final int KEYCODE_Y = 53;
/** Key code constant: 'Z' key. */
public static final int KEYCODE_Z = 54;
/** Key code constant: ',' key. */
public static final int KEYCODE_COMMA = 55;
/** Key code constant: '.' key. */
public static final int KEYCODE_PERIOD = 56;
/** Key code constant: Left Alt modifier key. */
public static final int KEYCODE_ALT_LEFT = 57;
/** Key code constant: Right Alt modifier key. */
public static final int KEYCODE_ALT_RIGHT = 58;
/** Key code constant: Left Shift modifier key. */
public static final int KEYCODE_SHIFT_LEFT = 59;
/** Key code constant: Right Shift modifier key. */
public static final int KEYCODE_SHIFT_RIGHT = 60;
/** Key code constant: Tab key. */
public static final int KEYCODE_TAB = 61;
/** Key code constant: Space key. */
public static final int KEYCODE_SPACE = 62;
/** Key code constant: Symbol modifier key.
* Used to enter alternate symbols. */
public static final int KEYCODE_SYM = 63;
/** Key code constant: Explorer special function key.
* Used to launch a browser application. */
public static final int KEYCODE_EXPLORER = 64;
/** Key code constant: Envelope special function key.
* Used to launch a mail application. */
public static final int KEYCODE_ENVELOPE = 65;
/** Key code constant: Enter key. */
public static final int KEYCODE_ENTER = 66;
/** Key code constant: Backspace key.
* Deletes characters before the insertion point, unlike {@link #KEYCODE_FORWARD_DEL}. */
public static final int KEYCODE_DEL = 67;
/** Key code constant: '`' (backtick) key. */
public static final int KEYCODE_GRAVE = 68;
/** Key code constant: '-'. */
public static final int KEYCODE_MINUS = 69;
/** Key code constant: '=' key. */
public static final int KEYCODE_EQUALS = 70;
/** Key code constant: '[' key. */
public static final int KEYCODE_LEFT_BRACKET = 71;
/** Key code constant: ']' key. */
public static final int KEYCODE_RIGHT_BRACKET = 72;
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下面还很多...
根据英语的提示，我们可以很容易地识别出，这些按键的类型，下面举个例子；
@Override
2 public boolean onKeyDown(int keyCode, KeyEvent event) {
3 // TODO Auto-generated method stub
4
5 if(keyCode==KeyEvent.KEYCODE_BACK){//back，返回键
6 Toast.makeText(this,"你按了返回键",LENGTH_LONG).show();
33 }
35 return super.onKeyDown(keyCode, event);
36 }
上面是简单的一个按键时间。一般要处理很多事件的时候，用 switch - case
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
// TODO Auto-generated method stub
if(event.getRepeatCount()==0) {//限制重复次数，防止用户过多点击
switch(keyCode) {
case KeyEvent.KEYCODE_SEARCH: break;
case KeyEvent.KEYCODE_CAMERA: break;
case KeyEvent.KEYCODE_DPAD_CENTER: break; default:
break;
}
}
return super.onKeyDown(keyCode, event);
}