Context
Context是维持Android程序中各组件能够正常工作的一个核心功能类。
弹出Toast、启动Activity、启动Service、发送广播、操作数据库等等都需要用到Context。
TextView tv = new TextView(getContext());
ListAdapter adapter = new SimpleCursorAdapter(getApplicationContext(), ...);
AudioManager am = (AudioManager) getContext().getSystemService(Context.AUDIO_SERVICE);getApplicationContext().getSharedPreferences(name, mode);
getApplicationContext().getContentResolver().query(uri, ...);
getContext().getResources().getDisplayMetrics().widthPixels * 5 / 8;
getContext().startActivity(intent);
getContext().startService(intent);
getContext().sendBroadcast(intent);
Bundle savedInstanceState
onCreate方法的完整定义如下:
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
}
从上面的代码可以看出,onCreate方法的参数是一个Bundle类型的参数。Bundle类型的数据与Map类型的数据相似,都是以key-value的形式存储数据的。
从字面上看savedInstanceState,是保存实例状态的。实际上,savedInstanceState也就是保存Activity的状态的
eg. SnakeView in SnakeGame :
private int[] coordArrayListToArray(ArrayListcvec) {
int count = cvec.size();
int[] rawArray = new int[count * 2];
for (int index = 0; index < count; index++) {
Coordinate c = cvec.get(index);
rawArray[2 * index] = c.x;
rawArray[2 * index + 1] = c.y;
}
return rawArray;
}
public Bundle saveState() {
Bundle map = new Bundle();
map.putIntArray("mAppleList", coordArrayListToArray(mAppleList));
map.putInt("mDirection", Integer.valueOf(mDirection));
map.putInt("mNextDirection", Integer.valueOf(mNextDirection));
map.putLong("mMoveDelay", Long.valueOf(mMoveDelay));
map.putLong("mScore", Long.valueOf(mScore));
map.putIntArray("mSnakeTrail", coordArrayListToArray(mSnakeTrail));
return map;
}
在snakeActivity中实现
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.snake_layout);
mSnakeView = (SnakeView) findViewById(R.id.snake);
mSnakeView.setTextView((TextView) findViewById(R.id.text));
if (savedInstanceState == null) {
// We were just launched -- set up a new game
mSnakeView.setMode(SnakeView.READY);
} else {
// We are being restored
Bundle map = savedInstanceState.getBundle(ICICLE_KEY);
if (map != null) {
mSnakeView.restoreState(map);
} else {
mSnakeView.setMode(SnakeView.PAUSE);
}
}
}
并重写onSavedInstanceState(),此方法会在activity结束时,调用.
@Override
public void onSaveInstanceState(Bundle outState) {
//Store the game state
outState.putBundle(ICICLE_KEY, mSnakeView.saveState());
}