Android应用开发经常使用知识

在其它站点看到的，Mark一下

1、近期打开的应用不在近期任务列表中显示

android:excludeFromRecents="true"

设置为true,则排除在近期任务列表之外，不在近期任务列表中显示

2、推断一个一个String str 是否为NULL或者是否为空字符串

TextUtils.isEmpty(str)

3、android:imeOptions="actionSearch|flagNoFullscreen"的使用方法

在做一个把EditText放到到ActionBar中作为搜索框的功能时，设置EditText的属性为android:imeOptions="actionSearch"，会遇到一个问题。当在横屏时。EditText的宽度会填充掉屏幕上除了软键盘之外的地方，与需求不符，改为android:imeOptions="actionSearch|flagNoFullscreen"后就OK了。

4、改变图片亮度的方法

1、使用image.setColorFilter(Color.GRAY,PorterDuff.Mode.MULTIPLY);能够使图片变暗。然后使用image.clearColorFilter();清除滤镜，恢复到原来的亮度。
2、使用
int brightness = -80;
ColorMatrix matrix = new ColorMatrix(); 
matrix.set(new float[] { 1, 0, 0, 0, brightness, 0, 1, 0, 0, 
brightness, 0, 0, 1, 0, brightness, 0, 0, 0, 1, 0 }); 
v.setColorFilter(new ColorMatrixColorFilter(matrix));
但这样的方法会使颜色不太正常。图片留有黑边。

5、用Handler来实现有时间间隔事件的推断

看到Android中GestureDetector.java是用以下代码实现手势的单击和双击推断的：

public boolean onTouchEvent(MotionEvent ev) {
    ……
        case MotionEvent.ACTION_DOWN:
            if (mDoubleTapListener != null) {
                boolean hadTapMessage = mHandler.hasMessages(TAP);
                if (hadTapMessage) mHandler.removeMessages(TAP);
                if ((mCurrentDownEvent != null) && (mPreviousUpEvent != null) && hadTapMessage &&
                        isConsideredDoubleTap(mCurrentDownEvent, mPreviousUpEvent, ev)) {
                    // This is a second tap
                    mIsDoubleTapping = true;
                    // Give a callback with the first tap of the double-tap
                    handled |= mDoubleTapListener.onDoubleTap(mCurrentDownEvent);
                    // Give a callback with down event of the double-tap
                    handled |= mDoubleTapListener.onDoubleTapEvent(ev);
                } else {
                    // This is a first tap
                    mHandler.sendEmptyMessageDelayed(TAP, DOUBLE_TAP_TIMEOUT);
                }
            }
    ……
}
 
    private boolean isConsideredDoubleTap(MotionEvent firstDown, MotionEvent firstUp,
            MotionEvent secondDown) {
        if (!mAlwaysInBiggerTapRegion) {
            return false;
        }
 
        final long deltaTime = secondDown.getEventTime() - firstUp.getEventTime();
        if (deltaTime > DOUBLE_TAP_TIMEOUT || deltaTime < DOUBLE_TAP_MIN_TIME) {
            return false;
        }
 
        int deltaX = (int) firstDown.getX() - (int) secondDown.getX();
        int deltaY = (int) firstDown.getY() - (int) secondDown.getY();
        return (deltaX * deltaX + deltaY * deltaY < mDoubleTapSlopSquare);
    }
 
    private class GestureHandler extends Handler {
        GestureHandler() {
            super();
        }
 
        GestureHandler(Handler handler) {
            super(handler.getLooper());
        }
 
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
            case SHOW_PRESS:
                mListener.onShowPress(mCurrentDownEvent);
                break;
                 
            case LONG_PRESS:
                dispatchLongPress();
                break;
                 
            case TAP:
                // If the user's finger is still down, do not count it as a tap
                if (mDoubleTapListener != null) {
                    if (!mStillDown) {
                        mDoubleTapListener.onSingleTapConfirmed(mCurrentDownEvent);
                    } else {
                        mDeferConfirmSingleTap = true;
                    }
                }
                break;
 
            default:
                throw new RuntimeException("Unknown message " + msg); //never
            }
        }