Android踩坑日记：点击变暗效果的ImageView实现原理

很多时候我们使用ImagView显示图片，无论是Gilde，Fresco等图片显示框架，比如设置中心更换头像，网格相册点击预览，选择等情况，会遇到点击变暗的交互需求。

• 源码分析
      我们想的办法是自定义一个ImageView，当点击图片时，是不是有回调方法来同时改变图片的滤镜或者蒙版等。
      特意去看了View.java的源码（ImageView继承View），想看看View被点击之后是是否有回调函数可用。

View的onTouchEvent()方法
case MotionEvent.ACTION_DOWN:  
            mHasPerformedLongPress = false;  
           if (performButtonActionOnTouchDown(event)) {  
                        break;  
              }  
          // Walk up the hierarchy to determine if we're inside a scrolling container. 
          boolean isInScrollingContainer = isInScrollingContainer();  
          // For views inside a scrolling container, delay the pressed feedback for 
          // a short period in case this is a scroll. 
           if (isInScrollingContainer) {  
                 mPrivateFlags |= PFLAG_PREPRESSED;  
                 if (mPendingCheckForTap == null) {  
                            mPendingCheckForTap = new CheckForTap();  
                   }  
                     postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());  
                   } else {  
                        // Not inside a scrolling container, so show the feedback right away 
                        setPressed(true);  
                        checkForLongClick(0);  
                    }  
                    break;  

意思就是case: MotionEvent.ACTION_DOWN,按下去的时候事件发生时，检测View是否被点击了，如果点击了就setPressed(true);把状态标记为已点击
对应的case: MotionEvent.ACTION_UP,松开手的时候会检测是否是unpressed，如果是就setPressed(false);把状态标记为未点击。

setPress(boolean pressed)这个方法，定义如下

  /** * Sets the pressed state for this view. * * @see #isClickable() * @see #setClickable(boolean) * * @param pressed Pass true to set the View's internal state to "pressed", or false to reverts * the View's internal state from a previously set "pressed" state. */
    public void setPressed(boolean pressed) {
        final boolean needsRefresh = pressed != ((mPrivateFlags & PFLAG_PRESSED) == PFLAG_PRESSED);

        if (pressed) {
            mPrivateFlags |= PFLAG_PRESSED;
        } else {
            mPrivateFlags &= ~PFLAG_PRESSED;
        }

        if (needsRefresh) {
            refreshDrawableState();
        }
        dispatchSetPressed(pressed);
    }

就是说View按下时会调用这个方法改变view的状态，那么我们就可以在这个方法中做文章，重写这个方法。当参数是true时，使用颜色矩阵ColorMetrix来改变drawable的滤镜，当参数是false时，还原图像

/*点击变暗效果的ImageView*/
public class MaskImageView extends ImageView {
    private boolean touchEffect = true;
    public final float[] BG_PRESSED = new float[] { 1, 0, 0, 0, -50, 0, 1, 0, 0, -50, 0, 0, 1, 0, -50, 0, 0, 0, 1, 0 };
    public final float[] BG_NOT_PRESSED = new float[] { 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0 };

    public MaskImageView(Context context) {
        super(context);
    }

    public MaskImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MaskImageView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }
    public MaskImageView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    @Override
    public void setPressed(boolean pressed) {
        updateView(pressed);
        super.setPressed(pressed);
    }

    /** * 根据是否按下去来刷新bg和src * * @param pressed */
    private void updateView(boolean pressed){
        //如果没有点击效果
        if( !touchEffect ){
            return;
        }
        if( pressed ){
            /** * 通过设置滤镜来改变图片亮度 */
            this.setDrawingCacheEnabled(true);
            this.setColorFilter( new ColorMatrixColorFilter(BG_PRESSED) ) ;
            //此为src，背景用getBackground()
            this.getDrawable().setColorFilter( new ColorMatrixColorFilter(BG_PRESSED) );
        }else{

            this.setColorFilter( new ColorMatrixColorFilter(BG_NOT_PRESSED) ) ;
            this.getDrawable().setColorFilter( new ColorMatrixColorFilter(BG_NOT_PRESSED) );
        }
    }
}