Android踩坑日記：點擊變暗效果的ImageView實現原理

很多時候我們使用ImagView顯示圖片，無論是Gilde，Fresco等圖片顯示框架，比如設置中心更換頭像，網格相冊點擊預覽，選擇等情況，會遇到點擊變暗的交互需求。

• 源碼分析
      我們想的辦法是自定義一個ImageView，當點擊圖片時，是不是有回調方法來同時改變圖片的濾鏡或者蒙版等。
      特意去看了View.java的源碼（ImageView繼承View），想看看View被點擊之後是是否有回調函數可用。

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,按下去的時候事件發生時，檢測View是否被點擊了，如果點擊了就setPressed(true);把狀態標記爲已點擊
對應的case: MotionEvent.ACTION_UP,鬆開手的時候會檢測是否是unpressed，如果是就setPressed(false);把狀態標記爲未點擊。

setPress(boolean pressed)這個方法，定義如下

  /**
     * 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);
    }

就是說View按下時會調用這個方法改變view的狀態，那麼我們就可以在這個方法中做文章，重寫這個方法。當參數是true時，使用顏色矩陣ColorMetrix來改變drawable的濾鏡，當參數是false時，還原圖像

/*點擊變暗效果的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);
    }

    /**
     * 根據是否按下去來刷新bg和src
     *
     * @param pressed
     */
    private void updateView(boolean pressed){
        //如果沒有點擊效果
        if( !touchEffect ){
            return;
        }
        if( pressed ){
            /**
             * 通過設置濾鏡來改變圖片亮度
             */
            this.setDrawingCacheEnabled(true);
            this.setColorFilter( new ColorMatrixColorFilter(BG_PRESSED) ) ;
            //此爲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) );
        }
    }
}