Android踩坑日记：点击变暗效果的ImageView实现原理

Posted 2022-11-25 tuke_tuke

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) );