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);}@Overridepublic 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) );}} }

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