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) );
}
}
}