自定义View中测量流程
自定义View
View的测量是从子View开始的，具体代码顺序是：父容器的onMeasure方法会遍历到每一个子childView -->然后调用measureChild()方法
作者：征途n_y_q
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
getChildMeasureSpec方法如下：
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
measureChild方法中最后的child.measure(childWidthMeasureSpec, childHeightMeasureSpec)会调用child的onMeasure方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
至此子view测量完毕，并setMeasuredDimension方法记录。
自定义View为什么设置了wrap_content，仍然撑满父view 代码如下
public class MyTextView extends View {
private Paint paint;
private Rect rect;
private String text;
private int color;
private float textSize;
public MyTextView(Context context) {
this(context, null);
}
public MyTextView(Context context, @Nullable AttributeSet attrs) {
this(context, attrs, 0);
}
public MyTextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.MyTextView);
text = typedArray.getString(R.styleable.MyTextView_text);
color = typedArray.getColor(R.styleable.MyTextView_textColor, context.getResources().getColor(R.color.white));
textSize = typedArray.getDimension(R.styleable.MyTextView_textSize, 10);
initPaint(context);
}
private void initPaint(Context context) {
paint = new Paint();
paint.setColor(color);
paint.setAntiAlias(true);
paint.setTextSize(textSize);
rect = new Rect();
paint.getTextBounds(text, 0, text.length(), rect);
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawText(text, 0, rect.height(), paint);
}
private int getStringHeight(String str) {
Paint.FontMetrics fr = paint.getFontMetrics();
return (int) Math.ceil(fr.descent - fr.top) + 2; //ceil() 函数向上舍入为最接近的整数。
}
}
此时显示效果
看View中的onMeasure方法：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
从getDefaultSize方法中可以看到当childMeasureSpecMode=MeasureSpec.AT_MOST或MeasureSpec.EXACTLY时候result=childMeasureSpecSize，那么对应的childMeasureSpecSize到底是多少呢，再看ViewGroup中的getChildMeasureSpec方法
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
可以看出来，
1、当childMeasureSpecMode=MeasureSpec.AT_MOST时候，对应的childMeasureSpecSize=Math.max(0, specSize - padding),其实就是parentMeasureSpecSize-padding，即为父容器除了padding外的所有空间，这样就导致子view充满了父view；
2、当childMeasureSpecMode= MeasureSpec.EXACTLY时候，childMeasureSpecSize=childDimension (此时是子view布局文件里面属性是固定的dp值) 或者当子view布局属性是MATCH_PARENT时候那么childMeasureSpecSize=Math.max(0, parentSpecSize - padding) 即充满了整个屏幕。
##怎么解决
重写自定义view的onMeasure方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
if (heightMode == MeasureSpec.AT_MOST) {
heightSize = getStringHeight(text);
}
if (widthMode == MeasureSpec.AT_MOST) {
widthSize = rect.width();
}
setMeasuredDimension(widthSize,heightSize);
}
private int getStringHeight(String str) {
Paint.FontMetrics fr = paint.getFontMetrics();
return (int) Math.ceil(fr.descent - fr.top) + 2; //ceil() 函数向上舍入为最接近的整数。
}
这样当childMeasureSpecMode=MeasureSpec.AT_MOST时候，重新设置宽高，及文字的整体宽高，这样即可解决。
效果如下：
作者：征途n_y_q
