剖析API Demos中的LabelView - JAVA

ureSpec.getSize(measureSpec);

// 如果view的宽度是确定值，那么直接获取此确定值
if (specMode == MeasureSpec.EXACTLY) {
// We were told how big to be
result = specSize;
} else { // 如果宽度不是确定值，就需要计算下
// 测量text的宽度
result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
+ getPaddingRight();
if (specMode == MeasureSpec.AT_MOST) {
// Respect AT_MOST value if that was what is called for by measureSpec
result = Math.min(result, specSize);
}
}

return result;
}

/**
* 决定这个view的高度
* 注意：字体的高度＝上坡度＋下坡度＋行间距
* ascent和top都是负数
* @param measureSpec A measureSpec packed into an int
* @return The height of the view, honoring constraints from measureSpec
*/
private int measureHeight(int measureSpec) {
int result = 0;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);

mAscent = (int) mTextPaint.ascent();
if (specMode == MeasureSpec.EXACTLY) {
// We were told how big to be
result = specSize;
} else {
// Measure the text (beware: ascent is a negative number)
result = (int) (-mAscent + mTextPaint.descent()) + getPaddingTop()
+ getPaddingBottom();
if (specMode == MeasureSpec.AT_MOST) {
// Respect AT_MOST value if that was what is called for by measureSpec
result = Math.min(result, specSize);
}
}
return result;
}

/**
* Render the text
* 绘制相应的界面，这里是drawText
* @see android.view.View#onDraw(android.graphics.Canvas)
*/
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawText(mText, getPaddingLeft(), getPaddingTop() - mAscent, mTextPaint);
}
}
上面代码里涉及到MeasureSpec对象，该对象包含了Measure's Mode和Size两个属性:

关于谷歌怎么把两个属性包装在一个int里，怎么在一个int里解包获取Mode和size属性，请看MeasureSpec源码

这样做可以节约空间，可以减少更多的对象的创建。

[java]
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;

public static final int UNSPECIFIED = 0 << MODE_SHIFT;
public static final int EXACTLY = 1 << MODE_SHIFT;
public static final int AT_MOST = 2 << MODE_SHIFT;

public static int makeMeasureSpec(int size, int mode) {
return size + mode;
}
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}

public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;

public static final int UNSPECIFIED = 0 << MODE_SHIFT;
public static final int EXACTLY = 1 << MODE_SHIFT;
public static final int AT_MOST = 2 << MODE_SHIFT;

public static int makeMeasureSpec(int size, int mode) {
return size + mode;
}
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}

onMeasure(int widthMeasureSpec, int heightMeasureSpec) 方法，系统在绘制对象时，首先得确定对象在屏幕上占用多大的范围，因此在这个方法中，必须得确定好控件的尺寸然后通过一个特定的函数接口（setMeasuredDimension(width, height)）去通知系统有关该控件的尺寸信息。系统传递进来的两个参数是一个约束条件，控件到底占据多大的尺寸由这两个参数决定，每一个参数其实一个Me

剖析API Demos中的LabelView (五)