首先，我们来了解Android是如何绘制页面的view的。
一、Android中View的绘制过程
measure (int widthMeasureSpec, int heightMeasureSpec)和layout (int l, int t, int r, int b).
measure()方法会计算view所需要的空间，在调用结束，会储存尺寸信息在自己的view中。而layout()则是计算view的布局信息，包括位置，边距等。
当一个View的Measure()方法返回的时候，它的getMeasureWidth()和getMeasureHeight()值必须已经被设置了，并且其子view的对应属性也一并被设置了。同时，一个view测量的尺寸必须与父view的所给出的限制条件。也就是说，子View在计算自身所需要的空间大小的时候，必须考虑父View的一些对应的限制条件。这样，对于每一个View，其子View的所有测量结果也已结束。
注意：一个parent可能不止一次的调用其子View的measure()方法。这是由于在第一次调用的时候，如果所有子view所测量的结果都是未指明具体的大小，可是parent是希望能够确切的知道其所希望的大小，所以这时候会再次调用measure()方法，不过这次parent会增加一些限制条件，以便能得到具体的大小值。
在计算View所需要的空间大小的时候，会用到LayoutParams，对应的不同的Layout，所用到的LayoutParams也会有所不同。在计算View所需要的大小的时候，就只需要一下三个属性：
1、MATCH_PARENT(FILL_PARENT) 尽量和parent一样大
2、WRAP_CONTENT 根据自身所展示的内容来计算大小
3、具体的数值
二、onMeasure()方法
onMeasure()是呈现一个组件与其上层容器至关重要的一个方法。此方法是测量view和其内容所需要的空间大小，这个方法是由measure()方法调用的，只需要覆盖onMeasure()方法，为其提供更准确的测量值。在复写onMeasure()方法的时候，必须调用setmeasureDimension()来设置计算出来的width和height值，这样才能储存view所包含的的测量数据。如果不调用此方法，将会抛出异常。
int widthMeasureSpec, int heightMeasureSpec)
其中的widthMeasureSpec和heightMeasureSpec是parent所能提供的空间大小。
MeasureSpec来解析，获取到对应的数据；
MeasureSpec包含了一个Size值和一个Mode值。Size值就是具体的大小，重点说明一下Mode。
总共有三种模式：
UNSPECIFIED：parent没有做任何限制，子view可以用所有的想要的尺寸；
EXACTLY ：parent为子view提供了一个绝对尺寸的限制，子view将被赋予这些限制；
AT_MOST ：parent会给子view一个尺寸上限，子View可以在这个上限以内定义值。
覆盖onMeasure()方法时，子类有责任确保这个view的最小height和width，也就是getSuggestedMinimumHeight和getSUggestedMinimusWidth();
大家可以参考文档：
https://developer.android.com/guide/topics/ui/how-android-draws.html
http://developer.android.com/guide/topics/ui/custom-components.html
http://developer.android.com/reference/android/view/View.html#onMeasure%28int,%20int%29
官方提供了一个例子，在APIDemo中，搜索LabelView（com.example.android.apis.view）
/*
* Copyright (C) 2007 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.apis.view;
// Need the following import to get access to the app resources, since this
// class is in a sub-package.
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;
import com.example.android.apis.R;
/**
* Example of how to write a custom subclass of View. LabelView
* is used to draw simple text views. Note that it does not handle
* styled text or right-to-left writing systems.
*
*/
public class LabelView extends View {
private Paint mTextPaint;
private String mText;
private int mAscent;
/**
* Constructor. This version is only needed if you will be instantiating
* the object manually (not from a layout XML file).
* @param context
*/
public LabelView(Context context) {
super(context);
initLabelView();
}
/**
* Construct object, initializing with any attributes we understand from a
* layout file. These attributes are defined in
* SDK/assets/res/any/classes.xml.
*
* @see android.view.View#View(android.content.Context, android.util.AttributeSet)
*/
public LabelView(Context context, AttributeSet attrs) {
super(context, attrs);
initLabelView();
TypedArray a = context.obtainStyledAttributes(attrs,
R.styleable.LabelView);
CharSequence s = a.getString(R.styleable.LabelView_text);
if (s != null) {
setText(s.toString());
}
// Retrieve the color(s) to be used for this view and apply them.
// Note, if you only care about supporting a single color, that you
// can instead call a.getColor() and pass that to setTextColor().
setTextColor(a.getColor(R.styleable.LabelView_textColor, 0xFF000000));
int textSize = a.getDimensionPixelOffset(R.styleable.LabelView_textSize, 0);
if (textSize > 0) {
setTextSize(textSize);
}
a.recycle();
}
private final void initLabelView() {
mTextPaint = new Paint();
mTextPaint.setAntiAlias(true);
// Must manually scale the desired text size to match screen density
mTextPaint.setTextSize(16 * getResources().getDisplayMetrics().density);
mTextPaint.setColor(0xFF000000);
setPadding(3, 3, 3, 3);
}
/**
* Sets the text to display in this label
* @param text The text to display. This will be drawn as one line.
*/
public void setText(String text) {
mText = text;
requestLayout();
invalidate();
}
/**
* Sets the text size for this label
* @param size Font size
*/
public void setTextSize(int size) {
// This text size has been pre-scaled by the getDimensionPixelOffset method
mTextPaint.setTextSize(size);
requestLayout();
invalidate();
}
/**
* Sets the text color for this label.
* @param color ARGB value for the text
*/
public void setTextColor(int color) {
mTextPaint.setColor(color);
invalidate();
}
/**
* @see android.view.View#measure(int, int)
*/
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(measureWidth(widthMeasureSpec),
measureHeight(heightMeasureSpec));
}
/**
* Determines the width of this view
* @param measureSpec A measureSpec packed into an int
* @return The width of the view, honoring constraints from measureSpec
*/
private int measureWidth(int measureSpec) {
int result = 0;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
if (specMode == MeasureSpec.EXACTLY) {
// We were told how big to be
result = specSize;
} else {
// Measure the text
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;
}
/**
* Determines the height of this view
* @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
*
* @see android.view.View#onDraw(android.graphics.Canvas)
*/
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawText(mText, getPaddingLeft(), getPaddingTop() - mAscent, mTextPaint);
}
}
