首先，我們來了解Android是如何繪制頁面的view的。
一、Android中View的繪制過程
Android的頁面結構是一個樹狀結構，是以在繪制的過程是按根節點開始逐級繪制的，在繪制每個節點，也就是每個View的時候，需要完成兩個計算：測量Measure和布局Layout。對應的方法就是measure (int widthMeasureSpec, int heightMeasureSpec)和layout (int l, int t, int r, int b).
measure()方法會計算view所需要的空間，在調用結束，會儲存尺寸資訊在自己的view中。而layout()則是計算view的布局資訊，包括位置，邊距等。
當一個View的Measure()方法傳回的時候，
它的getMeasureWidth()和getMeasureHeight()值必須已經被設定了，并且其子view的對應屬性也一并被設定了。同時，一個view測量的尺寸必須與父view的所給出的限制條件。也就是說，子View在計算自身所需要的空間大小的時候，必須考慮父View的一些對應的限制條件。這樣，對于每一個View，其子View的所有測量結果也已結束。
注意：一個
parent可能不止一次的調用其子View的measure()方法。這是由于在第一次調用的時候，如果所有子view所測量的結果都是未指明具體的大小，可是parent是希望能夠确切的知道其所希望的大小，是以這時候會再次調用measure()方法，不過這次parent會增加一些限制條件，以便能得到具體的大小值。
在計算View所需要的空間大小的時候，會用到
LayoutParams，對應的不同的Layout，所用到的LayoutParams也會有所不同
。在計算View所需要的大小的時候，就隻需要一下三個屬性：
1、MATCH_PARENT(FILL_PARENT) 盡量和parent一樣大
2、WRAP_CONTENT 根據自身所展示的内容來計算大小
3、具體的數值
二、onMeasure()方法
onMeasure()是呈現一個元件與其上層容器至關重要的一個方法。此方法是測量view和其内容所需要的空間大小，這個方法是由measure()方法調用的，隻需要覆寫onMeasure()方法，為其提供更準确的測量值。在複寫onMeasure()方法的時候，必須調用setmeasureDimension()來設定計算出來的width和height值，這樣才能儲存view所包含的的測量資料。如果不調用此方法，将會抛出異常。
onMeasure(int widthMeasureSpec, int heightMeasureSpec)
其中的widthMeasureSpec和heightMeasureSpec是parent所能提供的空間大小。
這兩個參數是按照View.MeasureSpec來進行編碼存儲的，可以通過MeasureSpec來解析，擷取到對應的資料；
每一個MeasureSpec包含了一個Size值和一個Mode值。Size值就是具體的大小，重點說明一下Mode。
總共有三種模式：
UNSPECIFIED：parent沒有做任何限制，子view可以用所有的想要的尺寸；
EXACTLY ：parent為子view提供了一個絕對尺寸的限制，子view将被賦予這些限制；
AT_MOST ：parent會給子view一個尺寸上限，子View可以在這個上限以内定義值。
覆寫onMeasure()方法時，子類有責任確定這個view的最小height和width，也就是getSuggestedMinimumHeight和getSUggestedMinimusWidth();
大家可以參考文檔：
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
官方提供了一個例子，在APIDemo中，搜尋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);
}
}
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