前言
android自定义view的详细步骤是我们每一个android开发人员都必须掌握的技能，因为在开发中总会遇到自定义view的需求。为了提高自己的技术水平，自己就系统的去研究了一下，在这里写下一点心得，有不足之处希望大家及时指出。
流程
在android中对于布局的请求绘制是在android framework层开始处理的。绘制是从根节点开始，对布局树进行measure与draw。在rootviewimpl中的performtraversals展开。它所做的就是对需要的视图进行measure(测量视图大小)、layout(确定视图的位置)与draw(绘制视图)。下面的图能很好的展现视图的绘制流程：
当用户调用requestlayout时，只会触发measure与layout，但系统开始调用时还会触发draw
下面来详细介绍这几个流程。
measure
measure是view中的final型方法不可以进行重写。它是对视图的大小进行测量计算，但它会回调onmeasure方法，所以我们在自定义view的时候可以重写onmeasure方法来对view进行我们所需要的测量。它有两个参数widthmeasurespec与heightmeasurespec。其实这两个参数都包含两部分，分别为size与mode。size为测量的大小而mode为视图布局的模式
我们可以通过以下代码分别获取：
int widthsize = measurespec.getsize(widthmeasurespec);
int heightsize = measurespec.getsize(heightmeasurespec);
int widthmode = measurespec.getmode(widthmeasurespec);
int heightmode = measurespec.getmode(heightmeasurespec);
获取到的mode种类分为以下三种：
setmeasureddimension
通过以上逻辑获取视图的宽高，最后要调用setmeasureddimension方法将测量好的宽高进行传递出去。其实最终是调用setmeasureddimensionraw方法对传过来的值进行属性赋值。调用super.onmeasure()的调用逻辑也是一样的。
下面以自定义一个验证码的view为例，它的onmeasure方法如下：
@override
protected void onmeasure(int widthmeasurespec, int heightmeasurespec) {
int widthsize = measurespec.getsize(widthmeasurespec);
int heightsize = measurespec.getsize(heightmeasurespec);
int widthmode = measurespec.getmode(widthmeasurespec);
int heightmode = measurespec.getmode(heightmeasurespec);
if (widthmode == measurespec.exactly) {
//直接获取精确的宽度
width = widthsize;
} else if (widthmode == measurespec.at_most) {
//计算出宽度(文本的宽度+padding的大小)
width = bounds.width() + getpaddingleft() + getpaddingright();
}
if (heightmode == measurespec.exactly) {
//直接获取精确的高度
height = heightsize;
} else if (heightmode == measurespec.at_most) {
//计算出高度(文本的高度+padding的大小)
height = bounds.height() + getpaddingbottom() + getpaddingtop();
}
//设置获取的宽高
setmeasureddimension(width, height);
}
可以对自定义view的layout_width与layout_height进行设置不同的属性，达到不同的mode类型，就可以看到不同的效果
measurechildren
如果你是对继承viewgroup的自定义view那么在进行测量自身的大小时还要测量子视图的大小。一般通过measurechildren(int widthmeasurespec, int heightmeasurespec)方法来测量子视图的大小。
protected void measurechildren(int widthmeasurespec, int heightmeasurespec) {
final int size = mchildrencount;
final view[] children = mchildren;
for (int i = 0; i < size; ++i) {
final view child = children[i];
if ((child.mviewflags & visibility_mask) != gone) {
measurechild(child, widthmeasurespec, heightmeasurespec);
}
}
}
通过上面的源码会发现，它其实是遍历每一个子视图，如果该子视图不是隐藏的就调用measurechild方法，那么来看下measurechild源码：
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);
}
会发现它首先调用了getchildmeasurespec方法来分别获取宽高，最后再调用的就是view的measure方法，而通过前面的分析我们已经知道它做的就是对视图大小的计算。而对于measure中的参数是通过getchildmeasurespec获取，再来看下其源码：
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);
}
是不是容易理解了点呢。它做的就是前面所说的根据mode的类型，获取相应的size。根据父视图的mode类型与子视图的layoutparams类型来决定子视图所属的mode,最后再将获取的size与mode通过measurespec.makemeasurespec方法整合返回。最后传递到measure中，这就是前面所说的widthmeasurespec与heightmeasurespec中包含的两部分的值。整个过程为measurechildren->measurechild->getchildmeasurespec->measure->onmeasure->setmeasureddimension,所以通过measurechildren就可以对子视图进行测量计算。
layout
layout也是一样的内部会回调onlayout方法，该方法是用来确定子视图的绘制位置，但这个方法在viewgroup中是个抽象方法，所以如果要自定义的view是继承viewgroup的话就必须实现该方法。但如果是继承view的话就不需要了，view中有一个空实现。而对子视图位置的设置是通过view的layout方法通过传递计算出来的left、top、right与bottom值，而这些值一般都要借助view的宽高来计算,视图的宽高则可以通过getmeasurewidth与getmeasureheight方法获取，这两个方法获取的值就是上面onmeasure中setmeasureddimension传递的值，即子视图测量的宽高。
getwidth、getheight与getmeasurewidth、getmeasureheight是不同的，前者是在onlayout之后才能获取到的值，分别为left-right与top-bottom;而后者是在onmeasure之后才能获取到的值。只不过这两种获取的值一般都是相同的，所以要注意调用的时机。
下面以定义一个把子视图放置于父视图的四个角的view为例：
@override
protected void onlayout(boolean changed, int l, int t, int r, int b) {
int count = getchildcount();
marginlayoutparams params;
int cl;
int ct;
int cr;
int cb;
for (int i = 0; i < count; i++) {
view child = getchildat(i);
params = (marginlayoutparams) child.getlayoutparams();
if (i == 0) {
//左上角
cl = params.leftmargin;
ct = params.topmargin;
} else if (i == 1) {
//右上角
cl = getmeasuredwidth() - params.rightmargin - child.getmeasuredwidth();
ct = params.topmargin;
} else if (i == 2) {
//左下角
cl = params.leftmargin;
ct = getmeasuredheight() - params.bottommargin - child.getmeasuredheight()
- params.topmargin;
} else {
//右下角
cl = getmeasuredwidth() - params.rightmargin - child.getmeasuredwidth();
ct = getmeasuredheight() - params.bottommargin - child.getmeasuredheight()
- params.topmargin;
}
cr = cl + child.getmeasuredwidth();
cb = ct + child.getmeasuredheight();
//确定子视图在父视图中放置的位置
child.layout(cl, ct, cr, cb);
}
}
至于onmeasure的实现源码我后面会给链接，如果要看效果图的话，我后面也会贴出来，前面的那个验证码的也是一样
draw
draw是由dispatchdraw发动的，dispatchdraw是viewgroup中的方法，在view是空实现。自定义view时不需要去管理该方法。而draw方法只在view中存在，viewgoup做的只是在dispatchdraw中调用drawchild方法，而drawchild中调用的就是view的draw方法。那么我们来看下draw的源码：
public void draw(canvas canvas) {
final int privateflags = mprivateflags;
final boolean dirtyopaque = (privateflags & pflag_dirty_mask) == pflag_dirty_opaque &&
(mattachinfo == null || !mattachinfo.mignoredirtystate);
mprivateflags = (privateflags & ~pflag_dirty_mask) | pflag_drawn;
/*
* draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. draw the background
* 2. if necessary, save the canvas' layers to prepare for fading
* 3. draw view's content
* 4. draw children
* 5. if necessary, draw the fading edges and restore layers
* 6. draw decorations (scrollbars for instance)
*/
// step 1, draw the background, if needed
int savecount;
if (!dirtyopaque) {
drawbackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewflags = mviewflags;
boolean horizontaledges = (viewflags & fading_edge_horizontal) != 0;
boolean verticaledges = (viewflags & fading_edge_vertical) != 0;
if (!verticaledges && !horizontaledges) {
// step 3, draw the content
if (!dirtyopaque) ondraw(canvas);
// step 4, draw the children
dispatchdraw(canvas);
// overlay is part of the content and draws beneath foreground
if (moverlay != null && !moverlay.isempty()) {
moverlay.getoverlayview().dispatchdraw(canvas);
}
// step 6, draw decorations (foreground, scrollbars)
ondrawforeground(canvas);
// we're done...
return;
}
//省略2&5的情况
....
}
源码已经非常清晰了draw总共分为6步;
绘制背景
如果需要的话，保存layers
绘制自身文本
绘制子视图
如果需要的话，绘制fading edges
绘制scrollbars
其中 第2步与第5步不是必须的。在第3步调用了ondraw方法来绘制自身的内容，在view中是空实现，这就是我们为什么在自定义view时必须要重写该方法。而第4步调用了dispatchdraw对子视图进行绘制。还是以验证码为例：
@override
protected void ondraw(canvas canvas) {
//绘制背景
mpaint.setcolor(getresources().getcolor(r.color.autocodebg));
canvas.drawrect(0, 0, getmeasuredwidth(), getmeasuredheight(), mpaint);
mpaint.gettextbounds(autotext, 0, autotext.length(), bounds);
//绘制文本
for (int i = 0; i < autotext.length(); i++) {
mpaint.setcolor(getresources().getcolor(colorres[random.nextint(6)]));
canvas.drawtext(autotext, i, i + 1, getwidth() / 2 - bounds.width() / 2 + i * bounds.width() / autonum
, bounds.height() + random.nextint(getheight() - bounds.height())
, mpaint);
}
//绘制干扰点
for (int j = 0; j < 250; j++) {
canvas.drawpoint(random.nextint(getwidth()), random.nextint(getheight()), pointpaint);
}
//绘制干扰线
for (int k = 0; k < 20; k++) {
int startx = random.nextint(getwidth());
int starty = random.nextint(getheight());
int stopx = startx + random.nextint(getwidth() - startx);
int stopy = starty + random.nextint(getheight() - starty);
linepaint.setcolor(getresources().getcolor(colorres[random.nextint(6)]));
canvas.drawline(startx, starty, stopx, stopy, linepaint);
}
}
图，与源码链接
示例图