关于自定义组件-以apidemo中的LableView进行讲解

对于自定义组件，可以有两种方式，一种是继承View，重写里面的onDraw（）函数，或者继承ViewGroup或者子类，自定义组合类型的组件。

下面以apidemo为例子进行讲解

下面是代码，我又进行了重新注释，应该好懂点了

1.对于构造函数，单个参数的构造函数是代码new的时候使用的，第二个构造函数是系统反射成对象用的，要想再xml中使用必须有两个参数的那个构造函数。

2.requestLayout();这个方法的作用是把新布局应用上去，但是界面还没刷新。

3.invalidate();这个是用来告诉界面刷新的，调用了这个方法不一定会立刻刷新，会在将来的某一刻刷新，刷新时调用onDraw()方法。

具体的一些细节还没有写完，先写到这里，有时间了再补充上去

了解一下这些api有利于明白自定义组件的过程

view

http://note.youdao.com/share/?id=1fcf0de6c7bbfec507fdb2b0962708ab&type=note

MessureSpec

http://note.youdao.com/share/?id=b992b2b83cd8ff3245ad5609a9499966&type=note

Paint

http://note.youdao.com/share/?id=735734a14ff01d96690f5c845f18ec7a&type=note

/* * 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 int mAscent;    private String mText;        /**     * 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() {    //create text paint to setting text        mTextPaint = new Paint();        mTextPaint.setAntiAlias(true);//设置是否抗锯齿        // 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;        //Call this when something has changed which has invalidated the layout of this view. This will schedule a layout pass of the view tree.         //当有些东西已经改变使这个view的布局已经无效使调用此方法，这个方法会把一个新的布局安排到视图树中去。        requestLayout();        //Invalidate the whole view. If the view is visible, onDraw(android.graphics.Canvas) will be called at some point in the future. This must be called from a UI thread. To call from a non-UI thread, call postInvalidate().         //调用这个方法使当前的整个view显示无效，当这个view可见时，将来的某一时刻会调用onDraw(android.graphics.Canvas)，这个方法必须在UI线程中调用，如果从非UI线程中调用postInvalidate()方法，从而换成新的显示。        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) {    //This mehod must be called by onMeasure(int, int) to store the measured width and measured height. Failing to do so will trigger an exception at measurement time.        //这个方法必须在onMeasure(int, int)中被调用，去存储测量的宽度和高度。如果不调用的话，在测量期间会抛出异常。    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;        //返回测量模式        int specMode = MeasureSpec.getMode(measureSpec);        //返回测量的大小        int specSize = MeasureSpec.getSize(measureSpec);        //如果模式是准确模式的话，大小只能是指定的大小，即通过传过来的模式得到的大小。        if (specMode == MeasureSpec.EXACTLY) {            // We were told how big to be            result = specSize;        } else {            // Measure the text        //计算这个view的大小：size = 左右空隙的大小+字符串的大小            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            //在AT_MOST模式，即最大模式的情况下，size只能小于等于指定的大小，大小取指定大小和计算出的大小中的一个最小的。                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();//返回字体基线上部的大小，返回的是负数        //关于字体的基线，可以看下我的这个翻译http://note.youdao.com/share/?id=735734a14ff01d96690f5c845f18ec7a&type=note        if (specMode == MeasureSpec.EXACTLY) {            // We were told how big to be            result = specSize;        } else {            // Measure the text (beware: ascent is a negative number)        //高度=基线上部的大小+基线下部的大小+上下的空隙            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);    }}