View programming (5): Research on source code of custom view_01_apidemo

In apidemo provided by Android, there is an attrs. xml file under/RES/values.

The content is as follows:

<?xml version="1.0" encoding="utf-8"?><!-- 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 athttp://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.--><resources>    <!-- These are the attributes that we want to retrieve from the theme         in app/PreferencesFromCode.java -->    <declare-styleable name="TogglePrefAttrs">        <attr name="android:preferenceLayoutChild" />    </declare-styleable>        <!-- These are the attributes that we want to retrieve from the theme         in view/Gallery1.java -->    <declare-styleable name="Gallery1">        <attr name="android:galleryItemBackground" />    </declare-styleable>         <declare-styleable name="LabelView">        <attr name="text" format="string" />        <attr name="textColor" format="color" />        <attr name="textSize" format="dimension" />    </declare-styleable></resources>

At that time, I was very confused. I don't know what this is ???!!! But today we need to use this knowledge, so let's look at it.

In apidemo, The customview1.java file in the com. example. Android. APIs. View package is as follows:

setContentView(R.layout.custom_view_1);

So let's take a look at this routine provided by Google!

1. customview. Java

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 com.example.android.apis.R;import android.app.Activity;import android.os.Bundle;/** * Demonstrates creating a Screen that uses custom views. This example uses * {@link com.example.android.apis.view.LabelView}, which is defined in * SDK/src/com/example/android/apis/view/LabelView.java. *  */public class CustomView1 extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.custom_view_1);    }}

The code is simple, that is, loading an XML file as a display view.
2. custom_view_1.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"        xmlns:app="http://schemas.android.com/apk/res/com.example.android.apis"        android:orientation="vertical"        android:layout_width="fill_parent"        android:layout_height="wrap_content">        <com.example.android.apis.view.LabelView            android:background="@drawable/red"            android:layout_width="fill_parent"            android:layout_height="wrap_content"             app:text="Red"/>        <com.example.android.apis.view.LabelView            android:background="@drawable/blue"            android:layout_width="fill_parent"            android:layout_height="wrap_content"             app:text="Blue" app:textSize="20dp"/>        <com.example.android.apis.view.LabelView            android:background="@drawable/green"            android:layout_width="fill_parent"            android:layout_height="wrap_content"             app:text="Green" app:textColor="#ffffffff" /></LinearLayout>

We can see that the XML file uses the custom control labelview, but you can see that there is a strange tag app. What is this?

It turns out that this is a custom property file. So we can customize attributes. However, the property file must be attrs. xml under Res/vaules.

Note:

 xmlns:app="http://schemas.android.com/apk/res/com.example.android.apis"

The app comes from here.

3. attrs. xml

<resources>    <!-- These are the attributes that we want to retrieve from the theme         in app/PreferencesFromCode.java -->    <declare-styleable name="TogglePrefAttrs">        <attr name="android:preferenceLayoutChild" />    </declare-styleable>        <!-- These are the attributes that we want to retrieve from the theme         in view/Gallery1.java -->    <declare-styleable name="Gallery1">        <attr name="android:galleryItemBackground" />    </declare-styleable>         <declare-styleable name="LabelView">        <attr name="text" format="string" />        <attr name="textColor" format="color" />        <attr name="textSize" format="dimension" />    </declare-styleable></resources>

4. labelview. Java

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);        mTextPaint.setTextSize(16);        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) {        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);    }}

Running effect:

Call the callback method:

D/mark    ( 2924): LabelView(Context context, AttributeSet attrs) is invoked!D/mark    ( 2924): LabelView(Context context, AttributeSet attrs) is invoked!D/mark    ( 2924): LabelView(Context context, AttributeSet attrs) is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onDraw() is invoked!D/mark    ( 2924): onDraw() is invoked!D/mark    ( 2924): onDraw() is invoked!

Note:

D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!D/mark    ( 2924): onMeasure() is invoked!

That is, the onmeasure () method is called back six times. So why? Please pay attention to view programming (6): Custom view_02_apidemo source code research blog.