Android的UI元素都是基于View(屏幕中单个元素)和ViewGroup(元素的集合),Android有许多自带的组件和布局,比如Button、TextView、RelativeLayout。在app开发过程中我们需要自定义视图组件来满足我们的需求。通过继承自View或者View的子类,覆写onDraw或者onTouchEvent等方法来覆盖视图的行为。
创建自定义的组件主要围绕着以下五个方面:
举个栗子,假设我们想创建一个图形允许用户点击的时候改变形状(方形、圆形、三角形)。如下所示:
我们创建一个ShapeSelectorView继承自View,实现必要的构造器,如下所示:
public class ShapeSelectorView extends View { // We must provide a constructor that takes a Context and an AttributeSet. // This constructor allows the UI to create and edit an instance of your view. public ShapeSelectorView(Context context, AttributeSet attrs) { super(context, attrs); } }
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" > <com.codepath.example.customviewdemo.ShapeSelectorView android:id="@+id/shapeSelector" android:layout_width="match_parent" android:layout_height="match_parent" android:layout_alignParentRight="true" android:layout_alignParentTop="true" android:layout_alignParentLeft="true" /> </RelativeLayout>
接下来我们定义一个命名空间app,这个命名空间允许Android自动解析而不需要指定具体的包名。
视图可以通过XML来配置属性和样式,你需要想清楚要添加那些自定义的属性,比如我们想让用户可以选择形状的颜色、是否显示形状的名称,比如我们想让视图可以像下面一样配置:
<com.codepath.example.customviewdemo.ShapeSelectorView app:shapeColor="#7f0000" app:displayShapeName="true" android:id="@+id/shapeSelector" ... />
为了能够定义shapeColor和displayShapeName,我们需要在res/values/attrs.xml中配置:
<?xml version="1.0" encoding="utf-8"?> <resources> <declare-styleable name="ShapeSelectorView"> <attr name="shapeColor" format="color" /> <attr name="displayShapeName" format="boolean" /> </declare-styleable> </resources>
对于每个你想自定义的属性你需要定义attr节点,每个节点有name和format属性,format属性是我们期望的值的类型,比如color,dimension,boolean,integer,float等。一旦定义好了属性,你可以像使用自带属性一样使用他们,唯一的区别在于你的自定义属性属于一个不同的命名空间,你可以在根视图的layout里面定义命名空间,一般情况下你只需要这样子指定: http://schemas.android.com/apk/res/<package_name>
,但是你可以使用 http://schemas.android.com/apk/res-auto
自动解析命名空间。
<?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="match_parent" > <com.codepath.example.customviewdemo.ShapeSelectorView app:shapeColor="#7f0000" app:displayShapeName="true" android:id="@+id/shapeSelector" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_above="@+id/btnSelect" android:layout_alignParentLeft="true" android:layout_below="@+id/tvPrompt" /> </RelativeLayout>
在前面我们定义了shapeColor和displayShapeName两个属性值,我们需要提取这两个属性值来用在自定义的视图中,可以使用TypedArray和obtainStyledAttributes方法来完成,如下所示:
public class ShapeSelectorView extends View { private int shapeColor; private boolean displayShapeName; public ShapeSelectorView(Context context, AttributeSet attrs) { super(context, attrs); setupAttributes(attrs); } private void setupAttributes(AttributeSet attrs) { // Obtain a typed array of attributes TypedArray a = getContext().getTheme().obtainStyledAttributes(attrs, R.styleable.ShapeSelectorView, 0, 0); // Extract custom attributes into member variables try { shapeColor = a.getColor(R.styleable.ShapeSelectorView_shapeColor, Color.BLACK); displayShapeName = a.getBoolean(R.styleable.ShapeSelectorView_displayShapeName, false); } finally { // TypedArray objects are shared and must be recycled. a.recycle(); } } }
接下来添加一些getter和setter方法:
public class ShapeSelectorView extends View { // ... public boolean isDisplayingShapeName() { return displayShapeName; } public void setDisplayingShapeName(boolean state) { this.displayShapeName = state; invalidate(); requestLayout(); } public int getShapeColor() { return shapeColor; } public void setShapeColor(int color) { this.shapeColor = color; invalidate(); requestLayout(); } }
当视图属性发生改变的时候可能需要重新绘图,你需要调用invalidate()和requestLayout()来刷新显示。
假设我们要使用前面的属性画一个长方形,所有的绘图都是在onDraw方法里执行,使用Canvas对象来绘图,如下所示:
public class ShapeSelectorView extends View { // ... private int shapeWidth = 100; private int shapeHeight = 100; private int textXOffset = 0; private int textYOffset = 30; private Paint paintShape; // ... public ShapeSelectorView(Context context, AttributeSet attrs) { super(context, attrs); setupAttributes(attrs); setupPaint(); } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); canvas.drawRect(0, 0, shapeWidth, shapeHeight, paintShape); if (displayShapeName) { canvas.drawText("Square", shapeWidth + textXOffset, shapeHeight + textXOffset, paintShape); } } private void setupPaint() { paintShape = new Paint(); paintShape.setStyle(Style.FILL); paintShape.setColor(shapeColor); paintShape.setTextSize(30); } }
这段代码就会根据XML里设置的shapeColor来画图,根据displayShapeName属性来决定是否显示图形的名称,结果如下图:
更多画图的教程可以参考这里 Custom 2D Drawing Tutorial
为了更好的理解自定义视图的宽度和高度,我们需要定义onMeasure方法,这个方法根据视图的内容来决定它的宽度和高度,在这里宽度和高度是由形状和下面的文本决定的,如下所示:
public class ShapeSelectorView extends View { @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { // Defines the extra padding for the shape name text int textPadding = 10; int contentWidth = shapeWidth; // Resolve the width based on our minimum and the measure spec int minw = contentWidth + getPaddingLeft() + getPaddingRight(); int w = resolveSizeAndState(minw, widthMeasureSpec, 0); // Ask for a height that would let the view get as big as it can int minh = shapeHeight + getPaddingBottom() + getPaddingTop(); if (displayShapeName) { minh += textYOffset + textPadding; } int h = resolveSizeAndState(minh, heightMeasureSpec, 0); // Calling this method determines the measured width and height // Retrieve with getMeasuredWidth or getMeasuredHeight methods later setMeasuredDimension(w, h); } }
宽度和高度都是基于MeasureSpec来讨论的,一个MeasureSpec封装了父布局传递给子布局的布局要求,每个MeasureSpec代表了一组宽度和高度的要求。一个MeasureSpec由大小和模式组成。它有三种模式:UNSPECIFIED(未指定),父元素未给子元素施加任何束缚,子元素可以得到任意想要的大小;EXACTLY(完全),父元素决定子元素的确切大小,子元素将被限定在给定的边界里而忽略它本身大小;AT_MOST(至多),子元素至多达到指定大小的值。resolveSizeAndState()方法根据视图想要的大小和MeasureSpec返回一个合适的值,最后你需要调用setMeasureDimension()方法生效。
如果想实现用户点击之后改变形状,需要在onTouchEvent方法里添加自定义逻辑:
public class ShapeSelectorView extends View { // ... private String[] shapeValues = { "square", "circle", "triangle" }; private int currentShapeIndex = 0; // Change the currentShapeIndex whenever the shape is clicked @Override public boolean onTouchEvent(MotionEvent event) { boolean result = super.onTouchEvent(event); if (event.getAction() == MotionEvent.ACTION_DOWN) { currentShapeIndex ++; if (currentShapeIndex > (shapeValues.length - 1)) { currentShapeIndex = 0; } postInvalidate(); return true; } return result; } }
现在不管什么时候视图被单击,选择的形状的下标会改变,调用postInvalisate()方法后会显示一个不同的形状,接下来更新onDraw()方法来实现更改形状的逻辑:
public class ShapeSelectorView extends View { // ... @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); String shapeSelected = shapeValues[currentShapeIndex]; if (shapeSelected.equals("square")) { canvas.drawRect(0, 0, shapeWidth, shapeHeight, paintShape); textXOffset = 0; } else if (shapeSelected.equals("circle")) { canvas.drawCircle(shapeWidth / 2, shapeHeight / 2, shapeWidth / 2, paintShape); textXOffset = 12; } else if (shapeSelected.equals("triangle")) { canvas.drawPath(getTrianglePath(), paintShape); textXOffset = 0; } if (displayShapeName) { canvas.drawText(shapeSelected, 0 + textXOffset, shapeHeight + textYOffset, paintShape); } } protected Path getTrianglePath() { Point p1 = new Point(0, shapeHeight), p2 = null, p3 = null; p2 = new Point(p1.x + shapeWidth, p1.y); p3 = new Point(p1.x + (shapeWidth / 2), p1.y - shapeHeight); Path path = new Path(); path.moveTo(p1.x, p1.y); path.lineTo(p2.x, p2.y); path.lineTo(p3.x, p3.y); return path; } // ... }
现在每次点击都会显示一个不同的形状,结果如下:
接下来添加一个获取形状的方法:
public class ShapeSelectorView extends View { // ... // Returns selected shape name public String getSelectedShape() { return shapeValues[currentShapeIndex]; } }
当配置发生改变的时候(比如屏幕旋转)视图需要保存它们的状态,你可以实现onSaveInstanceState()和onRestoreInstanceState()方法来保存和恢复视图状态,如下所示:
public class ShapeSelectorView extends View { // This is the view state for this shape selector private int currentShapeIndex = 0; @Override public Parcelable onSaveInstanceState() { // Construct bundle Bundle bundle = new Bundle(); // Store base view state bundle.putParcelable("instanceState", super.onSaveInstanceState()); // Save our custom view state to bundle bundle.putInt("currentShapeIndex", this.currentShapeIndex); // ... store any other custom state here ... // Return the bundle return bundle; } @Override public void onRestoreInstanceState(Parcelable state) { // Checks if the state is the bundle we saved if (state instanceof Bundle) { Bundle bundle = (Bundle) state; // Load back our custom view state this.currentShapeIndex = bundle.getInt("currentShapeIndex"); // ... load any other custom state here ... // Load base view state back state = bundle.getParcelable("instanceState"); } // Pass base view state on to super super.onRestoreInstanceState(state); } }
一旦你实现了这些保存和恢复的逻辑,当手机配置改变的时候你的视图能够自动保存状态。
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