Android自定义视图教程

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。

创建完全自定义的组件

创建自定义的组件主要围绕着以下五个方面:

  • 绘图(Drawing): 控制视图的渲染,通常通过覆写onDraw方法来实现
  • 交互(Interaction): 控制用户和视图的交互方式,比如OnTouchEvent,gestures
  • 尺寸(Measurement): 控制视图内容的维度,通过覆写onMeasure方法
  • 属性(Attributes): 在XML中定义视图的属性,使用TypedArray来获取属性值
  • 持久化(Persistence): 配置发生改变时保存和恢复状态,通过onSaveInstanceState和onRestoreInstanceState

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:

定义视图类

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);
    }
}

添加视图到布局中

<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。

自定义属性

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>

应用自定义属性

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。

画图

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

计算尺寸

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。

不同形状之间切换

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];
  }
}

保存视图的状态

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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