再Android中，自定義控制元件是一個非常重要的知識點，而自定義控制元件對Android開發者是一個難點，考驗開發人員對View原理的理解，對於繼承View的自定義控制元件，一般都需要重寫onDraw方法，而且往往需要開發人員能夠掌握Paint這個類。
簡介
The Paint class holds the style and color information about how to draw geometries, text and bitmaps.
Paint：該類儲存了繪製幾何圖形、文字和點陣圖的樣式和顏色資訊。也就是說我們可以使用Paint儲存的樣式和顏色，來繪製圖形、文字和bitmap，這就是Paint的強大之處。接下來我們使用Paint來繪圖，並且看看該類有哪些樣式和顏色。
Paint的使用
使用Paint之前需要初始化
mPaint = new Paint();
設定筆（Paint）的顏色和alpha值：
mPaint.setColor(Color.BLUE);
mPaint.setAlpha(255);
複製程式碼注意alpha的範圍是[0..255]，而不是[0..1]，是一個int值。
設定畫筆的樣式：通過mPaint.setStyle()來設定樣式。
public enum Style {
/**
* Geometry and text drawn with this style will be filled, ignoring all
* stroke-related settings in the paint.
*/
FILL (0),
/**
* Geometry and text drawn with this style will be stroked, respecting
* the stroke-related fields on the paint.
*/
STROKE (1),
/**
* Geometry and text drawn with this style will be both filled and
* stroked at the same time, respecting the stroke-related fields on
* the paint. This mode can give unexpected results if the geometry
* is oriented counter-clockwise. This restriction does not apply to
* either FILL or STROKE.
*/
FILL_AND_STROKE (2);
Style(int nativeInt) {
this.nativeInt = nativeInt;
}
final int nativeInt;
}
複製程式碼總共有三種畫筆的樣式
FILL：填充內容；
STROKE：描邊；
FILL_AND_STROKE：填充內容並描邊。
設定畫筆的寬度
mPaint.setStrokeWidth(50);
設定畫筆的線帽
通過mPaint.setStrokeCap來設定線帽，總共有三種線帽
/**
* The Cap specifies the treatment for the beginning and ending of
* stroked lines and paths. The default is BUTT.
*/
public enum Cap {
/**
* The stroke ends with the path, and does not project beyond it.
*/
BUTT (0),
/**
* The stroke projects out as a semicircle, with the center at the
* end of the path.
*/
ROUND (1),
/**
* The stroke projects out as a square, with the center at the end
* of the path.
*/
SQUARE (2);
private Cap(int nativeInt) {
this.nativeInt = nativeInt;
}
final int nativeInt;
}
複製程式碼BUTT：沒有線帽，預設模式
ROUND：圓形
SQUARE：方形
三種線帽對比：
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
mPaint.setColor(Color.BLUE);
mPaint.setAlpha(255);
//設定畫筆的樣式
mPaint.setStyle(Paint.Style.FILL_AND_STROKE);
//畫筆的寬度
mPaint.setStrokeWidth(50);
mPaint.setStrokeCap(Paint.Cap.SQUARE);//方形
mPaint.setStrokeJoin(Paint.Join.BEVEL);//直線
Path path = new Path();
path.moveTo(100, 100);
path.lineTo(300, 100);
canvas.drawPath(path, mPaint);
mPaint.reset();//重置
mPaint.setColor(Color.RED);
mPaint.setStyle(Paint.Style.FILL_AND_STROKE);
mPaint.setStrokeWidth(50);
mPaint.setStrokeCap(Paint.Cap.ROUND);//圓形
mPaint.setStrokeJoin(Paint.Join.BEVEL);//直線
Path path1 = new Path();
path1.moveTo(100, 200);
path1.lineTo(300, 200);
canvas.drawPath(path1, mPaint);
mPaint.reset();//重置
mPaint.setColor(Color.GREEN);
mPaint.setStyle(Paint.Style.FILL_AND_STROKE);
mPaint.setStrokeWidth(50);
mPaint.setStrokeCap(Paint.Cap.BUTT);//沒有
mPaint.setStrokeJoin(Paint.Join.BEVEL);//直線
Path path2 = new Path();
path2.moveTo(100, 300);
path2.lineTo(300, 300);
canvas.drawPath(path2, mPaint);
}
複製程式碼上面程式碼中有個重置畫筆，這時候需要重新設定畫筆。
設定Join
使用setStrokeJoin方法來設定Join，Join有三種型別：
BEVEL：直線
ROUND：圓角
MITER：銳角
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
mPaint.setColor(Color.BLUE);
mPaint.setAlpha(255);
mPaint.setStyle(Paint.Style.STROKE);//設定畫筆的樣式
mPaint.setStrokeWidth(50);//畫筆的寬度
mPaint.setStrokeCap(Paint.Cap.BUTT);//線帽
mPaint.setStrokeJoin(Paint.Join.BEVEL);
Path path = new Path();
path.moveTo(100, 100);
path.lineTo(300, 100);
path.lineTo(100, 300);
path.close();
canvas.drawPath(path, mPaint);
mPaint.reset();//重置
mPaint.setColor(Color.RED);
mPaint.setStyle(Paint.Style.FILL_AND_STROKE);
mPaint.setStrokeWidth(50);
mPaint.setStrokeCap(Paint.Cap.BUTT);//圓形
mPaint.setStrokeJoin(Paint.Join.ROUND);//圓弧
Path path1 = new Path();
path1.moveTo(100, 400);
path1.lineTo(300, 400);
path1.lineTo(100, 700);
path1.close();
canvas.drawPath(path1, mPaint);
mPaint.reset();//重置
mPaint.setColor(Color.GREEN);
mPaint.setStyle(Paint.Style.FILL_AND_STROKE);
mPaint.setStrokeWidth(50);
mPaint.setStrokeCap(Paint.Cap.BUTT);//沒有
mPaint.setStrokeJoin(Paint.Join.MITER);//銳角
Path path2 = new Path();
path2.moveTo(100, 800);
path2.lineTo(300, 800);
path2.lineTo(100, 1100);
path2.close();
canvas.drawPath(path2, mPaint);
}
複製程式碼以上就是Join三種型別對比。
設定防鋸齒
mPaint.setAntiAlias(true);
如果設定防鋸齒，會損失一定的效能
抖動處理
使用mPaint.setDither()方法，設定是否使用影象抖動處理。會使繪製的圖片等顏色更加的清晰以及飽滿，也是損失效能。
使用Path繪製圖形
點組成線，線組成面，這樣Path可以繪製各種各樣的圖形，可以說是無所不能的了，但是Path也提供了很多方法，來繪製圖形。
文字繪製
上文中，介紹了Paint畫筆，和繪製了一些圖形。但是介紹Paint的時候，我們知道它可以繪製圖形，文字和bitmap，所以Paint是非常強大的了，我們看下Paint是如何繪製文字的。
設定字元之間的間距
setLetterSpacing
設定文字刪除線
mPaint.setStrikeThruText(true);
是否設定下劃線
mPaint.setUnderlineText(true);
設定文字大小
mPaint.setTextSize(textSize);
設定字型型別
mPaint.setTypeface(Typeface.BOLD); // Style public static final int NORMAL = 0;//常規 public static final int BOLD = 1;//粗體 public static final int ITALIC = 2; //斜體 public static final int BOLD_ITALIC = 3;//粗斜體
字型型別有以上四種型別可以設定。
載入自定義字型
Typeface.create(familyName, style)
文字傾斜
mPaint.setTextSkewX(-0.25f);
文字傾斜預設為0，官方推薦的-0.25f是斜體
文字對齊方式
mPaint.setTextAlign(Align.LEFT)
有三種：
public enum Align {
/**
* The text is drawn to the right of the x,y origin
*/
LEFT (0),//左對齊
/**
* The text is drawn centered horizontally on the x,y origin
*/
CENTER (1),//居中
/**
* The text is drawn to the left of the x,y origin
*/
RIGHT (2);//右對齊
private Align(int nativeInt) {
this.nativeInt = nativeInt;
}
final int nativeInt;
}
複製程式碼計算制定長度的字串
int breadText = mPaint.breakText(text, measureForwards, maxWidth, measuredWidth)
注意：字元長度、字元個數、顯示的時候是真實的長度
Rect bounds獲取文字的矩形區域（寬高）
mPaint.getTextBounds(text, index, count, bounds)
mPaint.getTextBounds(text, start, end, bounds)
//獲取文字的寬度，和上面類似，但是是一個比較粗略的結果
float measureText = mPaint.measureText(str);
//獲取文字的寬度，和上面類似，但是是比較精準的。
float[] measuredWidth = new float[10];
//measuredWidth得到每一個字元的寬度；textWidths字元數
int textWidths = mPaint.getTextWidths(str, measuredWidth);
mPaint.getTextWidths(text, start, end, widths)
複製程式碼使用drawText繪製文字
public class PaintView extends View {
private Paint mPaint;
private String text = "你是我世界之光，我心另一半";
public PaintView(Context context) {
this(context,null);
}
public PaintView(Context context, @Nullable AttributeSet attrs) {
this(context, attrs,0);
}
public PaintView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init(context, attrs, defStyleAttr);
}
private void init(Context context, AttributeSet attrs, int defStyleAttr) {
mPaint = new Paint();
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
mPaint.setColor(Color.BLUE);
mPaint.setStyle(Paint.Style.STROKE);//設定畫筆的樣式
mPaint.setStrokeCap(Paint.Cap.BUTT);//線帽
mPaint.setStrokeJoin(Paint.Join.BEVEL);
int top = 100;
int baselineX = 0;
mPaint.setTextSize(50);
mPaint.setTextAlign(Paint.Align.LEFT);
canvas.drawLine(0, top, 2000, top, mPaint);
//文字Metrics
Paint.FontMetrics fontMetrics = mPaint.getFontMetrics();
float baselineY = top - fontMetrics.top;
canvas.drawText(text, baselineX, baselineY, mPaint);
}
}
複製程式碼繪製文字時，還有一個很重要的知識點就是基線的確定,有關drawtext的基線，可以參考《DrawText 基線的確定》這篇文章。