概述
本文主要分享仿今日头条文字渐变效果，效果图如下：
Screenrecorder-2020-07-15-15-47-30-244.gif
实现要点：
- 绘制水平垂直居中的文本
- 采用画布裁剪实现渐变效果
绘制水平垂直居中的文本
文本水平居中
文本水平居中有两种方式：
- 调用Paint的setTextAlign(Paint.Align.CENTER)方法
- 控件宽度 / 2 - 文字宽度 / 2计算文本的x轴坐标
文本垂直居中
文本垂直居中重点在于计算基准线，大致过程如下：
- 获取View的中心位置
- 中心位置下移半个字体的高度
- 上移descent，达到文字的最终位置即BaseLine的位置
代码实现如下：
Paint.FontMetrics fontMetrics = mTextPaint.getFontMetrics();
float baseLine = (getHeight() + fontMetrics.descent - fontMetrics.ascent) / 2 - fontMetrics.descent;
为了方便理解，baseLine的计算，可参考下图：
966618-20170921162908009-1156220555.png
画布裁剪实现渐变效果
文字渐变主要是通过画布裁剪实现的，裁剪渐变部分与不变部分再进行叠加，从而达到渐变的效果，这种实现方式有一个好处是防止过度绘制。
关键代码如下：
//渐变部分
private void drawGradientText(Canvas canvas) {
//保存当前画布
canvas.save();
//文字水平居中方案二：x = getWidth() / 2 - 文字宽度 / 2
float textWidth = mTextPaint.measureText(mText, 0, mText.length());
float x = (getWidth() - textWidth) / 2;
//文字垂直居中：BaseLine的计算 = getHeight() / 2 + 文字高度 / 2 - descent
Paint.FontMetrics fontMetrics = mTextPaint.getFontMetrics();
float textHeight = fontMetrics.descent - fontMetrics.ascent;
float y = (getHeight() + textHeight) / 2 - fontMetrics.descent;
mTextPaint.setColor(Color.RED);
//先裁剪渐变区域文字要显示的区域
float right = x + textWidth * mPercent;
canvas.clipRect(x, 0, right, getHeight());
//裁剪完成后再绘制文字
canvas.drawText(mText, x, y, mTextPaint);
//恢复画布
canvas.restore();
}
//不变部分
private void drawNormalText(Canvas canvas) {
canvas.save();
float textWidth = mTextPaint.measureText(mText, 0, mText.length());
float x = (getWidth() - textWidth) / 2;
Paint.FontMetrics fontMetrics = mTextPaint.getFontMetrics();
float textHeight = fontMetrics.descent - fontMetrics.ascent;
float y = (getHeight() + textHeight) / 2 - fontMetrics.descent;
mTextPaint.setColor(Color.BLACK);
//对不变区域文字进行裁剪，防止过度绘制(这个很重要)
float left = x + textWidth * mPercent;
canvas.clipRect(left, 0, x + textWidth, getHeight());
canvas.drawText(mText, x, y, mTextPaint);
canvas.restore();
}
仿今日头条文字渐变效果实现思路
知道了文本绘制居中与画布裁剪后，实现仿今日头条文字渐变效果就很简单了，大致思路是通过监听ViewPager的滚动获取滚动的百分比，再通过滚动百分比计算出渐变区域与不变区域并分别进行裁剪，最后通过图层叠效果就出来了。
渐变控件关键代码：
public class ColorChangeTextView extends AppCompatTextView {
private int mTextSize = sp2px(30);
private int mTextColor = Color.BLACK;
private int mTextColorChange = Color.RED;
private TextPaint mTextPaint;
private int mDirection = LEFT;//文件渐变方向
protected static final int LEFT = 0;
protected static final int RIGHT = 1;
@IntDef(value = {LEFT, RIGHT})
@Retention(RetentionPolicy.SOURCE)
public @interface Direction {
}
private float mProgress;//文字渐变的进度
public float getProgress() {
return mProgress;
}
public void setProgress(float mProgress) {
this.mProgress = mProgress;
invalidate();
}
public void setDirection(@Direction int direction) {
this.mDirection = direction;
}
public ColorChangeTextView(Context context) {
this(context, null);
}
public ColorChangeTextView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public ColorChangeTextView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
initAttr(context, attrs);
init();
}
private void initAttr(final Context context, @Nullable final AttributeSet attrs) {
TypedArray ta = context.obtainStyledAttributes(attrs,
R.styleable.ColorChangeTextView);
mTextSize = ta.getDimensionPixelSize(
R.styleable.ColorChangeTextView_text_size, mTextSize);
mTextColor = ta.getColor(
R.styleable.ColorChangeTextView_text_color, mTextColor);
mTextColorChange = ta.getColor(
R.styleable.ColorChangeTextView_text_color_change, mTextColorChange);
mProgress = ta.getFloat(R.styleable.ColorChangeTextView_progress, 0);
ta.recycle();
}
private void init() {
mTextPaint = new TextPaint(TextPaint.ANTI_ALIAS_FLAG);
mTextPaint.setColor(mTextColor);
mTextPaint.setTextSize(mTextSize);
}
@Override
protected void onDraw(Canvas canvas) {
switch (mDirection){
case LEFT:
drawLeft(canvas);
break;
case RIGHT:
drawRight(canvas);
break;
}
}
private void drawLeft(Canvas canvas){
//绘制底部不变部分
canvas.save();
String text = getText().toString();
float textWidth = mTextPaint.measureText(text);
Paint.FontMetrics fontMetrics = mTextPaint.getFontMetrics();
float textHeight = fontMetrics.descent - fontMetrics.ascent;
float x = (getWidth() - textWidth) / 2;
float baseLine = (getHeight() + textHeight) / 2 - fontMetrics.descent;
float left = x + textWidth * mProgress;
canvas.clipRect(left,0,x + textWidth,getHeight());
mTextPaint.setColor(mTextColor);
canvas.drawText(text,x,baseLine,mTextPaint);
canvas.restore();
//绘制顶部渐变部分
canvas.save();
float right = x + textWidth * mProgress;
canvas.clipRect(x,0,right,getHeight());
mTextPaint.setColor(mTextColorChange);
canvas.drawText(text,x,baseLine,mTextPaint);
canvas.restore();
}
private void drawRight(Canvas canvas) {
//绘制顶部渐变部分
canvas.save();
String text = getText().toString();
float textWidth = mTextPaint.measureText(text);
Paint.FontMetrics fontMetrics = mTextPaint.getFontMetrics();
float textHeight = fontMetrics.descent - fontMetrics.ascent;
float baseLine = (getHeight() + textHeight) / 2 - fontMetrics.descent;
float x = (getWidth() - textWidth) / 2;
float left = x + textWidth * (1 - mProgress);
float right = x + textWidth;
canvas.clipRect(left,0,right,getHeight());
mTextPaint.setColor(mTextColorChange);
canvas.drawText(text,x,baseLine,mTextPaint);
canvas.restore();
//绘制底部不变部分
canvas.save();
right = x + textWidth * (1 - mProgress);
canvas.clipRect(0,0,right,getHeight());
mTextPaint.setColor(mTextColor);
canvas.drawText(text,x,baseLine,mTextPaint);
canvas.restore();
}
static int sp2px(float sp) {
return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, sp, Resources.getSystem().getDisplayMetrics());
}
}
ViewPager滚动的关键代码：
mViewPager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener(){
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
if(positionOffset > 0){
ColorChangeTextView left = mTabs.get(position);
ColorChangeTextView right = mTabs.get(position + 1);
left.setDirection(ColorChangeTextView.RIGHT);
right.setDirection(ColorChangeTextView.LEFT);
left.setProgress(1- positionOffset);
right.setProgress(positionOffset);
}
}
});
完整代码实现
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